Greetings, I have a 56 ft Pump-boat with a Mitsubishi 4DR5 diesel motor and the hull is pretty lightweight being a wood-fibreglass composite design. Can anybody advise the best prop-size for her? She currently runs an 18" 3 blader but I feel that there is more speed to be had.
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The 4DR5 engine would have come from a Mitsubishi jeep or light truck originally. This engine is basically a "grunt" engine, "built for comfort, not for speed, as we used to say.
" but would have developed around 80 HP at 3,500 revs, when new.
Bottom line, as it's a land vehicle engine, there will be no recommended/best/optimum matched prop size for marine use. Everything would be a trade off.
Hopefully it has been properly prepared for the marine environment, or it probably won't last very long. Replacement engines in our area (Bataan) are around PhP 70,000 to 100,000. Cheaper up in Zambales.
Here is what I would do in your situation.
Looking on the internet, there are free service manual downloads, so it would be possible to work out what the gearbox output drive shaft RPM should be, at 3,500 engine RPM.
I would then look at the pumpboat gearbox (is one fitted, or is it a direct drive?) output shaft with a rev counter strobe, or get my friendly mechanic to do it, and see what we're getting under running load with an 18" prop. If it is near the value of the original gearbox spec, then any increase in prop size may not give any appreciable increase in speed, and could cause mechanical problems down the line. Greater stress on the output shaft/couplings/gearbox etc. etc.
If I still wanted to go with a bigger prop, I would try to "borrow" a bigger one, 20" or 22" say, and see what the difference (under load) in shaft revs are, with the engine running at 3,500 RPM, and select accordingly.
Of course, that is what I would do, others may disagree.
Peter
Topic: Prop for a Banka (Read 19523 times)