Propulsion system drag

Discussion in 'Powerboats' started by CAS, Nov 18, 2003.

  1. CAS
    Joined: Nov 2003
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    CAS New Member

    I have an ongoing "discussion" with a friend of mine about the differences in underwater drag between inboards and outdrives. I believe that the outdrives have less drag than the inboards. I have found some articles that seem to support my opinion but none were from indisputable sources. He has also gotten some opinions for the opposite case. This comparision does not include such factors as shaft angle which we both agree the outdrive has the advantage. We are stictly talking drag due to underwater gear (shafts, rudders, struts, lower units etc). Opinions are welcome but reputable articles/test results are needed for me to win the "discussion"..
     
  2. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Just do a comparison of wetted surface for rudder(s), shaft, strut, skeg, etc, versus the outdrive unit - that should prove it one way or the other....
     
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Surface alone does not account for the shaft's drag. Because it is turning it creates extra turbulence. Also, to calculate surface drag, you need to add the extra speed caused by its turning. A shaft surrounded by a tube has less drag than an exposed one.
     
  4. SailDesign
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    SailDesign Old Phart! Stay upwind..

    The point is, gonzo, that the area of the stuff listed above is still going to be greater than the area of the outdrive. Unless, of course, it is a bloody great outdrive that is waaay too big for the boat. ;-)
     
  5. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    What these guys are trying to say is that you're righht and your buddy is wrong:D
     

  6. Corpus Skipper
    Joined: Oct 2003
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    Corpus Skipper Hopeless Boataholic

    Boat Test or some such magazine did a side by side comparison of two Sea Rays, one with twin i/o, one with twin inboard pocket drives, same hp mercruiser engines on both. The i/o spanked the inboard in speed and fuel consumption. That pretty well sums it up i reckon! Just don't leave them Bravo drives to rot in the salt water!
     
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