Definition of Planing

That's a fair point. There's no need for a definition to compare things that can be observed side by side.

When comparing to published boat tests, to answer "how fast does it plane" or "is it faster than others to plane", a decision has to be made when to stop the stopwatch.

 

An example of the back end on the boat with a slight rise is Bryan Jackson from New Zealand who owned a boat called the Caracat pictured in the attachment, we talked some years ago which by chance he showed me his Caracat he designed that he used to win on many times an ocean race against other launches which was timed and also had the best gas mileage recorded for each boat.
This Caracat middle part between of the twin hulls gradually sloped to the rare bottom end, then it had a slight raise the last meter or so which the water run level on this planning that gave better performance and was like the hulls I was making for the landing barges.
Later I saw him again and he said he could not understand why no one does this to smaller catamarans which I showed him a design I had of a 4.8 model catamaran I designed and he said when he saw it “that’s it”
It’s one of my future projects amongst many other designs I will build.
With the 60 mile ocean boat race he broke the speed record by miles as well as used the least fuel.
I have been trying to find the history and can only find this picture of the Caracat in the back ground of his caravan manufacture yard.
It’s a simple design that allows less horsepower to start the boat planning which keeps the back bottom level with the water surface so it does not bog down.

 

Very interesting design of the superstructure on that Caracat! It could be considered modern even by today's standars. Thanks for that great pic, Myark!

 

yes, I think so, but if the weight of the volume is more than the dynamic lift, than the water-level in a pool must climb a little bit higher if a model starts to go faster, is that so???

it seems we do not understand planing verry well, but there are also questions for the displacment mode of a boat,

one is: can a diver on the ground of the water feel a boat that is navigate over him, will the pressure at his place change in this case???

is the answer different if this boat is navigate in displacment mode or planing???

 

If you make a hull plane in a pool (probably a long one) the wake will reach the sides and break over it. That will disperse some of the energy. The bow wave produces some lift, but there is a lateral component too.

 

Don't think I know the answer but I do think that if I am in a ship passing over a pressure activated mine, I want to be going either very fast or very slow.

On the issue of whether the mean water level changes if a boat is in displacement or planing, I also have no guarantee. But, since the force required to support a boat is identical (neglecting aero forces), I think the mean water level should remain the same whether the boat is in displacement mode, planing or sitting still. Tonight is good for guesses.

 

You can calculate the sea-bed pressure using Michlet. If you are very nerdy,
you can design a mine to detect a particular ship, and a ship to try to avoid
the mine.

 

There was estimates and analysis done during WWII to determine the change in air pressure that resulted from an aircraft flying over. It's a similar problem to the ship in the water. What they found was that the increase in air pressure was similar to the sound pattern, that is if you could hear the airplane, you could measure the pressure increase of the passage.

For a boat in a tank of fixed volume, the tank level with the boat planing should be lower than if it is sitting still. The rationale is that there is a pressure increase in the water caused by the passage, and since water is incompressible, that force acts to lift the boat out of the water, and therefore there is less total fluid displaced.

 

Maybe so but it does not seem obvious to me. Any hydrodynamic force lifting the boat up has an opposite downward force on the water. Why would this not act to keep the water at its mean level? Don't see how incompressibility of water enters into the equation.

 

A boat lifts in the front then naturally is a down ward force in the back so what I design is a slight rise in the back end allowing this force to run level with the water surface while planning.
An example pictured with the Myark folding barge 7.2 X 3.6 has two 15 hp for stroke out boards with the smaller 5. X 2.4 Myark folding barge has a more than needed at 40hp.
The 8.landing Myark landing barge pictured has two 25 hp out boards that produces a good speed along side economical on fuel.

 

Tom,

Think of it as a force over an area. Eventually all of the forces have to balance. To support the boat in the proverbial tank, the force of the fluid on the bottom has to equal the mass of the boat, plus the mass of the water. If there is a pressure force that increases the bottom pressure, then you need less height of water to get equilibrium. Since the fluid is incompressible, the height of the column of water is lower. All just a theory of course, but it does make sense.

Clearly the amount of water displaced by the hull is reduced. The direct hull displacement goes down, and we can all agree on that. The only question is, does the volume of water go somewhere else? You can make the arguement that there is a volume of water that is behind the hull that is also displaced downward, and that volume could be equal to the amount of displacement of the hull, but it would take a lot of calculation to prove that. I can see it either way.

 

The dynamic lift is produced by the inertia of the water as opposed to the displaced volume of water.

 

That doesn't help much. Both are just pressure forces integrated over the hull. All forces must be in balance.

Let's think it another way around. Think of a tank of water on a scale. The scale will read the weight of the tank + water. Now put a boat into the tank. Obviously the reading of the scale will go up the weight of the boat. As long as the weight of the boat is somehow supported by the tank (and thus water) the reading of the scale will be the same.

How does the weight of the water and boat transfer to the bottom of the tank and then to the scale? By pressure of course. So a planing boat will increase the average pressure on the bottom of the tank just as much as a displacement boat. Thus the average water level will not change due to planing.

 

ok, lets start a helicopter from the surface of the tank with fixed volume, before the start the wight of the heli will swimm like a catamaran with his two swimmers in the water(archimedes), if the swimmers are lifted one micro-meter out of the water the water sweps into both holes under the swimmers and we think the level will be lower a bit around. but at the same time you can messure the pressure of the rotor on the surface of the water, as you has discribed it in your text, while water is nearly incompressible the higher pressure from the rotor forms a new hole in the surface of the water

truly if the heli is flying a longer time at the same high over the surface (hoovering)

I think, at the side of the tank/pool fare away from the downwash of the rotor the level will be as before +/- 0!

the volume of the new hole will be the same as before, only the form has changed,

maybe if the heli climbs to the clouds the hole will be very large and veryvery little deap

IMO for the diver at the ground of the tank the pressure of the lower water and the higher air pressure must be together the same as before , with his pressure gauge he can not deside if the heli is swimming or flying, (nor if he is diving in free water or under a floating objekt at anker)

is there a difference if the lift is generated from a rotor (or wing) over the water or from the dynamic lift of a flat bottom of a gliding boat with a trim angle (or lifting foil...)??? plays it a role that the water has not enough time to move to the side to build the new hole, is that the reason for the higher pressure under a gliding boat?

 

I arrived at it another way round but my conclusion is the same. The mean water level cannot change due to planing. May be a little off the main topic but an interesting brain exercise. I included any vertical asymmetry in the wave propagated in the mean water level.

Need to forget about compressibility of water. It has no bearing on this problem.