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Thread: Birdwing double dory catamaran

  1. #1
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    Default Birdwing double dory catamaran

    Quote Originally Posted by TerryLL View Post
    Best of luck Ken. Is this really a retirement, or just a pause before you jet off on another venture?
    I have been looking at this double dory catamaran model I designed and built for twenty-seven years now and lately I've been thinking about actually building it full sized. Since its just two dory hulls strapped together, it should go together quickly and cheaply.



    In 2008 I showed the model to Phil Bolger at the Mystic Seaport WoodenBoat Show outside the restaurant right before we all went in to the Bolger tribute dinner. Mr. Bolger said that my double dory reminded him of his catamaran oyster carrier. He asked about the sunshade and I explained how easily I get sunburned. He said that with all that wetted surface you may not need any daggerboards. He also said, “I like it.” As other people demanded his attention and he turned, I floated away on cloud nine with his last three words. My longtime hero naval architect had said he liked something that I had designed. That is something that no one can ever take away from me.

    In truth, I can’t take much credit for the design because basically it is just two instant boats strapped together. It was my “cheapness” that decided to use the 2’ high sides because that’s basically what you get when you rip a ply sheet down the middle. The boat, after that decision, pretty much designed itself. I just stepped down the bow and the stern decks somewhat to eliminate some of the windage. I am certainly no naval architect – just an inventive model builder. Here it is from an old black and white print when it still had it's sail rig.



    Many is the night that this double dory model has successfully navigated the storm tossed covers of my bed. What we are talking about here is only seven or eight sheets of plywood and all the connecting arms are straight solid laminations (fir). I could buy everything I need for the build here in town. With a nod towards James Wharram and the Polynesians, she could be lashed together for flexible strength. Using the sail rig from another boat (I think I have a birdwing mast laying around somewhere) and local marine fir, I should be able to build the basic boat for about $2000 – thus the name Down and Dirty Double Dory. I also have this crazy idea that my life is more important than the life of a cheap plywood boat so I'm inclined to pour enough foam into the design to make her unsinkable and self-bailing. She could live on a mooring and be ready to sail at the drop of a hat.

    It would be neat to have both the boat and the rig be my design. I really think I should build it. It should give Ray's Slider a good run for the money. I'm retired. What else am I gonna do???



    Quote Originally Posted by James McMullen View Post
    Add in a little bit more rocker aft and then build that thang! As Captain Aubrey might say, "Waste not a moment!". If it turns out you need a daggerboard after all, which I suspect you do as Bolger was far more tolerant of mediocre windward performance than many of us have grown used to, then you can certainly add that in later.
    Hey James,

    Thanks for the encouragement. I agree she would tack better and sail better in light air with more rocker aft but I'm interested in that funky physics that multihulls exhibit where they sail faster than their hulls should allow – that slippery gray area that no one can quite explain. I'm also hopeful that in the right conditions she will plane with one hull flying but we will see about that. As things are, the sides are 2' x 16' with no need to cut on the bottom edge at all. The top edges are notched down at the bow and the stern to reduce windage. Also I have the idea to build it exactly like the model because that's what Bolger said he liked and I want to find out how she would perform as designed. With my little 4HP four-stroke running, it would also be interesting to see how well it would motorsail. Can't let her heel too much with the motor running because it's a fourstroke and you don't want to starve the valves of oil but sailed flat with a motor assist would probably yield some efficient numbers for gasoline mileage. And many times, early morning when the fish are biting, there ain't no wind anyways. Anyone know how much heel angle a little 4HP four-stroke can take and still operate correctly (getting oil to the valve train)?

    There will definitely be daggerboards in use for windward work. With the poured in foam on the inside of the bottom, a daggerboard box can be ripped apart on a hard grounding and not affect the boat's ability to float.

    The decision I'm really struggling with is what kind of plywood to use. I wanted to do the boat entirely in North American native wood (Douglas fir) but I'm having trouble finding 1/4" marine fir. I could do the whole boat in 3/8" fir ply but by the time I pour the foam in on the inside and glass the outsides, 1/4" fir ply would have been plenty stiff enough and better at keeping the weight down. I may switch back to mahogany ply because I really like how my existing tan bark sails look with sapelle mahogany and I already have six sheets or so of sapelle laying around from the last boat I almost built.
    Last edited by kenjamin; 12-09-2010 at 10:59 AM.

  2. #2
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    Default Re: Birdwing double dory catamaran

    Why pour in foam? Watertight bulkheads with access hatches will give the same effect, plus some dry stowage and all at less weight. Also, you'll be able to inspect the entire hull inside and out. That my only comment. Now just build the thing.

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    Default Re: Birdwing double dory catamaran

    Hey Woxbox,

    It's the old surfer dude in me that likes foam. There's something tremendously positive and undeniable about foam flotation. I just believe that the darn boat will outlast me even with the poured-in foam that complicates maintenance. If this boat were to be only asked to daysail in protected waters then I think closed-air/storage chambers would be fine for that but I may be wanting this boat to go out pretty far in the Atlantic to harvest grouper so I want to know for sure no matter what I hit in open water and no matter how hard I ground out a daggerboard – the darn thing will float me above the threat of hypothermia and sharks. It's not that I don't like sharks. I just don't want the darn things munching on me.

    You're sure right about the other thing, though, I should just shut up and build it.

    The garage is almost all cleaned up now and ready and most of the plywood and epoxy is just sitting there waiting. My cottage deal in St. Augustine fell through for now (went into probate) but I will wait for it to come out of probate and then purchase it but it looks like I'll have at least a couple of months or more here in Tallahassee to do nothing except build a boat. How fast do you guys think I can slap it together? It will be basic dory construction with fir chine logs with epoxy, lots of ringed bronze boat nails and some bronze screws. The beams holding it all together will be solid fir laminations with many, slightly less than 3/8" thick x 5.5" x ~8' layers. The deck and beams are flat to accept a small tent for camping. Should be fun! Ray will probably want to race me in his Slider. They should end up being quite similar.
    Last edited by kenjamin; 09-19-2010 at 04:47 PM.

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    Default Re: Birdwing double dory catamaran

    Have you sailed in Ray's Slider yet? I bet that would be wonderfully instructive in refining your plans since that really does seem to be a quite similar sort of boat. Ray seems super friendly--I'd be surprised if he didn't welcome the chance to help you out in making your model into reality.
    If this post did not meet all of your needs, please consult this thread for more options.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by James McMullen View Post
    Have you sailed in Ray's Slider yet? I bet that would be wonderfully instructive in refining your plans since that really does seem to be a quite similar sort of boat. Ray seems super friendly--I'd be surprised if he didn't welcome the chance to help you out in making your model into reality.
    I've spoken with Ray before about his boat and the possibility of using a birdwing mast on it. He didn't want to because I think it was too experimental for him to put on a already experimental boat. I've read with great interest much of what Ray had to say about his boat and I've even watched some of his videos of Slider sailing. It would be great fun to take both boats out to compare them. As I said before, I have sentimental reasons and curiosity about building it exactly as Mr. Bolger saw it. He said he liked it and that's good enough for me. It may motorsail better than it sails, it may not tack well, it may not be able to carry much but it will build quickly and I'll have fun adapting Xena's sails to it. As with all boats, it will have a unique set of compromises that make up its design. It will be interesting to see what it does best. It should be somewhat faster than Xena when there's plenty of wind to work with. That's my major hope for the design. Plus it will be one heck of a stable platform to fish from and darn efficient at trolling speed. The fact that it will be unsinkable and self-bailing should add to its charm.

  6. #6
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    Default Re: Birdwing double dory catamaran

    k,

    I think your model is quite interesting and eminently doable.

    The one thing that struck me was your airfoil/sunshade structure. If the vertical elements aren't serving some important secondary purpose, I'd be thinking about replacing them (even as cool as that whole thing looks) with something more lightweight, supporting a larger (more shade) canvas top.

    I'm also an old surfer dude, but would still be disinclined to pour in foam for flotation. It has too great a track record of rot-promotion. My attitude is that, no matter how 'quick&dirty' my intentions are, I still want a boat that'll be the best I can make it given the constraints of time & budget. Part of 'best' is long lived. Who knows... this could end up being a vessel of historical significance!

    But yes... settle the details and build the thing. I'll be quite interested to see/hear how it turns out.
    David G
    Harbor Woodworks
    http://www.harborwoodworking.com/boat.html

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  7. #7
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    Default Re: Birdwing double dory catamaran

    Looks good, though the sunshade will definitely be unnecessary weight. Alternately, you might build a solid dodger with sitting headroom if your planning on taking it a little ways off the coast, but that would add windage...

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    Default Re: Birdwing double dory catamaran

    Just an idea for you.
    In 1944 a children’s book club sent a volume about penguins to a 10-year-old girl, enclosing a card seeking her opinion.
    She wrote, “This book gives me more information about penguins than I care to have.”
    American diplomat Hugh Gibson called it the finest piece of literary criticism he had ever read.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by David G View Post
    k,

    The one thing that struck me was your airfoil/sunshade structure. If the vertical elements aren't serving some important secondary purpose, I'd be thinking about replacing them (even as cool as that whole thing looks) with something more lightweight, supporting a larger (more shade) canvas top.

    I'm also an old surfer dude, but would still be disinclined to pour in foam for flotation. It has too great a track record of rot-promotion. My attitude is that, no matter how 'quick&dirty' my intentions are, I still want a boat that'll be the best I can make it given the constraints of time & budget. Part of 'best' is long lived. Who knows... this could end up being a vessel of historical significance!

    But yes... settle the details and build the thing. I'll be quite interested to see/hear how it turns out.
    The sunshade thingie will probably have several functions. I may use it to hoist and lower daggerboards and it may also help step the main mast. I want the sunshade to be hollow and air-foil shaped and stiff enough to be well secured even in a blow. From how the model flexes, the sunshade also plays a part in stiffening the whole boat.

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    Default Re: Birdwing double dory catamaran


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    Default Re: Birdwing double dory catamaran


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    Default Re: Birdwing double dory catamaran

    If you're going to reinvent the wheel, start with a round one. Study the Wharram designs thoroughly. They're well thought out both in concept and in detail.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    ... I wanted to do the boat entirely in North American native wood (Douglas fir) but I'm having trouble finding 1/4" marine fir. I could do the whole boat in 3/8" fir ply but by the time I pour the foam in on the inside and glass the outsides, 1/4" fir ply would have been plenty stiff enough and better at keeping the weight down....
    Plenty of 1/4 inch marine fir around here. Glass both sides and it's very stiff...Woods' Janus uses dory hulls




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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    ...It's the old surfer dude in me that likes foam. There's something tremendously positive and undeniable about foam flotation. ...
    --- Or, install low-cost inspection hatches in the watertight chambers, and put in chunks of closed cell foam through the hatches -- thus they are removable for inspection. Or, inflate a float bag inside the chambers (easy in, easy out). Glue in small strips of wood to hold any inuslation from much contact with the floor and the airspace helps ventilate any trapped moisture when you store the boat and open the inspection hatches. I do this on my outrigger's vaka and ama. My center chamber is big (my vaka is much the same proportions of your proposed catamaran hull) with a Bomar hatch that is big enough to easily stuff a sleeping bag and large duffle through, so flotation chunks or small anchor buoys (dual-use flotation) could go in there fairly well if not packed for cruising. My ends are dedicated float-chambers being too narrow for much storage except for some spare line and light stuff. -- Wade

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    Default Re: Birdwing double dory catamaran

    I see the difference between using foam or air chambers as a study in priorities. With air chambers, the ultimate survivability of the boat is the priority. While air chambers can be planned so that they contain flotation, a major rip could possibly release some of that flotation. Of course with a catamaran, you'll likely to still have a perfectly good hull to float you but a hull breach could put a major dent in your day. Quarter inch thick plywood is not that difficult to puncture or rip open when there's just air on the other side of it. With foam, the human life aboard is the priority. The boat may die sooner than me but at my age (61), it will be a neck and neck race. There's also the fact that the foam would be structural to the boat adding stiffness throughout its length and a major rip or puncture to the hull will have very little effect on the performance of the boat. As someone who can't resist a good experiment, I should probably build one hull with air chambers, storage and inspection ports and pour some foam in the other.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by JimConlin View Post
    If you're going to reinvent the wheel, start with a round one. Study the Wharram designs thoroughly. They're well thought out both in concept and in detail.
    Good advice. I have studied the Wharram catamarans. They are very able craft obviously. The old trick of lashing multihulls together to give them a flexible yet solid unit strength is a good one.

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    Default Re: Birdwing double dory catamaran

    So...are you going to get after it? I'm doing a similar build once my football season is over (4-0 so far) with a pair of Tamanu hulls by Dierking. I think you might be in an interesting position regarding your dory hulls there. the flatter the run the better powerboat it will be. The more rocker you throw in there, the better sailboat it will be. Which do you favor? With Xena's rig she won't be too powerful compared to most cats, so you might think about where you want to end up. Terrible light air performance would make for a bummer of a way to find you wished for more rocker.

    Dan

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    Default Re: Birdwing double dory catamaran

    I'm starting a chant:

    Rock-er!
    Rock-er!
    Rock-er!
    Rock. . . .

    Don't make a boat
    That's shaped like a Block!
    If this post did not meet all of your needs, please consult this thread for more options.

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    Default Re: Birdwing double dory catamaran

    I'm not wearing my glasses today - didn't spot any numbers on this thing. Got to start with the numbers: how long, how wide, both over all and individually, draft, displacement, estimated load, estimated built weight.

    There isn't anything all that interestingly new about a dory shaped multi. Which might mean that it would be beneficial to stay within the proven parameters unless there is a dramatic reason to do otherwise. For flat bottoms I normally design as close to circular shapes as possible, which is to say the draft would be in the area of half the beam. The greats, like Newick and John Marples have tended to design in more proportional draft, for instance:

    http://www.seaworthysolutions.net/f/...tudy_Plans.pdf

    Getting the ability to exceed hull speed is mostly a mater of minimizing bow wave formation which is done by making the shape as slippery as possible, planing is not the easy way there. Generally the multihull advantage is that they don't need to plane to go fast. Funky boats with poor likely lift/drag are really taking the hard route if they want to plane.

  20. #20

    Default Re: Birdwing double dory catamaran

    I'll have to agree with James that a little more rocker would be helpful.

    With Slider, I needed extra rocker over what might be considered optimal for a fast multi, just to get enough displacement for two people and camping outfit in under 16 feet. This turned out a lot better than expected, because it made Slider unusually handy. She tacks as reliably as a monohull, even under main alone. It doesn't seem to have hurt her speed under sail; she's turned out to be pretty fast for a heavy little cat with a very modest rig.

    I guess Slider is more dory-like than dory. For comparison, here's the first page of the plans, which I put online in lieu of study plans.




    If you follow the link to the larger image, you'll see that I think 4mm ply would do fine for planking.

    If you take a similar approach, take the advice here not to use pour-in foam. The stuff is evil. Slider has 4 compartments separate from the main cockpits, and even if a shark ripped off the bows of both hulls, she'd still float pretty high. Remember that we're talking about a wooden boat with no ballast, so how's it going to sink?

  21. #21
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    Default Re: Birdwing double dory catamaran

    Guys,

    It may be that my little (16') double dory sails terribly. It's strength may be the way it responds to my little 4HP motor. I already have a Caledonia Yawl which sails on a hint of a breeze so the sailing characteristics of this boat are not what interest me most. Try to think of this double dory as more of a motorsailing fishing machine and then maybe you could be a little more excited about the set of design compromises that the boat offers. I'm pretty sure that the boat will sail good enough to troll for mackerel. I'm pretty sure that early in the morning when there's no wind anyway, that my double dory will respond well to the little four horse four stroke Yamaha that I already have. I'm pretty sure that inspection ports for air chambers would give access to low storage and if the air chambers were loaded correctly, they would probably always keep the boat floating. The thing is if I poured the foam in, I know it would always float because it wouldn't have a choice in the matter. There is a difference between "it will probably float" and "it will definitely float" especially if your foolish enough to be twenty miles out into the Atlantic in this boat.

    The problem for a fishing boat is that it has to carry a lot of extra gear that a typical daysailer would not. Tackle boxes are heavy and you need to carry extra ice to ice down your catch. There will most certainly be a live well or two so that's in effect adding water ballast that has be taking into account. So these dory hulls have quite a bit of volume to them to serve better in the role of fishing boat. Also the hulls are cheap. They could be made into flower planters while the next set of hulls are being built and tested. It's all about curiosity. We already know that more rocker will probably make them tack better and sail better but what do we know about these hulls with less rocker? Will they motorsail better? Will they provide a very stable fishing platform? Why did Phil Bolger look carefully at the model of this double dory and then conclude, "I like it"? These are the questions I want answered.

  22. #22

    Default Re: Birdwing double dory catamaran



    My daughter after a day of fishing in Slider.

    Fishing in Slider

    She's been fishing for a long time.



    Just for fun.

    Nothing wrong with a motorized cat for fishing, but if that's the way you'd want to go, it might be more rewarding to go all motorboat, and avoid the frustrations of trying to make a motorboat sail well.

    On the other hand, I can't really understand the degree of optimism it takes to go out on the Gulf with a single outboard, so I like having a way to get back if the mechanical gods frown upon my endeavors.

  23. #23
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    Default Re: Birdwing double dory catamaran

    i
    Quote Originally Posted by slidercat View Post



    On the other hand, I can't really understand the degree of optimism it takes to go out on the Gulf with a single outboard, so I like having a way to get back if the mechanical gods frown upon my endeavors.
    Hey Ray,

    You hit the nail on the head! The more ways to power yourself and crew home, the better. I've got a couple of extra tall oarlocks and a pair of 11' 6" oars to borrow from Xena so I'll probably play with them later to see how well they can be applied to moving my double dory. Sure wouldn't be pleasant rowing but if the wind dies down to nothing and the motor breaks, maybe rowing would be more comfortable than paddling.

    Thanks for all the great information on fishing from a sailboat. I've fished all my life from the beach and from small outboard powered dory/skiffs but not so much from a sailboat. I did catch a nice sheepshead from Xena one time but I don't think we even took the mast with us that day.

    Hopefully I'll be moving to St. Augustine by the end of the year so it will be a little harder to get your boat and mine together for some comparison testing. Even though they will be similar, there will be quite a bit of difference to the hulls with Slider's having more rocker and less volume to sail better. Then there's the birdwing thing that I'll be dealing with. I'll probably use stays on the double dory so I can more safely tap into the power of large headsails and maybe even a spinnaker. I'm looking forward to playing with it all. Any chance you could make it over to St. Augustine for some sailing, photography and video? Heck, we might even catch some fish!

  24. #24
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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    Guys,

    It may be that my little (16') double dory sails terribly. It's strength may be the way it responds to my little 4HP motor. I already have a Caledonia Yawl which sails on a hint of a breeze so the sailing characteristics of this boat are not what interest me most. Try to think of this double dory as more of a motorsailing fishing machine and then maybe you could be a little more excited about the set of design compromises that the boat offers. I'm pretty sure that the boat will sail good enough to troll for mackerel. I'm pretty sure that early in the morning when there's no wind anyway, that my double dory will respond well to the little four horse four stroke Yamaha that I already have. I'm pretty sure that inspection ports for air chambers would give access to low storage and if the air chambers were loaded correctly, they would probably always keep the boat floating. The thing is if I poured the foam in, I know it would always float because it wouldn't have a choice in the matter. There is a difference between "it will probably float" and "it will definitely float" especially if your foolish enough to be twenty miles out into the Atlantic in this boat.

    The problem for a fishing boat is that it has to carry a lot of extra gear that a typical daysailer would not. Tackle boxes are heavy and you need to carry extra ice to ice down your catch. There will most certainly be a live well or two so that's in effect adding water ballast that has be taking into account. So these dory hulls have quite a bit of volume to them to serve better in the role of fishing boat. Also the hulls are cheap. They could be made into flower planters while the next set of hulls are being built and tested. It's all about curiosity. We already know that more rocker will probably make them tack better and sail better but what do we know about these hulls with less rocker? Will they motorsail better? Will they provide a very stable fishing platform? Why did Phil Bolger look carefully at the model of this double dory and then conclude, "I like it"? These are the questions I want answered.
    All the reasons you outlined--light air performance, being able to take weight well in a dory hull form, only using a 4 horse motor, etc ALL argue for more rocker. Now if you were going for 20 hp, a long straight run would be preferable at the detriment for slow trolling or sailing. Since you will not be setting any speed records with this machine, throw a bit of rocker in the hulls and go fishing. As for the foam, I'd skip it. If you insist, wear some safety gear as you don't want that toxic goop on anything & try to keep the stuff dry! Remember trying to surf waterlogged tankers? Performance is killed by that extra poundage.

    As for why Bolger liked it? It's a cool model. Did you ask him how rocker in a sailing/low power cat might affect performance? If not he might have been saying it's a cool model. Since these boats have been built since dimensional lumber came into existence in Hawaii, there's a lot of information about what makes one go. If you are looking at 20' or so, check out Richard Wood's new design at www.sailingcatamarans.com for some ideas about rocker. Remember the more weight you throw at it, the bigger the rocker needs to be/wider the hulls need to be to handle that weight. Here's his assymetric skoota hull. His 21' Acorn design is viewable on his latest designs page as well.


    Perhaps come at the problem in reverse. Weigh your gear, weigh yourself and estimate your companions, estimate your boat weight, and have a go at one of the free programs that will tell you if it'll float that weight. Draggy transoms are not going to help you either with a 4 hp or sailing. Either way, Build it! Will you be trailering it or docking/mooring it?

    Dan
    Last edited by Dan St Gean; 09-23-2010 at 10:10 AM.

  25. #25

    Default Re: Birdwing double dory catamaran

    Kenjamin, I love St. Augustine, but haven't been there in a long time. It might be a nice trip.

    But by the time you get the dory cat built, I might have another dory cat to sail besides Slider. I've been scribbling up a 20 foot cat with cabins and sliding beams, and yesterday I bought a rig from a Nacra 5.2, so as soon as I get the back porch roof rebuilt, I'm going to start butchering wood.

  26. #26
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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by Dan St Gean View Post
    All the reasons you outlined--light air performance, being able to take weight well in a dory hull form, only using a 4 horse motor, etc ALL argue for more rocker. Now if you were going for 20 hp, a long straight run would be preferable at the detriment for slow trolling or sailing. Since you will not be setting any speed records with this machine, throw a bit of rocker in the hulls and go fishing. As for the foam, I'd skip it. If you insist, wear some safety gear as you don't want that toxic goop on anything & try to keep the stuff dry! Remember trying to surf waterlogged tankers? Performance is killed by that extra poundage.

    As for why Bolger liked it? It's a cool model. Did you ask him how rocker in a sailing/low power cat might affect performance? If not he might have been saying it's a cool model. Since these boats have been built since dimensional lumber came into existence in Hawaii, there's a lot of information about what makes one go.
    Dan
    Hello Dan,

    Thanks for your input. I agree that if I were worried about light air sailing performance that I should add more rocker. I agree that adding more horsepower would certainly make it plane and probably be a more practical fishing boat. Here's the thing. I already own a 4 HP motor and it is about as large a motor as I want to mess with. At 47 lbs. I can lift it off by myself and carry it anywhere. It sips gasoline like I was the president of the Sierra Club. I think the 4HP is light enough to carry with me and still give me a decent sailing experience under just sail alone and I'm very interested in what this boat will do in the motorsailing mode. It's unexplored territory and I want to explore it. I could always add more horsepower to get her to plane and each hull is basically three sheets of plywood so they're not a big deal to do over if I decided I really wanted more rocker. As for my 4HP, it is a short shaft so I'm probably going to have to invest in a swing down motor mount but it will still be cheaper than buying a new long shaft motor and I won't ever have to handle more than the 47 lbs. of my present motor.

    It's been said that there's no bad boats, only inappropriate uses of boat designs. Here I have a design that I'm exploring. I want find out what it can do and what it can't. I want to learn from it. At $2000, it's a cheap education. My hope is that it will look neat, need no more than a 4HP motor, be stable to fish from, rowable in a pinch and have an unusual hard sunshade that is structural to the boat and helps keep this very white skin of mine from burning. It only has to take an old fart fishing, fulfill a twenty seven year old desire that the model of it be built full sized, and be an unusual original boat design for an unusual original sailing rig.

    I'll eventually buy a trailer for it but I will get it to the water initially in pieces – one hull on my truck's racks, one hull on my existing boat trailer, and everything else in the truck bed. The hulls will be self bailing so it could live on a mooring but I'd rather buy it its own trailer so I can keep an eye on it better. Hopefully, if all goes well, I'll be living around the corner from a county maintained boat ramp right on the Matanzas River near the Route 312 bridge.

    Thanks again for your comments.
    Last edited by kenjamin; 09-23-2010 at 03:08 PM.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by slidercat View Post
    Kenjamin, I love St. Augustine, but haven't been there in a long time. It might be a nice trip.

    But by the time you get the dory cat built, I might have another dory cat to sail besides Slider. I've been scribbling up a 20 foot cat with cabins and sliding beams, and yesterday I bought a rig from a Nacra 5.2, so as soon as I get the back porch roof rebuilt, I'm going to start butchering wood.
    So the St. Augustine trip is at least a possibility? That would be great! I used to be a licensed tour guide to the city and I was born and raised there so I could really show you around, plus we could get some good action shots of our boats. Stay tuned in.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    It's been said that there's no bad boats, only inappropriate uses of boat designs.
    I don't know who said that, but they were wrong. Totally and unequivocally wrong. Some boats are better than others. Some boats are very much better than some others. Some boats are unbelievably dreadful and deserve nothing more than to be euthanized before they can murder somebody. There is such a different range of proportions and shapes that are optimal compromises for competing features that I can see how someone who is untrained and uncritical in evaluating such things might be fooled into thinking that "anything goes", but a bit of study will demonstrate that that is really very much not the case.
    If this post did not meet all of your needs, please consult this thread for more options.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    ...The thing is if I poured the foam in, I know it would always float because it wouldn't have a choice in the matter. There is a difference between "it will probably float" and "it will definitely float" especially if your foolish enough to be twenty miles out into the Atlantic in this boat.
    ....
    I've seen the damned stuff sink. - so much for know.
    Complicated problems usually have simple solutions - which are almost always wrong.

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    Default Re: Birdwing double dory catamaran

    For floatation, I've been thinking you could drop a fishing net in the space, and fill it up with empty water and soda bottles, then tie the net closed. If you got holed, the net would keep the bottles from floating away. Plus, you could call the boat green because you'd be using recycled materials. ;-)
    -- Tom (boating blog)

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by James McMullen View Post
    I don't know who said that, but they were wrong. Totally and unequivocally wrong. Some boats are better than others. Some boats are very much better than some others. Some boats are unbelievably dreadful and deserve nothing more than to be euthanized before they can murder somebody. There is such a different range of proportions and shapes that are optimal compromises for competing features that I can see how someone who is untrained and uncritical in evaluating such things might be fooled into thinking that "anything goes", but a bit of study will demonstrate that that is really very much not the case.
    James, that quote came from Annie Hill's book, "Voyaging on a Small Income". She mentions the guy's name who said it but I'm not sure what page it's on. I totally agree with you about good, better, and best boats. Thank God and Iain Oughtred that I'll have a Caledonia Yawl to compare to my little my double dory. I agree with you to that they are actually bad boats too. Raw Faith comes to mind.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by tomlarkin View Post
    For floatation, I've been thinking you could drop a fishing net in the space, and fill it up with empty water and soda bottles, then tie the net closed. If you got holed, the net would keep the bottles from floating away. Plus, you could call the boat green because you'd be using recycled materials. ;-)
    Now that is a damned good idea!

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    Now that is a damned good idea!
    And lots lighter than foam, won't rot, could be used for storage of other items in a pinch, etc.

    You keep saying that this will be a motor sailor, and your motor is a 4 hp. Why not design your hulls not to drag their transom? That'll eat up a lot of HP right there and you don't have much to spare. I love the design in other respects though. if you pull in the transom to nearly double ender, it may not matter. On my double Tamanu it didn't matter that we were not trimmed great or stern down since we were on a double ender.

    Check http://wikiproa.pbworks.com/Texas-200-report for a few shots of that 20' boat. In other situations a transom is a nice thing to have though--just so it's not dragging too badly.


    Dan

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    Default Re: Birdwing double dory catamaran

    [QUOTE=Tomcat;2723444]The greats, like Newick and John Marples have tended to design in more proportional draft, for instance:

    http://www.seaworthysolutions.net/f/...tudy_Plans.pdf

    Getting the ability to exceed hull speed is mostly a mater of minimizing bow wave formation which is done by making the shape as slippery as possible, planing is not the easy way there. QUOTE]

    Two of the Seaclipper 20 hulls would be a great little power dory with big capacity. The Wood's Acorn or Skua might be a good place to check for rocker as well. Another thing that made me wonder is all the crossbeams on your model. That'll really break up your cockpits.

    Dan

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by Dan St Gean View Post
    You keep saying that this will be a motor sailor, and your motor is a 4 hp. Why not design your hulls not to drag their transom? That'll eat up a lot of HP right there and you don't have much to spare. I love the design in other respects though. if you pull in the transom to nearly double ender, it may not matter. On my double Tamanu it didn't matter that we were not trimmed great or stern down since we were on a double ender.
    Check http://wikiproa.pbworks.com/Texas-200-report for a few shots of that 20' boat. In other situations a transom is a nice thing to have though--just so it's not dragging too badly.
    Dan
    [QUOTE=Dan St Gean;2725169]
    Quote Originally Posted by Tomcat View Post
    The greats, like Newick and John Marples have tended to design in more proportional draft, for instance:
    http://www.seaworthysolutions.net/f/...tudy_Plans.pdf
    Getting the ability to exceed hull speed is mostly a mater of minimizing bow wave formation which is done by making the shape as slippery as possible, planing is not the easy way there. QUOTE]
    Two of the Seaclipper 20 hulls would be a great little power dory with big capacity. The Wood's Acorn or Skua might be a good place to check for rocker as well. Another thing that made me wonder is all the crossbeams on your model. That'll really break up your cockpits.
    Dan
    Hey Dan,

    I may find that I want more horsepower but like I said, I have nostalgic reasons for wanting to build the model exactly as it is full sized. I've been looking at the damned thing for twenty seven years (honest) and it's what Phil Bolger said he liked at the 2007 Woodenboat Mystic Seaport Show. One thing that is difficult to describe but seems to work well is the way the model flexes in my hands. The hard sunshade feature which the model was designed around, does seem to add to the stiffness of the entire boat. Also the way the connecting beams are distributed logarithmically rather than linearly gives a sort of hard spot where the mast is stepped but allows for more movement aft. This I think will allow the boat to adjust somewhat to a wave as it moves through the boat without hurting the stiffness of the rig. You can feel how this works together when flexing the model – very stiff in the leading edge where the mast is stepped and progressively more flexibility towards the stern. Keep in mind that this cat is an experiment. Experiments do not have to succeed to be successful, they only need to yield data to increase understanding so that eventually a successful design can be achieved. You can crunch numbers until you're blue in the face but until you have a working prototype in real world conditions, it's really difficult to see how it will all work together. I see all the crossbeams as seats to fish from so I don't mind their numbers. There may eventually be panels that drop in between the crossbeams and add to the deck space but they may add to much weight. We'll just have to wait and see on that. Of course there will already be enough deck space to pitch a small tent for camping.
    Last edited by kenjamin; 09-25-2010 at 04:57 PM.

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    Default Re: Birdwing double dory catamaran

    Ken I cant tell from your models photos, are the transoms dory transoms, in other words are the hulls double ended on the bottom? this may alieviate some of the need for the rocker every one's mentioning. one of the concerns with a flat bottom is the transom will drag, if it's double ended on the bottom this wont be so bad. The Alpha dory has about 2 inches of rocker in her bottom so a relatively flat bottom for a dory is not new and not necesssarily terribly detrimental, but if the transom has any width at the bottom like a skiff then history shows rocker is a very good idea.

    also Boldger would likely have tweaked your design a bit if you hired him to consult on it so dont' be affraid to change things a little on good advice

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by Daniel Noyes View Post
    ...dont' be affraid to change things a little on good advice
    Ditto/+1/Amen.wink.gif

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by Daniel Noyes View Post
    Ken I cant tell from your models photos, are the transoms dory transoms, in other words are the hulls double ended on the bottom? this may alieviate some of the need for the rocker every one's mentioning. one of the concerns with a flat bottom is the transom will drag, if it's double ended on the bottom this wont be so bad. The Alpha dory has about 2 inches of rocker in her bottom so a relatively flat bottom for a dory is not new and not necesssarily terribly detrimental, but if the transom has any width at the bottom like a skiff then history shows rocker is a very good idea.

    also Boldger would likely have tweaked your design a bit if you hired him to consult on it so dont' be affraid to change things a little on good advice
    Thanks guys for your comments,

    Just measured and took some new pictures of the transoms. They appear to be 7 inches across the bottom of each transom. Not double ended but pretty skinny nonetheless. The sides flare out to 15 inches across at the tops of each of the transoms. The total height of each transom is 20 and 1/2 inches – that's along the vertical centerline.

    I did think ahead a little bit when I built the model and drew a grid on the inside of the sides every one inch. This will help me reproduce the model accurately and faithfully full-sized which is still the plan at present. It's not that I'm afraid to change it, I just don't want to. I've been wondering all these years how this catamaran would perform if built full-sized and looks like I'll be able to find out this year. If curiosity kills this cat then you guys get to say, "I told you so" but with a fairly flat run across the length of the bottoms, I'll probably be able to adjust the size of the outboard to convince her to plane. I'll be trying to get by with my existing 4 HP fourstroke but my fishing buddy has got an 8HP if it's clear it needs more power. Maybe a 6HP would be perfect. Testing with the 4HP and then with the 8HP should tell me what I need to know about that. The performance with the outboard is more critical than the sailing performance because after all, it's going to be a fishing boat. The sail only needs to get her to troll for fish with the motor off and get us home if the motor breaks. Many times, early early in the morning when we go fishing, there's not much wind anyway. Also curious about how the boat would handle with my little 4HP working together with a big genoa in a motorsailing mode.

    I'll try to post the new pictures of the model sometime tomorrow, Monday.
    Last edited by kenjamin; 09-26-2010 at 07:52 PM.

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    Default Re: Birdwing double dory catamaran

    with out board power transoms in the water and hence the rocker is not a concern, . Thing with a cat sailing is all the weight goes to one hull so that hull submerges to the point it is displacing all the weight of the boat rig and passengers, if there is a wide transom on the hull and it submerges the transom will really drag water and slow the boat down... you know how easily double ended Zena rows, now imagine rowing a wide stern motor boat... now imagine sailing that motor boat!

    My father built a 16' plywood catamaran about 1955 the hulls were 1 foot wide 16' long v bottom with a chine that ran high on the bow and down to nearly flat bottoms at the stern. The hulls looked like destroyers.
    He sailed the boat and it developed a reputation for running the bows under in good wind, pitch poled to the point of droping the anchor off the deck but never went over.

    he rigged the boat for outboard power and it would do about 9 mph with a 4.5 hp. Merc.
    Last edited by Daniel Noyes; 09-27-2010 at 08:54 AM.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by Daniel Noyes View Post
    with out board power transoms in the water and hence the rocker is not a concern.

    My father built a 16' plywood catamaran about 1955 the hulls were 1 foot wide 16' long v bottom with a chine that ran high on the bow and down to nearly flat bottoms at the stern. The hulls looked like destroyers.
    He sailed the boat and it developed a reputation for running the bows under in good wind, pitch poled to the point of droping the anchor off the deck but never went over.

    he rigged the boat for outboard power and it would do about 9 mph with a 4.5 hp. Merc.
    Got any pictures of your dad's boat? That would be interesting to see.

    I love my Caledonia Yawl for sailing but I am hoping to get more speed out of my little catamaran whether with outboard, sail, or both. Plus it will be fun to play with a boat where all of the hull design and rig are mine, mine, mine! When you get to be my age there's that thought in the back of your mind that at some point you've got to stop dreaming and just somehow make it happen. Thanks for your interest in my little experiment.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    Plus it will be fun to play with a boat where all of the hull design and rig are mine, mine, mine! When you get to be my age there's that thought in the back of your mind that at some point you've got to stop dreaming and just somehow make it happen. Thanks for your interest in my little experiment.
    Sounds great, now for some build pictures!

    Dan

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by Dan St Gean View Post
    Sounds great, now for some build pictures!

    Dan
    Hey Dan,

    Tonight is the big party to kick off the build of my catamaran. It will be mostly beer drinking, eating massive amounts of tuna casserole and boat building talk but we should be able to at least rip some of the ply down to the 2 foot width that I will need for the sides. I am tempted to just use a block joint splice for the sides so that tonight we could actually bend some plywood around the central spreader and get a preview of what a hull might look like. My Caledonia Yawl, Xena, will make a guest appearance on the trailer in the driveway and we'll probably hoist some sail on her to let guests know they've arrived at the party. Should be fun!

    Thanks for your interest in my little experiment.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    Hey Dan,

    Tonight is the big party to kick off the build of my catamaran. It will be mostly beer drinking, eating massive amounts of tuna casserole and boat building talk but we should be able to at least rip some of the ply down to the 2 foot width that I will need for the sides. I am tempted to just use a block joint splice for the sides so that tonight we could actually bend some plywood around the central spreader and get a preview of what a hull might look like. My Caledonia Yawl, Xena, will make a guest appearance on the trailer in the driveway and we'll probably hoist some sail on her to let guests know they've arrived at the party. Should be fun!

    Thanks for your interest in my little experiment.
    I'm interested 'cause I'm doing a similar build right now with two Tamanu hulls, but I'm leaning towards a folder with 11' of beam. The Tamanu BTW has 24" side panels made of 4mm with a butt block--works fine! I did a hasty version for the 2009 Texas 200 using all Hobie 18 gear, but I'd like to refine the setup and make it fit into my garage.

    Dan

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    Default Re: Birdwing double dory catamaran

    Interesting thread -- looking forward to more. I am with the crowd who says "get with the build". The shade platform reminds me of the poling platform on those skiffs down in the Keys that people use for hunting tarpon, bonefish and the like. I agree with you -- this project is something you can learn from at a relatively low cost -- and enjoy and have fun to boot.

    I am refurbishing an 80's Hartley12. On the foam issue -- I hate the stuff. I recently opened (for my first time since getting the boat in Jan) the floor boards from stem to stern. The guy who built this TS12 put pieces of foam throughout under the flooring -- and I mean pieces -- like the foam that come with your new TV set etc. Luckily most of the bilge looked good but in the aft section of the bilge I found a main plank/keel with about 18" of dry rot. So I have my own fun and learning experience coming up. I do not know if the foam sheets/pieces contributed to the rot but I know I still do not like the stuff. I also do not know if the original plans call for this type of floatation or not -- the boat has two large sealed air chambers built in to the p/s sides. I have been plannng on using litre-sized plastic milk bottle instead of the foam. The netted-bag idea mentioned above sounds like a great idea.

    This looks to be an interesting build/thread you got coming up--will be looking forward to it. Thanks, Gene
    If there are no dogs in Heaven, then when I die I want to go where they went. Will Rogers

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by yesiam View Post
    Interesting thread -- looking forward to more. I am with the crowd who says "get with the build". The shade platform reminds me of the poling platform on those skiffs down in the Keys that people use for hunting tarpon, bonefish and the like. I agree with you -- this project is something you can learn from at a relatively low cost -- and enjoy and have fun to boot.

    I am refurbishing an 80's Hartley12. On the foam issue -- I hate the stuff. I recently opened (for my first time since getting the boat in Jan) the floor boards from stem to stern. The guy who built this TS12 put pieces of foam throughout under the flooring -- and I mean pieces -- like the foam that come with your new TV set etc. Luckily most of the bilge looked good but in the aft section of the bilge I found a main plank/keel with about 18" of dry rot. So I have my own fun and learning experience coming up. I do not know if the foam sheets/pieces contributed to the rot but I know I still do not like the stuff. I also do not know if the original plans call for this type of floatation or not -- the boat has two large sealed air chambers built in to the p/s sides. I have been plannng on using litre-sized plastic milk bottle instead of the foam. The netted-bag idea mentioned above sounds like a great idea.

    This looks to be an interesting build/thread you got coming up--will be looking forward to it. Thanks, Gene
    Thanks for your interest in my catamaran build. A friend and I have actually done some ripping of the plywood and suitable scraps of 7 ply sapelle have been found for the transoms. I 'm also using some waste cut-off scrap from my last laminated birdwing mast for the bow stems which will save a lot of work. I'll slap together the central spreader for the build this morning and then start splicing the sides together to get the seventeen foot lengths I need. Progress is being made and I have more pictures to post but I've got to learn how to post them because I have a different account now and it's a completely different situation. A friend from my old job at FSU Physics is going to help me with posting the new pictures but it will probably be Monday before that happens.

    As far as the foam controversy goes, I may just hold off on the foam until after the boat is built and tested. I'll be able to calculate the weight of the foam, load the boat with that weight, and see just how it will affect performance. Then I can decide my priority – performance or the boat's inability to sink. Longevity of the boat is not really an issue for me because the boat is very likely to outlast me no matter what I do to it. As a fisherman there is also a desire for a very quiet form of flotation and poured in foam will probably be quieter than bagged up recyclables. I still think that a net bag (like what oranges come in) is a great way to contain your floaty items in air chambers but I'm still leaning towards foam because it can actually be a structural part of the boat and it's weight will be low in the boat where it will help with the overall balance and add momentum to the hulls for punching through chop.

    Thanks again for your interest and I post some new pictures tomorrow.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by Daniel Noyes View Post
    Ken I cant tell from your models photos, are the transoms dory transoms, in other words are the hulls double ended on the bottom? this may alieviate some of the need for the rocker every one's mentioning. one of the concerns with a flat bottom is the transom will drag, if it's double ended on the bottom this wont be so bad. The Alpha dory has about 2 inches of rocker in her bottom so a relatively flat bottom for a dory is not new and not necesssarily terribly detrimental, but if the transom has any width at the bottom like a skiff then history shows rocker is a very good idea.
    Daniel,

    I realized that I had a couple more pictures of my double dory that show the transoms, a more recent side view, a front view heeled so here they are:





    Last edited by kenjamin; 11-09-2010 at 06:32 PM.

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by kenjamin View Post
    Daniel,

    I realized that I had a couple more pictures of my double dory that show the transoms, a more recent side view, a front view heeled so here they are:





    What are the bits under the hard deck? You may want to go with a v bottom to your motor pod to avoid slapping depending on how low it is. Both Gary Dierking and Richard Woods have done some dory shaped outboard cats and found a fairing on the outboard to help speed. I think they are using higher HP motors though.

    Dan

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by Dan St Gean View Post
    What are the bits under the hard deck? You may want to go with a v bottom to your motor pod to avoid slapping depending on how low it is. Both Gary Dierking and Richard Woods have done some dory shaped outboard cats and found a fairing on the outboard to help speed. I think they are using higher HP motors though.

    Dan
    Hey Dan,

    Those bits under the deck were to mount a couple of swing down leeboards utilizing the lower most portion of the sunshade structure (option 4). I think I will be going with daggerboards instead but there are several options available as you can see from this figure:



    I'm leaning towards option 1 these days as it offers the most efficiency but probably would mount it somewhere between option one and option three. I really like option 2 also but it sure would be tricky to build. The black dots are block and tackle for raising and lowering the daggerboards but don't know if I can make the daggerboard boxes slick enough for that to work. Sure would be nice if it did. Another good thing about pouring foam in the bottom of the hulls is that the daggerboard boxes could be built lightly because if one of them exploded on a hard grounding, the boat could keep going with little change in performance – there would still be the other daggerboard available to keep sailing. And the damaged hull would float about the same due to the foam. Of course in that situation you'd have to sail her flat and not fly the only hull with an operable daggerboard. As for the motor pod, I suppose I could add a vee-shaped fairing to its bottom if slapping proved to be a problem. The false bottom created by the added v to the motor pod would be a good place to secure emergency stuff if this cat ever turtled and you needed to live on an up-side-down boat for a while.

    Thanks, Dan, for your interest in my project. I hope to have my first hull spliced up and on the building jig by the end of today.
    Last edited by kenjamin; 11-09-2010 at 06:40 PM.

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    Default Re: Birdwing double dory catamaran

    Hey Ken,

    Good to hear you are getting the shop door knob sticky again.

    I like option 1 best, you get the most support for the board in the down position.

    Getting the board down while the boat is moving with any speed could be a challenge. I'd bet you will need some kind of positive down haul.

    With all the shallow water in FL might conventional centerboards, just like Xena's with up and down haul lines, be a better idea?
    Denny Wolfe
    www.wolfEboats.com

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    Default Re: Birdwing double dory catamaran

    Quote Originally Posted by mcdenny View Post
    Hey Ken,

    Good to hear you are getting the shop door knob sticky again.

    I like option 1 best, you get the most support for the board in the down position.

    Getting the board down while the boat is moving with any speed could be a challenge. I'd bet you will need some kind of positive down haul.

    With all the shallow water in FL might conventional centerboards, just like Xena's with up and down haul lines, be a better idea?
    Hey Denny, how are you doing? Xena takes a daggerboard but it runs through a cartridge that allows it to give a little if the daggerboard grounds out. I like the efficiency that daggerboards offer but yeah centerboards would be better for exploring new waters. Phil Bolger used a centerboard on a fishing sailboard of a boat in his book "Small Boats". That centerboard was shaped like a quarter of a pizza pie and it had enough weight to lower itself but not so much that it wouldn't kick up out of the way if it hit anything. Since I'm saying this catamaran will be built for fishing, it would make a lot of sense to make the boards self-tending like that. The downside to centerboards is a little more weight and on catamarans the weight is so critical. When looking at the performance of a catamaran, daggerboards can offer cheap, light and effective lateral resistance. That fact that you have to watch where you're going when using them is a pain but if they help you point a little higher and go a little faster then maybe it's worth it. I used a polyethylene liner in my birdwing mast's luff groove and polyethylene slugs to slide in that groove so very slippery with the plastic sliding on plastic. I was thinking about a low friction daggerboard box using more polyethylene so that the daggerboards would deploy using just their weight and then a line to pull them up using my sunshade structure that I know I need but nobody else thinks I do. If I need down haul lines as well, it won't be the end of the world. It's a lot more fun building a boat for yourself than building one for somebody else. Good hearing from you. Come to Florida. We'll go fishing!
    Last edited by kenjamin; 10-10-2010 at 09:55 AM.

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