Duoprop F7 is too big by 400+rpms

[sony2001]

Hi. My 5.7L is running a F7 duoprop but I'm only turning 4200rpm with 2 people on board.
So I would like 400 to 500 more rpm, and to confuse you all, I want to move over to aluminium props. What size ''D'' props should I get? For the hell of it, what rpm will a F6 give me?

 

[Scott Danforth]

D6's or F6's

 

[sony2001]

Thanks. I think you are right on. If I did F5 what would be the top end rpm?
And would F5 be better for water skiing?

 

[Scott Danforth]

F5's would be better for skiing, however you will need to watch the top end RPM's

 

[sony2001]

Scott I found D6s never used and ten years old, and will install them in the Spring.
A lingering problem is the RPM needle falls to zero after about 15 to 20 mins. Since the gauge has the hour meter I want to try and fix it. Do you think the threads of the connections have oxidized? Or is it a fault of the alternator? I have a second RPM that I could piggy back to the connectors to see if they both fall to zero.

 

[Scott Danforth]

you are most likely loosing a ground or you are loosing the grey wire at the tach. start troubleshooting.

 

[Scott06]

You may want to verify the tach is reading correct at 4200 rpm before you make decisions on prop pitch. I would think if you are only hitting 4200 rpm due to too much pitch it would be lugging the engine on hole shot.

 

[Scott Danforth]

You may want to verify the tach is reading correct at 4200 rpm before you make decisions on prop pitch. I would think if you are only hitting 4200 rpm due to too much pitch it would be lugging the engine on hole shot.

i tend to think its accurate enough. even with my old AQ series, the B6' props would be 4600 RPM, so jumping to the B7's would have given me about 4200. other than the different hubs, the B6/C6 pitch and diameter is really close the the D6/F6