Transom Drag

Discussion in 'Hydrodynamics and Aerodynamics' started by jesdreamer, Dec 14, 2015.

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Boat tail cars have less drag so maybe we need car tailed boats?

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daiquiriEngineering and Design

Yes you are absolutely right, Leo.
It's just that I saw boat lengths mentioned in that post, so I thought that Jesdreamer was using them as a reference for the calculation of Froude numbers.
Cheers

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Mr EfficiencySenior Member

Has jesdreamer lost interest in defeating the transom drag demon ? Just think of it as the back end of a bus, it has an unavoidable complication of creating eddies behind it, but what kind of a bus has the fish-tail end ? A not very practical one.

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Leo LazauskasSenior Member

Both are required for some modelling calculations. For example, the
length-based Froude number will largely determine the nature (e.g. crest,
trough or between) of the hull wave at the stern. The stern depth Fr will
determine whether steady flow is possible, and some other considerations.
It can get even more complicated with multihulls where there is cross-flow,
and the bow-wave from one demihull cuts across the stern of another hull.

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jesdreamerJunior Member

SLR instead of Fn vs wet transom drag

Perhaps Fn is not the easiest key to looking at drag of a partially wet transom at low speeds -- Savitsky in his paper on high speed monohulls applies his many years of observing tank tests while managing the tow tank at Stevens Inst of Tech. He reports observing that transoms typically go dry by an SLR of 0.9 or so -- thus near Hull Speed, by which time transoms generally are dry. So this does focus the situation as a problem which is gone fairly early as a hull speeds up.

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Leo LazauskasSenior Member

That's why most people use Fnt, i.e. the Froude number based on transom
stern draft, as you should have seen in the papers I attached earlier.

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jesdreamerJunior Member

Summary of the situation??

I think this thread has just about burned itself out and would like to summarize where my thinking is as a result of the expert and more practical observations posted -- (I am not looking for the kind of debate the "Definition of Planing" thread has generated -- just a better grasp of the situation) -- Might the following be "acceptable"??

1) Wet transom exists at low speed only, & is generally gone by the time boat reaches Hull Speed --
2) Partially wet transom under motion generates strong vortex wake coming off all submerged transom edges which can produce a drag higher than "Transom Drag" (drag resulting when transom goes dry and related hydrostatic forward support is lost) -- Vortex pressure effects dissipate as vortex flow reaches water surface but the vortex action is continuously replenished as the hull moves forward --
3) The vortex action can yield high relative drag, but friction is low and wavemaking is almost non-existant at the low speeds under which the situation can exist -- so the overall magnitude of this induced drag can be really quite low --
4) Even though the drag result can be quite low, it is still high enough to be a major factor in a transom hull showing 2x-3x or more drag than a similar hull with canoe stern in displacement mode at speeds below Hull Speed --
5) The transient nature and other characteristics of this drag are very difficult to explore mathematically, and are disregarded in most research work on hull and wake hydrodynamics --

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BarrySenior Member

My comments below refer to planing hulls

Re 1) Hull speed pertains to displacement hulls and your original post was referenced to a square transom hull and applicable drag. The shape, width and depth will determine when the transom ventilates, for a planing hull, is normally around 9 - 10 knots. Whether a planing is 20 feet long or 100 feet long, with the same transom crossection, the hull will ventilate at about the same speed.

Re 2) This has not been proved by anything in the posts that this "partially submerged hull produces higher than the max drag created when the transom ventilates. If any position of the wake exhibits sustained pressure below atmospheric it will ventilate.

Re 3) The vortex action is a result of the lower pressure developing at the back of the transom , ie lowering of hydrostatic transom pressure and as 2) cannot create pressures that are lower than atmospheric as 2) ( there might be localized velocity induced lower pressures but these will be aft of the transom/water interface and not increase drag above the ventilated transom drag value.

Re 4) Of course a canoe, ie displacement hull exhibits lower drag at slower speeds.

Re 5) Mathematically, it is difficult to precisely determine the transom drag between the lowest value at say .0001 knots up to the maximum drag that will occur at transom ventilation. (beyond the transom ventilation speed, you will get an increase due to air drag ) BUT, an array of pressure sensors on the transom could easily determine transom pressure acting at various speeds without too much difficulty.

Without providing the quote from Van Oossenan about drag being twice that of a partially submerged transom, WITHOUT knowing what he was referencing, planing hulls vs displacement hulls or anything in between, it is difficult to prove or disprove this quote.

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jesdreamerJunior Member

No added drag with partially wet hull??

1) Per your #1 above -- Savitsky's observations from tank tests show full ventilation of transom sterns at SLR of 0.9 which is below hull speed. I could find no evidence that his observations were limited to slim displacement hulls. I am curious as to where your statement of planing hull transoms ventilating at 9-10kts comes from. I don't disagree with this range for ventilation but what about a high trim "bad attitude" planing hull vs the well balanced one with a transverse transom rail attachment to augment aft support??

2) illustrations posted in this thread by Daiquiri suggest vortex action is generated at least as fast as it might dissipate at water surface, and this action seems reasonable, thus it seems reasonable there could be related drag due to dynamics of the actions involved. It seems generally concluded that vortex action is caused by sudden change in flow direction as water separates from the hull at the submerged transom edge, which has nothing to do with any reduced hydrostatic force due to reduced draft at transom due to forward motion of the hull (to split hairs, hydrostatic pressure might be somewhat lower in this region due to forward motion and this lower pressure would tend to make any vortex action more severe) --

3) At any static speed with partially wet transom, the related transom draft would remain constant -- thus I question why the Transom Drag as conventionally defined would be changing. Also it might be argued that lower pressures could be a result of the vortex action rather than the other way around.

4) Many displacement hulls have square transoms and some such hulls could well have lower drag than a typical canoe when in displacement mode. OP was concerned with similar hulls but w or w/o a transom in displacement mode before hull speed is reached -- Question was not per a typical wide planing hull with straight buttocks vs a canoe -- it was per a partially wet transom regardless of the balance of hull configuration.

5) The maximum drag that will occur upon transom ventilation is readily accepted as that which is induced by the loss of hydrostatic support (negative drag) under forward motion -- your statement is to the effect that there can be no transom related drag effect other than this -- there seems to be other evidence that other water action drag effects can exist at speeds below ventilation speed -- and this phenomenon is what I have been trying to explore.

Barry, I really do appreciate your feedback but it has reinforced my feelings as summarized -- Can I be that far off base??

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Mr EfficiencySenior Member

You mean trim tabs ?

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jesdreamerJunior Member

trim control

Trim tabs would do the job but at significant drag penalty -- I meant an interceptor strip which generates lots of lift w/o much drag component

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Mr EfficiencySenior Member

The way I am understanding jesdreamer's 'concerns', is he is fixated on the idea that at a crucial point when water falls away from contact with the transom, there is a drop in pressure against the transom, and therefore an increase in resistance of the hull to forward motion. Right ? Well, unless you want a boat that is going to be running at that speed where the transom would remain wet, it is not something to be too concerned about, and were that the chosen speed range, the deeply immersed transom would not be designed into a boat anyway, so I don't understand why it is a bother.

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Mr EfficiencySenior Member

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Mr EfficiencySenior Member

Are we wanting one boat that is as efficient and effective as boats designed for specific speeds, at all speeds ? The thinking cap will need to be glued on. Not saying it can't be done, but it would require a boat whose shape was alterable according to speed, and that would create headaches galore.

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jesdreamerJunior Member

Water falling away from transom??

I accept or acknowledge the "transom drag" as associated with loss of hydrostatic pressure (forward) when a transom goes dry -- and this really has nothing to do with my original question as posted in beginning. I am interested in drag which may or may not be introduced by vortex flow at a partially ventilated transom (a stage of motion when part but not all of the hydrostatic thrust has been lost).

As to boat design significance and or efficiency, etc, etc -- this is a forum for discussion of hydrodynamics which is theory related and for discussion of technical issues which may or may not have any practical application at any particular speed -- My original question was/is theory related directly to hydrodynamics of hull/water reaction during displacement speeds up to hull speed -- and may or may not have design significance for boats operating at higher speeds

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