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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.

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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.

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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)

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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.
    My take is that if you poke someone with a sharp stick they'll get annoyed, if you smile and shake their hand they will be your friends.

    John Welsford

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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!

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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.

  27. #27
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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.

  28. #28
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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.

  29. #29
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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.
    Someday, I'm going to settle down and be a grumpy old man.

  30. #30
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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. ;-)

  31. #31
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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.

  32. #32
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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!

  33. #33
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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

  35. #35
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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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