15"x17P to 15-1/2"X15P too much?

[zchornyak]

I have a 1998 Starcraft Islander 2001 with a mercruiser 3.0L in it. Its a 21' boat. Boat and all fuel/gear/motor probably weigh about 3500 lbs. The boat used to have the 3.7 when the previous owner switched it out with a 3.0 after it died. I believe the prop on it now was the prop being used with the 3.7.

I can only get to 30 mph, by myself on the boat and the tach is reading approx 3000rpm. I have yet to check the tach for accuracy. So, i'm hoping to get a prop that will help taking off and getting up to plane, and help get me in the higher rpms in the power curve to get some more top speed.

I was thinking possibly going from the 15"x17P currently installed to a 15-1/2"X15P. Any comments about that? Any other ideas? Trying to do this once without buying multiple props just like everybody else!

Thanks in advance!

 

[steelespike]

Re: 15"x17P to 15-1/2"X15P too much?

Most likely the gear ratio is wrong for the 3.0.I think the tach is about right.Assuming motor is in good health and timed correctly and throttle opening all the way you need at least 1400 rpm.Rule of thumb is"about" 200 rpm per inch of pitch so it would appear you need "about" a 7" reduction in pitch. 10" pitch.
What you really need to do is get the right gear ratio.3.0 gear ratio is 2,00:1
I don't find a 3.7 only 4.3,1.80:1 and 5.7 1.47:1.
The lower numbers make it harder for the motor to turn the prop.
Even if you find a 10" the performance will still be comprimised.
Your gear ratio is usually stamped on the outdrive.You can check your ratio by removing the plugs and rotate the flywheel on revolution and count the prop rotation.

 

[zchornyak]

Re: 15"x17P to 15-1/2"X15P too much?

Right....didn't even cross my mind about the gear ratio.....thanks! However, I believe it is the 2:1 gear ratio as the previous owner dealt with that when he replaced the motor. The motor is new not rebuilt and is very healthy running..timing etc....

I'm still leaning on my tach being off but I'm definitely going to double check the gear ratios. Thanks!

 

[Bondo]

Re: 15"x17P to 15-1/2"X15P too much?

Ayuh,... Unless the PO replaced the gearing, You're Way over geared...
The 470s used the 1.84:1 ratio, Not the 1.98:1 the 3.0l Needs...

They don't make props small enough to make up the difference...