Technical query

This one by Kevin Ellway...

I never said it was a reasonable definition.  It is however the only definition of planing to which you can provide a simple answer to the question.

As you know better than I do Jim there are far more factors than just length that affect the onset of planing (rocker profile, rise of floor, beam, trim, heel just to name a few).  Apart from that there is a continuum from 100% of the lift required to stop the hull from sinking coming from displacement to 100% coming from dynamic lift (if you can ever actually achieve this). 

Where on this continuum do you define planing as starting?  The answer is that you can't define it without an arbitrary or subjective imposition of a border between the two.

Depends on the class I think. Merlins, for example, have been seeking earlier planing for the last decade and will now, in very favourable conditions, plane upwind.

AFAIK the definition of planing refers to the hull being raised above its wavetrain by hydrodynamic lift. Planing is, as you pointed out, not the only way hull speed can be exceeded. Very long narrow hulls such as multihulls and some warships can also exceed hull speed without planing.

Lift is defined by the equation:

L=0.5CLñsv2

NOTE: ñ should be Rho the greek letter P but the symbol won't work on here!

where CL is the coefficient of lift, ñ is the density of water, s is the surface area and v is the velocity squared

Lift will be close to the displacement weight at the point of planning, i.e is the boat and crew weighs 300kg then 300kg of lift would take it out the water but something less will start to lift it out.

Rearranging for speed gives

v = square root (L/0.5CLñs)

The density of water is fixed and the coefficient for lift ahould be assumed relatively constant so the only thing that affects the speed is surface area.

This is why in all RYA learn to sail bits it tell you to lean back when its windy on a reach.  This pulls the bow up and effectively reduces surface area and hence the potential to plane.

If you had a boat with flat sections all the way down the hull it is hard to reduce surface area even when hanging of the back.  Clever designers produce hull shapes that allow the surface area to be reduced the faster the boat goes.  Think of a jet plane wing.  Next time you take off look at the wing.  They extend the trailing edge to increase surface area at lower speeds to increase lift.  Once in the air and speed increase they retract the trailing edge to give a smaller surface area and therefore the ability to fly faster.  If only it was this simple in a dinghy

Look at RIBS and you will see that most have a 'shallow' V hull i.e. relatively flat, which is good for getting onto the plane but then difficult to reduce wetted area easily.  This is why it has spray rails to allow the boat to climb progressively up the rails and hence reduce wetted area.  Offshore RIBS tend to have 'deep' V hulls so that the wetted area can quickly be reduced to give higher speed potentials.

So our friend Froude quite rightly says that a long boat will go faster than a shorter one but a shorter one can plane faster if the hull design allows wetted area to be reduced and has the available power to maintain the increases in speed required.