Pointing ability

Discussion in 'Hydrodynamics and Aerodynamics' started by WoodCrossSailor, Nov 25, 2012.

  1. WoodCrossSailor
    Joined: Nov 2012
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    WoodCrossSailor New Member

    As I understand, when optimizing a heeled, relatively heavy mono-hull for pointing there are two schools of thought: Some designers go for a symmetric hull-waterline at around 15° heel, thereby minimizing hull drag and using efficient appendages for lift.
    The others go for an asymmetric hull-waterline (chines etc.) at 15° heel and consider the hull as a lifting foil. This has consequences for drag, cross-flow and especially design, size and area distribution between centreboard/keel and rudder.

    What is current thinking in 2012? Can somebody explain the consequences for designing the appendages considering both hull design concepts?

  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi WoodCross Sailor,

    Welcome to the forum. First, it is extremely difficult to get a true symmetric hull shape even at 15° heel. We get close, but other factors regarding hull shape and interior volume come into play.

    Second, there has been some discussion on this forum regarding asymmetric hull shapes and chined hullforms, and in my opinion to inconclusive results. I dare say some people are going to post replies on here that will advocate that chines do indeed make the hullform truly asymmetric an create lift like aerofoils. Personally, I am not convinced.

    Both types of hullforms, of course, have been around for centuries, so really, in this regard, for design practices for full-bodied displacement hullforms, there is nothing new in 2012.

    That said, there are other factors that come into play, and very specifically, the general shape and center of area of the flotation waterplane. A few years ago I participated in a discussion thread here regarding the design ratios of hull design and how they affect boat performance. The very first chapter in that thread, which ended up being a lecture series, so to speak, that I collected into a booklet called "The Design Ratios", addresses this very question. I post the document again here, which you may download and keep, and feel free to pass it around to your associates and friends.

    Finally, it is fairly easy to create a chined hullform that is every bit as weatherly as a round bottomed (non-chined) hullform, so chines are not necessarily the governing issue. And asymmetry of the hull, independent of chines, can hurt you as much as help you. The IMOCA 60s that run the Vendee Globe and other around the world races are classic examples of hulls that have very asymmetric heeled waterlines that cannot sail diddly-squat to weather. They are basically downwind-running hullforms. So these are really two independent characteristics--hull shapes and chines that affect weatherliness (being able to sail well to weather). The controlling factors are more fundamental. The Design Ratios will give you further insight.


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  3. bruceb
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    bruceb Senior Member


    Eric, Thank you for posting that- one stop shopping:cool:
  4. fcfc
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    fcfc Senior Member

    I do not think chine change anything on hull side force. See paper in http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/where-get-papers-45292.html

    I would say chines are kind of fashion and rating rules. To increase speed with "open" style rules, (mini, class 40, IMOCA 60), everything has a limit (draft, lwl, sail area, etc...). The only thing remaining is to increase stability. You are also at ballast structural limits. The last thing is form stability. ie chines.

    Cruisers follow the racing fashion, and incidentally see that chines also add space for aft cabins. So latest beneteau cruisers do have chines.
  5. Petros
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    Petros Senior Member

    it is hard to say what is the current "trend" because sailboats are designed around different priorities. It appears to me the varous ideas are still all over the place, it just depends what class of racing you are considering. If you look at pure racing monohulls it appears to me that the designers are going for narrow hulls with laterally swinging keels to keep the mast as upright as possible. I am not sure anyone would want to cruise in such a boat.

    I think one of the issues is how the designers intended the sailboats to be used, I have done some limited racing in both small and large sailboats, and you use the boat very differently when racing than when cruising. Even the same boat used in a race is not a pleasant or relaxing activity, you push hard and make more tacks and complicated maneuvers you would not do while cruising. this is also generally why good racing boats generally do not make good cruisers. And good cruisers are two slow and heavy to be good racers unless they are competing handicapped or in one design.

    Like anything else, it is up to the designers intended use, you compromise one type of use for the sake of the other. You would not want to drive around town in a formula 1 race car, after the novelty wears off, it would not be pleasant. Same is true for sailboats as well.
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  6. CT249
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    CT249 Senior Member

    As Petros noted, there is not really any "current thinking" in a part of the sport that encompasses boats as varied as the Mini 650s and Open 60s (very beamy, often chined canters), IRC specials (often quite heavy, narrow bodied and conservative especially at the smaller end, beamier and lighter at the top end), "open influenced" production boats in between, one designs, etc.

    With respect to Petros, comparatively few "pure racing monohulls" use canting keels. For example, in the last Fastnet race there were only 6 canters, of which at least 3 can be called "racer cruisers" rather than flat out racing machines. In contrast, there were a few dozen fixed keel "pure racing" boats. Most canters, BTW, are probably the very beamy "Open" style boats. And many "pure racing" owners buy production boats just for IRC/ORC racing, with no intention of cruising them.

    Most of us who race DO in fact use racer/cruisers and we DO in fact find it "a pleasant activity" - that's why we do it! Some of us also relish dual-purpose boats. Some of us love getting off the rail bashing upwind at night with a #3 and a reef and going down below (or even just looking at) a comfortable interior, and love cruising a boat that is fun to sail.

    The ultimate example, perhaps, is a boat that has been national offshore racing "Yacht of the Year" six or seven times, represented its nation internationally in team events, taken 8 class or overall victories in the biggest offshore races in three major sailing countries (and we are talking classes of 12-50 boats or so) AND been the sole home of its owners for 30 years and cruised the Pacific, the Horn and Alaska. That is an outstanding racing record AND an outstanding cruising record.
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  7. Mikko Brummer
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    Mikko Brummer Senior Member

    For what it's worth, here's asymmetric waterlines with a large chine of the Star. Red areas are all wet, blue is air and the colours inbetween are a mixture of air & water. The still pic is at rest, in the animation you can see how the wet area changes when in motion. This is in flat water, with waves the wetted area will of course vary much more.

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  8. rowboat70
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    rowboat70 Junior Member

    Double Enders

    The scow is a double-ended boat in profile, rather than in plan view,
    When the scow heels over, the waterlines develop well-balanced asymmetry.
    The scow's waterlines are so well balanced that it develops no weather helm when it heels over.
    The tiller is in the center of the boat until the mast hits the water.
    The efficiency of the scow is best demonstrated by comparing a 20-foot C scow to a 20-foot Flying Dutchman ("the world's fastest monohull"). The Portsmouth Yardstick shows nearly identical ratings for both boats. The 650-Lbs C scow is a catboat with a rotating mast and no head sails at all. The 375-Lbs Flying Dutchman is a sophisticated sloop with a genoa jib and a spinnaker.
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