Planing speed

From David
"The velocity of the water where it separates from the transom is higher than the freestream velocity, and this increase in speed corresponds to a decrease in pressure which offsets the increase in pressure of the water due to being below the undisturbed free surface height. The two effects offset and result in the pressure of the water being atmospheric."

Give this another go as I am not sure what you are referring to as the freestream speed. Say the boat is going 30 knots relative to a fixed point on earth. Are you saying that the water coming down the boat is going faster than 30 knots as it goes down the side?

David says
"This is an example of why gravitational effects on pressure should not be neglected when analyzing the flow around a boat on the surface."
Not sure what you are saying here,

 

From the perspective of an observer on the boat the freestream velocity of the water is 30 knots. From the same perspective the water changes speed as it goes past the boat, in some areas it is faster than 30 knots, and in other areas it is slower than 30 knots.

From the perspective of an observer in a position fixed to "earth" who takes an instantaneous snapshot of the water as the boat passes, the water near the boat in some areas is moving in the direction opposite the direction the boat is traveling while in other areas it is moving in the same direction as the boat.

The analysis can be worked from either perspective but one may be more convenient than the other.

If the effects of change in elevation on pressure of the water are neglected then erroneous conclusions can be reached about the flow of water around a boat.

PS You might want to start using the "Quote" featuring when replying to a post rather than cut and pasting. The Quote feature shows what is being quoted versus what is the new content.

 

Yellowjacket says
There isn't enough pressure at that shallow a depth to "shoot the water up like a rooster tail immediately after the transom" like you are thinking it should.
Barrys reply:At a 12 inch depth, the pressure from buoyancy should be 5 1/2 psi, and this would be enough <if the buoyant forces were there> to accelerate the water up extremely quickly. And I am not thinking that it should, because I do not think that there is a buoyancy component due to the fact that the hull is not immersed by definition if all of the sides below the water line are not encapsulated by water.


Yellowjacket states:
"There is a pressure distribution under a planing hull and the pressure can actually go well below atmospheric and drag the back of the hull down into the water."

The hull is planing, the chine, keel, are parallel, . There is no negative pressure under the hull due to speed. In a displacement hull yes, in a planing hull, no. As long as you have the planing surfaces in a bow up attitude to the water flow, all pressures are positive.

 

Re-check that 5.5 psi figure, more like 0.5 psi I think.

 

Why are you certain that the pressure under a hull which is planning is always positive? (I assume by positive you mean above atmospheric.)

 

Mathematical modeling of longitudinal dynamic pressure
distribution on planing hulls
Sasan Tavakoli, Parviz Ghadimi*, Abbas Dashtimanesh, Seyed Reza Djeddi
Department of Marine Technology, Amirkabir University of Technology
*Corresponding author E-mail: pghadimi@aut.ac.ir
David
There are quite a few graphs in this paper as well as the book on Performance of High Speed Planing Hulls.
Sorry I could not pull the link but googling this should get you there, if not come back to me on it.

Mr E
right, missed a decimal point

 

Of other importance is the fact that all the graphs show that the pressure at the transom is zero.

 

If the hull has any amount of rocker in it, or, more commonly, the hull has increasing deadrise as you go forward, then the pressure can go negative if the cg is far enough forward. In that case the front of the hull would have positive angle of incidence but the rear can go negative and the suck the back of the hull down. If the hull is a simple flat surface then no it can't, but in practice it absolutely can.

With only a half a psi how much rooster tail do you expect to generate? The answer is not very much, and if you look behind any planing craft there is a "hump" behind the hull. The faster you go the further back it is from the transom, but it's there none the less, that pesky momentum thing making it happen further behind the boat the faster you go.

Air separates off the back of a car at speed and it is much the same as a boat, and this happens at a very low speed in the boat due to the high mass of water, but the effect is the same in that it is simply flow separation, and it isn't related in any way to planing.

 

Shouldn't be to hard to get answers to the pressures question with strategically placed pressure sensors, flush with the hull. But the sensor won't know what the mix of dynamic and buoyant lift is, making up that pressure, only the amount of pressure.

 

And that is the problem, is there buoyancy forces at work in a planing hull when the transom ventilates, when there is no pressure on the back of the transom to create an immersed hull which is held up by buoyant forces.
I have been trying to come up with a solution to putting a sensor on that would not impact the flow around it. I had considered drilling small holes in an aluminum hull and hooking up manometers to measure pressure but I think that the drill hole would catch the water like a scoop and give a false reading.
Or conversely, the manometer tube would fill and the water that is going down the hull would lower the pressure reading because of the shear forces at the interface of the inlet.
I understand that there are pressure mats but have been unable to find them that could be glued on to a hull and become then the hull to water interface. Any suggestions?

Yellowjacket
"With only a half a psi how much rooster tail do you expect to generate"
I do not expect any, I originally said that years ago I expected dynamic and buoyant forces acting on the bottom edge of the transom. But the data that I have seen, from people who have quantified this pressure seem to say that it is zero. So if it zero then there are not buoyant forces.
Re your comment that the back can be sucked down due to negative pressure in a parallel chine/ keel planing hull, I cannot believe this if the hull surfaces are flat and the attitude is up, irrespective of the velocity of the water. There seems to be a belief that the faster the water flows, the lower the pressure. If this were true, then a planing hull would not reduce wetted surface as it climbs higher out of the water as speed increases. For curved surfaces, foils, displacement hulls, of course. But the higher the angle of attack, the higher the dynamic pressure will be on a flat surface.

 

Pressure measurements on a planing hull http://www.researchgate.net/publication/44063766_Planing_hull_model_tests_for_CFD_validation

"Hull pressures on the model were measured using 9 pressure taps mounted flush to the hull bottom at various locations. Several of these pressure taps malfunctioned during tests while others encountered relatively high levels of noise. The final results could not therefore be relied upon for specific quantitative information of the pressure distribution on the hull. They can, however, be used to show the range of pressure on the hull and identify certain trends that developed with increasing model speed. The most notable of these are shown in Figure 10."

 

The hull pressure graphs from the paper above

 

The fact that the hull is in motion does not effect the buoyancy component created by the displacement of water. The buoyancy component is there if water is displaced. As noted by others there is a dynamic pressure on the hull surfaces based on the momentum component (planing angle and velocity) and there is a component due to the velocity (Bernoulli's principle).

Your assumption that the static displacement force goes away because there is no local pressure force at the back of the hull is not correct. As I said previously, look at semi-planing hulls and you will see that the hull still has a displacement component, and as the hull lifts that is decreased. However even in a full planing hull there will always be a displacement component since the hull surface is always slightly immersed in the water. Here is a discussion of the forces on a planing hull that explains that the displacement component is always there.

http://books.google.com/books?id=0V...l lift displacement planing summation&f=false

You are confusing a local pressure that has various components (dynamic lift and velocity effects) with an overall force on a body. The displacement component of lift never goes away and is always equal to the the amount of water displaced.

In short unless the hull is skimming on the surface and the planing surface depth is very small (and can then be ignored) the calculation of lift must include the effects of displacement.

 

Gee, the hull local pressure at the aft location went negative at higher speed...

As Gomer Pyle used to say "SURPRISE, SURPRISE!!!"

 

Hardly appropriate as the link that I put above shows graphs that show zero, not negative pressure at the transom.

So if we decide to accept a lower pressure in the back of the boat, two things would happen
1) as the boat goes faster the back of the boat would go down-----------this does not happen, higher speeds result in a flatter attack angle

2) if you drill a one inch hole in the bottom of the boat near the transom, then no water would rush in--------------we have had leaks around jet pump intakes within inches of the transom, and surprise surprise, water rushes in, which would be impossible if the pressure is lower. Please explain this.

As I said a little earlier, how would you install a pressure sensor whose physical penetration of the hull could cause some accuracy readings. Within this paper, the researcher expressed concerns as to Noise caused by sensor as well as saying 7 of the nine sensors failed. Why? because of high positive pressure or??

And if there is negative pressure at the transom, then there is no buoyancy in this area. ie buoyancy is a positive pressure