chriscraft240

Running a 24' Chris Craft 14-1/2x21 S.S. prop. max rpm 4200 at 52.4 gps.Normal load, 1/2 tank of fuel, a little bit of trim. I liked the attitude of the boat pretty much neutral.Took a little to get on plane, but went great once there.The cruise was 30 at 3000 and that made the motor sound like it was in its sweet spot. Wide open should be 4800. Question now is should I just drop to a 19" pitch, or try a 4-blade 18 or 20. How much rpm will a 4 blade kill. What would either do to my cruise which I like now? Thanks

zt260

You need to drop to a 19 pitch 3 blade or an 18 pitch 4 blade. The 4 blade will help you out of the hole. The 3 blade should give you a little more top speed.

cooltoys61

My .02,,if you went to a 4 blade 21 you would be surprised by the diff in hole-shot and would not hurt your cruise to much.But if you want 4800 ,like you said,you'll have to gone down in pitch. I'd guess about what T260 suggested.It's all a compromise if you can't shift gears,you will loose cruise speed.

chriscraft240

I know changing to a 3 blade 19" would get me closer to my rpm goal, but how much rpm difference does a 4 blade make say 18" or 20"

chriscraft240

If my data is right a 14 1/2x18 4 blade would give me 4700 rpm? does that look right?

zt260

You get about 400rpm from each prop size change. But with the extra HP you have at higher rpm 500rpm is possible. If you can try both before you buy, go that way.

chriscraft240

Thanks for the info zt260 I'll try them all !!!

cooltoys61

I think you'll only see 200 per inch. Going from 3 to 4 blade might be another 100 but depends on type. There are some good calculators at go-fast.com
go to data knowledge base.

zt260

the pitch between props is 2" so 400rpm is right on the money. ie. 21 to 19= 400rpm

A 19" 3 blade should spin about the same rpm as a 18" 4 blade.

chriscraft240

Like you said earlier though I'll be higher up in the rpm range and more HP. Sooo..a 20"4-blade should do the trick. Better yet I'll try them both and let ya know. Thanks