Med vs South Florida boat Performance

Discussion in 'Hydrodynamics and Aerodynamics' started by klims106, Feb 17, 2020.

  1. klims106
    Joined: Jul 2011
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    Location: Fort lauderdale, FL

    klims106 Junior Member

    We have a Power catamaran we tested last summer in the Mediterranee and had no problem running at 10 knots with 120 kW and reached 17 knots with 300 kW. The boat appeared to start planning when we reached speeds above 13 knots.
    We shipped the boat to south Florida and we cannot pass the 10 knot mark. 11.5 in the best of condition with the same 300 kW.
    At same RPM the boat is in average 20% slower now in Florida than in the Med. Power, speed and torque are accurate within 1%.
    Yes - the Hull and the propeller are clean
    Yes - it is the same boat weight. In fact we tested with and without fuel or water here in Florida and had marginal speed difference (9.5 to 11.5 knots).
    Yes - we had no current or wind when we tested in the mediterranean or in South Florida.

    Could the water salinity explain the difference?

    Thanks for your Input
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Klims106

    It could be one of several factors:
    1 Water depth
    2 ambient temperature
    3 ambient humidity
    4 fuel quality

    or a combo of the above.
     
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  3. Richard Woods
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    Richard Woods Woods Designs

    Most likely the water depth. If you tested it in Biscayne Bay then I'm not surprised you cannot get the speed on a semi displacement powercat. Its been discussed before on this forum.

    My 28ft Skoota powercat runs at 16 knots with twin 20hp outboards in deep water but struggles to get to 9knots if water depth is under 8ft

    So how big is the boat? Where did you test it? I suggest going out into the ocean

    And if outboard powered remember the US uses much lower octane rating and possibly higher ethanol fuel

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    That is a huge difference, hard to imagine in 8 feet of water, for such a small boat.
     
  5. Richard Woods
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    Richard Woods Woods Designs

    We didn't really need an echosounder in the Bahamas. We knew how deep it was by our speed. Without changing the throttles to speed would increase as we got into deeper water. Same is true anywhere of course, but not many places can you cruise for 100 miles or more in water under 10ft deep and so really notice it

    RW
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    It is the magnitude of the slow down that surprises, would it have been less an issue with bigger motors, in terms of the speed difference ?
     
  7. DCockey
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    DCockey Senior Member

    Octane rating generally does not affect engine power. Octane rating indicates how resistant the fuel is to knocking. Running too low octane can cause engine durability problems. An exception to octane rating not affecting engine power is if the engine has a knock detection system which changes ignition timing, possibly mixture and/or retards boost for turbocharged/supercharged engines which reduce power.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Not when shallow water effects are understood.
    One can typically experience an increase in the hump resistance of around 25-30%. And since the engine cannot produce any more power...it struggles and labours and eventually doesn't get over the hump.
     
  9. messabout
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    messabout Senior Member

    The difference in salinity is not the likely difference. The difference in water density (saline or other mineral content) in the Med or the Atlantic is not enough to cause a significant difference in your boats performance.

    If you were testing in or near the Florida Gulf Stream then you should be going faster when headed more or less north and slower when headed in the opposite direction. I doubt that you have encountered the current differential if you are operating near shore. Yours is indeed a dilemma that should elicit some commentary from our members.

    You have stated that your performance measurements are accurate within one percent....................Forgive me for being skeptical about that degree of accuracy. On the other hand the difference between 17 knots and 11.5 is a significant difference that suggests that something else is going on
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Hence why I asked him if the modest engine power of Richard's boat had amplified the problem. What do you think ? As to the OP's boat, the thing could be a planing cat. Richard's is a displacement or semi-displacement.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Makes no difference.
    The prismatic hump has the same effect for all boats - it is basic hydrodynamics. The only difference between each boat is the %'age increase in resistance owing to the length-displacement ratio.
    But every boat, no matter whether displacement, semi-displacement or planing, is influenced by shallow water.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds like taking short cuts across shallow banks has another drawback, but I can't say I've noticed much difference in planing boats.
     
  13. Richard Woods
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    Richard Woods Woods Designs

    Twin 200hp outboards and only giving 17 knots implies to me its a big heavy boat. (Over 40ft over 8T??) So almost certainly semi displacement (or semi planing if you prefer). And thats the boat most prone to being "sucked down" in shallow water. Obviously displacement boats won't slow down or squat noticeably, although they do - remember the QE2 hitting a rock? Equally obviously a planing boat with enough power can lift up and basically the water depth is then largely irrelevant.

    But until the OP replies we don't know what boat he actually has

    Richard Woods
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Of course, and find out what water depth was involved.
     

  15. messabout
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    messabout Senior Member

    The OP lists his location as Fort Lauderdale. That location is about 30Nm north of Biscayne bay. and its shallow water. The water depth question can be resolved if Kim will take the boat east for a few minutes. The edge of the Gulf Stream is near and the water depth is a plenty.
     
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