[PUP] Keel Cooled, dry exhaust electronic engine question

[Kevin Kearney]

Hi, I have been repowering this winter and struggling with the engineering, I had simular experiences with Deere Corporate engineering. From a fundemental perspective  displacement boats require x power to do hull speed, then the propeller appature can only be fitted with a prop diameter that is within 10% of that space, a 34 inch prop requiring 3.4 inches of clearance for example. That prop, whatever size has a tip speed that should not be exceeded-and usually should be limited to 400-500rpm to achieve hull speed without cavitation--that also requires x power. Knowing these x power requirments will help you pick the right engine, Gerrs Propeller Handbook would be that reference.
   
  Every engine has a torque peak, which usually is where the best fuel economy is achieved.
  with the Deere engine I chose 1200-1400rpm is the torque peak and fuel use is sub 1 gal per hour, That represents 50HP or about 45 shaft HP, my 43ft waterline and 45,000 pounds requires 28Hp to achieve hull speed., 17HP is adaquate "reserve".
   
  The Deere engine I am using is rated at 80HP/2500rpm/and 180ft pounds torque/4.5 gal per hour at those speeds. I have chosen to operate it at 12-1400rpm/220ft pounds of torque. 50HP/1 gal per hour-getting the gear reduction right to turn 400-500 prop rpm-and the size of the prop right to fit in the appature is key.
   
  So my best advice is to pick an engine relative to prop size, gear the prop correctly, figure your hull power needs-then match the engine to those requirments, it will take less than you think-most boats are overpowered . A big prop moving slowly is the most efficient
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