Definition of Planing

For every planing depth you can have ripples/waves that are higher than 50%. And nothing exceptional is going to happen unless the waves become long enough to spoil the steady state situation. Thus I don't believe that is the reason for the inaccuracy of Savitsky you have noticed.

As I said I could get very low resistances from my Savitsky solver for D runabout, if I just set the LCG way back. LCG=0.25 m shows 55 hp at 89.5 mph (beam=0.65 m, disp=240 kg, propeller efficiency 65%). Of course there are other parameters for aerodynamic drag, drive unit drag etc. which affect the LCG which will give that power. . Most likely LCG is not that small. At LCG=0.4 the power jumps to 75 hp and LCG=0.15 m => 41 hp. So the model is very sensitive to LCG and thus trim. The trims for these three cases are between 0.68 and 1.09 degrees, but that small change more than doubles wetted area (Lk 0.18->0.52 m), which is the reason for huge differences in drag.

Do you agree with the above results? If yes, the only problem is predicting trim correctly for that case. It may be that Savitsky model does not work at that small trim angles. It is valid only for 2-15 degrees. But more likely the problem is in the aerodynamic lift and moment, which can be quite complex when the flat hull is riding very close to water surface. Predicting flip over doesn't really show the model is accurate at smaller angles.

What kind of model have you used for aerodynamic lift and moment? Do you have some wind tunnel or CFD results for them?

 

Actually, one of the tricky problems in flight controls design is to know when the plane is flying and when it's on the ground, because the control system often has to behave differently for the two cases. For example, you don't want to drive the controls hard over trying to level out when the plane is just taxiing up a grade. There are usually weight-on-wheels switches in the landing gear that detect when the strut is compressed by the plane's weight. However, if one strut is compressed and the other not, is it flying or on the ground? When you compound this with the possibility that a sensor can be failed, then it really gets fun.

 

Then there is the question of whether the aircraft is in ground effect or not. My recollection is this can be especially critical for helicopters. And ground effect does not have a sharp, discernable boundary.

 

A really fast moving boat must be reaching the realms of flying.
Images show a boat going over the hump and on to planing..to more easily get on the plane a boat needs to be bow heavy so it falls down the surface of the power wave like a surfer hanging 5 or ten if you have them.

 

Yes it does. Flying in ground effect, at least. But the ones in your pics do not look particularily fast. THESE ones, for example, are fast:




Cheers

 

While you can find a ripple/wave height equal to 1/2 the planing depth, the problem is how many wave lengths are in your planing surface. If you have a small ripple with a short wave length, you can get 3 or 4 wavelengths within the calcualted planing surface length. If there aren't that many wavelengths, then the you are just jumping from one wave top to the next and that's an entirely different case. What I'm talking about here are small surface waves that the boat is riding on top of in a stable manner. As you noted, wave length is a key parameter. If the wave length is short enough that's when you get to this particular case.

You have directly confirmed exactly what I said about it not making sense. The CG IS at a .5 meters, and at that point you are calculating that it would take 75 hp to push this hull to these speeds. Your numbers are confirming my analysis very accurately. But the boats don't have nearly that much power, so something else is happening and this analysis method is clearly not accurate anymore. You are absolutely correct in that most of the drag is due to the wetted surface area, but something must be happening to reduce that wetted surface drag and Savitskly's analysis for sure doesn't explain it.

I agree with the above results because the confirm exactly what I've been saying, that someting else is happening.

As to the aerodynamic effects, I assumed a parabolic stagnation area ahead of the planing surface and applied a lift force over that area that was equal to the stagnation pressure for that speed. I applied a lift force equal to the free stream static pressure on the upper surfaces, to simulate the pressure over the top of the hull. I know that't not totally accurate in that there is some acceleration over the top of the hull, but it's a conservative approximation.

for the aero drag I used a CD for the hull of .28 since there is little stagnation other than the area under the hull, and the transom area is relatively small (the hull height tapers down to a low transom height on the sides, so it is pretty decent. I then added a CD for the driver (who is behind a fairing) of .8, multiplied by the frontal area abovce the fairing. The aero drag forces were applied at the centroid of the respective frontal areas.

While the aero forces are an approximation, I'm comfortable that they are in the ballpark and shouldn't be that far off.

 

thrust is below water surface and drag is at the water surface. This torque would increase the trim angle at significant amounts in the very high speed range. Maybe enough to go from 0.68 degrees to 1.09 for example?

 

That is taken into account already in the 1964 Savitsky model. Thrust is not that big compared to weight of the boat. 55 hp at 40 m/s (89.5 mph) gives only 660 N thrust (65% efficiency). That compared to 2350 N gravity force is not that much. You would need almost four times longer arm for thrust for the same moment. These boats have a small surface piercing propeller and propeller shaft close to keel line, thus thrust has a clearly smaller arm than gravity.

 

Exactly, the prop shaft is .75" below the planing surface (by rule), the props are typically 8" in diameter (or thereabouts), the depth of the thrust vector is about 5" below the planing surface, and typically these motors are actually run with a bit of "tuck", that is the prop shaft angles up by one or two degrees, which lowers the thrust moment relative to the CG.

Also, as the speeds get higher, the thrust is actually lower. Power is force times speed, and since power is constant, the higher the speed, the lower the thrust is.

 

So if thrust is 660/2350=.28 and if the thrust line goes 5" below the planning surface the change in moment is going to be equal to a change in COG of 0.28*5" = 1.4"

Given Joakims claim "So the model is very sensitive to LCG and thus trim." the thrust moment is significant.

Also, if it weren't significant why would Savitsky have felt the need to take it into account?

Joakim, what was the thrust line input you used?

 

I have used VCG=f=0.3 m, thus at the keel line, which is of course not correct. Setting it 0.127 m lower changes the trim to 0.72 degrees at LCG=0.4. Setting it 0.5 m lower (f=0.8 m) results to 0.83 degrees. Thus not so big changes in trim angle, but quite substantial changes in power: 75, 69 and 56 hp.

Now setting LCG to the real 0.5 m results to 0.74 degrees/66 hp at f=0.8 and 0.64/81 hp at f=0.427.

Thrust force is easy to model, thus there is no reason to leave it out. And it can have much bigger effect in some other cases.

Yellowjacket:

5" is probably center of the propeller area below water level. But what would be the thrust vector direction? Most likely not the same as prop shaft angle for surface piercing propeller. I have used e=0, but setting +- a few degrees doesn't really change much.

 

I agree that it probably isn't along the propshaft angle for sure, but who knows??? Different props have different amounts of rake, and the thrust vector could be all over the place, some props create lift aft (which would decrease the trim angle) while others create bow lift, which would increase trim angle. There are setups that are all over the place depending on the preference of the driver, the roughness of the water and the phase of the moon (ok, only joking about the phase of the moon)... It's not likely to be a big change in thrust angle, but I don't think anybody really knows what is happening, they only know through testing what seems to work, and there isn't that much science in it.

 

This is a really really fast boat.. a World Champion and they strangely call them Hydroplanes when these days they must surely be flying across the water with all the power that can be installed.Not much use for every day use.
I doubt if a fast deep V racer could ever be called a planing boat.

 

Where are we now !!!

Planning all the way through this thread people keep coming up with answers and then another senario get thrown in the pot and simmers away !!
Deep vee must be plainning even right to the very point of the vee untill it get to the point where the whole boat is eventually supported by air pressure and suspended above the waters surface !!
Hypotheticly is it possible to plane on a knive edge ????:?:

 

Perhaps someone will come up with a logical definition soon. Perhaps tradition is standing in our way, and rules and regulations in boat sport and racing.
It is difficult to create innovation in anything if you have specs set for you which have out lived their usefulness.