Sailing hull Shape

Discussion in 'Boat Design' started by BarendGrobler, Oct 29, 2008.

  1. BarendGrobler
    Joined: Mar 2007
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    BarendGrobler Junior Member

    Hey all
    All my experience is in high speed hull design.
    Now I want to design my first sailing hull. More specifically, a sailing cat.
    Let me just say beforehand that I know absolutely nothing about sailing, you're welcome to laugh, but just keep that in mind!!!
    Now my first question is; i'm trying to figure out why all sailing hulls have such massive rocker (sure this is not the correct term in sailing), you know, the classic Cl curve of sailing boats. If I wanted to design a planing (20kn+) hull, I would be tempted to have a straight Cl aft of amidships.
    Now tell me, why is this a no-no?
    I'm guessing, with the low boyancy of the fine sailing bow, the up-sloping Cl (aft of amidships) is nesacery to prevent a bow-down attitude (at rest and under way), but surely this can be countered with a aft shift in LCG?
    ok, enough for one question.
  2. daiquiri
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    daiquiri Engineering and Design

    The question you have asked tells me that there are some hard times in your future. ;)
    You seem to not understand the basics of low-speed ships hydrodynamics so here is my best advice: do not learn from internet forums. You can use them to get some hints on the best solutions regarding this or that particular issue, but only if you allready know what you are doing.
    Here you are asking the basics. You are asking kind of "why do houses have roofs?" thing.

    If you don't have time or can't afford a course in yacht design, then buy yourself at least 3 or 4 good books about the physics and design of sailyachts. Then you should get on board a sailyacht and... go sailing. That's how you will get (though just a basic) first-hand experience with the flow around a sailyacht hull and how it interacts with boat handling characteristics. There are many things which can be understood only in this way, unless you work in some test basin.

    Do that, study a lot, resist the temptations of immediately creating some fancy, eye-catching but nonsense hullforms with some 3D design software and, if you preseverate, you will be able to make a decent project within 3-4 years.
  3. fastwave
    Joined: Jan 2007
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    fastwave Junior Member

    Hi BG,
    I will give you a slightly more constructive reply than Daiquiri.

    From your questions I think your hydrodynamics knowledge is just fine. And also questioning why you should go against your gut feeling when it is telling to go against the norm is also good.

    No reason not to go for the flat rocker at those speeds. It would actually be better. The problem is how to get to those speeds to make it work. Most of the time you would be below those speeds or never make it to 20kts due to the exces drag at lower speeds.

    Check out the BMW Oracle Trimaran to see that they have much flatter rocker
  4. ancient kayaker
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    ancient kayaker aka Terry Haines

    Rocker varies all over the place in sailboats, depends what they are for and to a certain extent when they were designed. You simply cannot generalize.

    Rocker in a sailing dinghy provides for nimble turning at the bouys, in an ocean going boat it is part of a complex design package to provide speed when heeled over on a reach with the ability to take a following sea and lots more. Not all sailboats have oodles of rocker, the Paper Jet 14 has very little by sailboat conventions, so does my buddy's sailing canoe.

    Daquiri may have been a bit negative but he's right, you're not going to master either the science or the art quickly. But who am I to speak? My first 2 boats were pretty but pathetic in the water; however number 3 is great and I learned a bit by doing. If that's the way your made, go for, but keep the first one or two small!

    An afterthought: if you're interested in out-and-out speed for its own sake maybe you should look at foilers ...
  5. daiquiri
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    daiquiri Engineering and Design

    Ok, sorry if my reply had a rancid taste, didn't mean to make it sound so. And cancel that "3-4 years", you can read "2-3 years" instead. It is more realistic.
    But I stay behind the keyphrase: don't learn just from internet forums.
    Understanding (but really understanding) hydrodynamics is not that easy.
  6. Tim B
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    Tim B Senior Member

    I think there's a simple expression around the corner here...

    Few people get it right first time, and those who do get it right have done it several times before.

    Of course, you have to do it the first time, to improve it the second...

    Cheers all,

    Tim B.
  7. Guest625101138

    Guest625101138 Previous Member

    Sailing boats designed to do 20+kts will have straight run aft, sit on foils or have very narrow hull/s. It is not any easy outcome to achieve though.

    The fastest sailing hulls have very little to no rocker.

    The design rules for ocean going yachts that caused full body shapes rewarded volume. Seems a bit silly when most were used purely for racing. But we still have the situation where none of the big time ocean races allow multi-hulls.

    Joe Adams was one of the first yacht designers I know of to get away from the rules and build fast sailing boats. The Adams 10 design dates back to the 1970s and was often first home in fleets having much larger yachts. Possibly his most famous design was Helsal 2. It is 66ft and on downhill runs was able to better 96ft big volume boats. Helsal 2 is still competitive today for line honours.

    So the big rocker hulls you see evolved from coal carrying vessels where volume counted if you wanted to make money. The shape has nothing to do with speed.

    Rick W
  8. tom28571
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    tom28571 Senior Member

    Daiquiri was correct about all but the 20kt thing. 20 kts in a sailboat will indeed require a flat run in the aft bottom lines. Any rocker at all will be a detriment at this speed. Fastwave is also correct that you have to get up to 20 kts before you can actually sail at that speed. No location I am familiar will have the wind and water conditions that will regularly allow sailing at that speed. Very - very few sailboats of any description will ever reach such speeds.

    A flat run is effective in small boats at much lower speeds though, which is why many will have bottoms like that. Look at the boats with the faster potential like the Paper Jet and Laser and you will find that they tend toward flatter bottoms. The skipper can modify the immersed bottom shape by shifting crew position and/or heeling the boat one way or the other. One true thing is that boats have gotten flatter and flatter over the years since I started sailing. Older boats tended to be of more traditional construction and heavier so spent nearly all of their time at or below "hull speed". Introduction of lighter construction methods, composites and other exotics have brought the possibility of higher speeds and so hull design progresses along with that.

    It will take some time and effort to learn this and you will never know it all. I don't understand why such advice as Daiquiri gave is labeled negative. It's just good sense and good advice to say that an internet forum is not the best place to learn the basics of any form of boating or playing a musical instrument or hitting a baseball. There are good books written by very knowledgeable people who know how to explain things better than most of us here. You can read, mull over it a while and read again until the light turns on. Not knocking anyone here but some weird advice and opinions find their way into the forums. Some basic knowledge is needed to be able to sort it all out.

    In the end, it takes practice on the water to really gain a halfway decent understanding of it. A boat sails in three elements and the instantaneously varying combinations of all three. I think this is more complicated than what any other human carrying vehicle has to deal with. That's why it remains as much an art as a science.

    My 2 cents and I approve of this message. I heard that statement somewhere recently although I'm trying to forget it.:D :D

    Edited to say that (as usual) I appreciate what Rick said while I was slowly typing and he posted his answer. His thoughts deal mostly with larger vessels while my experiences are mostly with smaller racers that took quite different paths to get to nearly the same answers, but got there sooner. I think John Spencer's Infidel moved in the high speed direction for large vessels pretty early also. Garry Mull's Windward Passage was in the 1960's. I would never forget a short time aboard Bill Lee's Merlin either. Try a book by Frank Bethwaite to see some very original thinking along these lines in small boats. Not new but great reading.
  9. BarendGrobler
    Joined: Mar 2007
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    BarendGrobler Junior Member

    sailing hull basics

    Thank you all for your replies, its exactly what I was looking for! I certainly believe (as daiquiri said) that experience is extremely important; problem is, experience is expensive and takes a long time.
    Fortunetaly there is a less expensive, not quite as informative but also much quicker way; and thats scale-model testing. I'm planning on building a couple of moddels with varying hull shapes, so I can see affect on performance and how the hull handles and responds to different sea conditions. I am also at the same time designing a wing-sail system which I will then be able to test.
    I have experience with towing tank testing of high speed boats, but nothing on sailing craft, I also don't have access to a towing tank anymore (should have done this when I was still studying!!) so I'll have to make some alternative plans, perhaps radio-cotrolled.
    Any advice or commets, please!!!

    On the rocker, thanks all for your coments sofar. I was thinking having slight rocker (2-3 degrees)just for the aft 25% of the hull, to encourage trimming up. Coments?

    And then finally, why don't you ever see sailing boats with spray rails?
  10. daiquiri
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    daiquiri Engineering and Design

    Uhm, towing tests are the least expensive way for you? Then you are a lucky guy. ;)
    Barend - the least expensive approach is to find and read as many books, technical papers, research reports etc, as you can. Internet is an excellent and free source for finding many of them.
    The Mankind has been performing scientific work on displacement hulls for 300+ yrs by now. I guess that two lifespans of model testings wouldn't be enough for you to equal the knowledge that has been gained in this long process. You have 99.9% probabilty of discovering things that have allready been discovered. Don't start from scratch and don't waste your time if you can benefit the results of other people's hard work.
    Then, after you've seen what (and why) has already been done, if you feel it can be done better you can start refining and modifying the existing hulls, propulsion systems etc.
  11. rob denney
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    rob denney Senior Member


    I design multihulls with no rocker. See the video at to see one cruising effortlessly at windspeed of 10 knots and later (5:15) at 15 knots.

    I have found no rocker is faster except in light air when additional wetted surface slows you down. However, my boats are half the weight of conventional catamarans, so this effect is not noticable. The boat in the video weighs 3 tonnes, is 15m long and is fitted out for cruising. There is a similar boat in Holland which weighs 2.2 tonnes. These are cedar/glass hulls, nothing fancy.

    I have been sailing for 50 odd years, but all my design expertise has come from building what looked right, and the most obvious thing to me was that water would flow easiest in straight lines. Every time I follow the books, the boat goes slower and takes longer to build.

    Another big advantage of zero rocker is that it is very simple to build. I am now building all our hulls from a single panel which can be laid up in a couple of days and shaped in a couple more.

    Where you may have trouble with the cat is tacking. This can be resolved by having the rudders in one hull, ie front and back rudders. Also means you don't need daggerboards.

    My advice is to build something big enough to handle your weight easily. Make it as rough as you like so you will not mind chopping and changing and go out and see what works and what doesn't. Much more fun and much more likely to get the fastest boat than any amount of reading books.

    Any questions on your cat, feel free to ask.


  12. BarendGrobler
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    BarendGrobler Junior Member

    Rob, I like the way you think! both about design and testing!
    How on earth do you get your boats so light? What are they fitted out with in terms of gear? If you say they are fitted for cruising, I can hardly emagine a 15m boat weighing less than 12 tonnes?
  13. Guest625101138

    Guest625101138 Previous Member

    It is good when practice meets theory. If you go to this thread:
    and look for post #38 you will see a Godzilla generated catamaran that I produced quite a while ago now. There are two sketches. One shows the boat in static condition the other shows it in full speed sailing trim. In the latter condition it will be doing 20kts with something like a 50ft mast. With a typical rig this would require 30kts of wind. (I think that was apparent!!)

    Anyhow if you are looking to do 20kts in a sailing cat this is a reasonable starting point. Its advantage over a planing hull at this speed is marginal but in lighter winds it will certainly be better than a hull with a broad transom.

    The Cp is 0.87 and Cb is 0.66 in the full speed state. Both would be regarded as high numbers but if you push a slender hull well above its hull speed then this is what turns out to be optimum. Would be always more efficient to go longer but then there are practical issues about marina fees and possibly pilotage.

    Would be interesting to get performance data from a model of similar proportions. Needs to be about 5ft long so the weight is not a build constraint.

    Rick W.

  14. rob denney
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    rob denney Senior Member

    How? A number of ways, but some are:

    Start with the least possible boat and then figure out how to sail and control it.
    Put a rig on it that localises the stresses to the area around the mast so the entire boat does not need to be strong enough to take the loads from the stays. The fact that this rig is so much more user friendly than a conventional one is a bonus.
    Combine the rudders and daggerboards into just 2 foils and mount them on the beams so you don't need cases and all the beefing up they require.
    Engineer the hulls, beams, rudders and mast from first principles rather than from design standards.
    Build it carefully. Use carbon where it is cost effective. Design it as a sailing boat (fast, simple to use) so big motors are not needed.

    The cruise configuration was for coastal crusing on the East coast of Australia, maybe a fortnight between shops.

    Godzilla and harry are very close to identical. They are also the easiest hulls shapes to build. Surprising that there are not more hulls like these.

    Should have some 5m/15' models to play tow tests with in the near future, will let you know. Meanwhile if anyone wants to build a model, I am happy to send them a lines plan.

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