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BF75a Prop selection

bryanpferry

New member
hi i have a 15.5 livngston that is a small cat type hull. I have a bf75a on it with a 13.75x15p prop on it. I'm only getting 5200rpm out of this. the boat seems pretty heavy. mabye 1500+lbs. Should i keep this prop or switch to another? I don't know where the bottom of the hull is. it might be the bottom of the dual hulls or in the middle? Anyone have suggestoins. Thanks.
 
When propping a boat... Make sure the engine is the right height. The engine is in good mechanical condition. The hull is in good condition. The boat is loaded, ie: full tank of fuel, maybe another person with you. I like to prop the boat so it makes near it's wide open throttle speed when the boat meets the previous conditions. So I can't really say if you should change the prop or not. You need to take certain steps first.
 
thanks for the reply, i have done all that. at top end i only get around 5200rpm and 28-29 mph. this might be it but i'd love to get a few more mph out of it.
 
Outboard mounting on a CAT can be tricky, and the Livingston design hull design can be even more tricky. The best test is that the anti-cavitation plate shoud be right at the water surface, or just slightly below at WOT (Given all the conditions stated by Homdamech, above.) If the anti-cavitation plate is too far below the water surface, raise the motor before changing the prop. Then, at WOT, on smooth water, play with the trim until you max out the RPM's. If you are still significantly below 6000 rpm's (say less than 5700), then consider going to a 1/4" or 1/2" smaller prop with the same pitch, or drop the pitch to 14, keeping the 13.75 diameter. As a rough rule of thumb, on heavier boats, a 1" drop on pitch will get you about 200 - 300 rpm increase. But I don't have any experience with anything like a Livingston hull, so you will need to experiment once you are assured that the anti-cavitation plate is in the right place, and you have optimized the trim. Your ideal should be to get as close to 6000 rpm as possible.
 
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