Definition of Planing

There is potential energy, since the water "tries" to have a flat surface and you have deflected it. This potential energy is internal to the fluid.

 

You need a force to keep that plate in place. That might be the weight of the plate or an extrernal force. For a planing boat that force is equal to its weight and thus its static displacement.

 

Mistake on my part. I'll correct it.

 

Another way to create a "hole in the water" though not planing. Place a thin plate (negiligable displacement) vertically in the water and then move it normal to the surface of the plate. The surface of the water will be higher ahead of the plate and lower behind. My initial thought is the size of the "hole" behind the plate will be related to the energy required to move the plate.

 

I think that the energy of the dynamic lift at planing should be equal to the difference in volume of water displaced.

 

Hi latestarter, Welcome to the discussion. Your statement is not true for a moving plate. The displacement is equal to the water displaced and the plate thickness has no bearing on it.

 

As for the energy injected into lateral waves, I was trying to make the problem simpler by eliminating some variables like that. In this case, as in much common engineering analysis, I considered a flat plate of infinite width to eliminate edge effects and side waves for a V bottom hull. If we can understand this simple case, maybe it can be extended to the more complex ones. For the infinite width plate, or rather a section in the middle of it, the only "hole" is aft as the trough in the standing waves.

I also don't like having the experiment in a tank of finite size along with its messy side or bottom issues.

 

Tom, my definition of displacement in this instance is the volume of the vessel, in this case the plate.
Water is displaced when the plate moves but that is a different matter.

 

We can't have a private definitions if we are to agree on anything. We are discussing the dynamic case of planing displacement. If we used your definition, none of the full planing hard chine boats would have any displacement when planing since they all clear the water at the chine edge just like plate of small thickness. Same thing as a water skier.

 

It was not my intention to have my own definition, normal one of the displacement of the water when the vessel is floating suits me fine.

We all seem to agree that the reduced displacement when planing is the static displacement less the lift created by the planing force.

What I was trying to do was to cut through the confusion by isolating the planing force from the rest of the factors.

If the planing force is high enough no displacement force would be required to support the boat e.g. a hydroplane or your example of water skis.

 

Assuming a planing force dictates that there is some displacement or else the boat would not be in contact with the water to generate that planing force. If the hull is reacting with the water by contact, then there is some displacement. That is apparently accepted by everyone. All hydroplanes I know of and especially water skis do have displacement when planing. If water is displaced, there is displacement. In the case of a waterskier, at rest displacement could be less than the weight of the skier and skis, so they sink. Because of the small planing surface of a ski, there is usually quite a lot of displacement, even at high speed.

 

What definition of "displacement" while the boat is planing is is meant by folks who are using the term?

I can think of two alternatives. There might be others.
1) Hull volume below the average height of the free surface?
2) Hull volume immersed in the water while the boat is planing, with immersed defined as below where the surface of the water is in contact with the hull. This volume may be difficult to define precisely?

Please note, I am not saying either is the correct definition.

 

David, How could it be other than #2? The displacement while planing is dynamic and changes with every motion of the hull relative to the water. Lift is also dynamic and any difference between the sum of these two and at-rest displacement results in the vertical accelerations that are so common in boats. This is a bit simplistic but still covers the basics while ignoring any other forces like drag.

I think you are correct that it is difficult to define planing displacement in other than as a steady state in perfectly calm water with a perfectly balanced boat running straight at a constant trim angle.

It is best to understand the simplest case in any event before adding complications that confuse the issue.

 

even for slow displacement boats it will be not possible to define "the surface of the water is in contact with the boat" what is the high of the surface under the middle of the bow? the same as at the side left and right? or higher?

we can truly messure the pressure under each part of the hull and design a volume of equivalent waterhight at this place inside the surface of the hull, and say this volume has for a displacement boat the same volume like the underwater-hull , (and if the boat lays at anker it has the same form as the underwater-hull )

if we do this with a planing boat we can also messure the pressure at each part of the hull with watercontact, but we can not seperate the statik pressure from the dynamic pressure since pressure is pressure,

we can only calculate the difference to a reference pressure and made a definition of the dynamic-part and statik-part

maybe in the way the reference pressure is only statik and the differenz is +/- dynamic

but if we use the level of the next free water at the side of the hull, this will bring no god results

so I think it is only possible to use the pressure of the average free water as reference----->1.)

 

first thought: if the boat starts to plane you messure a high peak for the time in which the boat is climbing, and a low peak for the landing phase, (the repulsion of the vertical acceleration)

but for a longer constant planing phase the whight must be the same as for a long rest phase

so the boat need added power for the climb , more than later for holding the higher level....


second thought: it depends on the form of the new hole in the water behind the boat


definition planing: planing starts if the hole in the water leaves the contour of the hull.......