This article seemed to annoy some folks over on some other forums, but they were somewhat mixed up in their thinking – as we all are if we accept the normal paradigm of planing, semiplaning and hullspeed.

Whatever way you look at it – it doesn’t hold water except as a very broad classification – the boats really don’t behave that differently at all.

One of the nice emails I got was from Tom Lathrop who designs easily driven powerboats that perform nicely with small powerplants.

Tom said

I read your thoughts on the planing phenomena and agree that the emphasis on “hull speed” is an arbitrary idea and a deterrent to understanding what is going on under a boat. Since all my power cruising boats are intended to operate in the transition range normally assigned to semi-planing or semi-displacement or whatever it might be called, I have spent some time in looking at this area. I collected and published my thought on my website under “Planing Boat Theory”. http://www.bluejacketboats.com

I look at the phenomena from the aspect of the water rather than from the boat. This disturbs some hydrodynamicists who want to see everything as flow streamlines but it think it allows a clearer look and a better understanding for non professionals like us.

My reply after reading Tom’s article above

Darn, Just read your article – really nice. Two main shocks … that the diagram from Elliason (an excellent writer and educator) is silly and the elegance of your mirror image diagram.

With Elliason’s diagram see how the plate is above the oncoming water – it’s flying and would not be in contact with the water to initiate this flow.

Actually I am a crap counter … three … the same annoying statements prompted you to write the article too and you refuse to label!

One hesitation only … that there are two responses of a “pea” … that is to approach the plate and then be co-erced into going either forward parallel with the plate of aft. No bounce. Otherwise there would be air gap! :) I remember from first year uni we played with a Pelton wheel – a simple water turbine with very cupped blades. The maximum power was when the velocity of the wheel (in an instantaneous calculus sense) was half that of the water flow as the 180 degree cup extracted double the momentum from the water and dumped the water downwards with little forward or reverse momentum. In the planing plate the water (OK guessing from this point) would impart momentum from the direction change to move parallel with the plate and only a minor amount from a velocity change .. but I have no idea of the proportions.

I don’t agree at all with Bolgers Pea arguments about what happens at chines for the most part … doesn’t fit observable facts. Never a vortex off a Goat or other efficient boat chine despite the greatly different curvature rates and angle of incidence of side of boat vs bottom. These are unavoidable in lighter displacement boats with low (or no) deadrise. I think it is something about minimising the energy state of the system. The water goes with the flow. Your engineering instincts would tell you if I’m kindof right or kindof wrong on this. I haven’t thought about it enough yet.

Just want to get people thinking rather than accepting nonsensical stuff.

Good one, MIK. I think you’re onto something. One assertion that bothered me, however, was that a shallow hull can’t generate a deep trough in the middle–due to its shallow hull shape.

I wrote a reply there, but I will put it here too as it may bug some other people too…

I guess it is a matter of perspective.

Is it more that a shallow boat is light so that when the trough forms under the ends don’t need to sink down much to compensate? The comment about shallow hulls not being able to create deep waves is from Bolger. Maybe I accepted it roo easily.

I think it is at least partially right at least. At hullspeed a deep heavy boat will have a deep trough but a light one won’t. It is at least partially due to the effect of the deeper volume. Wavelengths will be the same if course.

Both those explanations are the same one I think … just from slightly differing perspectives