I can provide some comments on ideas for hull shape.
I have been working on a proa design for single handed use in protected water per the rendering here:
The total displacement is 250kg. Static condition is 200kg on ww hull.
The shape of the windward hull is the lowest drag displacement hull for 6kts carrying 200kg having a flat bottom.
The ww hull is the lowest drag displacement hull for 200kg at 16kts with a flat bottom although at that speed and load it will be planing so the displacement aspect is somewhat meaningless at that point but it has to get there and above.
The ww hull ends up longer than the lw hull for this size boat based on my starting point.
The shapes are distinctly different and highlights the advantage of the proa configuration over a catamaran. The fine entry and long waterline length on the ww hull reduces wave drag in displacement mode.
The full bow and deep draft of the ww hull works to reduce wetted surface when viscous drag dominates overall drag at high speed. The full bow gives high KMl for the waterline length and the wave pressure at the bow creates buoyant lift once under way. This nose-up attitude assists transition to planing.
Both hulls have narrow flat bottoms that will plane at shallow angle of attack once speed gets over 10 to 12 knots. This means the lift to drag is high once in planing mode. There is no hump to planing just a gradual transition.
I have shown a centreboard and two rudders under the lw hull. This overcomes some of the vortex shedding issue I envisaged with two larger bi-directional rudders intended for leeway prevention. The centreboard is cambered with small radii edges. The intention is that it will work over a very narrow range - the goal is to have it at zero angle of attack. It needs to be elevated and lowered to achieve this to balance leeway forces from the wing and hulls. The rudders do not do much work so are also bi-directional with small radii edges but non-cambered. All the underwater control surfaces are under the hull to lower risk of ventilation and sudden changes in control. I expect the leading rudder will ventilate in waves but this should only result in a slight reduction in drag not sudden changes in control forces.
I have shown a wing over a flat elevated deck that works as a bottom end plate for the wing and will increase L/D over a wing without end plate.
It is intended the hulls can be transported side-by-side with cross beams removed. Wing would be light enough for a person to remove with the lw hull on its side.
As a proa is scaled up the relative lengths of the hulls will change. However with displacement mode there is a point where increasing length is detrimental to overall drag. There may be some benefit for drag reduction in waves through less pitching but this depends on the hull length relative to the waves encountered. There is a point for any hull where pitching will increase drag. Low reserve buoyancy in the ends with wave piercing deck seems the best way to reduce this drag factor as this reduces tendency to pitch. The bridge clearance needs to be high to avoid it ploughing through waves that the hulls slice through.
I have done polars for this design and it looks quite good. There is a performance advantage in having small rudders and cambered elevating centreboard over two large rudders. I also expect the control will be more stable as the centreboard has a set level of immersion and is not likely to ventilate easily.
Rick
On 11/05/2011, at 10:12 AM, Rob Denney wrote:
Doug,
Not sure about the ultimate skinniness. There are two pluses of long and skinny. One is top speed, the other is resistance to nose diving and pitching. How skinny is too skinny depends on the weight to be carried and the engineering complexity. Suggest you start with a calculation of the final weight of the boat, then the best length for handling, marina, build space etc and design around that. Your lee hull requirements are correct. Also needs to get the beams clear of the water, and on some boats have storage/sleeping space.
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