Froude and planing

If you ever run across a paper giving numbers on this, I would be happy if you would forward it to me. Planing hulls.

So if we were to place a planing hull in a towing tank, suppressed the trim angle to say 10 degrees up on the carriage, and kept the hull at the same depth in the water, you are saying that there is more drag on the hull up to where the transom ventilates than after due to flow issues ahead of the transom.?

 

Sounds like a concept from the theory of inviscid flow around an airfoil is being applied to planing hulls. As Joakim pointed out shapes with blunt rear ends can develop lift.

My recollection is the analogy of a planing hull as an airfoil is based in part that the speed of the planing hull is sufficiently fast enough so that wave generation can be neglected. This is usually a much higher speed then the speed at which planing begins. The validity of the analogy decreases at slower speeds.

 

Gentlemen, hang on for a moment please - before this turns into a fight.
The misinterpretation of my words are probably due to bad or incomplete wording of my previous post. I have said basically the following three things:

- which is evident by the fact that there is an area of recirculating flow for all regimes of wet or partially-wet transom. The word "separation" indicates that the flow at some point detaches from the surface of the body (as Joakim has correctly pointed out) and locally reverses the direction. That is what happens along the transom edge when the transom is wet.

- incomplete phrase which is probably the cause of your doubts. When I say that the hull resistance is increased with recirculation due to wet transom, the comparison is with the cruiser or canoe-stern hull, where the flow smoothly leaves the stern (again, as Joakim has pointed out). It has been explained more in depth here: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/transom-drag-54748.html#post760066 (see the great contribution by Leo too, and my CFD example in the post #12).

- this is a self-explaining fact. No recirculating vortex, no relative resistance component.

Hope this makes things more clear.
Cheers

 

I have troubles understanding your question. Do you mean a situation with different speeds, one just below the transom is dry and one above?

If you keep the trim angle and vertical position constant, the water level at the transom will gradually lower with speed. I don't think there is any sudden point of going from fully wetted to dry.

Since drag increases with speed, you will most likely get higher drag once the transom is dry.

If you would alter the trim with the same displacement so that the transom becomes dry, you would likely get lower drag with dry transom at the same speed. This will depend on hull form, but is a well known thing among all racing sailors and works also with hulls not that far from planing boats, e.g. sailboards and dinghies. This applies to displacement speeds.

 

Your comments below
"Since transom is vertical, only pressure on it can change the drag. Having water there will naturally increase the pressure and thus "push" of the transom. But that's not all. Energy is needed to keep up the backflow and turbulence at the wet transom. Thus it will change the pressure and flow field elsewhere as well. That change can be much bigger than the missing "push" of the transom."


I took your comments above as meaning that the boat overall has more drag with a wet transom due to other drag that is induced by "needing to keep up the backflow and the turbulence at the wet transom.

So I assumed then from your comments that if you towed a hull, and I should have been more specific, at just before a completely ventilated transom, and just after, that there should be a higher drag before transom ventilation

But to remove some variables, I thought to constrain the attitude of the boat and immersion at this point in time, pre to post, as then the drag should drop immediately and significantly due the additional drag from "keeping the backflow and turbulence at the wet transom" would disappear when the transom ventilated

I was inviting a paper to see the mechanics of this. I was wondering where the additional drag would come from and how it would attach itself to the hull or cause an increase in the drag due the horizontal pressure component of the hydrodynamic forces at the front wetted area of the hull. Or the pressure distribution under the hull.

 

W9GFO got one of them right - a waterski. In my days beginner's skis had "transoms"; i.e. square tips.

The other example is a sinker windsurfer; ie a short board that has insufficient volume to support the sailor without dynamic lift. They are less popular now due to other design developments, but in the '80s when boards were tiny it was not all that uncommon to sink to your waist of you were not sailing quickly. You could then sometimes gradually get the board to surface by dynamic lift, despite the fact that the whole board was under the water. Some of them had 'squashtails' which are essentially transoms.

These two examples prove that the transom being dry cannot be a necessary condition for what we call "planing".

At the other end of the scale, there are any number of yachts that have a "dry transom" at displacement speed. Therefore the performance of the transom seems to be irrelevant as far as a useful definition of planing goes.

 

Sorry, where is that definition written down?

 

But if you're using an old beginner's waterski (or a fence paling, which some guys I skied with did) or an old sinker windsurfer there is surely backflow up the transom at low speeds where you are developing dynamic lift.

In a sinker windsurfer, your centre of gravity can lift a couple of feet due to dynamic lift, but you may only be doing a 2-4 knots at a guess and there's no discernible separation at the transom. The angle of attack is quite extreme, of course.

This vid shows a much larger board with much more volume which floats much higher than the old mini sinkers, but it's still developing dynamic lift while underwater with what feels and looks like attached flow over the top and bottom of the "transom".

 

The edge where the transom meets the bottom should be 'square'. Lift is developed there via the Kutta condition: sharp separation of flow from the bottom with no backflow up the transom.

 

Yes, fair enough, a broader view is required rather than thinking only about "typical" craft.

I agree, although in most cases you can rely on the transom ventilating before planing occurs.

I think sanhammaren05 may be talking specifically about a foil fully immersed in a single fluid, rather than a general requirement for lift in all cases (although it seems like he may not currently realise it).

 

Thanks, Rasta, I'll 'sandwich' my comments in yours.

I'm talking bout boat bottoms. Glastron, Allison, racing tunnels, etc. 'Partial planing' is still plowing. So long as buoyancy carries a significant fraction of weight then the boat is still plowing after lift has developed, i.e., after the flow has separated from the bottom at the transom so that there is no backflow up the transom. The transom is dry after lift develops, which is presumably what you folks are unnecessarily labeling as 'ventilated'. I recommend the shorter, clear word 'dry'. Again, see Newman bout the flat plate shedding the starting vortex in the kitchen sink.

 

I was referring to your insistence that there can be no lift before the Kutta condition holds.
You appear to think it's a requirement wherever lift exists, which of course isn't the case.

"Dry", "ventilated"...I don't think anyone will care much which is used.

 

I care. 'Dry' is simple and clear. 'Ventilated' brings to mind channeling air along the boat bottom, very misleading.

 

I don't believe I've given any labels to anything.

This statement by you seems to be stepping away from "no lift without Kutta".

I would call that "streaking", but to each his own.

"Ventilated transom" is not incorrect.