Originally Posted by w32honu
Would a smaller h.p. outboard provide a similar amount of thrust (smaller prop at higher rpm) as a larger h.p. Deisel at a slower rpm??
Would a 6h.p. outboard equate to an 18h.p. deisel?? ....etc.
In general, a larger diameter, slower turning prop will produce high thrust at a low boat speed. Similarly, a small diameter, fast turning prop- if it is absorbing the same power as the big prop- will be best for producing lower thrust at higher boat speed.
If you try to use the small, fast prop to push a large, heavy, slow boat, the advance velocity (the speed at which the prop is moving through the wake) is very low, while RPM is high. This combination produces a high Bp, and thus a very inefficient prop. Most of the power is wasted thrashing the water around in circles, with relatively little power going towards useful thrust.
A large, slow-turning prop on the same large, heavy, slow boat will have the same advance velocity and the same shaft horsepower, but the lower RPM allows a relatively low Bp and thus a more efficient conversion of shaft power to useful thrust.
If you have a light, fast boat, the advance velocity is high, so a low Bp can be achieved with a relatively high RPM. Steep reduction gears and large diameter props no longer have an advantage, as we move to lighter, faster boats.
A propeller, frankly, doesn't know or care what's on the other end of its shaft. It could be petrol, gas, electric
, hamster wheels. All that matters, as far as the prop is concerned, is the torque on the shaft and the speed of the shaft (and, therefore, the power transmitted through the shaft). With a suitable gearbox
, any 100 hp engine can be made to look like any other 100 hp engine, as far as the prop is concerned.