Transom Drag

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

  1. jesdreamer
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    jesdreamer Junior Member

    High drag of partially submerged transom

    No No No -- Barry, we originally questioned a post on one of the planing threads quoting a Van Oossanen comment that a partially wet transom of a boat underway can have "transom related" drag effects as much 2x as with the same boat at enough speed to yield a dry transom (which would exhibit the loss of hydrostatic support, which might be viewed as a negative drag since it is opposite to friction, residual, and similar motion-related drag forces) -- And BTW I don't think anyone will agree that a transom goes dry almost immediately when the boat gets underway -- most references indicate transom goes dry at somewhere around 2.5-3 Fn which might be somewhere below theoretical hull speed but certainly not immediately --

    And No No No -- Daquiri, you were on the right track earlier. And I believe your vortex drag effects can be the source of high transom drag as you were describing. We were questioning whether a partially wet transom of a boat underway could have transom ("related") drag as high as 2x the transom ("related") drag of the same boat at higher speed with a dry transom.

    I never made any kind of statement along the line of a dry transom exhibiting 2x the drag of a wet transom (though at some lower speed with a wet transom and little or no turbulence, the related drag might well be lower than the transom related drag when the boat has a dry transom at higher speed, and thus at some low speed might well have a transom related drag around half that of the same hull at higher speed with a dry transom)
     
  2. daiquiri
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    daiquiri Engineering and Design

    Ok. Since I am failing to see the practical scope or point of this thread (for the reasons described in my previous post), I am leaving my seat to someone more motivated to engage in this kind of discussions. :)
    Cheers
     
  3. Barry
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    Barry Senior Member

    I never suggested that a boat underway immediately creates a dry transom. Please read my post. What I did say was that when the transition occurs I suspect that it is almost instantaneous
     
  4. jesdreamer
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    jesdreamer Junior Member

    Daiquiri please don't go

    Daiquiri, please don't go -- Your posts were the only ones supporting the thought that a wet transom can show more drag than the loss of hydrostatic support when the hull goes dry. And the illustrations clearly showed the problem to be vortex effects -- Might you have any insight as to magnitude of the forces vortex action can cause at a flat transom??
     
  5. Easy Rider
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    Easy Rider Senior Member

    The value in the answer to this question is in actually knowing the difference in hull drag between a typical trawler (SD hull) and a full displacement hull like a Fisher or Willard. And knowing at what speed the drag is the same. Of course all FD and SD hulls are different but a ballpark number for typical hulls would be useful.

    It looks like nobody's read the thread "Planing hulls at disp speeds". A very good thread and well related to this one. Haven't re-read the whole thread but as I recall it was more or less decided that it did NOT take twice as much power to drive a SD hull at displacement speeds. My gut feeling still tells me it does though so I need to re-read the whole thread.
     
  6. daiquiri
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    daiquiri Engineering and Design

    It looks like you haven't read this thread, Easy. And you have also missed the topic of the thread, just like I did. :)
    The link to "Planing hulls at displacement speeds" thread was given at the previous page, but it seems that Jesdreamer didn't want to discuss the difference between FD and SD hulls. Instead, he is apparently interested in the origin of the difference in drag between wet and semi-dry transoms, and how to calculate it. A rather odd topic from the practical side, but that's what it is.
     
  7. daiquiri
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    daiquiri Engineering and Design

    In the CFD simulation shown in the previous page, the pressure drag component of the transom-stern body was 94% of its total drag - if that can help you.
     
  8. jesdreamer
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    jesdreamer Junior Member

    resistance of partially submerged transom

    As suggested earlier, I read through the thread on planing hulls at displacement speeds which really was not pertinent -- Daiquiri has the question in focus -- same hull, same weight, etc. but apparently partially wet transom can generate more drag than the "Transom drag" (as defined as net change in total drag, due to lack of hydrostatic pressure, when transom goes dry) -- I am interested in whether this can be related to suction effects of vortex flow at rear, or is it just related to trim angle and resultant change in wetted area -- If there is a chance this can be related to vortex action, I would like to learn more (since the trim angle approach is rather obvious and I had assumed the 2x statement to be related to just the effect of wetted transom w/o considering any other drag effects) --
     
  9. daiquiri
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    daiquiri Engineering and Design

    But where does this claim come from?
    This whole discussion seem to revolve around this issue, but we still don't have the source of it. :)
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Where are you getting these ideas from ? A transom can be, but not always, a truncation of a boats underwater lines, such that they end abuptly below the static waterline, rather than rise up gradually to the static waterline. That has effects on the overall resistance of the hull at various speeds, generally at the low speed end it adds to it, at higher speeds it likely will reduce overall resistance. If a boat has no pretensions to the speeds where boats typically benefit from a submerged transom, there is little reason to have any transom submergence, but a transom above waterline is still entirely valid. You have to make up your mind what speed range your boat is required to operate in, to establish the wisdom or otherwise of the submerged transom, there is no "ideal" solution that fits all applications. Seems to me you are concentrating on the transom, and not considering the boat as a whole. The useful information given by daiquiri was that at "crawl" speed, a submerged transom can cause a large percentage of the total drag, which explains why sailing boats wanting to operate in light airs, typically avoid anything resembling submerged transoms.
     
  11. rwatson
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    rwatson Senior Member

    After barely any thought, the question seems to be pointless unless you specify the other parameters like Mr E pointed out.

    The drag component will vary dramatically under different speeds. So to state any number without speed, conditions etc, is like asking the proverbial string length.

    eg here is a detailed flow analysis
    http://www.dtic.mil/dtic/tr/fulltext/u2/a499067.pdf

    The drag force graph shows a drag range of between 100 and 300 lbs, over a speed range of 5 to 9 knots - certainly more than a doubling effect, on this particular model.

    Pick a speed, water conditions etc, and take your pick.
     

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  12. jesdreamer
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    jesdreamer Junior Member

    Has Daiquiri Nailed It?? (see his post #12)

    Late last night I read back through a few of the posts & actually examined the illustrations Daiquiri generated by his CFD analysis. His summary states the case but really hadn't sunk into my brain -- The illustrations tell the story quite vividly since the other drag factors are eliminated from his analysis -- 390N vs 180N at 3kts -- so at the kind of low displacement speeds appropriate per my original question, a flat transom can generate well over 2x the drag of a smooth transition. This is the first time I have seen actual numbers relating to the question of transom drag which is usually hidden within the overall drag figures. And in Doctors and other papers on transom effects, the focus is on higher speeds and the drag effect of a transom going dry. The Hydrostatic support is easy to calculate and it's loss upon a transom going dry represents a minor part of drag at speed, on the order of perhaps 5 or 10% at most. With this in mind, I had questioned the popular statements that a square stern hull can have 2 or 3 times the (OVERALL) drag of a hull with canoe stern -- It seemed to me that this major increase in overall drag had to be directly related to the flat stern. So I wanted to explore a flat stern at low speeds. I had suspected vortex action as the culprit but had never seen a paper with data in this area. The Van Oossanen reference in one of the threads on planing was the closest I could find where anyone discussed actual wet transom drag -- in the VO case of a partially wet transom at low speed generating 2x the drag of a dry transom and it's related loss of hydrodynamic support -- Daiquiri's post #12 very clearly addresses the specific question.

    I really now end up with many more related questions but at top of my list is a desire to put this into perspective -- I guess we can assume that a canoe stern might contribute little drag for a small % of overall resistance. And we now see that a flat stern gives more than double the transom drag at low speeds of a similar hull but with smooth transition, but just how significant is this fact?? Is it a major component of overall drag at low speeds with a wet transom or is it a minor % of overall drag under these conditions?? Can we get some numbers on this from Daiquiri or someone else?? With all of the stories of transom hulls showing 2 or 3 times the low speed drag of a canoe stern, the effect must be significant --
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    jesdreamer, you are looking at this like a horse through blinkers, you aren't seeing the full picture. What kind of boat are these cogitations focussed on ? There is no blanket approach that gives the right answer to all situations. Everyone knows that a submerged transom causes extra drag at low speed, and it has already been indicated just how much of total drag at those speeds can be due to a submerged transom, but it was also indicated by daiquiri that overall resistance under those circumstances is quite low anyway. If you want a boat that tootles around at 3-4 knots, you don't want a submerged transom, although there is little value resistance wise in choosing a canoe end over a transom stern, clear of the water. If, however, you develop a speed craze, the canoe stern or non-submerged transom will become a complete party-pooper, as resistance goes through the roof without it. Again, what is the boat you envisage, required to do, and especially in terms of speed ?
     
  14. daiquiri
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    daiquiri Engineering and Design

    In general, it is not possible to isolate the effect of the transom alone. Such analysis would require keeping everything else (except the transom) equal, which is geometrically not possible.

    It is not possible to construct two hulls, one with the canoe stern and the other with the transom stern, and call them "similar". All the most important hydrodynamic parameters (displacement, hull form coefficients, wetted areas, length-beam-draft ratios etc.) are intimately related to each other - the variation of one of them requires a variation of all the rest of them, so the resulting two hulls will never be sufficiently similar from the hydrodynamic point of view to allow you to isolate the contribution of just the transom to the overall drag.

    For example, if you keep the same displacement, length and wetted surface, the transom-stern vessel will need to have either a smaller draft or a larger beam. And in both cases the latter will have higher block (Cb) and prismatic (Cp) coefficients. Higher block and prismatic coefficients will modify the wave drag of the hull.

    However, in the very low-speed range the wave drag is a small part of the total resistance - like 15-25%. That IMO allows us to assume that the difference in Cp and Cb between the two hulls will not have a significant impact on the overall resistance, and an approximate comparison of drag values between a canoe-stern and a transom-stern vessel could be indicative of the influence of the transom.

    So... I have bothered to perform another quick analysis of two simple hulls, one with the canoe stern, the other with the transom stern. Both are 12 m long, displace 12 cu.m of water, have the same draft and the same shape of the forward part.
    Hence, in order to keep the above parameters equal, the canoe stern hull has a larger beam and a lower L/B ratio:

    Hull models.jpg

    I have ran a simple 3-D CFD, 3 kt water speed, no waves. The results:

    Canoe stern.jpg

    Transom stern.jpg

    Considering that the wave drag should be expected to be on average 20% of the total drag in this speed regime, the above values of drag have to be multiplied by 1/0.8 = 1.25 , in order to obtain the ballpark value of the total drag. So:
    Canoe-stern vessel drag = 2190x1.25 = 2750 N
    Transom-stern vessel drag = 2630x1.25 = 3300 N
    (values rounded to 50 N for enhanced clarity, it is all aproximate anyways)
    In other words, according to these data, this transom-stern vessel has just 20% higher total resistance at 3 kt, compared to his canoe-stern counterpart. A far cry from the expected 100% increase. Evidently, the pure transom drag has been diluted by other hydrodynamic forces acting on the two hulls.

    I admit that this is quite a surprise for me. Under the simplifying assumptions which these calculations are based on, the drag difference is far from what I have expected. Therefore, it should be taken with a grain of salt.
    One thing that should be verified is the assumption that the wave drag is 20% of the total drag of the transom-stern vessel. It is on average true for canoe-stern hulls, but a transom-stern hull might have a somewhat higher wave component of the total drag, perhaps up to 35-40%.

    If we assume that the wave drag is 35% of the total drag of a transom-stern hull, and 20% of the canoe-stern hull, then the above drag summary becomes:
    Canoe-stern vessel drag = 2190/0.8 = 2750 N
    Transom-stern vessel drag = 2630/0.65 = 4050 N
    (values rounded to 50 N, as before).
    In this case, the transom-stern vessel has almost 50% higher total resistance at 3 kt, compared to his canoe-stern counterpart. Now that is something.

    As you can see, it is very difficult to get a definitive picture, even under all the simplifying assumptions of this case. The main problem is the lack of reliable experimental data for such low speeds, which cast a shadow on the validity of CFD results too. A result of CFD calculations is considered valid when it coincides with experimental (towing tank) measurements. If the experimental data is unreliable, the CFD output has to be considered equally uncertain.

    Now you have quite a lots of things to think about in these days. ;)
     

  15. jesdreamer
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    jesdreamer Junior Member

    Hypothetical design

    Mr Efficiency --
    I am really waiting for some further info from Daiquiri. I find his post stating the submerged transom pressure drag (is this transom drag??) was some 94% or so of the total drag at 3kts in the case he looked at to be quite enlightening. (but I don't know how one can get these figures in a 2D case).

    I am trying to explore and understand some of basic principles and their relationships -- but if you insist on a proposed design, and can break down the drag factors, I would like to see some numbers in the 3-4-5kt displacement range on friction, wave drag, and transom drag for a 14ft boat, perhaps 3ft beam, 250# displacement with a transom designed for planing. I can translate this into horsepower required (in displacement mode) but am most interested in the transom drag and it's % of total, as well as friction and wave drag figures (software I have been using has no way to break out transom drag as a separate figure) --
     
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