Transom Drag

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

  1. jesdreamer
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    jesdreamer Junior Member

    Suction acting before it can dissipate??

    My question in an earlier post which Daiquiri is referring to is --- "is the speed of everything taking place allowing suction to perform it's effect on drag before it can dissipate??" -- The question makes sense to me, it relates to earlier discussion of massive vortex drag at partially wet transom if the air couldn't get into the wake void, and to the discussion on a hull step which if it had no access to air would create massive suction drag --

    As with the hull step requiring air access to prevent vacuum drag, I question how vortex action at a partially wet transom at some speed can act to produce drag when the vortex is so close to surface and ample air is so immediately available?? I readily grasp how a non ventilated hull step could cause drastic vacuum drag, and I do understand how a vortex wake can add to transom drag but can not quite grasp how this all can actually take place with surface air so readily available -- why doesn't hull just ventilate a little more?? (and dissipate the vacuum) --
     
  2. jesdreamer
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    jesdreamer Junior Member

    Doctors paper - Resistance during unwetting??

    I had seen this paper before and it is focused on wake hollow length. The little transom related resistance data clearly show the loss of hydrostatic pressure as speed increases and transom wetted area decreases -- None of the data or discussion show any evidence of vortex effect increasing transom drag as transom progressively goes dry -- leading one to question whether any vortex drag actually takes place as transom gradually goes dry as speed gradually increases from idle through slow and slightly onward until dry transom condition exists?? (wouldn't the instrumation in these tests have detected a related increase in drag?? Or would such an effect just get hidden in the readout showing gradual loss of hydrostatic pressure -- and if so, it seems to me that any slow speed vortex action with partial wet transom has to be very small and of very little net effect -- I thought Daiquiri had shown the slow speed vortex action to be a major factor??
     
  3. jesdreamer
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    jesdreamer Junior Member

    The first statement referenced above is wrong if vortex flow does in fact generate a suction effect -- but I do agree that it is hard to believe that the vortex action can generate much if any suction with ambient air so close just above the depressed waterline at partially wet transom --

    Latter statement above just describes the loss of hydrostatic support and disallows or disregards and of the vortex suction action which I feel Daiquiri has quite adequately described --
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    They might well be very important at low-speeds, but I have never been
    interested in that regime. They would increase the eddy-resistance which
    can be thought of as part of the viscous drag. At very low speeds (actually
    Froude numbers) wave-making is very low. That is the justification for using
    methods like Prohaska plots to estimate form drag. (Wet transoms make that
    procedure much more uncertain, however!)

    The papers by Doctors and others don't include the effect of vortices.
    They use thin-ship codes which only use sources and sinks, and no vortices
    or dipoles.

    There is some re-circulation aft of the transom where wave-breaking is
    evident. See, for example, Fig. 15 and 16 in the attached paper.
    Some components of transom drag are shown in Fig. 18.

    I'm sure that you are beginning to see what a complicated flow regime
    this is, and that you are very unlikely to get simple answers to your
    questions. :)
     

    Attached Files:

  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Don't over-focus on transom drag, jesdreamer, it is just part of the overall "drag" picture, there is likely to be far more drag from wave-making resistance than transom drag, at the stage the transom "dries". It is more a minor inconvenience than the main impdediment. A furniture van barrelling down the road will experience drag from the blunt rear-end "suction", and will still experience it as it starts to climb a steep hill, but the hill-climb will become a greater source of resistance. Your boat experiences a similar situation with the hull generated transverse wave system changes, as speed is attempted to be increased.
     
  6. daiquiri
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    daiquiri Engineering and Design

    But it does ventilate. It just does it so little at low speeds, that we can assume it doesn't ventilate for the sake of simplified analysis. At very low Fn, the water-air interface behaves like a rigid wall, where either symmetry or slip-condition can be applied. That's what I did.

    So, once again: please bear in mind (and Barry too) that the CFD analysis in the post #29 refers to very low speed regime in which the following assumptions are considered valid:
    1) wave drag is very low, at limit neglectable
    2) the hydrostatic influence of surrounding wave elevation is neglectable
    3) the transom is fully-wetted.​
    If you try to extend the conclusions of that analysis to higher-speed regimes, you will get to wrong conclusions because the transom will start to ventilate in a non-neglectable manner, the wave drag will become more important and the wave elevations around the whole hull will have to be taken into account when calculating the hydrostatic drag component. These drag components will decrease the importance of the turbulent-wake drag.

    Referring again to the transom-stern boat in the post #29, the resulting calculated average gauge pressure on the transom is approx. 450 Pa, which corresponds to water surface lowering by 50 mm behind the transom, which is 8% of the static draft. It IMO confirms that the assumption of flat water surface is essentially correct for such ballpark analysis. And, anyways, I don't have so much free time to set-up and perform free-surface CFD calcs, so this is what I can offer. :p

    Cheers
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Your efforts are appreciated, if not really registering entirely with we dullards, but 450 Pa is tiny, negligible ?
     
  8. daiquiri
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    daiquiri Engineering and Design

    Yes, you are right. I don't know how did that "less than atmospheric" end up there. Guess I should not write replies while thinking about jobs awaiting to be finished. :)

    In fact, if you look at my reply #48, you'll see that we are saying the same thing.

    Thanks for noting the typo.
     
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  9. Barry
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    Barry Senior Member

    I think that all the post contributors gain a lot of knowledge when you offer your experience to the mix. So thanks for that.
     
  10. daiquiri
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    daiquiri Engineering and Design

    I didn't say it is negligible, I said that in retrospect it justifies the water surface modeled as flat, because it causes just 8% decrease in transom draft. :p
    450 Pa is an estimate, calculated from the difference in drag of transom-stern and canoe-stern hulls in the post #29. Since these two hulls have been tweaked in such way to have a nearly same wet area, displacement, length, draft and the shape of the forward half of the hull, it is reasonable to assume that at very low speeds the difference in drag is for the major part due to transom. But it actually has to be verified more in depth, it is too late here to do it now.

    Not thatt this particular is very important either. The main goal of those CFD sims was to provide an indication of the relative importance of the transom drag at low-speed regimes. I don't have neither time nor desire to perform an academic-grade study of this issue. The intention was to give some food for thought. Considering that the discussion has gotten quite alive after that post, I would say that the goal was reached. ;)
     
  11. jesdreamer
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    jesdreamer Junior Member

    Wet transom drag

    Post referenced above is #4 from very early in this thread. Subsequent posts by Daiquiri have added substantiation to the above feelings/observations/data that a partially wet transom at low speeds can have high (relative) transom drag as associated with vortex action in the adjacent wake and that resultant low speed total drag can thus be substantially higher than that of a similar hull having canoe stern -- We all realize that the drag forces involved in low speed displacement operation are smaller than when at higher speed, and that the transom-related vortex drag contribution goes away when transom goes dry as speed increases --

    later and most recent posts are starting to sound as though this effect might not really exist and/or the effect to be negligible. I agree that the effect goes away at more typical cruising speeds -- but I was trying to explore low speed partially wet transom. Do we all agree that most flat transoms will go dry at around Fn of 2.5 to 3.0 or so -- might this usually mean at speeds of 2-3-4Kts for a tupical hull in the 15-20ft range, with maybe 5-6-7Kts for a typical hull in the 30-40ft range??
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    jesdreamer, you are fiddling while Rome burns, at the stage the transom is drying, at around 10 knots in your 20 footer I'd suggest, the thing is well on to the 'hump', where wave-making has escalated, and there is little planing lift happening because the speed is not yet high enough. The transom drag is a minor player at that point. The real game is with the nasty transverse wave generation, made all the more onerous by the fact your hull has developed a squatting attitude because of its position in the wave trough of its own making. If by some magic the deep transom were suddenly to be raised, that squat would increase, and with it, the wave amplitude increase further.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Not quite. :)
    The length-based Froude number is defined as:
    Fn = V / sqrt(g L)
    where:
    V is the speed expressed in m/s
    g = 9.81 m/s^2
    L is the LWL expressed in m
    or:
    V is the speed expressed in ft/s
    g = 32.2 ft/s^2
    L is the LWL expressed in ft
    So, for a 20 ft boat, Fn = 2.5 means that the speed is
    V = Fn sqrt(g L) = 63.4 ft/s = 37.5 kt​
    and so for the other boat lengths and Fn's. ;)
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Many results in the papers and references I have supplied use the Froude
    number based on transom stern draft. Tuck and Vandenbroek found that
    steady waves cannot exist below about F_T = 2.23 for the 2D case.
    Experiments and other modelling shows that the limit is not much different in
    3D, but it depends a bit depending on the transom shape.
    A Reynolds number can also be defined by replacing the hull length with the
    transom depth. It plays a fairly minor role compared to the transom Froude
    number.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    jesdreamer is bent on dodging the devil of transom drag, even it it doubles total resistance, seemingly !
     
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