Round Bilge vs hard chine

Discussion in 'Multihulls' started by nickvonw, Apr 11, 2011.

  1. seaofmirth
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    seaofmirth New Member

    Doug:

    Thanks, I did repost.

    Dave
     
  2. groper
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    groper Senior Member

    Nice thread... its funny, ive spent countless hours trying to figure out the best way for me to build my cat... so many considerations and methods, each with their own pros and cons... in the end, ive settled on a method that quite a few modern builders seem to be going for...

    build the true round bilge hull "shoes" then laminate the CNC cut bulkheads to join the boat together. Lift the prelaminated bridgedeck up from underneath, add the insides and deck but the leave the outer topsides off until last. This way you have good access into the entire hull from teh side and makes fitting out the inside much easier. This is where the majority of time is spent on the build and its also the most uncomfortable in the cramped and poorly ventilated spaces.

    So a combination of flat panel and round molded peices looks to be one of the best for cat construction IMHO...
    [​IMG]
     
  3. ThomD
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    ThomD Senior Member

    You really have to look at two hulls. And leave out all the monohull comparisons. The wetted surface area argument is the main one, but it is often developed by comparing the Max station. There is more to a hull than that, and there are simply fewer options with chines than rounded sections, or to state the real issue with hulls constructed as rounded hulls are (HCRHA). HCRHAs include the universe of hard chine hulls, whereas plywood hulls can only be developed in so many ways, one of which is actually as near ideal parabolic sections.

    Multihulls, and recent developments in materials, computing, and methods, make the construction of HCRHAs far easier than in the past. Sometimes easier than a lot of bits an pieces, miles of fillets, and the extra materials required to stiffen flat panels. Unfortunately, one result of these great leaps forward has been that every part of a boat may be infinitely designed, and the complexity of a one-off has risen to what one only got in the odd boat with 35 molds from a GRP factory.

    I am currently building a smallest possible tri, underneath my 24 footer that will allow convenient solo trailer sailing. There are a number of things I wanted to make round that don't really make sense against a backdrop of a displacement and beam budget, that are heavily constrained by the needs of the vessel and of the folding mechanism.

    At the end of the day you can make generalizations, but the real work is in understanding your comprehensive needs and the best means to achieve them.
     
  4. upchurchmr
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    upchurchmr Senior Member

    ThomD

    Have you posted any information on your concept? I am wishful thinking in the same range.

     
  5. ImaginaryNumber
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    ImaginaryNumber An Imaginary Member

    As a general rule of thumb can a percent improvement be estimated between the ideal parabolic section and perhaps the worse case of squarish single chines, such as Bernd Kohler's KD860?
     

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  6. groper
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    groper Senior Member

    Wow, you should really think carefully before making such statements... A square hull section can actually be one of the lowest (if not THE lowest) resistance hull forms possible. The reason is because it gives maximum displacement for a given waterline beam. Waterline beam is just as important as wetted surface area, if not more important, as it directly influences wave making drag.

    The extra wave drag a wide hull will produce, usually far exceeds the small penalty of slightly increased wetted area, and by a considerable margin too...
     
  7. CatBuilder

    CatBuilder Previous Member

    Wouldn't the extreme case of this be a SWATH?

    Is a SWATH any better than a round hull section catamaran?

    They have the smallest beam possible at the waterline, they are not all round section and they have more wetted area.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ==================
    In the Moth class(11') it was found that the skinny square section was faster than the semicircular low wetted surface section for the reasons you state. On large multihulls it is possible to have a low wetted surface ,semicircular hull that is skinny with a L/B above way 10/1 which is why many performance multihulls are shape that way.
    Heres a rough illustration:

    click on image-
     

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  9. ImaginaryNumber
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    ImaginaryNumber An Imaginary Member

    Groper,
    I was being provocative when suggesting that square-chined hulls are a poor design. Kohler's cats are not slow! What I was really wondering was why more square-chined hulls are not used? Kohler seems to think his square chines help reduce leeway, compared to a rounded hull shape, to the extent that some designs don't have either a keel or a daggerboard, though he does use what he calls an anti-vortex panel on some cats.

    Doug,
    You pointed out that sometimes the need for a high L/B is more important than low wetted area, and other times the reverse is true. Is there a rule of thumb for when one characteristic is more important than the other characteristic? Many of your posts are related to go-fast boats. How would this rule of thumb (if one can even be stated) differ for sea-kindly cruising boats, where speed typically has to be compromised for more displacement? In particular, why don't cruising catamarans and trimarans have squarish hulls, to maximize both their displacement and their L/B ratio -- while accepting the necessary penalty of increased wetted area?
     
  10. Doug Lord
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    Doug Lord Flight Ready

    =================
    I'd think that you'd go with the narrow low wetted surface option as soon as you can as length and weight increases. For small high performance trimarans that seems to occur around 16'. I think there are a lot of squarish cruising tris around-at least it seems like that. There is more to it: the example I gave was relative to light weight ,high performance hulls. If you had a squarish HEAVY hull that was narrower than a low wetted surface lighter hull the example doesn't hold true-in fact I don't think it would hold true in comparisons of heavy boats at any time but I haven't really looked into that angle. Part of the reason is that on light boats like the Moth the SA/wetted surface ratio is still higher even with the skinny, higher wetted surface option. With cruisers I doubt that would be the case.
    I don't think a "rule" could be derived from the example I gave because as the boat gets bigger(and heavier) many more factors enter the equation. Each case has to be looked at from the standpoint of all the factors that influence speed and drag.
     
  11. Submarine Tom

    Submarine Tom Previous Member

    This is very interesting indeed.

    Rick W did extensive research on square vs round, even single SWATH style drive pod. He's done exceptionally well with his square human powered cat. Like world speed record well.

    -Tom
     
  12. Richard Woods
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    Richard Woods Woods Designs

    I write about various hull shapes in an article you can see on the Articles pages of my website

    I have a bit more to say on my FAQ's page on hard chine hulls

    "Although my Strike 16 and 18 trimarans both have hard chine hulls, when I started developing a new racier small trimaran design I initially drew a round bilge main hull. However I thought it would be worth comparing it to a hard chine hull shape, as obviously the latter is much easier to build and trail.

    Fortunately a computer makes it very easy to modify existing hulls. A couple of mouse clicks changed the spline tensions and converted a round bilge hull into a hard chine one with no other input from me, (although I did need to adjust the draft slightly to keep the total displacement the same)

    I was very surprised by the results.

    As I had kept the length and displacement the same the Slenderness Ratio stayed constant, as did the Prismatic Coefficent. These are two of the main factors affecting wave drag. The other major factor needed to reduce wave drag (and thus to increase top speed) is a high LWL/BWL ratio.

    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.

    As I say, that was for a small hull drawn essentially by the computer, not by me (so obviously the final design still needs my input). However logic says that these comparisons will also apply to larger boats. And it did show that a hardchine hull is better than people think, and that a double chine hull (as used on Flica, Romany etc) is better still. Certainly it has convinced me to use it for my new racing trimaran. "


    My 28ft Gypsy design has been a very successful budget ocean cruiser and has hard chine hulls. It isn't slow (12knots easy in flat water) and is seaworthy for a couple to ocean cruise. One has sailed Panama to Tahiti, another Vancouver to Mexico and back via Hawaii.

    My 22ft Janus design has proven fast and fun to sail with a surprising amount of interior space. There's a couple of good youtube videos of a Polish Janus both with and without a cuddy fitted. Again links are on my Videos page of my website

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  13. oldsailor7
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    oldsailor7 Senior Member

    All of the foregoing posts have only served to convince me that the wooden Buccaneer trimarans designed by Lock Crowther with multi chined bottoms are at least as good in all respects as the more expensive, and more difficult to build, round bottom designs. :cool:
     
  14. groper
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    groper Senior Member

    I think you find, that the reason cruising boats are more rounded, is because a round hull provides the most volume inside for accommodation space... Vertical sides on a square bottom with narrow waterline beam gives no room inside! But look at all the racing cats, you'll see alot of flat bottoms and vertical sides... I think the argument of multichined vs true round is of little consequence...
     

  15. oldsailor7
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    oldsailor7 Senior Member

    " I think the argument of multichined vs true round is of little consequence..."

    Except as a matter of aesthetics. :rolleyes:
     
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