dory catamaran hull

Discussion in 'Multihulls' started by frank smith, Feb 2, 2010.

  1. Guest625101138

    Guest625101138 Previous Member

    The conclusions are based on thousands of kilometers of testing under my own power. Nothing rushed about them. The Michlet/Godzilla/ Flotilla simply supports what I have measured. That is why I have confidence in the results that I get from them.

    Show me any hard data that confirms other than what I have stated so far on this thread about the performance of round chine versus hard chine on slender hulls in the operating regime discussed here.

    You are good on useless rhetoric but offer no evidence and I expect have zero experience on the matter in question. Why bother with you unhelpful comments.

    Rick W
     
  2. rayaldridge
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    rayaldridge Senior Member

    The reason I chose semi-dory hulls for Slider had nothing to do with turbulence or the lack thereof-- I had to use the hull form to get enough displacement in a 15 foot warterline for two people and a luxurious camping outfit. Even so, I had to overdo the rocker a little, but that turned out well, since Slider tacks like a monohull.

    I don't want to take sides, because both Chris and Rick know more about design than I do, but I'll relate an amusing fact. When I first started talking online about my idea for a tiny open cat with in-hull seating, and claimed that there were no examples of that kind of boat extant, the comment I heard most often was this: "You know, Ray, if it's true that there aren't any boats like that, there's probably A Very Good Reason why."

    My point is that there are always holes in conventional wisdom. It's certainly true that adhering closely to conventional wisdom is the surest path to success. With Slider, I stuck to conventional wisdom in almost every respect, and that's probably why the boat turned out so well, despite the small innovation of in-hull seating. With my most recent design, I tossed conventional wisdom completely out the window, and I'm having quite a struggle ironing out the bugs-- the boat may turn out a miserable failure.

    With all that said, the art of boat design isn't advanced by folks taking the the tried-and-true route to success. It advances through the efforts of the contrarians who look for the holes in conventional wisdom, and who can take the time to explore those holes. Most of the time, those efforts will end in failure, but failure has always been the price of progress.

    Anyway, bottom line: when someone starts telling me something I feel is incorrect, my first reaction shouldn't be "You're wrong!" It should be "Show me your data!"
     
  3. Richard Woods
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    Richard Woods Woods Designs

    For what it is worth, I have designed three dory hulled multihulls. The 22ft Janus and 28ft Gypsy, both catamarans, and the Strike 18, a trimaran.

    All three sail better than one might expect. But I chose the dory hull for ease of building, not ultimate speed. A flat bottomed hull is not only easy to build but it can stay upright unsupported, which eases trailering and assembly.

    I always think there are two sorts of designers, those who start with a genuine blank page (or screen) and those who modify or produce variations on what others have done before. I hope I fit in the former category.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  4. garydierking
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    garydierking Senior Member

    The main reason that commercial higher speed power cats have shallow vee shapes all the way to the transom is to make them bank into a high speed turn. A dead flat aft section in a high speed turn will actually heel out from the center of the turning circle. Ths may not only be less seaworthy but doesn't feel good either.
    I recently examined the rudders on a 65' fast cat and they have the two rudder shafts canted in opposite angles to increase the banking on turns.
    A sailing hull doesn't have this problem.

    Gary
     
  5. frank smith
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    frank smith Senior Member

    Thanks for the info,your Gyspy 28 was one of the boats I was thinking about
    when I started this thread. Although I think that a multi chine is not much more work , I wonder at the advantages of it over the dory form .
    Frank
     
  6. Guest625101138

    Guest625101138 Previous Member

    Richard
    What expectations did you have about sailing performance? How were the expectations formed?

    How does the dory style actually perform side-by-side with similar rounded section hulls.

    Rick W
     
  7. masalai
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    masalai masalai

    I do not know where others are coming from but I can understand where Rick Willoughby is coming from, with "runs-on-the-board" and for those who take the time to read, will well understand his frustration at continually "having to prove himself" to pontificators...

    A degree does not necessarily make an expert it is the application of the base skills and development of a strong knowledge base by research and analysis (doing it as well as theorising) - not necessarily a flash of brilliance followed by mediocrity reliant on the "awards of brilliance", like in my interest area, macro-economics, - A Nobel laureate, Mr Christian who is making a fool of himself at CFTC hearings etc...

    Mr Willoughby, appears to do the theory research and practical testing to support his ideas, as well as offer proof in tests.... YEAGHhhh - - - - - - Go read something useful in the link below... starting around the 20th March...
     
  8. Richard Woods
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    Richard Woods Woods Designs

    I don't think you should get too hung up about the accuracy of performance prediction. I write more about it on my FAQ's page, where I also compare the real speed of boats from different designers.

    I very much doubt if anyone can really predict speed to any accuracy for average boats crewed by average sailors. It is very different for those who have money-no-object race boats crewed by the worlds best sailors. Because only those boats have two of the main speed variables taken out of the equation, ie the quality of the sails and the skill of the crew.

    The Laser has been built for over 30 years and 190,000 approx have been sold. It is a strict one design class. So one would think that they would all sail at the same speed, after all that is what the computer would predict. But clearly they don't. The better sailors always win. And they only beat their rivals if they have a new sail (and in Laser terms "new" really does mean "new", not a week or a month old).

    Back on topic. Only the Gypsy is available with both dory and round bilge hulls, but I have never seen the two versions sailing against each other.

    Having said that, I doubt if there would be much difference in speed. I had a dory hull Gypsy myself and it was about the same speed as a round bilge Iroquois (which has a roughly similar rig size and weight).

    Bottom line, if you worry that a dory hulled Gypsy is not fast enough for you then you should look at a different design, making the hull round bilge won't help.

    Enough rambling for a Friday afternoon

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  9. Richard Woods
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    Richard Woods Woods Designs

    Having posted I thought I should re read the thread.

    Seems we are all getting way off topic. Frank Smith is looking at an 8000lb 12 knot power boat. So only Rayaldridge has posted a useful reply

    I would agree that you'd be looking at a 34ftish boat to get 8000lbs disp.

    And a powerboat hull form is very different from a sailboat. Partly because you know how fast you want to go, partly because engines make the sterns squat and you need more buoyancy aft.

    Having said that it is quite easy to get even a sailing hull to go 12 knots under power. The 36ft Mirage, seen on my youtube video, does 8.5 knots under power (with the mast up) using two 9.9hp Yamahas. Clearly bigger engines will increase speed. I have used twin 90hp outboards on a 35ft sailing catamaran Banshee, and that has towed a waterskier at 22 knots, yet also sailed at 19 knots. So a real motor sailer.

    The Mirage has chined hulls, the Banshee is round bilge.

    I see you want to build in India. One of my 34ft Romanys is being professionally built in Mumbai by a yard that previously built a Flica. If you want contact details of that yard please email me (not a PM as I don't read those)

    So back on track

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  10. frank smith
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    frank smith Senior Member

    Richard ,
    I have reread my own thread and you are right about it being for a power house boat, sorry . I did abandon thinking about it because I could not get useful disp. out of it. I am however also interested in sailing hulls. I have read
    all of the info at you site , and it has been helpful. Thanks
     
  11. Guest625101138

    Guest625101138 Previous Member

    Richard
    My previous comments have all been based on 12kts, 4t, 11m power cat.

    I was interested to see how dory verses round worked out in sail boats hence the question to you.

    I do my comparisons based on reasonably reliable engine data over many accumulated hours of testing. I also race the boats so I am interested in small gains.

    I have attached some recent data. Best comparisons come with calm conditions, as was the case for most of the attached, because they are easily discernible and repeatable on any calm day. But I keep comprehensive records that allow comparisons over a range of conditions.

    There are horses for courses and dory style hulls are not near the lowest drag hull in some cases but there is a good deal of misinformation about them that many think they are inferior displacement hulls in all circumstances.

    Rick W
     

    Attached Files:

  12. Chris Ostlind

    Chris Ostlind Previous Member

    And the interesting part of all this is that nobody has answered the germane question: Why do so many well-known and globally established designers not use flat bottomed dory hulls when they are available to them as a design solution?

    No real answers here on this thread save for a diatribe and some wandering monologue about hoping that one doesn't copy the work of others. Neither one of them answered the obvious and simply skipped over the point in order to launch into another comfy discussion without getting into the meat of the matter.

    The answer does not lie in the launching of an uncomfortable rant. Similarly, it does not reside in the wishing that one doesn't simply replicate and comes up with, on a very clean sheet, a whole host of never before seen solutions.

    Truth is gentlemen, there is very little uniquely different in the world of hull design. The best we can do is find a set of answers that meet specific goals and be happy with the outcome. You guys are kidding yourselves if you think it is otherwise.

    If one thinks their stuff is all that unique, then simply apply the design idiom across the board to other boats already done and long since explored. Learn something new there? Nope, well, then on to recognizing that one's uniqueness is somewhat lacking in originality and is basically a rehash of what already exists with one's own small twist on how it all works. This is design school 101. You know, the part where the professor steps in and lets you know that your stuff is not all that special. When you get out in the real world, you're supposed to know how to do that exercise on your own. The world of power cat design is functioning just fine right now and all anybody is going to do is, once again, rehash that which has already been explored many times over.

    Functionally, there are no monumental breakthroughs at present. It will require a major change in the technology of how boats are built and the materials used to leap to another form within the genre. There's only so many ways you can bend a sheet of plywood, or foam, or cedar strip. Soon enough you use up the available design potential and you're down to moving deck chairs to derive that last bit of difference between the forms. Call it what you will. Come up with some walloping good marketing phrase to describe this killer breakthrough... but the bottom line is... it's all been done before and you are simply going through the motions.

    Right now, the standard in the industry is not a flat bottomed, hard-chine hull. That may change in the future, but I seriously doubt it. All the rest is an effort to approximate the circumstance and come up with cool descriptors that look longingly to get one in the game.

    I use flat bottomed hulls for some of my multis. They make sense for the simpler desires of some backyard builders who are intimidated by multichine builds, even if the multichine designs look decidedly better as aesthetic exercises. It's pure pragmatism, but it's not really art. Guys, in case we missed it somewhere along the line in our rush to get all square and plywoody... rounded forms look nicer. Always have and always will.

    Water is not square in nature. It flows and circulates around rounded forms with ease and with a whole lot less ease around hard-edged constructions. Take a long look at the big rocks that line the sides of a fast moving river. Observe the form of stones left to their own devices in the surf line. Each and every object is smoothed from years of being abraded by the natural flow of water.

    Smooth and not squared. Smooth and not hard edged.

    Smooth.

    If water wants the objects in its path to follow that simple, rounded paradigm, then why do we insist on forcing hard edged forms into the water and calling them good? Yes, that's a rhetorical question, but come on, guys... what is it really that we are doing in this design world that we seek to discover more completely? Nature has written all the rules long before we found out about math as a describing exercise.

    You guys who are going on and on with the hard chine thing... what is it, really, that you'd like to say to nature and its infinite wisdom?... That man has a right to alter the flow of natural energy for his own uses and that he'll go to any ends to make it justified as an argument? Please.

    I challenge you guys to go out there on the Internet and see what kinds of solutions are being generated by all the well-known designers on this planet. That means you guys with your empirical software evidence, and you guys who want to think that you start with a clean sheet each and every time you go forth to create a new boat. Somebody went before you. They dosed out all the variables. Some of them are a whole lot smarter than the rest of us and there they go defining the process for the rest of us to follow. They gathered all the potentials and they miraculously came up with a very well defined set of norms. Bingo, cha-ching, whatever you want to call it, but the answers are there for all to see... if one simply looks.

    You want to operate outside the norms... please be my guest. I do it every once in awhile and sometimes it works. More often than not, it does not work and we are only kidding ourselves that it does. I'm not saying that we should not explore possible shapes and solutions, but come on, folks, the world does not revolve around plywood and its limitations.

    Want to know how this works in another design paradigm?... Go look at the newest design work for the coolest Porsche automobile, the Cayman S. Do any of you guys see flat surfaces anywhere, or do you quite obviously see a smoothly contoured form that speaks to the environment in which it operates?

    What we are arguing about here is a system to justify the use of a planar product... plywood. This is done in order to approximate the functions of what the environment really demands.

    Compromises all around for everybody, bartender.

    .
     

    Attached Files:

    Last edited: Apr 19, 2010
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  13. Richard Woods
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    Richard Woods Woods Designs

    My apologies Rick, I missed a post of yours. But I didn't understand the March 27th pdf. Is that a test on a 4T 34ft powercat??? and what is HR (hull resistance??)

    Richard Woods
     
  14. Guest625101138

    Guest625101138 Previous Member

    That is not what I am arguing about. Sure flat pack construction is easier to build with fewer panels but not the point I have been making.

    I am making the point that the lowest drag hull for the conditions stated back at post #8 will have closer to flat sections than round. In fact so close that if it is flat it will not make measurable difference to the lowest drag.

    The other arguments are about fads and what looks good. That is what drives the design rather than any great R&D effort as you suggested earlier. Your comments support the view that it is the result of what looks good and because others do it rather than any great R&D effort. The technical merit related to comparative drag really does not come into consideration

    Recreational boats of the future will be slower and require much less power. There are not many designers addressing this. In fact 12kts is a slow speed for a power cat. How many of the boats you looked at were designed for 12kts cruise?

    Rick W
     

  15. Guest625101138

    Guest625101138 Previous Member

    Richard
    That data is for a 95kg displacement flat bottom slender hull. The 140bpm is a little over 120W. I have similar data for a 100kg round chine hull. The hulls can be viewed here:
    http://www.boatdesign.net/gallery/showphoto.php/photo/14406/ppuser/18624
    http://www.boatdesign.net/gallery/showphoto.php/photo/15138/ppuser/18624

    Both of these hulls were products of Michlet/Godzilla and performed almost to expectations however the flat bottom moved easier above design speed than Michlet suggested it should. Both hulls have since been analysed with Flotilla and the flat bottom trims bow up more at higher speed and eventually lifts. Hence the reduction in drag below what it would be if there was no change in trim. The round chine just sinks slightly with almost negligible change in trim.

    I operate them around 6 to 7 knots so the operating regime is a little lower than 12kts on 11m. My displacement would be a fraction lower than 4t if scaled but the difference is not much.

    However I have looked at the shape of the lowest drag cat hulls for the conditions 12kts, 4t and 11m and the chine is closer to square than round.

    Rick W
     
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