Transom Drag

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

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Seeing this "stage" is one that is so transient for hulls accelerating to planing mode, and boats designed to spend much of their lives travelling at the sort of the speed it would be an issue, don't have the deeply submerged transom, well, it is like worrying about the efficiency of a hull going in reverse, and not being that concerned about its performance going forward, aka " a waste of time"!
    :D
     
  2. jesdreamer
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    jesdreamer Junior Member

    Partially ventilated transom a transient condition

    I agree that a partially ventilated transom can be a transient condition for boats powered to go much faster -- but there are a lot of boats that never get out of displacement mode. Some of these have transom sterns for a variety of reasons (such as load capacity volume, or stability) and if these boats might have added drag problems of a partially wetted transom, it might be an area to explore.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Plenty of displacement hulls do have transoms with shallow submergence, the main reason I'd suggest is to sneak an extra knot or two, by lessening squatting around "hull speed", and the extra resistance that would cause. A worthwhile trade-off that works for the speed the boat will most often be travelling at. At no-wake speed, it will create more resistance, but that is not the speed the boat was made for.
     
  4. jesdreamer
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    jesdreamer Junior Member

    I agree and feel some beam aft with a transom can be a good way to reduce drag due to trim angle relating to squat near hull speed -- perhaps one reason transoms find their way onto displacement boats.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Displacement cats are one category where the pro's and con's and general effects of submerged transoms is worth investigation, there being a wide range of speeds these boats operate at relatively effectively, with our without submerged transoms.
     
  6. daiquiri
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    daiquiri Engineering and Design

    I had made it pretty clear in several posts that my CFD calcs (and the conclusions stemming from them) are relative to the very low-speed regime, where the transom is fully wetted.
    I had also warned against extrapolating them to higher-speed conditions.
    So the point 2 is not correct, or at least not proven.
    Cheers
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Of course, the disadvantage of transoms due to higher drag for some
    Froude number regimes is likely to be completely outweighed by other
    benefits, such as better performance in waves and better arrangement
    of equipment.
    I feel a bit uncomfortable when I see non-mathematicians focussing so
    narrowly on one aspect of boat performance. :)
     
  8. HJS
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    HJS Member

    comfortable

    I also feel a bit uncomfortable when I see amateurs focusing so
    narrowly on one aspect of boat performance.

    When I design a boat for a wide speed range, it is important that the transom has the right area, width and depth.

    This is especially important in my boats designed for semi-planing speeds. I optimizes the area for the maximum speed, the FnL 0.7, the width so that the boat does not dive and the depth so that the transom is dry at the lowest acceptable speed, FnL 0.35. The depth I put around FnT 2.5. That way I'm sure the transom is dry and has low drag.

    Obviously, all other parameters are also optimized for this to be an easy driven and seaworthy boat.

    JS
     

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  9. jesdreamer
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    jesdreamer Junior Member

    Re - Jurgen Sass "uncomfortable" with interest of amateurs

    I feel Mr Sass's post deserves a response and I have pondered as to just what to say. I feel it a little unfortunate that Mr Sass is bothered when amateurs question some details of boat performance. It seems a little unprofessional for an expert of his stature to belittle the question of an amateur since most experts are dependent on amateurs for their livelihood.

    As to relative expertise, I am an engineer (RPI) with several honors and awards, having completed all of the coursework for PhD (Case & U of Del) but no Dissertation due to work issues. I have over 25 years advancing the state of art in Hydrodynamics (a very narrow focused area far from hull design), with several patents and international recognition for new developments in the field. I was called upon for help in development of classified new concepts for stealth submarines. But I may not be totally up to speed in hull design --

    Nevertheless, I have been designing and building boats of classic lines as inspired by the antiques on the River Thames in England (wood roof photo attached) as well as canvas canopied ones inspired by the Gentleman's Launches of the 1920 era by Fay & Bowen (other photo). while these semi disp boats fit neither the popular paddle powered mkt nor the planing speedboat mkt, they do occasionally find a buyer for whom price is no object. They have been the darlings of local Classic and Antique boat shows. Usually of 12-14ft lengths with beams of 24-32" our hulls fit almost exactly the specs Sass is so proud of having developed for his Keyhaven Skiff, as well as his " Easily driven rowboat" and "Fast rowboat" suggesting it might not need an expert to design via some common sense and logic.

    These hulls yield a calculated drag of around 10-12# when below Hull Speed. I began with canoe sterns (1st photo attached) but went to transoms to lessen trim with MinnKota or similar trolling motor power inboard just behind rear seat. And I am interested in drag of a partly wet transom at such speeds because I want more performance out of the 12V 55# thrust motors (which it seems might theoretically allow planing) -- I dislike the weight penalty of going to 24 or 36V for higher thrust because each 12V battery weighs close to 100#. (BTW if any reader might know of a potential buyer please PM me).

    So in view of the boats in question, I had hoped the Forum might accept validity of my "narrow, focused, impractical, and unrealistic" original query. I dislike the elitist tone of the Sass post, I dislike the badgering of Mr Efficiency as to the ridiculousness of my question. And I dislike Daiquiri seeming to back down after his posts on vortex pressures (or lack thereof). Actually I feel the submerged transom edge can perform much as the non-ventilated hull step which Daiquiri claims would create "tremendous" drag -- But since this is a short term transient condition for most hulls, I can find little or no research into the subject (but lots of wake hollow research) --
     

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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't know what you want, a law passed that lessens drag from partially submerged transoms ? You are looking for a solution that allows you to retain the benefits of this transom, but without the drawbacks, is that it ?
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Many of the papers that I cited also contain experimental data.
    The transom hollow idea might not appeal to you (I too have doubts
    about that type of model, e.g. how can a hollow ventilated to
    atmospheric pressure be a wave-maker!), but if you subtract off the
    skin-friction you will be left with the "residuary resistance".

    Several papers by Doctors et al show the lines plans, hull offsets
    or equations for the offsets, and other details of the hulls under
    investigation. They also give equations to calculate the extent of
    the wetness of the transom, or explicitly show the measured wetness.
    Therefore, you should be able to get a good idea of the effect on
    resistance of transoms, from their fully-wet to their fully-dry state.

    The free programs Michlet and Flotilla also provide estimates of
    transom stern resistance, including the effects of partial wetness.
    Flotilla does not use the "fictitious hollow" method for estimating
    transom stern wave resistance, but one I developed over the last
    few years.

    Validations are contained in the short reports at:
    http://www.cyberiad.net/flotilla/flotilla_mono.htm

    The program you can download contains input files for those hull series,
    and hence the digitised offsets.
     
  12. powerabout
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    powerabout Senior Member

    make it get up on foils?
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't think there is going to be much foiling going on with the minn kota ! A question to the "experts" though, would there be any advantage to having the transom vented to atmosphere along its lower edge, rather than just from above ?
     
  14. powerabout
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    powerabout Senior Member

    I believe every vessel with every weight and angle of attack has a different result
    The CFD if done with air is not representative of water in that situation
    To my mind the water is not being sucked back it is just gravity making the wash fall down into a cavity hence my comment earlier that at some speeds the falling wash is pushing the boat.
    I have spent many hours looking over the back of ski boats....
     

  15. daiquiri
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    daiquiri Engineering and Design

    Leo, now I recall that you have expressed this doubt several times in this forum, but this is the first time I have time to reply briefly and say my short opinion on this:

    The hollow itself cannot sustain a pressure difference and create wave, but I understand that the "hollow" is a convenient mathematical model of the effect of the transom edge, which is the actual wave-maker in this case.
    So by using the "hollow" model, the cause and the effect are just swapped and the calcs are done (probably faster or more easily - you will know that). This model gives no physical insight into what actually happens to the flow down there, and teaches nothing to those who are in search of explanations. But it evidently delivers the necessary numbers to the ignorant engineer who just has to get the job done. :)

    Cheers
     
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