prop physics, basic question

[pcrussell50]

new to boating here, but have just enough science knowledge to ask dumb questions...

so here's one: last night, I calculated a theoretical speed for a 21 pitch and a 24 pitch prop at 5500rpm. in the idealized world of calculations on a cocktail napkin, it came out to be 109 and 125mph respectively. real physical systems rarely behave as they do in hand calculations, and this is obviously no different. my question is more, is there a "rule of thumb" inefficiency factor that most folks apply to the ideal? like there is other areas. (15% loss when measuring automotive horsepower on a dynojet relative to crankshaft horsepower).

since a 21 pitch prop at 5500rpm should be 109mph, but isn't, maybe it would really be 40mph. (just pulling numbers out of you know what). that equates to something north of 60% inefficiency. is that typical?

-peter

 

[Dhadley]

Re: prop physics, basic question

With a 1.0:1 gear ratio that's about right. That's about the speed of some of the SST boats running a 19" prop if they were only turning 5500.

You have the right idea, just left out the gear ratio. Once you figure the prop rpm, not engine rpm, it'll come out a lot closer. Generally speaking for a planing hull we like to get 12% slip or maybe a bit better.

A typical V6 running at 5500 engine rpm will produce about 2957 propeller rpm.

 

[pcrussell50]

Re: prop physics, basic question

With a 1.0:1 gear ratio that's about right. That's about the speed of some of the SST boats running a 19" prop if they were only turning 5500.

You have the right idea, just left out the gear ratio. Once you figure the prop rpm, not engine rpm, it'll come out a lot closer. Generally speaking for a planing hull we like to get 12% slip or maybe a bit better.

A typical V6 running at 5500 engine rpm will produce about 2957 propeller rpm.

doh! didn't even consider the idea of the reduction gears in the lower unit! so, 12% give or take. that satisfies a big point of curiosity. thanks.

-peter