Transom Drag

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

  1. jesdreamer
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    jesdreamer Junior Member

    Thanks To Daiquiri

    DAIQUIRI --Thanks for your CFD help -- I just can't handle some of the math.

    You were surprised at the low relationship of a hull with transom to a similar canoe shape as suggesting the transom adds some 20% but with some slight mental math gymnastics it might add 35% --

    However if we were to make a couple of minor changes to the hull configuration criteria it might have a major effect on drag contribution of the transom -- Instead of keeping wetted area the same and accordingly widening the beam of the hypothetical canoe shape -- what might happen if we let the draft go greater on the canoe shape but not widen it's beam (for same displacement)?? -- Your thoughts as to how this might affect the outcome??
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Your skinny 14 foot planing-capable boat will need to be quite flat bottomed, or will be as tippy as all heck. And the dreaded submerged transom drag will vary wildly according to what weight is placed in the boat, and where. Some ballast in the bow would see it potentially abolished altogether. In any case, the penalty at 3 or 4 knots would amount to a cup-full of fuel for a day's outing, not worth fussing over. Theories are nice, but in practice there is no point in bothering with them, if all your assumptions go out the window when some bozo move back, rather than forward, in the little boat !
     
  3. jesdreamer
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    jesdreamer Junior Member

    Detailed flow analysis ref from rwatson

    I have read through the reference supplied by rwatson -- I don't feel it to be pertinent to our question of wet transom drag since much of the drag in these tank tests might be attributed to trim angle (graph included shows tests to have significant bow rise and stern sinkage w/o breaking out any trim related drag effects). However the speeds in the 5,7,9kts range for a 30ft model are probably in the range of our interest -- and the sonic return illustrations do show strong vortex action in the massive turbulence behind a partial wetted transom at 5kts with progressively smoother flow at higher speeds and almost laminar-looking separation and wake with dry transom at 9kts -- so the paper does give good visual credibility to our questions of high drag behind a partially wet transom --
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    It turns out that purpose-designed fast planing boats are more efficient, in mpg terms, at crawl speeds, than at what they were designed for, higher speeds. No-one is worrying about squeezing some extra mileage at the low speeds, because the savings would be miniscule over the life-time of the boat and the typical usage. The only people that use slow speed for sustained periods in planing boats are fishermen trolling, if you have no intentions of doing that, it is a pointless to worry about it.
     
  5. jesdreamer
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    jesdreamer Junior Member

    Hypothetical example

    I hesitated to get involved with an example because I am trying to explore basic principles. I did provide an example because you asked for one -- and I would have assumed the displacement to be evenly dispersed with idle hull sitting level. I realize the transom drag would go to zero if load moved forward enough to yield a dry transom. Daiquiri has come up with some significant drag levels attributable to a partially wet transom at related lower speeds -- I am looking for actual data on transom related drag and relationship to other drag factors, not guesswork on fuel consumption which we realize would be low at low speeds regardless of whether wet or dry transom --
     
  6. jesdreamer
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    jesdreamer Junior Member

    Efficiency vs speed

    Well, the hull certainly would generate less drag at low speed & thus help efficiency -- but I would bet that an engine and prop might also both show better performance at low speed --
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    There is more to boat design than minimizing drag. You might finish up with a fabulously efficient boat, but a disgruntled crew. For example, a flat bottom boat planes more readily, but gives everyone spinal compression when the wind comes up. You have to figure what the main objectives are, and not focus exclusively on what might slip along with the least resistance.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    There is typically only 7 or 8 knots between the most fuel efficient speed in a planing boat, and the least. And transom drag has not got a lot to do with that.
     
  9. Barry
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    Barry Senior Member

    At this point, what has been quantified by Daiquiri is that a planing hull transom, square, produces more drag than a displacement, canoe profile.


    But I have to differ in the above quote that "everyone knows that a submerged transom causes extra drag at low speed" if we are referring to a planing hull.

    At rest say the total water pressure on a particular square transom is say 100 pounds from hydrostatic pressure.

    At 1 knot, the water pressure is say 90 pound, at 2 knots the pressure is 80, and so on and then at 10 knots the water pressure is 0 pounds. And at that point, the transom becomes "dry". There is no water pressure.

    My point being is that when there is water against the hull
    there is pressure against the hull, and at low speeds the pressure is higher than at speeds as they approach the "dry' transition speed

    But I do have a question which is somewhat relevant to this post.
    Two options:
    When the water separates from the transom and becomes dry, is this caused by the inability of the water to fill the low pressure void.

    or does the water pressure at the lowest point of the transom hit atmospheric and the air just fills it in.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    What I meant by low speed, was before wave-making becomes noticeable. The drying of the transom typically occurs at the stage where wave-making resistance is at its peak, and the major retardant to the boat's progress. The resistance would be much higher it it wasn't for the deep transom in that mode.
     
  11. powerabout
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    powerabout Senior Member

    I guess the transom width plays an important part in this as well?
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    You have to look at the whole picture,with particular emphasis on what speed you want to be travelling at, most of the time, and optimize for that. The boat that can beat the field, resistance wise, regardless of the speed you pick, is a fantasy. And it is pointless in worrying about inefficiencies at speeds you may only spend 1% of the boat hours at, as in transitioning to plane.
     
  13. tspeer
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    tspeer Senior Member

    How about a wetted transom drag as an example of hydraulic jump?
     
  14. powerabout
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    powerabout Senior Member

    Many super computers working on IRC optimisation and the best they can do is make a winner at one wind speed
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Some precise pressure sensors on the submerged transom would tell the tale, otherwise it is guesswork.
     
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