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Thread: A 20 foot cruising catamaran

  1. #1

    Default A 20 foot cruising catamaran

    I hope folks will find this interesting. For my next attempt at boat design, I'm going to try to build a fast 20 foot cruising cat for two. The rig is a donor from a Nacra 5.2, and the boat folds to trailerable beam, but sailing beam is 12 feet.

    I've been posting a series of articles on the various design decisions I've been making. The drawing below is from one of the accommodations articles-- about the two single berths, which are to be in the starboard hull. A small galley, dinette, and space for a porta-potty are intended for the port hull.



    I'm trying for the minimal amount of cat that will allow two people to cruise the Bahamas. Among other design goals: good performance, trailerable behind a compact car, inexpensive, simple and quick to build, and maybe a little wholesome beauty.

    More info here:

    I should add that there will be no plans available for years, if ever. I'll have to build and test the boat extensively before I could consider offering plans.

  2. #2
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    Default Re: A 20 foot cruising catamaran

    Great start Ray. I've been enjoying watching the design process!

    Dan

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    Default Re: A 20 foot cruising catamaran

    Looking forward to more. The hulls seem to be the 'best of' the Wharram and the dory design but with more appropriate rocker to reduce Wharram cat hobby horsing (which I have only heard about, not experienced). -- Wade
    Last edited by wtarzia; 02-01-2011 at 09:23 AM. Reason: spelling

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    Default Re: A 20 foot cruising catamaran

    20 ft seems like a really practical size for a cruising cat, a catamaran "pocket cruiser".

    Short enough that cost should stay very low, cheap and easy to find rigs/etc., easy to trailer, yet large enough to be relatively seaworthy. But, you gonna be able to squeeze space into those hulls, even for single berths? Seems like they will be really tiny, unless you increase the width of the individual hulls alot, but then you have other problems... any other pics with some more specs and angles? What length/beam ratio are you going for on the hulls?
    “The difference between an adventurer and anybody else is that the youthful embrace of discovery, of self or of the world, is not muted by the responsibilities or the safety-catches of maturity.” Jonathan Borgais

  5. #5

    Default Re: A 20 foot cruising catamaran

    Thanks, guys.

    Wade, you're right about the excessive pitching you sometimes get with a Wharram. Our old Tane was a great boat, but in the right circumstances (say, light wind and leftover chop) she would pitch the wind right out of the sails. These hulls are very similar to Slider's hulls and Slider is really resistant to pitching, for a fine-hulled cat. There's a video here that shows us crossing a wake, and you can see how quickly the pitching damps out. I think the difference is that Wharrams are double-ended and Slider is not. The run aft flattens out and has a good bit of displacement all the way to the transoms.

    The rocker is a leftover from Slider, too. I had to add a little to get enough displacement for two people and a camping outfit, and still keep the hulls fine enough. But it didn't seem to hurt Slider's speed, and I hope it won't hurt the new boat's. I do think the extra rocker helps to make Slider as handy as she is.

    Peter, it isn't easy to get even modest accommodations into a tiny cat. What makes it possible in this case is that the new boat is dory-hulled. The flare gives it reasonable fineness at the waterline. These hulls are 10 to 1, which I regard as the minimum for a reasonably fast cat. However, the flare also allows single berths of reasonable width higher in the hulls. The forward berth shown in the drawing above is 28 inches wide at the head. Th lower berth, which is less luxurious, is 24 inches at the head. Both are wider than the absolute minimum width for a single berth, generally held to be 22 inches. Both taper significantly at the foot, but this is okay.

    This is a drawing that shows some more information:



    You could probably shoehorn a double berth into these hulls, if you were willing to build a knuckle into them, but I like the simplicity of build, lower weight, and (I think) drier ride of the dory hulls.

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    Default Re: A 20 foot cruising catamaran

    Have you got all the details of your folding system together Ray?

    It looks like the centre longitudinal section will go forward with the mast as the beams hinge at the hull and on the centre line. What about the decks?

    Andy.

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    Default Re: A 20 foot cruising catamaran

    Some things, Ray...

    I see that you have moved the daggerboard to one of the hulls, rather than the previously argued center beam structure. What prompted this deviation when you were so adamant about a surface piercing centerboard in previous conversations?

    I also see that the boat is just shy of six feet tall at its deepest point. I had this impression that it was going to have very low profile, slippery aero structures that did not create large drag signatures. The way these cabins are drawn, they turn out to be but a small difference from the Gato Especial after all is said and done. Could you tell us why you have such tall cabins considering the previous discussions on this topic?

    I, too, would be interested in the details of the folding system.

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    Default Re: A 20 foot cruising catamaran

    Looks very promising to me. If it were to be my boat, I might be more interested in a low aspect, simple rig like Slider's. I do also wonder where all the gear and goods for a cruise around the Bahamas would get stowed. Maybe a forward beam that's a chunky box section that could take odds and ends like anchors, fenders, dock lines, etc.? How about a boom tent? All that stuff that gets wet or grungy that you don't want to stow below.

  9. #9

    Default Re: A 20 foot cruising catamaran

    Thanks Wox. I think that weight is a more limiting factor than stowage. There is separate stowage in the buoyancy compartments for light stuff, and I'll probably put anchor chocks just forward of the forebeam. There seems to be plenty of volume in the hulls. The deal with the rig is that I acquired a complete Nacra 5.2 rig-- mast, boom, jib, main, blocks, travelers, sheets, standing rigging, halyards, etc. for less than half the cost of Slider's loft-made mainsail. The rig is from a 17 foot cat with an 8 foot beam, so I think at 20 X 12 Slinger will be pretty stable. I'll need to build a couple of reefs into the sail for cruising off soundings.

    Andy, I think I have the folding system worked out. I've never seen it before, and it might be patentable, so I plan to keep it under my hat for a while. The system I'd worked out previously required a fairly heavy central spine in order to work, which is why I'd considered putting the foils on the centerline. But the current idea can use a much lighter central spine, so I moved them back to the conventional positions.

    I can never tell when Chris is kidding. Here's the above-waterline profile of Slinger.



    I'm not seeing any resemblance at all, but maybe Chris will post a similar profile from his boat.

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    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by slidercat View Post
    Thanks Wox. I think that weight is a more limiting factor than stowage. There is separate stowage in the buoyancy compartments for light stuff, and I'll probably put anchor chocks just forward of the forebeam. There seems to be plenty of volume in the hulls. The deal with the rig is that I acquired a complete Nacra 5.2 rig-- mast, boom, jib, main, blocks, travelers, sheets, standing rigging, halyards, etc. for less than half the cost of Slider's loft-made mainsail. The rig is from a 17 foot cat with an 8 foot beam, so I think at 20 X 12 Slinger will be pretty stable. I'll need to build a couple of reefs into the sail for cruising off soundings.

    Andy, I think I have the folding system worked out. I've never seen it before, and it might be patentable, so I plan to keep it under my hat for a while. The system I'd worked out previously required a fairly heavy central spine in order to work, which is why I'd considered putting the foils on the centerline. But the current idea can use a much lighter central spine, so I moved them back to the conventional positions.

    I can never tell when Chris is kidding. Here's the above-waterline profile of Slinger.



    I'm not seeing any resemblance at all, but maybe Chris will post a similar profile from his boat.
    Your boat has a real workboat flavor to it in profile. I like the look. Have you thought of an interior chine flat to expand the accomodations a bit? It would add and extra step in the build, but the K650 has quite the interior volume for individual hulls. I sailed for a week on a Tennant Tourissimo 10 that was stretched to 36' and had inboard pods for the bunks. I think the port hull had a queen! You could have a twin in each hull with such an arrangement even on the short waterline. They'd have to be pretty far above the waterline to avoid slamming though...wasn't that the earlier working name?

    I'm loving the creativity and bringing yet another interesting multihull microcruiser into being. Continued success!

    Dan
    Last edited by Dan St Gean; 02-03-2011 at 09:21 AM.

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    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by slidercat View Post
    ... The deal with the rig is that I acquired a complete Nacra 5.2 rig-- mast, boom, jib, main, blocks, travelers, sheets, standing rigging, halyards, etc. for less than half the cost of Slider's loft-made mainsail...
    Presumably there is room to play with the rig as other builders might find deals on variously available sails.

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    Default Re: A 20 foot cruising catamaran

    who will be the first to slap a polytarp junk rig on this? lol
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  13. #13

    Default Re: A 20 foot cruising catamaran

    Dan, you're right about the flavor, I do like the looks of Western workboats a lot-- probably for the same reasons WoodenBoat is a popular magazine. One of the things that I'm trying to do with these designs is to try to meld that esthetic with modern catamaran ideas. A knuckle would give considerable more interior volume, but at the cost of some weight and complication in building. I also wonder about the behavior of boats with a pronounced knuckle in chop. It seems ideally placed to catch a wavetop and thump the boat a little. That may not be a real problem, but I think that for the same reason that bridgedeck clearance needs to be as high as possible, that knuckles may put additional stress on a hull. I'm probably wrong about that, as many excellent designers have taken that approach. I guess I'm committed to keeping the boat as light and simple as possible-- to have only enough stuff to take two people to the Bahamas, and no more.

    Jim, that's correct. Because the mast is stepped on the central spine-- in order to allow the rig to stay up when the boat is folded for a narrow slip or for dry-sailing-- the mast placement is independent of the crossbeam positions. You can see in the drawings that the mast is well aft of the forebeam. Anyway, what this means in practice is that the mast can stepped anywhere along the central spine. Should my various calculations go awry, and I have too much weather helm or even worse, lee helm, I can just move the mast, which is a much easier proposition than moving a daggerboard case. If it turns out to be a decent little boat, it would probably be ideal for someone who wanted to experiment with different rigs.

    I'm working on the drawings for the port hull, which contains the galley and the dinette. Hard to believe it can fit, but I think it will.

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    Default Re: A 20 foot cruising catamaran

    Having lived for years in various camps, I'm looking at the mundane logistics.

    You can probably fit a galley/dinette into one of the hulls, but won't it be pretty cramped with two people for reasonable comfort while cooking, talking, charting, etc.?

    What about bridging the house forward of the mast, so there's a communicating space between the two hulls? I don't think it'd add much windage, and it would certainly make the boat more liveable, i.e. you could pass food to the other hull, converse at anchor, and so on.

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    Default Re: A 20 foot cruising catamaran

    Along with what Chip-skiff mentioned, that could be a dodger/bimini arrangement a la Jim Brown's setup. Make each independent of the other yet it'll give lots of utility either way. Easy to fold up and stow as well! Little big boats seems to favor multis as long as one can keep them light. One of my faves in the category is Wood's Sango with it's slick folding system.

    Dan

  16. #16

    Default Re: A 20 foot cruising catamaran

    Chip, it's a folding cat with accommodations in the hulls, so there really can't be any cabin on the bridgedeck. The center deck is in two sections, hinged along the central spine, so they lift up and can be bungied together for folding.

    The dinette is almost 5 feet long, so you could squeeze 3 or 4 folks in for dinner. You'd have to put the shortest person in first, since headroom drops forward. The navigator will have to chart at the dining table, but it could be worse, I suppose. I'll put up some drawings of the port hull in a day or so, which may make matters clearer.

    With a cat this small, I think you really have to be disciplined as to what is actually necessary for a short passage. Otherwise you might wind up with a heavy monstrosity that doesn't sail well. I really wanted a built-in icebox, but I don't think it's in the cards.

    With a wide beam cat, there's a lot of room to spread out, when not on passage. I imagine a deck awning and some folding chairs and table would add greatly to comfort at anchor. Slinger's center deck is almost 5 feet across, and 10 feet long, so there's circulation.

  17. #17

    Default Re: A 20 foot cruising catamaran

    Just a couple quick sketches:



    And a section through frame #3"



    Details here.

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    Default Re: A 20 foot cruising catamaran

    I like your idea - have you published details of the folding arrangement?

    Perhaps you should discuss the internal accommodation arrangements with Tony Bigras, who travelled a great distance in a 16´cat, including crossing from Central America to Cuba and on to Florida (see http://turtleislands.net/tmc/default.html)



    I suppose someone who has lived aboard for so long would have some tips about what to avoid and what must definitely be observed when building a vessel of this kind.

    Gernot H.
    Last edited by 62816inBerlin; 02-03-2011 at 04:36 AM. Reason: I really should turn on the English spelling checker!

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    Default Re: A 20 foot cruising catamaran

    That'll work. Lots of small cats do it that way. I'd have some largish windows so you don't feel so claustrophobic while eating.

    Dan

  20. #20

    Default Re: A 20 foot cruising catamaran

    Thanks, Gernot.

    I'm being secretive about the folding mechanism, because I haven't seen it before, and I'm looking into whether or not it's patentable. I hate to do that, because it makes me feel like a jerk to be using the forum as a sounding board without letting everyone in on a crucial element of the design. But I can say that it's a variant on the kind of mechanism that folds at the hulls and at the central spine.

    Isn't Miss Cindy a great little boat? When Tony was on his big sail, I checked his blog every day, to see if there was a new update.

    Unfortunately, Miss Cindy is a completely different approach to the tiny cat problem. Tony made her with a fixed beam of 8.5 feet, so she could be transported easily on the American highway system. He put that low aspect biplane rig on her so that he could get 200 sq. ft. of sail, but still keep the center of effort low. Because he had fixed beam (no folding) he could use a bridgedeck cabin.

    Slinger will still have to be 8.5 feet for trailering, but she has a high aspect rig of 220 sq. ft. I had to figure out a way to fold her out to a wider sailing beam, which precludes having a central cabin. Richard Woods has designed some very clever folders that do have central cabins, but they have drawbacks for my purposes. One is that all accommodations are in the central hull. Another is that the boat cannot be folded with the rig up for narrow slips or dry sailing. One of the things I'm trying to accomplish with Slinger is a catamaran that's as easy and quick to fold as a Farrier trimaran. A couple of Farriers live on the canal across from my house, and their owners keep them out of the water in slings, rig up. Every time we take our little beachcruiser Slider out, we paddle past these boats, and I am fascinated. When the owners want to go out, it doesn't take them much more time to lower their boats into the water and fold out the floats than it takes us to get aboard and cast off.

    Dan, you're right. You can see the windows in the frame sketch, if you look close. Down here in the sunny semi-tropics, it's a balancing act for windows. You want the light, but not the heat. I think I'll err on the side of more light, for the very reason you suggest.

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    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by slidercat View Post
    Thanks, Gernot.

    snip--comments in bold

    Isn't Miss Cindy a great little boat? When Tony was on his big sail, I checked his blog every day, to see if there was a new update.

    Unfortunately, Miss Cindy is a completely different approach to the tiny cat problem. Tony made her with a fixed beam of 8.5 feet, so she could be transported easily on the American highway system. He put that low aspect biplane rig on her so that he could get 200 sq. ft. of sail, but still keep the center of effort low. Because he had fixed beam (no folding) he could use a bridgedeck cabin.

    A nice solution to a vexing problem. His cruise really made my winter speed by...

    Slinger will still have to be 8.5 feet for trailering, but she has a high aspect rig of 220 sq. ft. I had to figure out a way to fold her out to a wider sailing beam, which precludes having a central cabin. Richard Woods has designed some very clever folders that do have central cabins, but they have drawbacks for my purposes. One is that all accommodations are in the central hull. Richard has some add on cabin tops that remedy that issue. Another is that the boat cannot be folded with the rig up for narrow slips or dry sailing. I'm sure that could be dealt with using a baby stay and inner sidestays. One of the things I'm trying to accomplish with Slinger is a catamaran that's as easy and quick to fold as a Farrier trimaran. A couple of Farriers live on the canal across from my house, and their owners keep them out of the water in slings, rig up. Every time we take our little beachcruiser Slider out, we paddle past these boats, and I am fascinated. When the owners want to go out, it doesn't take them much more time to lower their boats into the water and fold out the floats than it takes us to get aboard and cast off. I'm wondering if one of Woods' folders couldn't deal with the folding in a similar manner--using a liftunder the bridgedeck if width is an issue. Still, I'm looking forward to seeing your build and solution to a vexing problem.

    Dan, you're right. You can see the windows in the frame sketch, if you look close. Down here in the sunny semi-tropics, it's a balancing act for windows. You want the light, but not the heat. I think I'll err on the side of more light, for the very reason you suggest. You can always meet in the middle with tinted glass allowing the view without the heat.
    Good conversation! I am enjoying where you are taking this little cat. A very different solution than Richard Wood's latest small cat Acron. As an aside, which Jones cat did you use to estimate your beams from?

    Dan

  22. #22

    Default Re: A 20 foot cruising catamaran

    Dan, I guess one of Richard's folder could be modified to keep the rig up, but most boats kept in slips must be antifouled. The way the hulls fold, if I'm remembering correctly, is that they rotate until horizontal. They'd grow a lot of barnacles on the topsides in my canal. That's why the Farrier owners have their boats up in slings. I'm trying to devise a system where the hulls stay upright. If you don't mind the hulls tipping on their side, Thomas Firth Jones has a nifty little cat, too. Brine Shrimp folds upward in the middle.

    I should hasten to add that this is no criticism of Richard. As far as I'm concerned, he is the foremost designer of small cruising multis in the world today. I wish I had even a fraction of his talent and experience.

    I think I will use tinted plexiglass. It does cut down some on the heat, though not as much as you'd hope. The main objection to tinted portlights is that they give a sort of gray cast to the scenery.

    For Slider, I used the beam scantlings of Jones' Weekender. For Slinger, I'm using box beams, which Jones did not use in his designs. I fear my box beams will be massively overbuilt, since I'm also using them to get flotation in the right place for a capsized cat. I hope never to have to test this flotation, but it would be foolish to ignore the possibility of capsize offshore.

  23. #23
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    Default Re: A 20 foot cruising catamaran

    Ray,

    On a week trip on Gilbert & Sullivan a 36' modified Tourissimo 10, the builder had matching homebuilt ice chests. He used thinly glassed ply with 4" of rigid insulation. With preprepared meals frozen along with some frozen water it lasted the whole week cruise. You could easily do smaller ones on either side of the mast on the flat hard deck. Its not as handy as being right next to the galley, but you don't want to open it much anyway. Once a day for the big chest and another for daily use works well.

    Dan

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    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by slidercat View Post
    Thanks, Gernot.
    Hi Ray,
    I'm being secretive about the folding mechanism, because I haven't seen it before, and I'm looking into whether or not it's patentable......
    I fully understand your situation.
    Ever since I sailed on a shakedown day outing with Jim Wharram on his original "Rongo" I have been a fan of catamarans and crewed on Shearwaters as a teenager. Since then my only hands-on experience is limited to a few hours' sailing a rented Hobie in a raging Mistral (30 years later) on the Mediterranean coast of France and a cruise on a party catamaran around San Francisco bay and under the Bridge.
    Limited docking space, unfortunately, makes owning a cruising catamaran problematic in Berlin which is why I have stuck to monohulls. A folding version that can be rigged up quickly and still has enough room to keep the best wife of all times (as Ephraim Kishon would say) happy would be great for messing about in the local lakes, rivers and canals, being "beachable".
    A relatively low aspect rig would also be an advantage in the sense that there would be less problems with a lowered mast when passing through locks and under bridges, which is why I was taken by Tony's idea. Unfortunately, unstayed masts do not go well with a folding arrangement.

    I wish you much success, keep us informed and post lots of photos!

    Gernot H.
    Last edited by 62816inBerlin; 02-04-2011 at 04:33 AM. Reason: slight addition to text.

  25. #25

    Default Re: A 20 foot cruising catamaran

    I've refined the design a bit more, in the incremental manner I like. One big issue that I was unsatisfied with was the seating on deck for the helmsman. At one recent point I's settled for the approach recommended by Thomas Firth Jones, which was for the helmsman to sit in one of the companionways.

    But I was dissatisfied with this for a number of reasons. Finally it occurred to me that a couple of removable seats would do the job, and could serve also as removable ice chest and cockpit stowage.

    Here's an updated plan view, with the removable seats in pink:



    Here's a section, to make the idea a bit clearer:



    Here's a post on the design process that led to these ideas.

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    Default Re: A 20 foot cruising catamaran

    Looks good Ray! Keep cracking at it. I got after the beam and attachment for my Tamanu today. Main beam laminated, MDO beam attachment plates cut out and a plan for the aft beam hatched.

    Dan

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    Default Re: A 20 foot cruising catamaran

    Yeah I'm replying to myself... I forgot to mention something though. You could easily do some of those folding chairs available with the ratcheting backs and sunbrella covers to both raise the backrest and give a nice cushion to the bum.

    Dan

  28. #28

    Default Re: A 20 foot cruising catamaran

    Dan, that's a good idea. Sometimes we put a folding chaise lounge on Slider, though not for the helmsman, because the steering line runs around the outside perimeter of the cockpits.

    The new boat will have a central tiller attached to the linkage bar, so it's within easy reach of either seat. I've thought about those little stadium seats atop the ice chest and cockpit locker-- the cabin side is inclined at the right angle to support them.

  29. #29
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    Default Re: A 20 foot cruising catamaran

    The central spine beam will need some bow in it and a bit more structural integrity. Make the hulls a little farther apart, cant the hulls outward 7-8 degrees and reinforce the edges of the craft to hold clamps/fasteners for the net. Take a close look at Jim Browns wing folding system.
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  30. #30

    Default Re: A 20 foot cruising catamaran

    Good suggestions, but the central spine is a box beam, more than strong enough to withstand the thrust of the mast. Because the bridge deck has almost 30 inches of clearance, the central box beam can extend well below the decks without too much concern for wave impact. It has to be over-engineered, because it carries the forestay-- a necessity given the design goal of being able to fold the boat without taking down the rig. This is to facilitate dry sailing, and parking the boat in a narrow slip. I believe a small protective nacelle for an outboard can be engineered at the aft end of the spine.

    I'm not a believer in canting catamaran hulls outward, though that works very well for tris. It may well be a good idea, but it wouldn't work well in this case, due to the folding arrangement I've devised. This arrangement also limits the maximum beam, unfortunately, so it couldn't be adapted to much larger cats.

    Jim Browns trimaran folding scheme is very clever, but not applicable here, because of the cabins, which project above the center deck, so that the hulls cannot be folded under the central deck, as the floats of Jim's new tri are. I'm also using hard decks, which are a lot more comfortable for cruising than tramps, in my opinion.

    That said, I have to admit that it was his 20 foot Seaclipper that got me thinking about this design. He had a lot of sensible things to say about little multihulls, and the possibility of recycling beach cat rigs. In a way, my new cat is an attempt to show that small cats make better cruising boats than small tris. If you want to compete with someone, why not compete with the best?

  31. #31

    Default Re: A 20 foot cruising catamaran

    I am a little curious how you arrived at a displacement of 1,900lbs (say a ton)? Even if you build lightweight, my hunch is that the bare hull + fittings would weigh half of that. Isn't the live load, (two people plus their provisions and gear), likely to weigh more than 1,000 lbs?

    I guess this question is a round about way to suggest that the bottoms of the sponsons look kind of narrow to my eye for a cruiser, and widening the bottoms would both increase the total displacement at that waterline, plus give a bit more foot room for the boaters feet.

  32. #32

    Default Re: A 20 foot cruising catamaran

    One other thought, after looking at this design. The nearly plumb stem would cause the side panels to be twisted in the forward 1/3 of the sides. It deserves some examination of the panel expansions whether this tortured plywood will be an issue during construction.

    Red shows the tortured plywood twist...


  33. #33

    Default Re: A 20 foot cruising catamaran

    Bruce, I'm hoping for a lightship weight of 800 lbs. If I make it, then I'll have 1100 pounds of payload, which is actually a surprisingly high number for a 20 foot cat. Many designers would tell you that this is an unrealistically high number for a cat of that size, if the boat had any pretension to performance.

    For comparison purposes, my 16 foot Slider has only about 600 lbs of payload. I frequently go beachcruising with two people and a fairly luxurious camping outfit. This includes a big tent, cots, chairs, big ice chest, and so forth. The times I've tried to go with three people, I think we were a little overloaded. Video here. Out in the bay that day there was a pretty good chop, and we took a little spray aboard, which almost never happens. It didn't slow us down appreciably, but usually Slider is perfectly dry, even in much bigger chop than we had that day. In order to get this much displacement on a 16 foot cat, I had to go with what many designers would call excessive rocker. I felt that even if the rocker slowed the boat, it wouldn't slow it as much as wider hulls.

    The big disadvantage that multihulls have, when compared to monohulls, is their lack of weight carrying capacity. This is just something you have to put up with if you want the speed and stability of cats. Were I to widen the hulls, then the waterline length to beam ratio would fall below the 10 to 1 magic mark that I feel is the cutoff point for high performance.

    Your point about developability is well-taken. I use Freeship for hydrodynamic analysis, and it also shows these problem areas, though not to the extent your approximation indicates. In addition, the outside of the frames are modestly cambered to add strength to the topside planking. However, this is 1/4" plywood planking, and as such can be tortured into shapes far more extreme than this. Slider, which uses almost exactly the same hull shape, also showed these problem areas, to the same degree, and the planking went on without any difficulty at all. I suppose if I get the first hull framed up and the planking won't go on, I'll be sad. But I think I have reasonable grounds to believe it will be okay.

    Hydrodynamically, the new boat is almost identical to Slider.. That the new boat is really just a modest evolutionary development of a successful boat is very reassuring, especially to someone like me, who has to give luck most of the credit for Slider's success.

  34. #34
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    Default Re: A 20 foot cruising catamaran

    Most cats don't have good load carrying ability. Those that do are like you suggest--complete dogs--as evidenced by the charter boats that are no faster than their mono bretheren. As always there is a tradeoff between priorities. fast multis are light, slim, and sport huge rigs. Charter monos have everything including the kitchen sink and all the toys in the garage. Essentially they are sweet floating condos that sail a bit. It's tough to have it all, and at 20' you just can't. Even with super wide hulls the carrying capacity wouldn't be that great and the performance would suffer.


    If you think like a kayaker, the weight isn't really a problem. The challenge is simply not bringing everything. The heaviest store is always water.

  35. #35

    Default Re: A 20 foot cruising catamaran

    Yes, water is a big part of the load, at least on the ocean. Let's just keep this between you and me, Dan, but sometimes I've wondered if it would be possible to cross the Atlantic in a cat this small. I guess it's been done in beach cats, but I think they had support vessels, though I could be wrong. Of course, a big problem would be that I'm starting on the wrong side of the pond for an easy passage. I'd need a watermaker and a spare, I guess. Lots of freeze-dried food.

    I'm probably not that crazy... but, I've had a good run, and my kids are mostly raised. Our youngest is off to college next fall, so it might be time for a death-defying adventure. Most of us seem to get more cautious with age, which doesn't make a lot of sense, because the older we get, the less remaining life we have to lose. Human nature is strange. When we're teenagers, we're perfectly willing to risk the loss of the 60 or 70 years we might have left.

    I just put up my temporary building shed. I'll post pics in a day or two. Moving along!

  36. #36
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    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by slidercat View Post
    I just put up my temporary building shed. I'll post pics in a day or two. Moving along!
    Let's hear it for moving along! As a father of two young ones, I'll keep my cruises decidedly in the beachcruising category for the time being. Still feels like I'm "out there" if I can't see land though.

    After doing the Baja Bash once in 9th grade on a 48' sloop, I'll skip the really tough slogs for now.

    Dan

  37. #37
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    Default Re: A 20 foot cruising catamaran

    ...cross the Atlantic in a cat this small
    Been done and then some: Cooking Fat

  38. #38
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    Default Re: A 20 foot cruising catamaran

    Ray,

    Your "minimal" cruiser will sail better if you stretch it and use round bottoms. Even the Nacra sail is relatively large compared to a Mono of the same length. A round bottom (strip planked) attached to your plywood sides would not be much more work. The combination would allow the payload you want with easier to drive hulls. 20 feet means 2 1/2 sheets of ply per side, why not use 3 and not have the wasted cutoff? (I know you can find a use for the 1/2 sheet, just trying to sell my own bias).

    I fully expect to see Dory style hulls on the final product, but take care with the Nacra mast on the spine. Even the Nacra at a much lower righting moment generates a significant mast down load. The beach cats have a dolphin striker and wire or strap to take these loads, and they are usually the thickest strongest member on the whole boat. I worry about you having an unsupported beam (the spine) with a huge load that can increase significantly with every wave. Previously I saw a thread that discussed a Warram cat which broke in two at the cross beams. A different issue I assume, but I don't want to see you in the same state halfway to the Bahamas.

    There is a simple way to strip plank a round bottomed hull which takes a lot less time. Everyone here probably knows how to do it.

    Marc

  39. #39

    Default Re: A 20 foot cruising catamaran

    That's true, Wox, but Slinger is just a wee bit smaller than Cooking Fat. And I'm evidently not as brave as Rory. I might have a modest advantage, despite Slinger's smaller size-- If she goes over, a folder should be easier to right using the tilting mast and airbag technique.

    Marc, I went back and forth with making the boat a 3 sheeter. It has a certain economic appeal, of course, and the boat might be faster, and would certainly have more load-carrying ability. But the downside is that the boat would weigh substantially more, and I wanted this to be a boat that could be hauled by a compact car. Because the boat is a folder, the cabins could have more volume, but little more usable space, due to the constraints on individual beam necessary to get the folded craft to 8.5 feet of overall beam. In addition, parasitic drag of the hulls would increase, but I'd be using the same rig, so I doubt that the boat would be better to windward.

    Round bottoms might be faster, though I'm not entirely sure about that. As someone pointed out on another forum, BMW-Oracle has dory hulls with softened chines. A researcher with whom I have corresponded has done extensive work with human-powered craft, and has found a semi-dory hullform to be lower in resistance than round-bottomed. All that said, I'll stay with flat bottoms for practical reasons related to ease of trailering, and the ability to take the ground without point loads developing.

    Your concerns about the central beam are valid, but simple beam analysis isn't rocket surgery. The rig of a Nacra 5.2 does have a small strap under the forebeam to spread the load from the bolt that the mast sits on. Both my main beams and my central spine will be massively stronger than the aluminum extrusion on the Nacra, as this is a heavier boat intended for offshore use. Also, compression loads on the mast will be much lower, due to the wider staying base.

    It's maybe not obvious, but I've actually spent a couple of years thinking about this design, so I hope I have the major stuff sorted, and at least some of the details.

  40. #40
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    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by slidercat View Post

    Round bottoms might be faster, though I'm not entirely sure about that. As someone pointed out on another forum, BMW-Oracle has dory hulls with softened chines. A researcher with whom I have corresponded has done extensive work with human-powered craft, and has found a semi-dory hullform to be lower in resistance than round-bottomed. All that said, I'll stay with flat bottoms for practical reasons related to ease of trailering, ability to take the ground without point loads developing.

    State of the art cat designers are actually doing something quite different from what is being put forth in this comment. Calling the BMW Oracle tri a dory-hulled design, with softened chines, is quite a long way from the facts. The hulls of the BMW-Oracle tri are very distinctly rounded surface, arced projections. To say otherwise is less than accurate.

    When Ray says that, "A researcher with whom I have corresponded has done extensive work with human-powered craft." he is talking about an Australian by the name of Rick Willoughby, who is a great guy, but has a special penchant for hard chined hull forms. As you will read below, the finest state of the art results from both computer analysis, as well as on the water, two boat testing, also reveal rounded hullform results when it comes to efficient forms moving through the water.

    Lately, there has been a lot of work going on in the F18 and A-Cat environments. Considerable time has been spent in the last couple of years with hundreds of design elements being tested to fulfillment through hydrodynamic RANSE simulations (Reynolds Averaged Navier Stokes Equations).


    Martin Fischer has worked on numerous bleeding edge design studies in order to exact the best forms for given applications. Consistently, he has been producing round hull forms and not sharp chined variants for the fastest boats on the planet. And just to make it clear… point loads will develop on any hull when rested on a ground surface, save for deep soft sand or squishy mud. Put a flat-bottomed hard-chined hull on a rocky beach and you have point loading wherever the hull touches the rocks for support. Put it on coral… the same thing. This is a non-issue.

    If one were to confine their argument to that specific aspect of the design equation, there'd be little to discuss. Venturing into performance arenas immediately pushes the hard chine form into a less than desirable region of capabilities. A Martin Fischer interview at this link: http://catsailingnews.blogspot.com/2010/05/cs-interview-martin-fischer.html





    F18 cat





    AC45





    F18 Capricorn...Martin Fischer design

    Quote Originally Posted by slidercat View Post

    Your concerns about the central beam are valid, but simple beam analysis isn't rocket surgery. The rig of a Nacra 5.2 does have a small strap under the fore beam to spread the load from the bolt that the mast sits on. Both my main beams and my central spine will be massively stronger than the aluminum extrusion on the Nacra, as this is a heavier boat intended for offshore use. Also, compression loads on the mast will be much lower, due to the wider staying base.
    While the term, rocket surgery, represents an interesting mixing of metaphors, it is still a mis-direct as to the business of running accurate calcs for a specific beam under load from the rig and all that goes with the process.

    If you read through the various descriptions of the boat being discussed here, you will see that there is an emphasis as to what gives a multihull its speed advantages over an equivalent monohull... one of those things being; how light it can be built and what that means to relative performance from a specific rig size. Over-building a boat invites one to take a look at just what it might be able to do against a similarly rigged boat that has been designed with something, shall we say, more than "rocket surgery". Rocket surgery and construction timbers from the Home Depot might be able to get you somewhere in the ballpark. But, how much easier could that boat be propelled through the water, how much more of its displacement could be aimed at other uses and how much less complicated could it be to maneuver onto the trailer in a congested and harried launch ramp environment, if it were lighter and stronger in the first place?


    Quote Originally Posted by slidercat View Post


    It's maybe not obvious, but I've actually spent a couple of years thinking about this design, so I hope I have the major stuff sorted, and at least some of the details.

    Just a quick observation to address this comment... Some time back in the very recent past, this same boat was being championed with a centrally mounted centerboard that was designed to swing from a single mounting bolt from the central beam. When the designer was told that a surface piercing foil of that length and side loaded leverage would start to look like a Main Battle Tank and that a foil of that type would quickly swing the design process away from a light and strong multihull to something else, there was an immediate argument and refutation.

    When the designer was told that he would suffer from huge surface piercing losses as entrained air literally leaped down his low pressure foil side, relegating the effectiveness to something a whole lot less than optimal, he said that he'd simply make it bigger, fatter and you guessed it... heavier. Later on, these same issues were put forth on the Yahoo Multihull Boatbuilder Group and he was told by well-respected multihull designer, Kurt Hughes, that the central, surface piercing foil was not a good idea at all and with his comments, the same issues were presented a second time from another, qualified source. Today, we see that the centerboard has been moved to one of the hulls, that it is a daggerboard now and next to nothing is being said as to why that major design switcheroo has happened.

    In short, this shines a very bright light onto the "rocket surgery" front beam discussion and the apparently intractable position of the designer to openly entertain other ideas.

  41. #41

    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by Chris Ostlind View Post
    In short, this shines a very bright light onto the "rocket surgery" front beam discussion and the apparently intractable position of the designer to openly entertain other ideas.
    I have to say that I've come to greatly enjoy Chris's weird obsession with my tiny design portfolio, as somehow he always manages to give me an opportunity to put my ideas forward more effectively. Case in point:

    Slinger is evolving

    A quote from the piece: "This leads to my prime directive when it comes to design– Never Make Up Your Mind Until You Must. This motto has earned me much ridicule from those who are never wrong, but it’s served me pretty well."

  42. #42
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    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by slidercat View Post
    "This leads to my prime directive when it comes to design– Never Make Up Your Mind Until You Must. This motto has earned me much ridicule from those who are never wrong, but it’s served me pretty well."
    I agree with that ray %100. We feel like we are in a rush to pick a particular solution to a particular design problem, and as grandpa told us, "haste makes waste". Sometimes we get emotionally entranced with a particular design attribute, and it takes a few weeks for pure "logic" to allow us to believe that it is, indeed, a bad idea. I like the fact that Ray puts his designs through all kinds of ridicule on these forums. He may not give in on a certain point right away, but down the line he seems very adaptive of others' suggestions.

    Chris, I think you're being a little hard on Ray's design. A camp-cruising 20 foot catamaran is necessarily going to have huge trade-offs, and will never have the advantages of a larger design. However, Ray has some compelling reasons to keep it this small, and he is not concerned, it seems, with high speed sailing. I'm sure as is, and even if he allowed a surface piercing cb to prevail, it would still sail right along with any other "workboat". Even if it never breaks 6 knots, it is still a worthwhile endeavor IMHO.

    Of course, if it were up to me, given Ray's requirements, I would build a 20 foot scow with an 8 foot beam, getting the best of both worlds I think (stability, carrying capacity, spacious deck, real cabins, cheap construction), or I would build a small trimaran, since according to chris white (and others), at under 30 ft loa tri's have the advantage for cruising over cats...
    “The difference between an adventurer and anybody else is that the youthful embrace of discovery, of self or of the world, is not muted by the responsibilities or the safety-catches of maturity.” Jonathan Borgais

  43. #43

    Default Re: A 20 foot cruising catamaran

    Peter, I appreciate the support. I will say, however, that if Slinger doesn't routinely hit the low to mid teens, I'll be one very disappointed amateur designer. Slider, a boat which gives up almost everything to comfort, has hit 10 knots, and she's a heavy 16 footer with a much smaller rig than Slinger-- and an archaic sprit-sloop rig at that. This level of unexpected performance is one of the reasons I'm sticking with Slider-like hulls for the new boat. You might be interested in this article I wrote about last year's Florida 120, where Slider surprised a lot of folks. In addition, Slider is the best-behaved boat I've ever sailed in almost 40 years as a sailor. She is very resistant to pitching, she tacks reliably with any combination of sails, and if you let the steering line go, she rounds up into the wind and sits there like a good little duck. I'm hoping these seakindly qualities carry over.

    I think the reason Chris White and others would give the nod to trimarans under 30 feet is a matter of internal volume, which can be enormous on big cats. On smaller boats, the single living space of a tri can be much more comfortable than the cut-up space of a cat. However (that's one of my favorite words, I guess) when you get down to the 20 foot LOA level, while the tri still has the edge in internal volume, it loses in overall usable space. A 20 foot cat can use an awning over its central deck to gain a huge living area at anchor, which is where cruisers really spend the majority of their time. A tiny tri can't do this effectively, because the lower displacement of the floats means that the side decks will tilt if someone tries to set up a bed or a folding table on one of the tramps. Here's a piece I wrote on why I prefer cats to tris in very small sizes.

    I think that tris get the nod on performance, too. Cats are just as fast in heavy air, for the most part, but in light conditions, the tri, with its lower wetted surface, will be faster. You get both light and heavy conditions in cruising, of course. That said, I've been astonished at how well Slider ghosts.

    This is another reason I'm sticking with the same hulls. Since I lack all training in design, I pretty much have to go by what has worked for me. I got really lucky with Slider, and I'm just not willing to spit in the face of Fortune.

    I'm getting ready to start lofting Slinger. Here's a pic of my temporary boatshed:



    I'd better have the hulls together enough to put on a trailer by hurricane season, I guess.

  44. #44
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    Default Re: A 20 foot cruising catamaran

    Good lighting in the temp shed. That's important.

  45. #45
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    Default Re: A 20 foot cruising catamaran

    Ray,

    Its a pity that our toys don't allow building Slider in different forms. It would be really interesting for those of us not scientists to see a direct comparison of a design feature. Round bottom verses dory style is my personal hot button, but others are obvious in this thread .

    I am impressed with your willingness to share and look at other concepts, and I do believe that you have thought about this for a significant amount of time. Your choices just would not be mine because of my biases, background and sailing experience (or lack of).

    Marc

  46. #46

    Default Re: A 20 foot cruising catamaran

    Marc, it would be very interesting. One of the major reasons I went with dory-like hulls with Slider was displacement. When I first drew up the hulls, I had double chines to approximate a round bottom-- the extra wetted surface is only a few percentage points more than semi-circular. But I found that I just couldn't get enough displacement in a 16 foot cat for two people and a camping outfit, using those hulls. Slider turned out so well that I just can't bring myself to abandon the shape.


    I have a sort of side project going, a car-toppable cat called Slipper. It has so far not been a notable success, I'm sorry to say. The idea was to cut down the freeboard, and a couple feet off the overall length, and have a boat with modules small enough to get on a roof rack, but big enough for a couple folks to daysail on.

    I wanted to see how simple I could make a cat and still have it sail modestly well. The hulls are very similar to Slider's-- and because Slider goes upwind fairly well even with her board up, I decided to see how far I could get with a boardless Slider-like boat, and just to make things even worse, I decided to try barndoor fixed rudders with the same draft as the hulls, so that there would be no fussing with boards and kick-up rudders. This did not turn out that well; details on the blog. I've eventually come to the conclusion that Slider goes to windward boardless, and tacks so reliably, because of her large kick-up rudders, with their 0012 NACA profiles. Anyway, it's mentally stimulating to have a little boat (with no investment to speak of) to experiment with. I finally got Slider back in her slip yesterday, so now I can spend a bit of time with Slipper. I think I'll try good rudders first, and see if I was correct.

  47. #47
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    Default Re: A 20 foot cruising catamaran

    Ray,

    I see an interesting parallel between using NACA 012 section rudders instead of "barndoor" rudders and using a round bottom hull instead of a dory hull. I think the boat would sail easier if you used a round bottom - which would require a wider max waterline to get the same displacement and you would have more volume in the hull above the waterline where you would be "living". That doesn't necessarily mean more weight, unless you fill it with stuff. Of course, you would have to keep a well shaped center board or dagger board, since the round hulls will be nearly useless in resisting sideways motion. You've been patient, no more comments about hull shape from me.

    If you are going offshore, you might want to reconsider 2 berths in one hull. Generally with 2 crew, one will be resting while the other sailing. The resting person should probably always be in the weather hull, to improve the righting moment. Small multihulls are sensitive to weight placement as I am sure you know. In a knockdown situation you wouldn't have time to adjust the 150#x boat width of righting moment. It might make a difference.

    Marc

  48. #48

    Default Re: A 20 foot cruising catamaran

    Marc, don't worry about it at all. I find this a very interesting discussion. As long as folks are civil, I think it's a good thing to have to defend one's ideas in public forums like this. If I can't defend my ideas, then I should certainly modify them.

    Were I to make my hulls round-bottomed, particularly in a very small cat like these, there are two ways I could regain the lost displacement. I could make the hulls wider, but my personal opinion is that catamaran hulls should not have a waterline to waterline beam ratio of less than 10 to 1. I believe that fat hulls slow down a boat more than hard chines, which do not develop as much turbulence in a long fine hull as in shorter fatter monohulls, due to the fact that the curves are much less extreme.

    The other way to go is to make the hulls deeper. This increases rocker and wetted area. Ideally, I think the half-breadth at the waterline, at least midships, should be not far from the draft-- a cube has less surface area. than a rectangular prism.

    As I said, Slider has proven be faster than she has a right to be, by the numbers. As an example, she seemed to be faster downwind than a couple of Windrider 17s in last year's Florida 120. This was a very unexpected result, because the little tris had the same sail area in a modern rotating rig, but were lighter, and had round hulls.

    While I don't know the reasons why, the dory hull serves enough other practical purposes in a very small cat that I want to stick with it. This could very well be the wrong decision, but I've enjoyed Slider so much that I want progress to be evolutionary rather than revolutionary.

    You make an excellent point about the offwatch crew in heavy weather. I think I'll make the dinette convertible, so that there is at least a short berth available if it begins to seem like a good idea to get both crew in the port hull. I don't know if you've read Thomas Firth Jones' account of his trip to Bermuda, in which he and his wife survived a hurricane at sea, in a 23 foot Wharram Hinemoa. He and his wife were both in the lee hull for much of the time the storm was building to its peak. However, at one point, they decided he'd better try to cross the deck and get into the weather hull, which he did. I never want to experience anything like that, but it would be good to have the option to shift the weight.
    Last edited by slidercat; 02-20-2011 at 06:52 PM.

  49. #49
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    Default Re: A 20 foot cruising catamaran

    Hey Ray,

    There's an idea that I've been knocking around in my head about adding roundness to flat-bottomed catamaran dory hulls such as you and I seem to like to build. My hulls for my down & dirty double dory catamaran are sitting stacked together in my backyard as they wait patiently for me to finish the remodel of both the kitchen and bath of my 1925 cottage in St. Augustine. I've had no time or energy for boat building but it hasn't stopped me from thinking about new possibilities. An idea I'd like to experiment with someday is adding roundness to my flat-bottomed dory hulls by simply gluing deadwood to them and shaping and fairing it. With Adobe Illustrator on my laptop I could take the length of the bottom and divide it up into 1 and a half inch sections that would represent thicknesses of cheap white pine 2x lumber that would be glued across the width of the bottom. So say for instance a 15 foot bottom length would have 120 sections of white pine 2x lumber glued to it. These sections could still maintain somewhat of a flat bottom midship (with rounded chines) but morph to more well rounded sections at the ends. It would be interesting, I think, to measure the boat's performance both ways – as a true flat bottomed dory catamaran and then as it would be more rounded with the deadwood added and shaped. Yes, the deadwood would add quite a bit of weight but it would also add quite a bit of stiffness. It would also create hulls which could ram just about anything and survive. It would also mean that daggerboards would break off like pretzels rather than harm the hulls. Anyway, seems like something that would be fun to do.

  50. #50

    Default Re: A 20 foot cruising catamaran

    That would be an interesting experiment. I might be inclined to use foam skinned in glass, as this would add little weight and increase displacement.

    One of these days I'll have to haul Slider over to St. Augustine.

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