Propeller options

[spareparts]

140 mercruisers should have a 1.98-1 drive ratio, unless its be changed to high altitude gear set ( 2.20). the other ratios available are 1.84, 1.65, 1.50, 1.32. The later Gen II's had different ratios, but the are very close to these

[ThePHNX]

The 1.98 still gives a 21% slip, on the high side of reasonable.
Kahuna, next time you're out try a proper speed trial, run the reciprocal: first run at whatever bearing convenient, second at 180 degrees opposite, take the average of both runs.
Also, what does your wake look like?
Spares, is it common for the gear ratio to be changed by the user? How can the ratio be verified, short of counting teeth? How many reduction points are there on a sterndrive, more than one?
HBH

[Kahuna]

PHNX,
The boat currently has a 13.75 x 21. Too much wheel. Need to drop down to get RPM's up. I'm going to have to go with the assumption that the gear is stock on the 1984 apha 1 drive. Agree, the number on the GPS is over the bottom, and not through the water. However in about 8' of water over a sandy bottom, with minimal tide effect, minimal wind, using a Tempo tachometer, the motor turns 3850 and the hull runs just short of 31knots trimmed properly just short of porpoising. Short of wind tunnel testing, or radar gun and putting a calibrated electronic tacometer directly on the motor, I feel the numbers I have got are close enough be able to make reasonable assumptions. I would like to go to a smaller wheel so that I'm turning 4500-4600 wfo with a light load. I spoke with a dealer adn found out that a 13.75 x 19 is not availabel. I don't have the available sizes for the alpha 1 in front of me here (think I left them at the office in Newport), but if I remember correctly, every time they drop two inches of pitch, they changed an inch in diameter. So the trick here I guess is going to be to try to guess right. The dealer did also say that I could take one, try it and as long as I didn't ding it up, I could bring it back, to ensure that I get the right one.

[Bigshrimpin]

I spoke with a dealer adn found out that a 13.75 x 19 is not available.

What's not available?

[spareparts]

i think you can get a 13.25 X 19, don't worry about the diamter too much, just use the pitch numbers to get you in the correct rpm( if you were running more hp at higher speeds it would be more of a concern), if you want to fine tune it you can change diameters, but I don't think you will gain enough to offset the price of propellers. Stay with the same brand of props while you're swapping them, I've seen measured pitches vary by manufactures by a wide margin

[Kahuna]

Ok, that makes sense, stay with the same brand when swaping. I have worked through the fact that every inch of pitch is good for 200rpm's on a gas motor, but was not aware of how the change in diameter would work into the equation. I guess that i just won't get too excited about it, just monitor closely when I do the. At this point, I think it is going to have to happen next season.