Re: Drive gear ratio's
A 3.7, 170 hp, 4 cylinder Is this a mercruiser with the GM engine.
If you have merc. with a 4 cylinder you probably have a 4.0, 135 or 140 hp with a 1.98 ratio. A 1988 should have a Alpha 1 gen1. However I caution you from experience the year of the boat does not mean you have an engine and outdrive that matches.
21 foot cheiftain, just like a islander 1986
Speed I get 26-28 mph at wot, sea level ocean with 4 people and fishing gear, 35 gallons of fuel also have a 25 hp kicker on the boat. Rpms are 3800 wot. I have a adjustable 4 blade prop from westmarine. 14-22 pitch adjustment. I think I have mine set at 15 pitch. I cruise at 2800 rpms and 19-20 mph, I think this gets me the best fuel economy. I do not like to run above 2,800 rpms for long periods of time. I normally have a 30-120 min run to the fishing holes. Most days the weather is poor enough I can only make 8-12 mph.
If I lower the pitch I gain rpms put loose topend speed and fuel mileage.
If I increase pitch I loose rpms but gain topend speed and loose hole shot.
I purchased a 1986 21 foot cheiftain, just about like a islander. It had the 140hp mercruiser, what I found out was it was a 1971 vintage motor and outdrive. Since then and during the past 5 yrs the old outdrive a motor gave up. 2 yrs ago motor broke timing gear, the engine block was questionable so I put a brand new motor (2007, 135hp, 4 cylinder chevy) back in the boat. Last year the outdrive went out, rust in the upper gears. Replaced with a complete SEI 106, ratio 1.98. Now I have 140 hrs on the engine, and 100 hrs on the outdrive and all is going well.
