Rpm Drop

[rathrbfishin]

I have a 18ft CC bay boat with an 90hp yamaha. I had a SS 17pitch prop which was running 5900rpm @ wot. I change to a SS 19 pitch because I was over my recommended rpm range. I thought that the 19 would drop my rpm's to around 5500 and I would gain a little more top end , but instead dropped them down to 5000rpms and made me even slower than before. My question is if i change to a 19 pitch aluminum prop , will that increase my rpm's and top end. I went from 41mph with the 17 to 33 with the 19. When I changed to the 19 pitch my motor was mounted all the way down (running 4800rpm's) raised to about 1.5" above bottom of boat , that when I started getting around 5000 rpms. It just seemed like a drastic drop in rpm's from the 17 to the 19 pitch. Could the 19 have more cup to it?

 

[WillyBWright]

Re: Rpm Drop

I'll bet that boat is rated for a lot more than 90Hp. That's when the steeper pitches make the most substantial changes in RPMs. In situations where the motor is close to the maximum boat rating, it's about 200rpm per inch change. But on a minimally powered boat, that number can easily be 400 or more.

SS props often have more cupping than aluminums. It's a curl on the trailiing edge of the blades. Cupping is usually figured into the pitch.

I assume the 19ss is yours for keeps one way or another. It can be tweaked. Pitch can be reduced by an inch and cupping can be reduced as well. There's probably a prop rebuilder somewhere around you. Tell him what you have and what it does and he can adjust accordingly.

 

[rathrbfishin]

Re: Rpm Drop

boat is rated for 115. I do not have a prop shop close that's why I was wondering if a 19 pitch aluminum would bring the rpm's up.

 

[rathrbfishin]

Re: Rpm Drop

If going from stainless to aluminum in same size, would that increase rpm's ? I would think aluminum would be lighter than stainless.

 

[WillyBWright]

Re: Rpm Drop

Stainless props have thinner blades. They spin through the water easier. That's why in many cases a stainless will spin faster than the same pitch aluminum. 90 isn't so far from 115, but is a cylinder. I was thinking max 150.