low rpms

[kwt600]

I have a 98 150 johnson on a 20' trophy dual console boat turning a 14 3-4x 19 prop at 4600 wot how much of an rpm gain if I go to 17 pitch

 

[sweebs]

Re: low rpms

lots of variables so guess is just that, a guess. I "lost" 1000 rpms and 10 mph by trying a 14 x 21 composite vs a 15 x 19 SS. I think that your main concern should be keeping your rpms in the reccomended range for your engine. Most liekly by dropping to a lower pitch, your holeshot should be improved and your rpms a couple hundred higher at WOT. Top speed may be slightly slower, the same or higher, but perhaps likely slower with a lower pitch, just my 2 cents

 

[rndn]

Re: low rpms

A general rule of thumb is 200 rpm's for each inch of prop change in either direction. Increase the pitch from 19" to 21" should reduce rpm's by 400. These are ballpark numbers but should be close.

 

[Silvertip]

Re: low rpms

Those numbers (200 rpm per inch of pitch) only apply if you are comparing identical props with the exception of pitch. A 19P aluminum prop from company "A" will probably not perform the same as a 19P stainless or aluminum prop from company "B". Some props are simply more efficient than others. Like shoes, there are many styles within a given size.

 

[gregtobin]

Re: low rpms

I want to switch to a 4-blade prop to improve the hole shot. When going from a 3-blade 17p to a 4-blade 15p, should I expect a better hole shot? Reason I ask is that I was just talking to someone who said that moving from a 17p to a 15p would make my hole shot worse. True? False?

Greg

 

[rndn]

Re: low rpms

All things equal moving from a 17" to a 15" should improve your hole shot and lower your overall top end assuming you are now running at the proper pitch. Becareful of changing pitch for hole shot improvement without first knowing what your engine should be running at WOT.