Should I switch to a 19P prop?

[Propeye]

Hi All.

I am working the kinks out of my new-to-me boat (and myself is getting acclimated to owning a I/O configuration). After a few outings with her now, I'm starting to wonder if perhaps the previous own(ers) had too much prop on the boat.

boat model: 1985 Thundercraft bowrider (about 18.5') with 470 and Alpha One outdrive

The current prop is 21P and the RPM's max out at 3,500rpm with the speedo indicating 40-42mph. The boat feels happy to plane at 30mph @ 2,700rpm. This data is with a full load of fuel and 4 passengers. I tried it solo last night, still with a full load of fuel and the result is the same.

So. Should I leave it alone or pursue a 19P? From what I've read this motor should be at 4,000rpm +/- 200rpm at wot. I haven't tried pulling tubes or skiers with it yet, but that is on my list of wants for this boat.

Thanks for the advice folks.

 

[achris]

Re: Should I switch to a 19P prop?

3,500 is Way to low... You're putting too much load on the engine (and that's through out the rev range, not just at WOT)... Each 1" of pitch should change the revs by about 100-150... You need to get that engine up to the 4200-4600 range, so reduce the prop to about 17"... Maybe even a 15"!

Chris...........

 

[Don S]

Re: Should I switch to a 19P prop?

You also need to confirm you have the correct gear ratio in your outdrive (1.84:1) and that your tachometer is correct.

 

[Propeye]

Re: Should I switch to a 19P prop?

You need to get that engine up to the 4200-4600 range

Is this true even when the manual states the max rpm should be 3800-4200? If so, is the logic here that the engine is not 100% loaded in the 3800-4200rpm window?

 

[Maclin]

Re: Should I switch to a 19P prop?

Propeye, go by what is in the manual for your engine regarding the RPM range.