Efficiency: Submerged transoms

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jun 3, 2021.

  1. dustman
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    dustman Senior Member

    Let's say you have a catamaran, 24'x12', 2000lbs displacement, mostly operating at between 6 and 15 kts. Would submerged transoms increase the overall efficiency?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Not at 6 knots, you would have to expect. But a submerged transom at speed might reduce pitching, even if costing a tad of speed, but at 15 knots could be beneficial and be less draggy, through reduced wetted area
     
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  3. dustman
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    dustman Senior Member

    So a submerged transom is more efficient at high speed because of reduced wetted surface, or because of the way it affects wavemaking off the stern, or both?
     
  4. fallguy
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    fallguy Senior Member

    from my understanding; you want a clean exit at higher speeds versus a draggier [sic] one, but its a hack answer
     
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  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Realistically, at low speed the submerged transom creates more drag, but when water breaks away cleanly at speed, that is no longer the problem. It is speed related, you really need to work out what your desired cruise speed is, or what speed you expect to be travelling at most of the time. It is OK to say between 6 and 15 knots, but if it is 6 knots 80% of the time, a substantial area of submerged transom is a handicap.
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    Well, that is a Fn range from 0.36 to 0.91, so the simple answer is... yes. The B/T at the transom has an effect of the resistance, for hull form circa 60% would be 'nominal'.

    Added into the mix your l/d ratio of 7.5 is good, so the 'effect' is not as great as a low l/d ratio.

    Kudos for the grammar :cool:
     
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  7. dustman
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    dustman Senior Member

     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    You're 60% is related to the draft T and breadth B....

    As for the other Q...aaahh.. now you're getting into the nuances and need to classify what YOU mean by 'efficiency'.
    Since this is a very loaded term as noted by other posters not understanding in the context of the question.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Saving sixpence by having a little less drag at some speed you have no intention of spending much time travelling at anyway, makes little sense. Design the thing for all-round satisfactory performance at the speed you will be travelling at, most of the time, if you take buoyancy out of the stern to avoid a submerged transom that might cause some drag, you could end up increasing pitching motions you don't like.
     
  10. dustman
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    dustman Senior Member

    I have already contemplated this, and I believe I will be spending 70-80% of my time at 9-11 knots. Low speed efficiency doesn't concern me as much as an efficient cruising speed and the ability to reach 15kts with 20hp.

    I should say that most of the time my displacement will be closer to 1500lbs than the originally stated 2000. I have been scolded too many times by people who don't believe I can achieve such a light displacement, so i am appeasing them.
     
  11. dustman
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    dustman Senior Member

    Oh yes, I was reading a study done on variations of models where they varied the B/T of the transom while maintaining area, the 16cm deep version had somewhat significantly lower drag than the 30cm one. Not sure how that affected hull shape at the stern or what else played into it.

    You are right, I guess it is important to state a speed at which you hope to achieve said efficiency gains. I am loath to include too much or too little information in my posts for fear of getting brow beaten one way or the other. Sometimes it seems like you can do no right on these forums and that you should be ashamed not to be a full fledged naval architect, in which case I wouldn't need to be on here in the first place. So I will say 10kts cruising speed, 30' catamaran, 1500lb displacement, 20 available horsepower(probably more like 7000 watts of actual propulsive power after losses), and if possible I would like to achieve 15kts as a top speed.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Are we to assume this prospective boat is for smooth waters ?
     
  13. dustman
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    dustman Senior Member

    Yes, for darting from place to place in the Bahamas.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    That's most likley because those posters that reply/respond to your posts are not naval architects. As such their only "value" to the thread is to shift the subject away from what you're asking and onto one they feel comfortable answering.. classic misdirection, and thus can unload and criticise at their own free will...as they now control the narrative. Im sure somewhere in some bar in some seedy part of town, they are going woohp woohp to their barfly buddies that they had a "win" today...:oops:o_O
    NA are engineers, we tend tor reply with binary black/white answers that also on the face of it.. can appear blunt/rude...but take the emotion out, and it is factual.
    As I tend to state endlessly it seems, engineering has no emotions, only facts. The person reading it.. they have emotions..but it doesn't alter the facts only their response to said facts...some go off the deep end and rant, as we have already seen, yet still never answer the Q at hand...!
     
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  15. Mr Efficiency
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    Mr Efficiency Senior Member

    haha, no emotion here, but at a practical level he can get the most "efficient" cat known to science, and have something that in actual use is quite unsatisfactory. I would be very leery of anything this small with 20:1 ratio on ruffled water, from the pitching viewpoint. On a river would be different
     
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