Planing speed

http://www2.worldpub.net/images/BL/maxum3100se600graph.jpg
This is a post that has a performance graph for a 31 Maxum SE, I could not find it for the 29 that Sedan owns. Unfortunately the mpg graph was not calculated but estimating some of the numbers and calculating a mpg figure, the mpg follows what I would expect
At low speeds, mpg is quite low, at 7.5 mph, 3 miles per gallon, then a drop around 10 knots to about 1.6 mpg then a better efficiency from 10 mph up to 20ish

 

A light boat with a transom which is submerged at rest can have the transom exposed at speeds below "hull speed" if the transom is not submerged too deeply at rest. Would you claim "no buoyant forces on the hull" in that case?

 

There is just pressure, regardless of speed, the pressure is affected by a number of factors underway, but only one when at rest, that being immersion depth.

 

From Dave
A light boat with a transom which is submerged at rest can have the transom exposed at speeds below "hull speed" if the transom is not submerged too deeply at rest. Would you claim "no buoyant forces on the hull" in that case?

The Froude number which sometimes is tossed about and sometimes applied to planing hulls , relates to displacement hulls. Much like the speed of sound prior to about 1945, the resistance of drag graphically was appearing to increase exponentially and approaching asymptotically 765 mph depending on temp, pressure etc. Of course this was broached. The Froude number suggests this wall but many displacement hulls operate above this "hull speed". Which could be due to a displacement hull achieving some lift and carrying it beyond the "hull speed limit"
So if in your comment that the transom is almost out of the water, it could be only their because the boat is extremely light, extremely wide or the lines do not follow my parameters for parallel lines of the keel and chines. As I was trying to keep the focus on a planing hull the topic of discussion, I tried to set some parameters. I would say that a planing hull acts like a displacement hull ( poorly designed as there is not a pressure recovery ability within the shape) until the water separates at the transom, then the hull is not immersed, because there is not water pressure on the transom.

From Mr Efficiency

There is just pressure, regardless of speed, the pressure is affected by a number of factors underway, but only one when at rest, that being immersion depth.

There is pressure but would you call it buoyancy?
If this is the case then all planing hulls are always in planing mode and displacement mode.

So would you say that when a hull is planing, by my definition, the transom ventilates, that you could take a cut horizontally through the hull, calculate the volume, then weigh of the water and say that this is the buoyant force acting on the hull. (though technically by definition, the body is not immersed which is a requirement of the term buoyant force. So a 5000 pound boat at the onset of the transom ventilating speed with 10 cubic feet of water below the water level adds 625 pounds of lift and the balance is dynamic loading or planing forces or 5000 - 625= 4365.

Or as I believe, but cannot substantiate, that once the hull ventilates, it does not have any buoyant forces because the hull is not immersed on all sides. A heavy hull such as the one I defined above at 5000 pounds is now 10,000 pounds, and of course sits deeper in the water. This hull sits lower in the water even at the transom ventilation speed but in order for the hull to hit my "planing speed" the hull is accelerating much more water and hence develops much more dynamic force due to the extra mass it is accelerating out of the way to carve a path that would be shaped like the transom up to the waterline. Ie accelerate more water, more force is developed

To define another concept
From reading some other threads, it appears that some would believe that planing lift is like an impingement of a stream of water that causes the dynamic force. ie like turning a water flow out of a hose on a hull and measuring the resultant force. But the force to move a boat in a planing mode is the force that is developed from moving the mass of water out the way in front of it. Say a boats keel at the back sits at 12 inches down from the water surface on plane. The top 3 inches (arbitrary but we need to capture a couple of depths) of water is easy to move, it is not constrained by much as the upper part because it is open to the atmosphere. At 6 inches, and of course a liquid is incompressible, you have to move a mass of water that is constrained by the water above and of course this creates a wave above the water level, which costs energy, and by the time you hit the 12 inch depth, the water is even harder to push out of the way as now there is 12 inches of water above it.

Just some thoughts.

 

A near-vertical immersed transom would not experience much "buoyant lifting force" to speak of, as the pressure is normal to the surface, and the component of it in the vertical plane is not much. A "boat" that consisted of a cube, or something shaped like a coffin would have the entire bouyant lift on the bottom only, the sides would be neutral, regardless of immersion, so far as lift is concerned.

 

True but the definition of buoyant forces is for an immersed object.
The picture that I am trying to paint is that when the transom ventilates their are no buoyant forces. There are forces that result from accelerating the water aside, the hydrodynamic planing forces. And the deeper the planing hull is in the water, the harder it is to push the water out the way of the boat, but these are not buoyant forces.

This just simplifies the process for a planing hull. They operate with buoyancy and planing forces until the transom ventilates, then only planing forces act on the hull.

 

Froude number applies to planning hulls as well as displacement hulls, and is commonly used in research and analysis of planing.

Perhaps you have confused Froude number and the "hull speed" concept.

A novel and as far as I'm aware unique concept: a hull is not immersed if the flow is separating from the bottom of the transom.

A paper on flow separation from transoms: http://www.iwwwfb.org/Abstracts/iwwwfb19/iwwwfb19_32.pdf The separation from the transom in the experiments reported occurred at Froude numbers based on waterline length well below any planing speed.

 

The pressure of the water flowing past a hull depends on the effects of gravity whether or not the transom is ventilated.

 

So if I fire a bullet into the water, and the water doesn't touch the rear end of the bullet till it slows down sufficiently, it isn't immersed in the water and subject to "buoyant forces" till it has lost all that velocity, and it is engulfed ? Sounds arbitrary to me.

 

Early attempts at fast boats were based on very long thin hulls. They found that they would start submerging at higher speeds. We know that pressure decreases as the velocity of a fluid relative to a surface increases. It is the principle that allows planes and birds to fly. So, there is a decrease in lift due to pressure, but the extra power applied to the system result in a total net lift. It requires a particular shape and geometry to work.

 

From David
"The pressure of the water flowing past a hull depends on the effects of gravity whether or not the transom is ventilated."
Not for planning hydrodynamic forces. The pressure is generated by accelerating a mass, which is the mass of the water that the boat has to move out of the way.
If you have a one pound mass on earth and accelerate it at 20 feet per second squared and the same one pound mass in zero gravity, and accelerate it at the same rate, the resultant force will be the same

Gonzo
I understand what you are saying but for planing hulls that move a trough of water out of the way as it moves along, the higher the speed the higher the pressure. Otherwise in a planing hull, as the speed increases the wetted surface would have to get larger not smaller. ie higher speed, higher pressure, higher lift.

If the direction of movement of water is parallel to the direction of the hull, with no impingement, depending on the form of the surface, you are right. And that is why displacement hull actually are pulled down as they speed up, barring wave making issues.
It is called the Coanda effect, best illustrated by taking a spoon, turning on a sink tap, holding the spoon at the end of the handle loosely, with the convex back of the spoon into the flow of water and watching it get sucked into the stream.

From Mr Efficiency
To many other things are in play here, a high speed of the bullet could cause supercavitation, the water at the back of the bullet would most certainly cavitate, and the back of the bullet would not be hooked to atmosphere, Complex
But the concept is interesting
A key point here though is you said if the water does not touch the back of the bullet. This is not the same as having the back of the bullet ventilated.

 

You can plane at any speed, it isn't related to hull speed. Given a properly oriented center of gravity, a hull will plane (generate lift, rise out of the water and then start to decrease planing angle with increasing speed) once the lifting forces equal the weight of the hull. A very light hull can generate sufficient lift to plane at a much lower speed than the same hull that is more heavily laden.

In addition, the center of gravity (as noted earlier) is a key parameter. Further forward and the hull will plane later, further aft and the trim angle will increase faster and the hull will actually plane at a lower speed.

The fact that the hull rises up and planes at a lower speed does not mean that it is actually more efficient just because it planned at a lower speed. For every total weight, hull form, hull width, and speed there is a optimal center of gravity that will result in the lowest drag. The faster you go the further aft that optimal cg is. If you want to travel efficiently at lower planing speeds, you want to move the cg forward so that the trim angle decreases to the optimum trim angle for that hull configuration. For a flat bottom hull the optimum trim angle for lowest drag is close to 3 degrees. If you are trimmed at a higher angle then you are not as efficient. If you are trimmed further nose down, you will also be less efficient.

The OP was asking if power had an effect and the answer is no, not so long as the cg remains in the same place. But if you have more or bigger outboard motors on the hull, then the CG is moved aft and that means that the most efficient planing speed will be higher because the CG is moved further aft.

And it really doesn't have anything to do with when the flow separates from the transom. That happens at a lower speed and by the time you are starting to calculate planing forces it is no longer a consideration.

 

From Yellowjacket
"And it really doesn't have anything to do with when the flow separates from the transom. That happens at a lower speed and by the time you are starting to calculate planing forces it is no longer a consideration."

The reason that I introduced the speed at which the transom ventilate is that prior to this point, there are buoyancy forces at work. The hull is immersed on all sides, and hence buoyant forces are evident, by definition and by actuality
When the water separates from the transom, buoyant forces cease to exist, or so I believe. I wanted to make this statement to get some response from learned contributors as to if this is the case.

At zero speed, planing hull, all buoyant forces
At speeds from post zero to the transom separation, some planing/ dynamic forces
After separation, zero buoyancy and the hull is supported by dynamic forces only.


The reason that I believe this but could be proven otherwise, is that I had read a naval engineering book many years ago that showed the pressure distribution on the bottom of a high speed planing hull and at the transom on the bottom of the boat, the pressure was shown to be atmospheric. I would have thought, at that time, that it should be higher because it is deeper in the water, ie pressure increases with depth.
But then if the water pressure is higher than atmospheric, the water should pretty much shoot up like a rooster tail immediately after the transom, and it does not.

Your thoughts

 

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.

This is an example of why gravitational effects on pressure should not be neglected when analyzing the flow around a boat on the surface.

 

NO, that is not correct. There is a always a buoyant component any time the hull is immersed below the surface of the water. There are two components to the amount of lift created. One is buoyancy and is ALWAYS equal to the amount of water displaced, and the other is dynamic lift, which is the result of the change of momentum of the water under the hull. Look as semi-planing craft. They are supported by some dynamic lift and some buoyant lift.

As DCockey noted the separation of flow from the back of the hull has nothing to do with buoyancy, it is simply the result of the flow momentum being high enough that it can't turn the corner and flow up the transom. 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. The water under the hull has been displaced downward by the action of the planing surface and it does rebound up after the boat passes, and that is seen behind the boat and out to the sides in the wake. The depth of planing is so shallow that there isn't much pressure to drive the water up like you are thinking it should.

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, but that is not related to the overall buoyancy, it is simply a function of the velocity under the hull. The overall pressure distribution on the hull is the total dynamic component and in order to plane more lift is created forward, and less (or even negative lift) can be created aft. That doesn't mean that there isn't a buoyancy component or that there is no buoyancy being created, it all comes out as an average of the pressure forces on all of the surfaces.