RPM vs. Speed (Gearing/Prop pitch)

[spectras only]

The OMC 800 5.7L had a 1.41:1 ratio.With this ratio it works out to 64.34 MPH.

[Tahiti350]

So the info I pulled from the other forum is not correct? I'll check the actual ratio on the boat when I get back to the states in Mid January. Thanks for the help. I need to check the prop again, I think it is actually a 14X21+ (twist blade).
Not sure I understand how a smaller, less aggressive blade would increase top end though. There's a lot I still need to learn. I have 15x17, and 14 1/2/19, both of which seem to pull better down lower, but let the motor wind too high for my taste. I use the 15x17 for pulling skiers, and it pulls 2 skiers comfortably at 2800-3000 rpm. The 14X21 loses a litle bottom end, but definately top ends better than the others.
I was told elsewhere (by someone who "knows") that I needed to increase the pitch until the motor would not pull past about 4800 rpm.
Thanks again for the help.
Tahiti350

[jackpunx]

The OMC 800 5.7L had a 1.41:1 ratio.With this ratio it works out to 64.34 MPH.
I guess I was close..

[burtandnancy]

Here is a formula that may help you:
rpm x pitch x ratio x .00094697 = theoretical speed, minus slip. you will have to find your own slip factor. Use radar or GPS to get your actual speed. If you have a Bravo with 1.50 gears, use .667 for ratio, if you have 1.36 gears, use .735. Hope this helps...

[75spectra]

The 800 series drive was not electric shift in 81. They changed to a mechanical push pull cable in 79.

[Tahiti350]

Thanks to all for the info.
Spectra,
I'm not sure what year the drive is (believe the boat may have been a conglomeration project), I know the lower is not the original for the upper (different colors, and striping), but I do have 2 wires going out thru the gimble to the drive, and was told by the mech at the boat yard where I bought it that they were for the electric shift . Guess I still have some investgation and research to do when I get home in January. Actual ratio will be the first factor to find out. Are there serial or id markings on each part of the drive that I could use to ID what I have???
Gary B.
"Tahiti350"

[mirvin]

So how come my 5.7 pushing a VP290 with a 21p prop is barely finding 50mph??? That's at about 5500 rpm.
I have no idea what my gear ratio is........1988 21 foot Crusader (in my avatar). I think it's a bit on the heavy side.
mirvin

[spectras only]

Mirvin, with a max 260HP SAE, and a heavy 21' boat 50 MPH sounds about right :mix:

[mirvin]

Mirvin, with a max 260HP SAE, and a heavy 21' boat 50 MPH sounds about right :mix:
That's what I was thinking...... :rollside: I keep thinking about messing with it but it runs so darn good and hey, it's a VP 290 for cripes sake. That thing gets hairy at about 45mph. I'd hate to see what it'll do at 70 :notam:
Thanks spectras :rollside:
mirvin