Prop diameter

[SeaDooSam]

Hi all.
So, all season I had a 17 p prop on my Chap 1830 with 190 hp 4.3 merc. I was getting around 4200 rpm out of it. Then just this weekend I put on a 18 pitch with a larger diameter. The rpms increased to 43-4400 and the boat went a few mph faster. Can someone explain this or is my boat just a rule breaker. I had thought bigger diameter would have dropped the Rpms. Is it possible the 17p with smaller diameter was just to small?

Thanks.

 

[Starcraft5834]

interesting, the performance you have reported is contrary to the norm, the part about increasing rpms that is.... more pitch means more speed usually

 

[Sea Rider]

What's the max wot rpm range for that engine ? In theory, with a 16 pitch would probably have gained same higher rpm, better hole shot and bit less top end speed.

Happy Boating

 

[SeaDooSam]

Yeah I thought the Rpms would decrease as well but they didn't! Very curious. Mechanic says should be around 44-4600 WOT rpm. I like the way it handles with this large diameter prop. I think I will leave it. I have to see how it does when the boat is loaded up!

 

[Sea Rider]

With increased wot rpm to 43-4400 and the boat running a few mph faster, the engine is revving next to min wot rpm lugging range. When combo is loaded up rpm will drop accordingly if wot test was done lightly loaded. If combo will usually be used loaded up towards heavy side , better dial a prop that runs middle to max wot rpm range for that load. Just for the record, post wot rpm achieved with current prop while loaded up.

Happy Boating

 

[SeaDooSam]

Yeah the boat doesn't seem to follow the rules. The boat initially had a 19p and went to 17p to increase Rpms. However, this had no effect that's right no change in max Rpms. 4400 is the best the boat has ever done under my ownership and I achieved it with this 18p prop with larger diameter. Mechanic says 4400 is good for the boat considering it is so heavy

 

[shaw520]

A 'cupped' blade will normally show less rpm than a non cupped