Predicting Hull Speeds

Discussion in 'Hydrodynamics and Aerodynamics' started by farjoe, Jan 7, 2015.

  1. farjoe
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    farjoe Senior Member

    It appears that the modern trend in hull design is to go for wider sterns generally including chines.

    Given that nobody would look favourably at a new boat which is slower than a 5 year old design it seems reasonable to assume that modern VPP's predict that these shapes are faster.

    Indeed IRC ratings are penalising such hulls with respect to older leaner shapes.

    In practice, or at least on the boat I race on, this does not appear to be case. One can argue this may be due to sailor competence but even in the most recent Sydney Hobart where we can assume that sailor competence is high at least on the top boats, the 10 year old leaner boat managed to beat the ultra modern wide stern carbon fibre beauty.

    What are the group's view on this?
  2. RHough
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    RHough Retro Dude

    You are comparing a mature racing organization with a decade of experience to a new this year program. Hardly enough data to draw a conclusion from.

    The newer wide assed boats are not sailed that way. The upwind hull profile is skinny. The big difference is off the wind where the wide stern has a lower drag shape when planing.

    Look at planing skiffs (non-foiling) ... that is where the shape came from. Build an 18 that looks like WO and you won't be at the pointing end of the fleet.
  3. farjoe
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    farjoe Senior Member

    That may well be but, putting these 2 aside, are wider sterns in racing and cruiser/racing yachts faster overall than narrower hulls in an upwind/downwind race?

    I don't think comparison with the 18's is good since these seem to have wildly different characteristics.
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    To answer your question, generally, no, fat-assed boats are NOT faster upwind, or rather, they could be faster in boat speed, but they don't point well. A narrower boat will point higher than a fat-assed boat upwind, and in upwind sailing it is speed-made-good-to-windward that matters. The reason that fat-assed boats don't sail well to windward because as they heel, they pick up buoyancy aft which causes the bow to sink. That orientation causes less angle of attack, maybe even negative angle of attack, on the keel. So the keel has less ability to pull the boat to windward. On a properly designed skinnier hull, as the boat heels, the stern will sink slightly which causes the bow to rise. This necessarily adds more angle of attack to the keel, which increases lift and power to pull the hull to windward. The skippers of fat-assed boats know this and to compensate, they have equipped their boats with adjustable asymmetric daggerboards to increase lift to windward. Actually, that technology is advancing pretty rapidly, especially in light of the results of the last America's Cup.

    Fat-assed boats are very fast downwind if they can get into a planing regime, which on a light boat with a lot of sail area is not hard to do. So the idea is that you try to spend more of your time on downwind legs than upwind legs in order to be competitive. In an upwind/downwind race, the bet is that faster downwind speeds will compensate for the poorer upwind performance due to lack of good pointing ability. What was the wind like in this year's Sydney-Hobart--was it more upwind than downwind? If so, that explains the difference.

    Note that VPPs cannot predict planing ability, so predictions of planing speed for sailboats have to come from other sources, such as model testing and CFD analysis.

    That's the perspective from my point of view.

  5. Mikko Brummer
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    Mikko Brummer Senior Member

    In static, bow down trim and heeling may indeed orient the keel towards the side of the heel. But at speed, the boat (and the keel) will then re-orient itself to the necessary, positive leeway angle on the keel, to produce the sideforce needed to cancel the heeling force. This means that the hull may travel more side-on through the water, but the keel will always seek its positive angle of attack. The leeway angle measured to boat centerline will then be larger than for the skinny boat, but their keels will travel at more or less the same angle through the water.
    1 person likes this.
  6. RHough
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    RHough Retro Dude

    Yes they are. The wide, skiff-like hulls are not sailed flat upwind. They are sailed at a higher angle of heel. They are very skinny *in the water* since they are sailed on edge. They can use the power and planing ability of a wide stern off the wind at shallow heel angles.

  7. Karsten
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    Karsten Senior Member

    First some data. Wo has a length to beam ratio of 6 and Comanche about 4. Both are not extreme. Ranger has 6.5 and the regular production boat about 3.3. Wo overtook Comanche in some light air. Wo has lower drag due to less weight and smaller wetted surface in these conditions. I reckon Comanche just doesn't have the sail are to keep up with Wo under these conditions and possibly a large part of the power the sails could provide is destroyed by worse motion of the boat.

  8. daiquiri
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    daiquiri Engineering and Design

    True. And the extreme development of that approach are the scows. They have an advantage of not changing the trim when heeled (the point discussed by Eric and Mikko), but are not suitable for rough seas because the large flat bow wants to dive into head-on waves.
    In any case, they are definitely impressive sailing machines. Check this video of the pure joy: :)
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