Trousers said...
So reading thru a previous post, I started to wonder, exactly what planing is. At what point can you say 'I am planing'?
Sure it's accademic, and knowing a technical description doesn't really change anything for all but the most technically-minded of sailors, but I'm keen to hear the internet hive-mind's thoughts on this.
I'll kick off...a quick google brought me to a yachting site where someone said;
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However if you take the definition of going faster than the speed of a wave of the same length of the hull ie the point afterwhich the hull will climb over the bow wave (hull speed)...
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So does that mean once you are going faster than the bow wave caused by your board pushing through water, you rise above it, get released from it's resistance and you're planing? Obviously lift factors in there somewhere.
There are many "pop" definitions (most of which are completely illogical - the idea that going faster than hull speed is planing completely ignores thin-vessel dynamics) but the most popular one for naval architects is that planing occurs when dynamic lift causes the hull to lift above its static centre of gravity.
To explain - as we all know, something floats because it displaces a volume of water equal to its weight. The height at which a vessel floats when stopped is related to its displacement etc.
When any vessel starts moving, it creates waves. The way that the waves work, and especially the way they work at high displacement speeds, is that they migrate to the bow and to the stern, leaving a hollow in the water level amidships. A boat moving at high displacement speed sinks into this hollow, and because there is less volume at the ends (where the waves are) and therefore it has to sink lower to keep the immersed volume at the amount that is required to support the craft by displacement forces. This tendency for a craft to sink when it moves at displacement speeds is called "squat".
A wave cannot move faster than a Froude value of 1 (equal in knots to 1.34??? times the length between crests IIRC). At a Fv of 1, the drag caused by the waves is very high- this is "hull speed". The waves are piled up into one big bow wave and one big stern wave.
When a boat moves faster than a Froud value of 1, it will move faster than the waves it creates and the stern wave will be left behind. Some people say that this point (where the wake off the stern is flat) is planing, but this can happen with some craft when they are not planing and it can happen with some planing craft at planing speeds.
What happens with a planing hull is that as it moves across the water, it shoves water particles down. This causes an equal and opposite reaction that lifts the hull higher. The hull can then lift out of the wave train that is slowing it by only moving at a Froude value of 1. The lifting of the hull also reduces wetted surface.
Because planing is created by moving water particles down, a wide hull (which shoves more particles down at the "stagnation point" where the front of the hull meets the water) is more effective at developing planing lift than a narrow hull (which has a smaller stagnation point, because much of its "planing area" is further back, where the water has already been "shoved down". The induced drag of a planing surface drops with the square of the width...unfortunately under planing speed, drag basically INCREASES by the square of the width and also a shorter hull, being limited more by the distance between bow and stern waves, hits its "hull speed" at a lower speed and therefore facing high drag at a time when it's not moving fast enough to create lots of dynamic lift and therefore pop onto the plane.
I haven't done a great job of expressing this, but this is the way I understand it from talking to naval architects and reading the texts. It certainly explains the way wide boats v skinny boards and longboards work. One point is that many of the effects of hydrodynamics and aerodynamics vary at the square of the speed (up and down) which is why a longboard can work so well at (say) 8 knots of wind compared to a Formula board, but the Formula board works so much better in 10 knots.
