# Transom Drag

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

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### jesdreamerJunior Member

In reading through the threads on planing and planing definition, I ran across a post which referenced a statement by Van Oossanen that a transom with partial submersion "can create a drag 2x that of a dry transom" --

Transom drag is usually discussed as associated with loss of hydrostatic pressure against a transom (which when present via wet transom, helps rather than retards forward motion, so when this pressure is lost via a dry transom there is an associated resistance or drag). My question is how one can get 2x the level resistance which comes from loss of hydrostatic pressure loss as transom goes dry -- can turbulent vortex flows actually produce a suction effect of this magnitude against the wet transom under speed??

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### BarrySenior Member

It appears that you are assuming that this statement is correct.
If the transom is dry, then the pressure against it would be about 14 psia. Acting on for simplicity's sake, from the water line down

If the comment is correct, then when there is water against a fully submerged transom if it were to offer twice the drag of a dry transom would be -14 psig or absolute zero.
Acting on the water line down

If the transom is ONE HALF partially submerged, ie the area of wet transom is HALF than the two areas as noted above and this water is producing a drag of - 28 psig.

This is not possible

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### SamSamSenior Member

I would think a dry transom has more or less zero drag.
If you were comparing a transom dragging air behind it to one dragging water behind it, 2x sounds believable.

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### daiquiriEngineering and Design

The most informed experts in this field would imo be Leo Lazauskas and Donald M. MacPherson, so let's hope they will jump in to explain.

As far as my knowledge goes, the effect of the transom on hull resistance is not yet fully comprehended, and no universally accepted physical model is available. For that reason the output of velocity-prediction programs is not reliable in the low-Froude number range (where the transom is wet or semi-wet).

That being said, it is true that a number of research papers point to the fact that the wet-transom drag is very high in the low-speed regimes, up to twice the drag of a canoe-stern vessel of similar geometry. It is important to emphasize that the term "very high" has to be intended in the relative sense, compared to the canoe-stern drag at the same speed. In the absolute sense, measured over a whole practical speed range of a given hull, both canoe-stern and transom-stern vessels have low drag at low speeds.

The fact that the transom-stern drag is much higher than the canoe-stern drag at low speeds is imo not very difficult to accept, if you consider that the total resistance of a canoe-stern hull at low speeds is very modest (since most of it comes from the friction, the wave drag accounting on average for some 15-25% of the total). The wet transom adds at least two important additional components to this base drag value:
- the turbulent wake - made of small and big vortex structures (the latter ones creating areas of reduced pressure on the transom) and
- the hydrostatic drag.
So it really isn't that difficult to double the friction drag at the end.

The turbulent wake, in particular, can imo account for a good part of the wet transom drag, since the wet transom essentially behaves like a transverse step in the flow - with the associated separation-induced large vortices.

I'll stop here, awaiting for the above-mentioned resident experts to come and say more on this issue.

Cheers

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### jesdreamerJunior Member

To clarify "transom drag"

Lets try to clarify this a little. We might all agree that a dry transom really creates no drag -- the resistance referred to as "transom drag" is really a lack of negative resistance which is provided by hydrostatic water pressure against a transom helping to propel the boat forward -- so when a transom goes dry at sufficient speed (Fn 2.5-3.0 range) this helpful force is lost and the related change in total drag is called "transom drag".

The way I interpret Van Oosanen's referenced statement is that a wet transom at some speed less than that yielding a dry transom can generate a drag as high as 2x the above referenced hydrostatic "transom drag" --

Daiquiri, in his post just above ("The turbulent wake, in particular, can imo account for a good part of the wet transom drag, since the wet transom essentially behaves like a transverse step in the flow - with the associated separation-induced large vortices...") gets into the only way I can see to explain such an increase in transom drag before transom goes dry -- and that is strong vortex flow and related pressures reduced below atmospheric.

Can this be the cause of a wet transom giving a drag level as high as 2x "transom drag" as per the VanOossanen reference??

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### Easy RiderSenior Member

I submit that it has little or nothing to do w air pressure on submerged or open air transoms.

The element causing drag is the hull bottom. The trailing edge (so to speak) is the transom/bottom junction .. where the water breaks away from the hull. A hull w a deeply submerged transom has 1/3 to 1/2 of the boat at a much higher angle of incidence (or angle of attack) and the bottom w a submerged transom pushes the water down all across the aft end of the boat. Whereas the water returns gracefully to the surface experiencing far less turbulence as most of the turbulence is just surface friction. The tumbling water aft of the transom at less than hull speed pushes on the transom a bit and perhaps even helps move the boat. As speed increases the boat runs away from the water at the transom and the angle of attack of the bottom on a planing or semi-planing hull is much greater than a boat w the bottom ramping up to the surface as in a full displacement hull. I think it's all about the aft end of the bottom of the boat.

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### jesdreamerJunior Member

Could it be Trim Angle effect??

I had been considering only the effect of water flow at the transom -- but could the Van Oossanen comment of "wetted transom having drag of (up to) 2x the level of "Transom Drag" caused by loss of hydrostatic pressure actually be related to the aft sinkage even at low speeds and it's related trim effect (thus the hull having to climb uphill)?? -- It would seem to me that any drag caused by increase in trim should have it's own name -- such as "Trim Drag"

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### BarrySenior Member

Jesdreamer
Perhaps you can dig out the thread where Van Oossanen made the statement so we can ascertain the parameters under which he made this statement. I was unable to find this under the search engine
Thanks

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### jesdreamerJunior Member

The original Van Oossanen reference

Good point -- before posting the question I had gone to the Van Oossanen web site and looked at their list of publications. I didn't see a title that looked directly relevant, read through a few papers but could find no reference to a 2x comment -- perhaps someone else can find it?? (I will also look through the many many posts on planing to see if I can find who posted the comment on one of those threads where I had first run across it) --

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### Leo LazauskasSenior Member

Very capable summary, Slavi!

I'll just add that stern wedges and flaps can help reduce transom drag.

There are many papers available on the net that might help.
(Most of these are for thin and slender hulls.)

Molland, A.F., Wellicome, J.F. and Couser, P.R.,
"Resistance experiments on a systematic series of high speed displacement
catamaran forms: variation of length-displacement ratio and breadth-draught
ratio",
University of Southampton, Ship Science Report 71, 1994.
http://eprints.soton.ac.uk/46442/1/071.pdf

Lawrence J. Doctors has been refining a model for many years. See, for example:
http://www.iwwwfb.org/Abstracts/iwwwfb21/iwwwfb21_10.pdf
http://people.eng.unimelb.edu.au/imarusic/proceedings/13/Doctors.pdf

Lawry's recent book has a lot on transoms, but it is quite expensive.
Hydrodynamics of High-Performance Marine Vessels - 2 Volumes.

Bill Vorus and Brendon Taravella have also developed a model of transom stern resistance.
https://www.navalengineers.org/ProceedingsDocs/FAST2011/FAST_C/C3-3.FAST2011.Taravella_et_al.pdf

Good luck!

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### daiquiriEngineering and Design

Check this (and possibly the rest of the thread too):
http://www.boatdesign.net/forums/boat-design/planing-hull-disp-speeds-31464-4.html#post405575

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### daiquiriEngineering and Design

I have performed a simple 2-D CFD analysis of the flow behind two bodies:
1) a body with a rounded stern and
2) a body with a cut-off stern.

The "hull" is 1 m deep, the water speed is 3 kt.
The water surface is modeled as a symmetry plane (or slip wall, if you prefer), given the very low water speed. In other words, it is assumed that surface waves and their influence on drag are negligible. The intented goal is to observe the qualitative behavior of just pressure and viscous forces in the aft part of the hull.

This is the main result:

- the water flow is from left to right, streamlines and colored pressure field are shown.

The 2-D canoe stern gives 180 N of drag force, while the transom stern gives 390 N in the same flow conditions. Hence, the analysis says that the transom-stern drag is approximately 2.15 times the drag of the canoe stern, under the assumed simplifying conditions. Which is in line with Van Oossanen's data in that other thread.

Let me underline the fact that this is a 2-D analysis of a simplified model of a submerged stern at low speeds and with no surface waves. Everything else being equal, a 3-D body will give lower numerical values of drag (the 3-D relief effect) but the relative ratio of transom/canoe stern drag forces should not be too much different.

What this analysis shows is, IMO, that the 2x difference between drag forces of a canoe-stern and transom-stern hulls at very low speeds could indeed be mostly attributed to the vortex-dominated wake behind a transom stern.

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### jesdreamerJunior Member

Well, we were not questioning the flat transom vs a canoe stern and Daiquiri did not explore this specific comparison -- but his post indicates the strong forces that might be related to vortex flow behind a transom --

Daiquiri, in his latest post graphically shows the turbulence behind a transom stern and calculates that it can have 2x the drag of a rounded stern (not a canoe stern but blunt back end similar to flat transom but with smooth curved up bottom, eliminating the the turbulence of the flat transom and very illustrative of the vortex drag situation) -- I have not quoted his whole post but his final statement just above suggests that the Van Oossanen reference to wetted flat transom able to show 2x the drag of "hydrostatic transom drag" (increase in total net drag when transom goes dry) might well be a reasonable observation -- Is this situation similar to the high drag related to turbulent flow and vortex action at rear of an automobile??

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### BarrySenior Member

This makes more sense and I doubt anyone would disagree with the drag of a transom stern hull being twice that of a canoe stern hull, especially at lower speeds.
The OP referenced a statement that a PARTIALLY SUBMERGED transom stern hull's drag would be 2x that of a dry transom.

After some consideration of the OP, I cannot remember seeing a transom stern boat with a partially submerged transom, underway, which, if the situation exists, would be immediately before full water separation to a dry transom.

IE I suspect that the transom goes from full wet to full dry almost instantaneously.

Can anyone offer a situation where a boat could run with a partially submerged transom.

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### daiquiriEngineering and Design

Ok, I must have misunderstood the sense of your opening post. In that case, you will have to provide the source of your claim about dry transom having twice the drag of a wet transom (intended as the same transom running dry at higher speeds), before going into further discussion. I cannot recall anything similar.

Anyways, at very low speeds the flow around the wet transom will look qualitatively like the one shown in the bottom pic of my previous post.

At high speeds, when the transom becomes dry, the water becomes replaced by air along the vertical wall, and the pressure is constant and equal to the atmospheric. How much the drag due to this single change of boundary conditions - I do not know that, and hence cannot say if it halves, doubles or else.

Such comparison would not be physically meaningful either, because in dry-transom conditions all the drag-related hull attitude parameters (like trim, wetted length and wetted surface) of the vessel would be completely different from the low-speed case. Hence it is not possible to isolate the influence of the transom ventilation only.

It is imo a futile theorical exercise.

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