Froude and planing

"Quote:
Originally Posted by tom28571 View Post
Rastapop
That is quite shocking to imagine. Can you give an example?

Originally Posted by W9GFO
A waterski?


No, that is an incorrect view of the situation. The displacement of any craft is the amount of water it displaces, whether the ski has that much volume or not is immaterial. All that extra displaced volume of water is caused by the dynamic motion of the ski but it is displacement whether static or dynamic.

This is a tricky situation but all of the above seem to be correct if the proper physics are applied.

Edited to add: By all the above, I mean the comments posted by others are correct if interpreted properly.
Sorry, where is that definition written down?"

Everywhere displacement is mentioned. To interpret that displacement must only be the water enclosing the surface of the boat is wrong. 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?

Not coming from a hydrodynamics background, I am unfettered by the conventions that seem to affect most people here. I am smart enough to realize one thing that I think clouds and confuses this discussion. 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. Every one is talking about flow with all kinds of equations and conditions and wind up arguing about the measurements rather than the physics. Granted that these formulas allow measurement and prediction of results, they mostly confuse a basic understanding of what is going on under the boat.

Application of Newton's laws is all that is needed to see what the water is doing. Until the plate or boat comes along, the water is just sitting there in static equilibrium. The positive angled plate strikes the water surface and accelerates it downward and forward. The water reacts normal to the plate and the result is lift from the vertical component and drag from the forward component. Lift is reduced toward aft on the plate because the water has been accelerated away by the more forward plate surface, giving the usual curve of lift distribution seen on drawings. The downward motion of the water continues aft of the plate until the momentum is lost and then rebounds to create standing waves oscillating behind the flat plate. The waves are moving forward at the speed of the plate or boat, but the water is only moving up and down. All this assumes the plate is actually planing.

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. I think much of the argument and confusion here is caused by assuming that the water has a horizontal momentum (other than that given it forward of the stagnation point), but the water has NO such horizontal momentum.

Deadrise and narrow beam changes things but the fundamentals remain true. I was very hesitant to get into this and expect lots of blowback but I'm sure that the above analysis is basically correct.

 

Lift on a hull is developed by pressure on the bottom. At the sharp edge of a dry transom the pressure is zero relative to the atmosphere so no lift is developed at the transom edge.

The Kutta condition is a concept from inviscid modeling of the flow around airfoils. Inviscid models of the flow around an airfoil need a condition or similar which determines the circulation around the airfoil. If the circulation is zero the lift is zero. If the circulation is non-zero the lift is non-zero. The Kutta condition is the standard condition used to determine the circulation; ie the circulation which causes the overall flow to satisfy the Kutta condition.

Added: How "square" is square? What if the edge is not square? Boats plane with rounded edges on the transom. In fact with measurements of sufficient resolution all physical edges can be seen to be either rounded or irregular.

 

Tom;

I actually agree with you about much of what you said about the mechanics, although I am only a layman. However, your definition of displacement is NOT referred to
"Everywhere displacement is mentioned". No source that I can find refers to displacement as including water that is affected by the movement of the vessel, as in my examples.

For example, Larsson and Eliasson define displacement as "the weight of the yacht" or "the volume of the immersed part of the yacht".
Foreship defines displacement as "Displacement, Displacement weight The weight (and center of gravity) of the vessel herself and vessel’s cargo, stores, passengers, crew, etc."
The US Navy defines displacement as 'the weight of a ship'. This was in accordance with standard naval definitions, such as those used in the Washington Treaty (see DK Brown's books).
Reeds "Reeds 21st Century Ship Management" defines displacement as "the weight of a ship's hull, machinery......"
Panbras says a ship's displacement "is its mass"
The Handbook of the Nautical Rules of the Road states that displacement vessels are "those that "float" or are supported by the STATIC buoyancy derived from the water that their hulls displace" (my emphasis) and states that "non-displacement craft" are "those that are supported by the dynamic lift of hydrofoils or other lifting surfaces, such as planing hulls"
"Hydrodynamics of High-Speed Marine Vehicles" by Odd M. Faltinsen contains no definition like the one you are claiming.

If displacement was the total volume of water moved by a moving vessel, then the displacement would change as a vessel moved over waves or sped up and slowed down. That is not a useful definition. It would mean that a hot-air balloon being blown close along the surface of a lake (which would cause a small movement of water) would have displacement, but the same hot air balloon stopped the same distance above the same lake would not.

 

I was wondering about that. If I take a square-tailed beginner's waterski and round off the back edge, does it stop planing?

 

Does a round-tipped prop stop satisfying the Kutta condition? In both cases, no. The water still separates from the blade at the trailing edge.

 

Rounded trailing edge may allow porpoising, and in any case the boat planes deeper at the transom case causing more drag. We use Bondo and/or sand to make the trailing edge of race boats as sharp as possible.

Allow me to become pedantic in order to help some of the folks to see the point. As long as there is backflow the transom is fully submerged, same as when at rest, and buoyancy carries all the weight, same as with rig at rest. As soon as the flow separates from the bottom at the transom (Kutta) then the submerged depth is less, and so is the buoyancy. That is the onset of lift. The inviscid flow treatment is correct, you may calculate the boundary layer thickness to convince yourself. In any case, in the eqn. for the net force the lift and form drag are given by a different integral than is the skin friction. So I don't know what you're worrying about.

 

Rounded trailing edge is less efficient for a planing boat, but that doesn't mean that there is no dynamic lift.

It is not at all true that the transom is fully submerged when there is backflow. The transom gets dry gradually not magically at one point.

Kutta condition is used in inviscid flow calculation to desribe what viscosity would do at the trailing edge. It is not required for full Navier Stokes nor in real life. And it is not a requirement of lift.

There is no onset of lift. There is always lift when an object is moving through fluid. Of course the lift may happen to be zero, e.g. when the object is symmetric at it's angle of attact.

There is no water flow on the upper side of a boat, thus it can't be symmetric. Still there can be situations of zero lift, but that would be a very special case and would only happen at a specific trim angle. At other angles there would be lift (postivive or negative). How big portion of the displacement this lift would be is another matter.

 

Thanks, sandwiched response below.

Simply take half the correct lift coefficient for the bottom only. Bottom half of a vortex sheet, mathematically. For 12-13 degree deadrise the lift coefficient for planing is c≈.96xboat bottom's trim angle.

Which brings to mind: so long as the boat plows with zero trim angle then the lift is zero-the lift coefficient is proportional to the trim angle.

 

Your "sandwhiching" makes it very difficult to follow and to know who has written what.

Many examples have already been given. E.g. water ski. It produces a lot of lift before it gets fully dry on top and at the transom. It would do the same if you made it so thick that is would have the upper edge of the transom above water level.

I also asked you earlier why do you think it would be necessary to have air at the transom and do you really think there is no backflow with a stalled foil or a foil with transom like thick trailing edge.

This old paper shows measurements of cut foils. Notice how well the foils with quite thick "transom like" trailing edges produce produce lift. Even better than the sharp one, but with much more drag. Do you think there is backflow at the "transom"?
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930082748.pdf

 

Here are some photos of the measured transom wetting at different speeds. Does the water level look constant before the transom is fully dry? Already the first pictures show water level lower than at still at the transom. http://www.iwwwfb.org/Abstracts/iwwwfb19/iwwwfb19_32.pdf

 

In this field, am also a layman although I have found that all engineering uses a common mind set in analysis of a problem. Some call it the "scientific method", which will do as well as any label. You are correct that the displacement changes at the boat runs over waves, but only when we look at the total water displaced at that time. Wave making is a common term but it is still water displaced by the boat.

I readily admit that looking at displacement like I am here takes some thinking outside of common usage, but I do think it is correct and leads to a better understanding. Most likely the term should always have a static or dynamic prefix to avoid confusion. The definitions you quote are all of a boat in static equilibrium. It does not require exotic thinking to realize that a boat in motion is neither static nor is the water it displaces. I am using the word in its true meaning. That is, water displaced by the boat, whether static or dynamic. So my definition of displacement is the vector sum of both static and dynamic values.

Only by looking at dynamic displacement can one visualize what is actually going on under a planing boat. Dynamic displacement can be many multiples of the static value. Static displacement is a fixed value and does not correlate to a boat in motion, especially in other than flat calm water. That is why the lift required to keep a planing boat in equilibrium will always be greater than that required to support its weight minus the buoyancy at that condition.

Dynamic displacement does correlate to the trim angle and is greater for higher trim angle. All that wave making force of a high trim angle results from the greater dynamic displacement and the greater lift required to overcome it and cause the trim angle to decrease at high speed. At high speed, dynamic displacement is minimal but is always greater than the weight of the planing boat. If there were some other term than dynamic displacement which explains the phenomena as well, I would use it but dynamic displacement is self explanatory, so I will use that.

I realize that there is no mention of Kutta, Bernouli, fluid flow or any of the common terms of hydrodynamics here. My thinking has evolved over the years and may change more if further evidence comes up. For any learned people who may be insulted by my simple conclusions, just ignore them.

 

Sorry about the sandwich. Thanks for the ref. I'll take a look when I have a chance. Waterskis, like hydrofoils, develop lift when fully or partially submerged. A planing ski will have the flow separate at the 'transom'. Thickness doesn't matter create lift. If water flows over the back of the ski then the speed has dropped and the skier is sinking. You can add a Bondoed 'bump' to the bottom at the trailing edge. If the boat was running too deep at the transom then it will afterward run dryer and pick up speed. Most likely, it will only force the trim angle lower, drag more boat, and lose speed.

To come out of the water and ski barefoot you start with your feet fully submerged. They develop lift as the boat pulls you and if you have the strength to
hold onto the rope then you'll come on top of the water and your feet will plan. Feet are not a very good planing surface.

I'm not talking about 'a lot of air'. I'm talking about the transom running dry when lift develops.

In hydrodynamics an object in a flow is either wet or dry. It is never 'ventilated'. If I store a boat in a barn and open the door then the transom (and indeed the entire boat) will be ventilated. There will be a lot of air around the transom but this has nothing to do with lift.

Maybe 'to plan' is wrong and 'to plane' is right. I do not have a nautical reference book with me now.

 

In hydrodynamics an object can be ventilated too. For example, hydrofoils can ventilate - especially when they are taken out of a barn, put in water and pushed to high speeds.
In hydrodynamic things can be aerated too, like aerated water.
They can also be wet like wetted area, and dry like dry transom.

Your personal preference for a particular wording doesn't make that particular wording a scientific standard, and doesn't make it a definition in hydrodynamics. Especially when it is defined, in hydrodynamics, exactly the way you claim it is not.

Transoms, bottoms, foils, struts and propellers can and do ventilate.

 

A transom is not ventilated like the core of the tip vortex in surface piercing.
I stick by my definitions as clarity and simplicity are my personal preferences. Struts, foils and props that pierce the surface have tip vortices that end on the water's surface, therefore are 'ventilated', meaning the core is air, not water. Aerated water is far afield of any hydrodynamics discussion here. How do you want to ventilate a boat bottom? Air injection?

 

The term "ventilation" (from latin word "ventus" = wind) indicates a process where an originally submerged surface becomes exposed to air, in general due to local reduction of pressure with consequent suction of atmospheric air.

To answer your question - yes, a stepped hull is intentionally ventilated (as you surely know better than me) in order to replace water with air behind the steps.