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Thread: A 20 foot cruising catamaran

  1. #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

  2. #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

  3. #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

  4. #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.

  5. #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.

  6. #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."

  7. #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

  8. #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.

  9. #44
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    Default Re: A 20 foot cruising catamaran

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

  10. #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

  11. #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.

  12. #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

  13. #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.

  14. #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.

  15. #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.

  16. #51
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    Default Re: A 20 foot cruising catamaran

    Shaping a ply box with glass over foam has been done for 20 years or more. It has its points, but in small boats you do loose interior depth that is usually wanted. Someone circulated plans for a dinghy built like that sometime back, with the added sales point that it was unsinkable.

  17. #52
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    Default Re: A 20 foot cruising catamaran

    Ray i have also considered doing a stripper hull in foam core. Mainly because it is super light and far less work than milling and fitting cedar strips. But the cost of the foamcore material is prohibitive, far more expensive than even wrc. Do u have a good source for the foam strips, or is that just the cost of a superlight composite boat? Haha for all the nostalgia it seems sometimes a wood boat is the cheapest way to build a boat one off even today!
    “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

  18. #53
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    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by peterchech View Post
    Ray i have also considered doing a stripper hull in foam core. Mainly because it is super light and far less work than milling and fitting cedar strips. But the cost of the foamcore material is prohibitive, far more expensive than even wrc. Do u have a good source for the foam strips, or is that just the cost of a superlight composite boat? Haha for all the nostalgia it seems sometimes a wood boat is the cheapest way to build a boat one off even today!
    Peter, I use a lot of foam core in my shop. I'm not sure what you are paying, but feel free to contact me about my sources and prices. Scott

  19. #54

    Default Re: A 20 foot cruising catamaran

    Peter, my knowledge of foam construction is pretty much zero. I think I have to agree that plywood is still about the cheapest (and fastest) way to build a one-off boat. I'm planning to use some cheap non-structural foam to add flotation to my center decks, but that's different from building a hull.

  20. #55
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    Default Re: A 20 foot cruising catamaran

    I've priced 2" thick transom foam that would be appropriate for the computer-generated cross sections that I would like to add to the flat bottoms of my dory styled catamaran hulls but found it to be quite pricey. Scott, I'd be interested in hearing about your source for marine foam as someday I'd like to play around with the stuff. As I said I'm in the middle of renovating my cottage so no time for boat building and no money for it either right now but soon as I have a functioning kitchen I'll be back at it again. And then as a retired guy on a fixed income, I'll have more time than money so the idea of making a flat bottomed boat round bottomed from scrap 2x white pine or spruce has its appeal even with the weight penalty. The dory catamaran hulls that I've built weigh very little as they are now so they could probably afford to gain some mass especially down low on the bottom. For my hulls, I'm not talking about a lot of material anyway – just enough to soften the chines and therefore the cross sections of the bottom. The sides I would not touch. Another added benefit for me is that the little wooden cross sections that I would be adding to get a more rounded bottom could get their final shape from my power planer so the transitions from one chunk of wood to the next would not have to be perfect. By measuring the before and after performance of the hulls, if the lighter flat bottoms performed better than the heavier more rounded ones, I could just plane off the dead wood cross sections that I had added and return nearly to the lighter flat bottoms that I had started with. Us old guys need to keep ourselves entertained.

    Scott, I think you owe me a welcome back to St. Augustine beer.

    Ray, you should trailer Slider over here as it would be a great excuse for me to take a break from all this house renovation and just go sailing.
    Last edited by kenjamin; 02-22-2011 at 06:36 PM.

  21. #56

    Default Re: A 20 foot cruising catamaran

    On the whole flat bottomed vs. round bottom front, I came across some speculation on Richard Woods' site that struck me as interesting. He has designed some dory-hulled boats, and seems to feel that they are pretty fast. Anyway, at the bottom of his FAQ page there was this:

    "So my first surprise was that the WL beam of the hard chine hull was less than for the round bilge hull, hence the LWL/BWL ratio is higher/faster. Much more surprising was the fact that the wetted surface area was essentially identical, and WSA is of course the main factor affecting low speed drag.

    So the implication is that a chined hull will have similar speed to a round bilge hull in light winds, and be faster in a blow. "
    I greatly admire Richard's designs, so I pay attention to what he says.

  22. #57
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    Default Re: A 20 foot cruising catamaran

    If the Acorn was out when I started building, I'd be on that boat this summer. Assembling a boat like that at the ramp soen't sound fun though.

    Dan

  23. #58
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    Default Re: A 20 foot cruising catamaran

    Scott, I think you owe me a welcome back to St. Augustine beer.
    I bought you one, but then drank it while waiting for you to show up. Give me a ring. My number is on the contact page of my website, www.seaworthysolutions.net , Scott

  24. #59
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    Default Re: A 20 foot cruising catamaran

    On the whole round bottomed add-on idea, I wonder if some reinforced Dow foam would do, glassed like a surfboard? Wouldn't weigh as much as solid wood I imagine, although the amount of glass that would be needed to reinforce that soft foam may make up for the weight savings... I suppose WRC wouldn't add too much weight, if an arc-bottom would suffice...
    “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

  25. #60

    Default Re: A 20 foot cruising catamaran

    I've pretty much finished lofting the new boat. This is my lofting table:



    I'm lofting at half scale.

    I've still got a few measurements to take and then I can start cutting patterns and putting together the strongback. The actual lofting of a hull this simple takes an hour or two, but figuring out where to place all the frames and stringers to make the internal fitting out as uncomplicated and light as possible takes a bit longer. As is my habit, I left a couple framing decisions to the last minute.

  26. #61
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    Default Re: A 20 foot cruising catamaran

    Looking good Ray! Keep cranking on it. Do take the time to figure out where the optimal bulkhead & stringer placement will be as it will save huge amounts of time later in the build. After building the decks on my Tamanu hulls, I now want to have a slightly different beam attachment method and am now forced to work through a hatch or worse, rip off the deck. Actually, ripping off a tiny bit of deck in the area of the beam location wouldn't be that bad... like you I like to mull over the design ideas a bit.

    I'd rather build to plan, but when there's no plan for what you want to build, you 'gotta improvise a bit.

    For example, I love the look of Richard Woods new acorn design & would have built it if it was available when I was building. However, I'm looking for a trailerable width and a hard deck aft, so even that really nice design isn't quite "perfect" for my needs. Which is also why you're designing such a different multihull....

    Ironically, I am closest in design to a 1959 design called the Pacific catamaran blended with a Hobie 21SC. It's fun fooling with it though, but with small kids I gotta say it seriously cuts into available building time (as does a cold garage & the need to totally clean up the school's shop if I use that).

    Press on man!

    Dan

  27. #62
    Join Date
    Feb 2008
    Location
    Berlin, Germany
    Posts
    681

    Default Re: A 20 foot cruising catamaran

    Quote Originally Posted by slidercat View Post
    ............. 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. .............
    Yes, it's been done, see : http://www.sail.ie/misc/cats_atlantic.htm

    But I quote:
    >>>>>. Pradel's feet, which had seldom been atop the wings and were always submerged in salt water, were just tattered flesh. The skin was torn away over most of their surface. Laurent had deep wounds and scars over his butt and thighs as well as craters on his feet a millimetre deep. Both men's hands were covered with wounds that had crusted and would not heal. Each cut, which never had a chance to dry and heal properly, was infected. Their circulation suffered the effects of blockage due to sitting and crouching in one position for hours on end and their hips and knees were paralyzed. Every movement brought tears to their eyes, but the worst wasn't over. They were almost in a state of shock. With their eyes glazed and the circulation problems preventing any feeling in their lower extremities, the pain was not nearly a horrible state, I remember that never as bad as it would become. Later in the evening of their first day on land, Pradel was wheeled to a restaurant to have dinner with friends while Laurent slept in his hotel room. Pradel's meal consisted of two large steaks, a plate of vegetables, noodles and six large pieces of cake. Then he too retired for the evening.
    The next day, both men could barely move. Pradel, despite being given tranquilizers, was tortured by the dressings on his feet, which began to come back to life during the night. Tears welled in his eyes for three hours. Groggy, he kept asking for someone to help him. Finally, when he managed to fall asleep, he felt himself aboard the boat, unable to stop the rolling movement or the hammering of the waves in his ears. In his dream he stretched his hand for a tool and some food only to have the waves wash them away. With infected third degree burns over his feet and during the whole trip did he once complain."<<<<<


    It sounds as if they were in wose condition than Steven Callahan was after his seventy-six days in an inflatable liferaft between the Canaries and the Caribbean.


    Gernot H.

  28. #63
    Join Date
    Apr 2010
    Location
    northern new jersey, usa
    Posts
    796

    Default Re: A 20 foot cruising catamaran

    I would like to see someone do it in a wa'apa.
    “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

  29. #64

    Default Re: A 20 foot cruising catamaran

    Hey Dan-- right now I'm shaping new rudders for the cartop cat, since my youngest son wants to borrow the boat for spring break-- he and some of his friends want to go down to Port St. Joe and camp.

    I sure hope this does the job. I'm thinking I'll just post plans on the web site for anyone who'd like to to try the basic design out-- I can't see charging anything for such a simple and modest boat.

    Anyway, here's the latest drawing for the 20 footer:



    Gernot, what an amazing story. Those two must be almost superhuman. I can't imagine what that must be like. Making the trip in a 20 footer with cabins would be incredibly luxurious in comparison. I think it is of cardinal importance to be able to stay dry and sheltered from the elements on any lengthy voyage.

    I guess, after reading that story, I have little excuse not to consider at least the possibility of such a crossing. My main concern would be that almost all the crossings in tiny cats have been east to west, the easier route from Europe to the Americas. I'd have to start out from the U.S. east coast, proceeding via Bermuda and the Azores. There is a strong probability of gales in the best season for that trip-- late May to early June.

    I hope that watermakers have improved. I've wondered if some simple water-driven pumping system could be contrived to work the watermaker-- it would slow the boat a little when in use, but you wouldn't have to use it constantly to get enough water for a couple of frugal people.
    Last edited by slidercat; 03-01-2011 at 03:05 PM.

  30. #65

    Default Re: A 20 foot cruising catamaran

    Well, as anyone who knows me might have guessed, the boat is almost nothing like it was a couple months back.

    I decided that I'd increase the length to 23 feet 8 inches. It may be that the Nacra 5.2 rig will not drive this larger boat to its maximum speed, but if not, I can always build a bigger rig, once I've seen whether or not the boat is a good one. I can also add a masthead forestay and upper shrouds, and fly a light air headsail to greatly increase sail area in light conditions.

    I've greatly simplified the accommodations, going for lightness, and maximizing the usability of the remaining amenities. No settee now, but two very comfortable single berths, a minimalist galley, plenty of accessible storage, and good sitting headroom over most of each berth. These changes allowed me to have very low houses, thus cutting down on windage.

    The biggest change, really, came about because I realized I was throwing away one of Slider's most lovable virtues-- her extremely comfortable seating. So now Slinger will have s small comfortable cockpit on each hull just at of the cabin, where the crew can be protected from the elements and yet be able to have good all-round vision. The crew won't have to worry about the boom while seated in the cockpits, and for a bad weather trip, dodgers could be added to give really good protection.

    Anyway, here's a profile:



    Another change is that I'm building her stitch and glue. The first hull is wired up, and I'm a little intimidated by her size:



    I had to lay a sort of scaffolding into the hull to get the second topside strake laced up. Luckily my balance is still good enough that i didn't fall out the side. I was quite surprised at how well all my edges matched up. The topside strakes are not quite developable, so there's a bit if convexity tortured into the hulls.



    I have to go out of town for a couple weeks, but when I get back, the hull ought to progress rapidly, with installation of bulkheads and stringers.

  31. #66
    Join Date
    Jun 2002
    Location
    San Pedro, CA
    Posts
    737

    Default Re: A 20 foot cruising catamaran

    What other meaningful specs did you change (e.g. beam) and why did you change them, if so, when you seemed so set on the boat in its earlier form? Seating, alone, drove the boat to be this much bigger?
    It looks like the aft beam could be moved considerably aft from its indicated position in the drawings above, allowing for the desired seating within the original LOA, but the sheeting angle for the rig would change accordingly. Is the choice of rig the issue that shaped your decision? If it is, did you consider a different rig configuration?
    Last edited by Chris Ostlind; 07-01-2011 at 03:07 PM.

  32. #67
    Join Date
    Feb 2008
    Location
    Berlin, Germany
    Posts
    681

    Default Re: A 20 foot cruising catamaran

    Having been on Jim Wharram's original Rongo as a kid on a shake-down cruise (felt like a real ship) and making acquaintance with a 20-footer last week:


    , which didn't even look too big in relation to my dinghy, I appreciate your wish for a few extra feet.
    How is the folding bridge arrangement coming on - patents applied for yet

    Gernot H.

  33. #68

    Default Re: A 20 foot cruising catamaran



    Of course, it's gone through dozens of changes. As I may have mentioned, my motto is to never make up my mind until I have no other choice.

    I had something happen to me that in retrospect is pretty funny. I had the first hull wired up, and as is my habit, I tabbed between the wires with little epoxy fillets. I removed the wires, and was starting to fillet and tape the hull at the bow, when I noticed I'd missed a couple of wires. As I started back toward the open transom to pull them, I heard a terrible ripping sound and I dropped about a foot. The only way out was the transom, so as I galloped aft, the hull opened up most of the way.

    More here:

    http://slidercat.com/blog/wordpress/?p=559

  34. #69
    Join Date
    Dec 2001
    Location
    Northern NSW Australia
    Posts
    59,919

    Default Re: A 20 foot cruising catamaran

    Here's my favourite Worry Cat , a 27 foot Tane Nui

    '' You ain't gonna learn what you don't want to know. ''
    Grateful Dead

  35. #70

    Default Re: A 20 foot cruising catamaran

    We had a Tane for many years. Great boat.

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