One thing to remember about the proas is that they have an even fore-aft weight distribution with the weight concentrated close to the center.  Thus, the boats are likely to respond as observed: bow-down while sailing, potentially slightly bow-up while motoring.

  Things are different on a cat or tri where the weight (mast, motor, crew) can be move farther aft, supported by a wide hull section, and buoyancy added farther forward (relative to the weight) with hull length.  Those boats will be more level when resisting sailing loads at a given weight.

  The slender wave-piercing hulls on the proa, though, aren't going to produce much lift or suction on either end, and they aren't going to balance out sailing loads as level as a single-direction hull.

  Fortunately, since the proa is going to have more speed at a given weight, and build cost, the non-optimum fore/aft balance isn't an issue.

        - Mike

 

George, 

I am sure there is a communications problem, from my end mostly. My background is in aeronautics. I have been sailing for less than three years and never considered sailing before then.

"Planning is when the hull bottom generates lift to substantially reduce the water displaced. It does require a somewhat flat bottom, sloping up to the bow, but it is rarely compatible with wave piercing bows, or with very narrow hulls."

All boats, just like airplanes, have to displace their weight at all times. A displacement boat displaces its weight by volume and a planing hull displaces its weight via pressure (like pointing a fire hose straight down).

"Planning is when the hull bottom generates lift to substantially reduce the water displaced."

No, planing is just using a smaller surface to displace the weight of the boat. Think of it as an integral in calculus.

"A long thin hull pushes the water more gradually and less far (for the same displacement) and so produces a smaller wave system and produces less wave drag. The downside is that the wetted area of a long thin hull is higher than a short fat hull, so viscous drag is higher."

I believe you are talking about the prismatic coefficient and yes it produces less wave drag. But the wetted area of a long thin hull is the same as a short fat hull if they are the same basic shape, flat, elliptical, or V etc.

"Also, no real boat hull will have laminar flow over much of its surface, so viscous drag is bigger than you might imagine on paper."

Yes viscous drag can be huge and like all drag its effects multiply with speed. A rudder that loses laminar flow greatly increases its drag in a stalled position.

"Hull rocker has very little to do with it, as any Bernoulli effect vertical suction is small."

I think this is the heart of the misunderstanding between us. I think that the wave action and squatting of boats is a direct visible indicator of the Bernoulli effect and it is caused by the shape of the hull.

Place a spoon flat against a stream of water coming from a faucet and you can see and feel how the spoon is sucked into the stream.  Then do the same with a knife blade it is almost unaffected.

A curved displacement hull is very similar to a spoon and the faster it goes the higher the Bernoulli effect and the more downward force is exerted on the boat.

But I suppose I see that differently too, just like I see the HP's bow down and stern up and Ricks pedal boat bow up and stern down.