Froude and planing

That's well fore of the transom. I've seen stepped hulls for ocean running recently. We had a racing tunnel with steps and I removed them best I could. Just extra drag.

Dry: not wet. As the with transom of boat on a plane.

 

Some interesting comments here

Your comment ["" When designing my first planing boat, I realized that there is no flow of water under the ideal flat plate of infinite span or wide enough to discount end effects. ""]

An infinite span is not applicable to determine the hull pressures OR the movement of water under a boat that might have a reasonable length to beam ratio of around 4
Ie 32 feet long and 8 feet wide. Water on a hull moves down and out to the sides (ignoring water moving forward ahead of the stagnation point)



Your comment "Every one is talking about flow with all kinds of equations and conditions "

I understand the point the you are trying to make. Ie the water has no momentum so there is no flow.
So what is the definition of flow, I doubt we want to go there as the "What is the Definition of Planing" thread ended up with upwards of 700 comments. Perhaps an extremely simple definition of flow is movement of water. Down a pipe, against a flat plate at an angle, in a river channel.

It is a given that when you accelerate a mass, water in this case, that you have a resultant reactive force. I doubt that anyone would argue against this point.

I think the most important point is that the plate is creating flow in the cross sectional area that it "moves out of the way" as it transits down the water surface. I was going to use the words "that it displaces as it transits down the water surface" but we do not need another displacement issue show up.

So this water trough that the flat plate creates a flow by my definition as "water moving " with respect to the hull. Flow

If you were to take your flat plate and stick it in a river with a 10 knot current. How could you say that there is no flow acting against the plate?

It is this water mass that is being accelerated to give a lift.

Your comment " I know that it is easier to compute results by using theory that looks at the event as flow but that does not mean that there is actual water flow present. In fact, there is NO water flow along the flat plate."

The water that the hull is moving out of the trough that it makes is in fact moving. If you were to throw a floating ball in front of the hull, to represent a small amount of water, the ball will be impacting the hull and running down the length of the hull and toward the chine.

Your comment "but the water is only moving up and down. "

This is not the case in either a hull with deadrise or a flat plate. Water move sideways as well. You mentioned the stagnation pressure distribution longitudinally. There is also a transverse pressure distribution what has the highest pressure in the middle of the plate going to atmospheric at the chines (hydrodynamic pressure, there would be a small buoyant force if the chine is submerged)

It is a fact that if you have a high pressure area and a low pressure area, the fluid in the high pressure area will move to the lower pressure area.

So from the center of the plate to the outside. Ie there is not just vertical up and down motion of the water.

 

Reference lengths I recall seeing used in research papers, books and monographs on planing hulls for calculating Froude number:

Static waterline length - Consistent with usage in slower speed boats. Frequently used in transition to planing. However at higher speeds the static waterline length does not represent the wetted portion of the hull because the forward portion of the hull is out of the water.

Beam across chines - Good for higher speeds. Good for prismatic hull studies where the wetted portion of the hull is prismatic.

Cube root of displacement - Good for comparing fundamentally different shape hulls, particularly when displacement rather than length or beam will determine the dimensions of the boat.

Square root of "projected planing surface" area with "projected planing surface" defined as the projected area bounded by the chines and transom.

There is no single reference length which is "best" for all purposes.

 

This is also problematic, since quite many higher speed vessels can have dry chines. I remember when I made my Savitsky calculator and tried to get reasonable results for the boat I used to have. It was rather light and small and had a top speed of 48 knots with standard 40 hp OB. I could not get even close small enough drag to predict such a speed. Then I realised that the standard Savitsky approach assumes wet chines at transom at all times resulting in way too big wetted area. I modified the model to allow automatic calculation of narrower wetted area at the transom. The current model gives wetted beam at transom only 50% of the chine beam and predicted propulsive power is about 20 kW (depending on the surface roughness), which is reasonable for a OB with measured power of 31.6 kW and not optimised cleaver.

 

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Some interesting comments here

Your comment ["" When designing my first planing boat, I realized that there is no flow of water under the ideal flat plate of infinite span or wide enough to discount end effects. ""]

An infinite span is not applicable to determine the hull pressures OR the movement of water under a boat that might have a reasonable length to beam ratio of around 4
Ie 32 feet long and 8 feet wide. Water on a hull moves down and out to the sides (ignoring water moving forward ahead of the stagnation point) Certainly Barry, an infinite flat plate was used only as the simplest and most easily understood example. All the books use that example in a drawing to illustrate the conditions but I argue that the flow they show does not exist and that common example is just wrong.



Your comment "Every one is talking about flow with all kinds of equations and conditions "

I understand the point the you are trying to make. Ie the water has no momentum so there is no flow.
So what is the definition of flow, I doubt we want to go there as the "What is the Definition of Planing" thread ended up with upwards of 700 comments. Perhaps an extremely simple definition of flow is movement of water. Down a pipe, against a flat plate at an angle, in a river channel.

It is a given that when you accelerate a mass, water in this case, that you have a resultant reactive force. I doubt that anyone would argue against this point.

I think the most important point is that the plate is creating flow in the cross sectional area that it "moves out of the way" as it transits down the water surface. I was going to use the words "that it displaces as it transits down the water surface" but we do not need another displacement issue show up. Yes, in a boat of normal beam but to ignore the displaced water beggars the point.

So this water trough that the flat plate creates a flow by my definition as "water moving " with respect to the hull. Flow Well, that needs more conditions explained.

If you were to take your flat plate and stick it in a river with a 10 knot current. How could you say that there is no flow acting against the plate? Absolutely, and the math works out the same but no boats I know of actually operate in that fashion.

It is this water mass that is being accelerated to give a lift. Yes, lift is the result of water being accelerated normal to the hull, the vertical component that is.

Your comment " I know that it is easier to compute results by using theory that looks at the event as flow but that does not mean that there is actual water flow present. In fact, there is NO water flow along the flat plate." In order to maintain the simplest case, I was still talking about the theoretical infinite beam where there can be no lateral flow.

The water that the hull is moving out of the trough that it makes is in fact moving. If you were to throw a floating ball in front of the hull, to represent a small amount of water, the ball will be impacting the hull and running down the length of the hull and toward the chine.

Your comment "but the water is only moving up and down. "

This is not the case in either a hull with deadrise or a flat plate. Water move sideways as well. You mentioned the stagnation pressure distribution longitudinally. There is also a transverse pressure distribution what has the highest pressure in the middle of the plate going to atmospheric at the chines (hydrodynamic pressure, there would be a small buoyant force if the chine is submerged) Yes, that is obvious but the whole point was related only to the theoretical flat plate.

It is a fact that if you have a high pressure area and a low pressure area, the fluid in the high pressure area will move to the lower pressure area. Certainly.

So from the center of the plate to the outside. Ie there is not just vertical up and down motion of the water. Infinite flat plate, please. In one example, I have watched standing waves behind a very large flat bottom hull that had only up and down motion except near the edges.

Thanks for the response Barry as no one else seems interested or willing to discuss these points. Only after agreeing on the theoretical flat plate can the discussion go on to hulls of normal beam or deadrise. Clearly the transverse waves from a hull are partially the result of sideways flow but they are mainly up and down as well.
Cheers …….Tom

 

The use of the chine beam for Froude number length I've seen was confined to cases where the displacement was sufficient to ensure the chines were wetted at the transom for the speed range of interest.

Joakim - does that boat have fore-aft strakes or steps? Any problems with chine-walking or similar?

 

Tom, would you also claim that 2D airfoil theory and data, which can also be considered as a constant section wing of infinite span, does not represent reality and "is just wrong"? 2D airfoil theory and data was the basis of aircraft wing aerodynamic design and analysis for most of the twentieth century and is still a fundamental tool today.

Also are you claiming that a boat needs to be analyzed in a reference frame fixed to the earth in which the flow around the boat at any point fixed in the reference frame varies with time instead as the boat moves relative to the reference frame? The standard reference frame for steady state hydrodynamics, aerodynamics, etc is a reference frame fixed to the vessel, aircraft, projectile, automobile, etc in which the flow at any point fixed in the reference plane does not vary with time.

 

Yes it had four pairs of strakes the middle ones were cut maybe 0.5 m before transom. No steps (except the ends of the cut strakes). It was porpoising very badly in 15-30 knots range, but became then very stable. At 45+ knots there was some chine walking, but it was easily controlled by steering. A few times there was quite wild "corner to corner" instability after hitting a wave not seen soon enough. Once I thought it will capsize, but didn't. It was an "offshore" boat and could be driven fast in a surprisingly big waves. The designer said he always drove with full throttle and the speed decreased enough since the propeller was airborne while jumping between waves. I was not as brave.

 

Please tell us which part of the pressure removed from a vertical transom, when it becomes dry, was previously aiding buoyancy.

You can't seem to let go of the idea that the Kutta condition and lift are basically the same event. They are not, except when talking about a foil immersed in a single fluid.

What will actually happen is that as speed increases from zero the transom will first ventilate (Kutta condition satisfied) while lift is still *negative*, and then later at a higher speed lift will become positive.

I see this every day on the "semi-displacement" hulls I perform CFD analyses on.
Here's a good little paper by Savitsky that may correct some of the errors still clinging stubbornly to the inside of your skull: http://www.sname.org/HigherLogic/Sy...tFileKey=62fe6b2b-b038-426c-9225-3206c600f567

 

Please expand on your comment of *negative* lift as it pertains to the planing hull discussion that we are having?

 

I tried, apparently without success, to establish the simplest reference point with the least number of variables from which to start looking at the movement of water under a planing hull. Nothing related to a foil immersed in a medium, water or air, was intended. I'm not satisfied that I correctly interpret all the issues you bring up so I don't think I am intending to make any claims other than those specifically mentioned under the limited conditions offered. There are several things "wrong" with the usual illustration/explanation of planing and lift in all the texts I've seen but I certainly don't make any claims that the supporting theories are wrong. That would be way above my pay grade.
t

 

This is an example where a word can have different but similar meanings. It should be easy to visualize that a boat moving through the water affects a much greater volume of water than it actually displaces - and I don't mean "displace" as in "relocated". That is not the correct usage of "displacement" when we are talking about boats.

The "displacement" of a hovercraft is the volume of water equal to the weight of the hovercraft, just as it is with a regular boat. It is NOT the volume of water that is relocated or "moved out of the way". The pressure underneath a hovercraft causes a shallow depression in the water, that depression will be equal in volume as the weight of the vehicle. As the hovercraft accelerates it will have to get "over the hump" and plane just as a regular boat will, although a bit easier due to the low friction and the large, low pressure surface area.

 

As in the hull being pulled down by hydrodynamic flow. Sandhammaren05 continues to appear to believe that the Kutta condition being fulfilled and the beginning of lift (positive) are one and the same event, in general.

 

That's an interesting paper. How would the heave (dynamic lift) curves look like for planing boat with several degrees positive trim while stationary or a water ski with 20-60 degrees trim at low speeds? Many planing boats optimised for high speeds have VCG far back and thus clear positive trim at all speeds (less at full speed than stationary).

Savitsky explains the sink being due to zero trim and acclerated flow near most parts of the hull. At higher trim angles the flow starts to decelerate near the hull and higher pressure is devoloped and thus lift.

 

Perhaps we should only use "displacement" attached to a prefix to eliminate confusion. Even in common usage by some NA's and far more of those who should know better, the term is misused in a list of specifications.

If you look up displacement in Websters, the 2nd definition is the one you claim as the only one describing a boat while the 1st definition is the one I use. Dynamic displacement is a real event and extremely important in looking a a boat in motion. What else should it be called that better describes it?

A couple of people got the picture concerning the waterski paradox correct. CT249 and W9GFO and maybe others, where the ski without significant thickness and one with sides that reach to the nominal water surface give the same results. In fact, if the skier with the high sided ski is replaced with lead weights and the tow point remains the same, it is impossible to see which example is being seen from the effect (I call it displacement) on the water.