Taldorwood,
Not sure where you get your hydrodynamics from. Maybe there is a communication problem, but your hull speed/displacement explanation sounds completely wrong to me.
I'm not an expert but this is my take:
A displacement hull is one that displaces a relatively constant amount of water whether moving or stationary - on a harry proa (without hydrofoils) the waterline doesn't change much when moving, so I call it a displacement hull, not planning.
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.
Wave-making drag is generated (as the name suggests) because making waves requires energy. Both the bow and stern of any object moving through the water surface will generate a wave, and thus some drag. Waves from the bow and stern (and indeed from separate hulls on multihulls) interact and can either enhance each other (more drag) or cancel each other(less drag) somewhat. How they interact depends on (among other things) speed. "Hull speed" is when bow and stern waves of a hull are enhancing each other, and producing a local maxima in the drag curve. In heavy displacement (typical monohull) boats this wave-making drag dominates as they get near hull speed.
For a given boat mass there are some ways to reduce wave-making drag, such as planing and hydrofoils (which use some induced drag to reduce the water displaced and thus reduce both wave drag and viscous drag) or by making the hull long and thin like a multihull.
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.
The net result is that for a multihull even at hull speed the viscous drag can be significant compared to the wave-making drag, and formulas which ignore this (valid for short/heavy boats) do not produce valid results.
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.
Hull rocker has very little to do with it, as any Bernoulli effect vertical suction is small.
George
I'm not an expert but this is my take:
A displacement hull is one that displaces a relatively constant amount of water whether moving or stationary - on a harry proa (without hydrofoils) the waterline doesn't change much when moving, so I call it a displacement hull, not planning.
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.
Wave-making drag is generated (as the name suggests) because making waves requires energy. Both the bow and stern of any object moving through the water surface will generate a wave, and thus some drag. Waves from the bow and stern (and indeed from separate hulls on multihulls) interact and can either enhance each other (more drag) or cancel each other(less drag) somewhat. How they interact depends on (among other things) speed. "Hull speed" is when bow and stern waves of a hull are enhancing each other, and producing a local maxima in the drag curve. In heavy displacement (typical monohull) boats this wave-making drag dominates as they get near hull speed.
For a given boat mass there are some ways to reduce wave-making drag, such as planing and hydrofoils (which use some induced drag to reduce the water displaced and thus reduce both wave drag and viscous drag) or by making the hull long and thin like a multihull.
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.
The net result is that for a multihull even at hull speed the viscous drag can be significant compared to the wave-making drag, and formulas which ignore this (valid for short/heavy boats) do not produce valid results.
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.
Hull rocker has very little to do with it, as any Bernoulli effect vertical suction is small.
George
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Posted by: George Brettingham-Moore <gbrettin@yahoo.com.au>
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