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Commander
- Joined
- May 31, 2002
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Re: V8 Merc outdrive on a 4cyl boat, time to experiment - opinions..
My 16' Mark Twain (Mercruiser 120 I/O, #1 outdrive) planed at 15 MPH and did 34 MPH top end with the 1.98 drive and 19" pitch prop. It also planes at 15 and does 34 MPH top end with the 1.50 drive and 15" pitch prop. A friend of mine swapped a 1.47 outdrive onto a V6 he revived from hurricane Katrina. Dropped 2" in pitch and the boat runs just like it did before it was sunk.
From Bond-O
The math above takes into account EVERYTHING. Drive ratio and prop pitch are where the "rubber meets the road" on a boat. It's just like calculating what new rear end gears you need if you put bigger tires on your truck. HP and torque don't know whether you have high (numerically) gears and big tires, or low gears and little tires. Same with a boat, HP and torque don't know if you have high (numerically) gears and a lot of prop pitch, or low gears and a smaller prop pitch. Same acceleration and speed in either case. My experience with boats has been that as long as you stay within the 15" to 23" pitch range they don't care what combination you use.
Maybe doing the same thing I did on a bigger motor would make more difference with cavitation. Possibly my 120 doesn't have the power to spin the 15" pitch up hard enough to cavitate. It's sitting in the back yard on the trailer, feel free to come try it out.
Sounds like you swapped a bad upper onto it.
My 16' Mark Twain (Mercruiser 120 I/O, #1 outdrive) planed at 15 MPH and did 34 MPH top end with the 1.98 drive and 19" pitch prop. It also planes at 15 and does 34 MPH top end with the 1.50 drive and 15" pitch prop. A friend of mine swapped a 1.47 outdrive onto a V6 he revived from hurricane Katrina. Dropped 2" in pitch and the boat runs just like it did before it was sunk.
From Bond-O
The math above takes into account EVERYTHING. Drive ratio and prop pitch are where the "rubber meets the road" on a boat. It's just like calculating what new rear end gears you need if you put bigger tires on your truck. HP and torque don't know whether you have high (numerically) gears and big tires, or low gears and little tires. Same with a boat, HP and torque don't know if you have high (numerically) gears and a lot of prop pitch, or low gears and a smaller prop pitch. Same acceleration and speed in either case. My experience with boats has been that as long as you stay within the 15" to 23" pitch range they don't care what combination you use.
Maybe doing the same thing I did on a bigger motor would make more difference with cavitation. Possibly my 120 doesn't have the power to spin the 15" pitch up hard enough to cavitate. It's sitting in the back yard on the trailer, feel free to come try it out.
Sounds like you swapped a bad upper onto it.
