Displacement Speeds - Number if Chines

Discussion in 'Boat Design' started by fredrosse, Mar 13, 2014.

  1. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    Many simple hulls are built with fully rounded hull shapes, or with an approximation of the original shape, built with plywood or other sheet material formed having a number of chines. One or two of these discontinuities is common, and there are some hulls built with 5 or more chines, progressing more and more to the shape of the original hull. In the limit, producing a sheet material hull with many many chines would approach the shape and performance of the original hull.

    For displacement type speed levels, has there been any real testing conducted to address the performance characteristics of the hull vs. the number of chines used to approximate the original hull shape? I understand there are many opinions here, what I am looking for is actual data produced in towing tanks or other reliable and quantified methods.

    Much thanks in advance for answers.
     
  2. Skyak
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    Skyak Senior Member

    I would like to see an answer as well but I will not be surprised if no scientific experimentally verified answers show up. The answer starts with "it depends..."

    Less surface area means less drag. The reduction in surface area of a rectangular hull going to a semicircular can be plotted and the drag change calculated.

    Sharp chines cause vortexes in any flow that has a perpendicular component (not straight along the chine). This is where it gets fuzzy because you don't know what the flow is without complex modeling. A well designed two chine hull might beat a poorly designed 10 chine.

    I don't like any vortex forward -to keep low drag laminar flow as long as possible, but aft a vortex might not be so bad or might even counter wave generation.

    Good luck with your request.
     
  3. SukiSolo
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    SukiSolo Senior Member

    A well designed National 12' if designed in 4 planks per side is a max of 3mm at any point different to a totally smooth shape.

    Bear in mind that a lot of 'smooth' shapes now have chines (a bit like powerboats) for specific reasons. Both bow and stern chines are used on different scale boats.
     
  4. philSweet
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    philSweet Senior Member

    Fred, you seem to be coming at this from the perspective that round chines are always best, and are always measurably better than chined hulls. This really isn't the case. Even if you look at cases where a round hull is really good at it's job, the best chined hulls would only be a tiny bit worse. One might travel 98% as fast on the same power, and that could be hard to measure on a small craft. In other cases, the chined hull might track better, or have a more desirable motion, and therefore might be preferable.

    I think cross-sections with the same area and beam all have pretty similar performance potential unheeled on a mill pond.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Building a boat with five chines sounds like a nightmare, no matter what material used.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The chine count has little bearing at displacement speeds. You can design efficient shapes with any number of chines. This said, if two designs where the same, one round bilge and the other multi chine, there would be a slight increase in drag on the multi chine, comparatively, but really difficult to quantify at these speeds. At higher speeds, this would become more of an issue, but not at the very modest S/L ratios, associated with displacement speeds.
     
  7. daiquiri
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    daiquiri Engineering and Design

    I don't think you will find any scientific research relevant to this issue. An eventual research would have to be done either by towing-tank measurements or by CFD computations.

    At very low speeds both have their issues regarding the reliability of the results, and hence the conclusions would be dubious. But the basic physics of the problem in this speed regime is well-known and the conclusions can be drawn through very simple considerations. At the low-speed range the resistance is overwhelmingly dominated by the friction. The wave drag can be typically less than 10% of the total, hence the objective of the design is to minimize the wetted surface, regardless of the number of chines. However, if the hull has just 2 chines, there will be areas in which the chine angle is too sharp and the drag due to flow vorticity will add to the total, as Skyak has noted.

    In the proximity of cruise and maximum speeds, the wave drag becomes important, if not predominant. The longitudinal distribution of submerged volume commands there, again regardless of number of chines.

    Cheers

    Edit: To clarify what I intended by 2 chines: 1 port side, 1 starboard side.
     
  8. DMacPherson
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    DMacPherson Senior Member

    There actually has been substantial research done over the years on chine hulls (called "simplified", "straight-framed", "multi-chine") and their powering penalty against fully rounded forms. It all depends on the hull type, the amount of simplification, and speed regime. For hulls where the drag is largely viscous (frictional), there does not seem to be any significant penalty for a double-chine hull against a fully rounded hull. For example, there was a large research effort in Korea in the late 1960's that compared equivalent double-chine hulls to the Webb Trawler Series. Qualitatively, there are slight humps and hollows in the drag curves, but quantitatively, they are equivalent. Prof. Tom Lamb form UMich was a big proponent of "design for producibility", and I believe that he, too, did some testing on "producible" geometries with the same conclusion for lower speed hulls. Of course, the game changes at higher speeds.

    Don MacPherson
    HydroComp
     
  9. fredrosse
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    fredrosse USACE Steam

    philSweet wrote: "Fred, you seem to be coming at this from the perspective that round chines are always best, and are always measurably better than chined hulls."

    No, I don't think I implied that at all, just looking for some real data that may be had.
     
  10. Petros
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    Petros Senior Member

    some years ago this issue came up a number of times on a now defunct sea kayaking mailing list. someone posted some water tank drag tests done on identical hulls, one smooth bilge, one multi chine. It was done at the lower speeds of about 3-4 knots that a kayak hull normally travels at. the results showed very slightly higher drag at the top end of the speed range tested for the multi chine hull, but it was small enough that it would not be measurable in normal use by a typical paddler.

    At higher speeds there may have been a larger difference, but it demonstrated that there was no practical reasons to have one agaisnt the other. I do not recall who tested it, it was a university with a NA program I think in Canada.

    I have noticed when paddling behind a kayak that you can get vortexes shedding off the chines in the aft part of the hull and it is possible that on a bluff aft configuration that hard chines could actually reduce drag by delaying the boundary layer separation if the aft part of the hull closes too rapidly. though that would be a special case. These vortexes on the aft part of the hull however would help directional stablity of a canoe or kayak hull, reducing the need for a skag or rudder, also possibly reducing drag. I also know of tank tests that show hard chines will also have lower drag in chop or heavy swell since it allows wave peaks to break away from side of the hull, reducing drag, however in smooth water the smooth sided hull had lower drag. this last one is not widly known nor ever published. these results were confided in me by the person that did the tests for a boat maker.

    So there are cases where hard chines could be beneficial. and if there is any chance of planing at higher speeds, a properly designed hard chine hull will have less drag and come up on plan easier. I also suspect that hard chines have a slightly higher dynamic resistance to rolling since there are also vortexes coming off the chines resisting the roll. this would only be instantaneous, and the effect is stronger when the hull is underway (you can actually feel the effect in a hard chine kayak, sitting still the hull is a lot more "tippy" than when underway). The static stablity will not be affected, you have to moving fluid over the chines for this effect to work.

    So it is a mixed bag. there are both advantages and disadvantages, if hard chines make it cost less and easier to build, than that enough is reason alone use them since any performance penalty is small and within fairly narrow operating limits.
     

  11. messabout
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    messabout Senior Member

    Interesting thread for those of us who tinker with little boats.

    Please comment on this imaginary kayak like boat....................
    It is 12'-6 LOA, 11"-6 LWL. The chine width at mid section is 24". Total displacement is 200 pounds and it will have a draft of about 4 inches. Ply construction with 4 and 6 mm Okumee. The bottom is flat despite the wetted surface penalty. Cp about 0.53

    The boat can have hard chines with about a quarter inch radius. Alternatively it can have a strip built chine area of perhaps 1.5 or 2 inch radius. I think someone on the forum has done this strip to ply chine thing. (It might have been Ancient Kayaker.)

    Q:1 Is the rounded chine option worth the trouble?

    Q:2 The kayak bottom, above, has a 4 or 5 foot section in the middle that is almost flat in elevation view, the ends have a bit of rocker. Both ends are pointy. That is pretty much a conventional layout.

    Now let me re-do the aft end. It now has a small transom with a bottom width of 9 inches. The aft rocker raises the transom end so that the bottom is at or very slightly above the WL. There is no flat section in the elevation view. The bottom has a continuous smooth curve with max depth just a bit forward of midships. Max width is slightly aft of midships to keep the section areas arranged to make a conventional area curve. We can build it with hard or rounded chines. We will use a skeg or rudder to help it go where we aim it.

    Now we have four different iterations of the same basic boat.....Yes, I know that I am just throwing darts but the target is invisible.

    Your comments will be carefully considered.
     
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