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Old 01-13-2006, 05:07 PM
Dan B Dan B is offline
1934 Ford Roadster
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Another thing to remember about boat engines is that the correct propeller must be matched for the engine. If your manual says max rpm is 4400 RPM then the correct propeller would let you run the boat wide open throttle and the engine will not exceed 4400 rpm. Let's say you have a prop with too much pitch (or cup) and at wide open throttle you only get to 4000 RPM... this is very, very bad because you are "overpropped" and the extra load on the engine will shorten it's life. It is just as bad if it is "under-propped" and at wide open throttle you exceed 4400 RPM. The engines are designed to get maximum speed at the stated max RPM, then you should typically run the engines at around 75% of the max (around 3200-3300) when you are cruising. Easy test, just go out and hit wide open throttle for a few minutes and see what it does. You want it to be within a 100 RPM of max otherwise there is either a problem with the prop, motor, or the hull.
If your engine can turn 4700 RPM then it has the wrong prop. Typical boat surveys include wide open throttle runs for about 5-10 minutes to check this and see if the temp and oil pressure meet specs. Some owners almost have a heart attack when we do this but if the combination is set up correct you can run for a long time at WOT with no harm done. This applies to your standard marine engine (Merc or Crusader) and their manual should be followed.

"Captain" Dan B

PS. What kind of boat/drive are we talking about here?
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