Froude and planing

You know what "displacement" means in the context of a boat in the water. If you need to describe the act of moving matter from one place to another then you are aware for the potential of miscommunication and should take steps to prevent it. Perhaps add the word "dynamic" so that it is clear what you mean.

 

The paper that you referenced deals with hull pressures below "planing inception" and many times state that at planing speeds the hull is supported by dynamic and buoyant forces.
Which with the exception of Sandhamaren,( who insists that there is not hydrodynamic pressure until the transom ventilates, oops drys) most agree.

Being supported by dynamic and buoyant forces when planing, is certainly most accepted.

Pre planing, now that is a different issue. Certainly I would accept that for a displacement hull, you could get squat so to speak. ( mainly due to pressures from streamline conditions, one might argue Coanda, or Bernoulli) For a planing hull with any trim up situation, it is pretty hard to say that when the entire hull is moving against the water and the water is being accelerated down and away to the sides that it would create suction.

It is unlikely that Newton is wrong.

If so when the transom ventilates, the air at the transom would then move forward to the "suction" less than atmospheric area ahead of the transom.

This has been discussed on a thread a long time ago, and I believe it is again commonly accepted for a planing hull the aft hull sinks due to the center of lift being far forward before planing,
Ie the minute the hull is moving, with any trim up angle, hydrodynamic forces exist and are positive

I wonder how they would consider measuring this because if at slow speeds, and the trim up attitude ie the volume of buoyancy would move rearward, because with trim up, there is not as much volume in the front in the water, then the rear drops and the buoyant pressure increases.

 

I did W9GF0, used that term exactly several times. Could not make it clearer.

 

All of my comments regarding "displacement" were based upon your following quote, I did not see any distinction made between static and dynamic displacement.

 

"Displacement is the water displaced (moved away) by the boat whether it touches the hull of the boat or not. Nothing else makes any sense. Otherwise, what is the displacement of a hovercraft?"

You should add Displacement "at rest"

to be completely clear !

 

Displacement in this context does not require the water to be moved out of the way. For example, when a drydock is flooded in order to float a boat, the water is not "moved away" rather the hull occupies the space that the water would have if the hull were not present.

If you lower a boat into the water there will be a larger volume of water "moved away" than the volume of water equal to it's displacement. It will displace (move away) more than it displaces (weight).

 

That exception of Sandhammaren was the reason for my post.

I am/was not trying to establish or share a fact or point of data and then use that to make general assumptions. The point of my post was to provide counter-examples that show Sandhammaren's general assumptions are not correct (in general).

For example (among other things) he seems to believe that achieving the Kutta condition and the coefficient of lift becoming greater than zero are essentially one and the same event, for any hull with a sharp edged transom. That is wrong, and I've given an example of when it's wrong, and provided a paper by possibly the most respected contemporary expert on the topic in the world that corroborates my example.

As for the rest of your post, if you're saying Savitsky is wrong, I'll let you hold that viewpoint on your own without my company - I would be very surprised if he is (Not implying Newton was wrong).

 

Furthermore Sandhammaren is applying the Kutta condition in a wrong manner. He thinks it is equal to dry transom. While the Kutta condiion is "A body with a sharp trailing edge which is moving through a fluid will create about itself a circulation of sufficient strength to hold the rear stagnation point at the trailing edge." Which essentially means there is sepration at the edge and the flow doesn't turn around the edge and follow transom. If there is backflow at the transom there is separation as well. So Kutta condition applies also before the transom is dry.

Kutta condition is used to force separation where it is known to be. Not the other way around. E.g. for an oval profile or a typical foil going backwards there would be no trailing edge. Just a rounded part. Then it would not be known where the separation will occur and the location will certainly depend on AoA and Reynolds number. So Kutta condition is not applicable, but still lift is generated. Even a lot of lift as the curve I linked earlier shows for a backward NACA 0012. Up to 10 degrees AoA NACA 0012 produces about equal lift backwards and forwards, but drag is more than order of magnitude higher backwards. So a bit like prismatic hull with rounded wet transom.

What Barry and I were asking is what would happen for a prismatic hull (or waterski) with a clear postitive trim at displacement speeds (and wet transom). The paper you linked had zero trim until "hull speed". Would there still be sink? I would think that at some postitive trim no.

 

Try post #86 and several others. Apparently everyone here is content to regard static displacement (weight of the boat) is all that matters so I am wasting time in trying to have a broader discussion. I do not wish to insult anyone .

 

My understanding of this thread was that we were discussing mainly planing hulls at planing speeds. This Swavitsky paper has much discussion about hull pressures below planing speeds and that was the point that I was trying to make. One of his hull side profiles shows positive lift all the way back to the transom with a small trim attitude and also contradicts some of his statements. Ie he says there is suctions at the rear of the bottom of the hull, to me suction is a pressure below atmospheric. ie negative PSIG or positive PSIA but below 14.7 PSIG
Then shows that the forces on a planing hull is positive back to the transom.
So when I re read the paper, a little more slowly, it became clear, by his wording that this suction that he CLAIMS to exist, exists below planing speeds.

He clearly states that the pressure on a hull is a combination of buoyant forces and hydrodynamic forces.


This is clearly opposes Sands opinion that there are no hydrodynamic forces present until the transom ventilates ( drys)


But further to this suction/negative PSIG pressure on a PLANING hull.

If we assume that he is correct and say there is negative pressure under the hull, which would be a combination of buoyant forces and hydrodynamic forces, that would mean that at whatever speed he says this happens, lets just pick 5 knots. (realising that this might not be the point that it happens. I contend then that if I were to drill a bunch of holes in the area that he says is negative pressure, then water would not come up through the hull.

I have read many published papers regarding hull pressure for PLANING hulls, and have never ran across another paper saying that there are negative pressures WHEN THE PRESSURE VALUES WERE DERIVED FROM EXPERIMENT at planing speeds. iE tow tank experiments. Below planing speeds, though they support the view of positive pressures, a combination of buoyant and dynamic forces, at low speeds to higher speeds.

 

Barry: Savitsky clearly talks about pressure without the hydrostatic component, which is common practise in fluid dynamics. So displacement force can be calculated just based on the volume below the water level and dynamic forces are calculated with pressure and friction integration over the hull.

So when he says pressure is negative he means that it is lower than far away at the same depth.

 

Ok, I did not get that from the paper but then are you saying that the combined pressure of dynamic pressure and static pressure acting on the hull is lower than pressure that is far away from the hull at that depth which means lower than hydrostatic pressure at that depth.

If so then, that means that dynamic pressure must be lower, which means that even though water is impinging on all of the hull bottom and moving water, ie accelerating water, that this acceleration which should be creating a positive pressure, Newton, is causing a negative pressure.

Again, does not seem to be intuitively correct

But for a displacement hull with curved lines, that is a different situation

But Figure 11, which shows bottom pressures with zero trim, there are not any negative values and Figure 12, Additional Bottom Pressures due to Trim Angle, again, positive pressures through out ( both figures are planing hulls)

Note that we are speaking about hull pressures below planing inception

 

As Bernoully equation tells accelerated flow will have lower pressure. The lift force on a planning hull is based on decelarated flow. For a flat bottom hull there is even a stagnation area with zero velocity and thus pressure 0.5*rho*V^2 over the normal hydrostatic one.

The Savitsky paper we are discussing measured the heave and thus lift, but the trim was zero until hull speed and even then it was very low. So this was not like a typical pleasure boat with some trim at rest and much more than 2.5 degrees near the hump.

So flow is accelerated at the curved bow and a higher velocity is kept all the way under the hull. Due to transom being dry or partly dry the hydrostatic pressure is lower than "normal" at the transom, which probably expalains the even lower pressures there in the CFD simulations shown.

The CFD simulations are with higher trim (5-7 deg), but still the boat is said to sink at Fdelta 1.3.

 

The paper had approximately static trim until hull speed, not necessarily zero trim.

I can't say where a planing hull would sit if there were stern down trim at lower speeds - it would depend on the balance of forces pulling down and those pushing up.

That is only reasonable at the interface between water and air, not anywhere deeper.

 

I know airfoil theory pretty well, the math, etc. I'm stating that the Kutta condition for planing is clean separation of the flow from the planing surface, there is no backflow. Marine propeller blades never have sharp trailing edges, and they satisfy the Kutta condition for lift/thrust in the sense that I've described. The mathematical Kutta condition is a sharp trailing edge on a wing; the physical Kutta condition for propellers and boat bottoms is a sharp separation of the flow from the blade or planing surface.

A plowing boat fully supported by buoyancy can gain a trim angle by riding up the bow wave. Du Cane discussed this in 1951. There is no hydrodynamic lift so long as the transom is wet. Du Cane went on to calculate what he called 'the hydrostatic lift' when the transom is dry and notes implicitly that this is buoyancy, not lift, because the result is speed-independent. Following his calculation you can see that the submerged depth of hull, with trim angle > 0, even after the flow has separated at the transom is d=Lxalpha where L is the wet length of bottom and alpha is the boat's trim angle, for small trim angles alpha: you can estimate the submerged depth from the wet length of boat.