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I got the bug to think about a new motor for my non-smirked 1973 Montauk. My curiosity is leaning toward a non-high-thrust 60-HP motor, but there are five manufacturers I can choose from. I started doing some comparisons. I assume weight of people, motor, and gear is 1700-lbs. With all five motors max rpm was about 6,000-RPM. Weight on all five was within 18-lbs of each other, and alternator output was 17 to 25-amperes. Displacement was 52.7- (864) to 61-cubic-inchs (998cc).
No big differences there but, I found that the number of cylinders and the gear ratio seem to have a relationship. The motor with least cylinders, two, has the highest numerical gear ratio and the motor with the most cylinders, four, has the lowest numerical gear ratio. I figure there must be a reason for that relationship. Does anyone know why?
Out of curiosity I decided to use a Propeller Calculator to determine what size propeller I would need for each motor in order to achieve 35-MPH. Using my existing motor and my prior performance data I determined that my average slip was 6-percent. So I used that value for comparison purposes.
The prop calculator uses RPM(6000) Gear Ratio(shown below) slip(6) and pitch(shown below) to arrive at speed in mph.
To achieve approximately 35-MPH you need.
Evinrude 2 cylinder, 2.67:1 ratio 17.5 pitch
Suzuki 3 cylinder, 2.27:1 ratio, 14 pitch
Honda 3 cylinder, 2.07:1 ratio, 13.5 pitch
Yamaha 4 cylinder, 1.85:1 ratio, 12 pitch
Mercury 4 cylinder, 1.83:1 ratio, 12 pitch
So is it better to have a smaller number of cylinders with a larger pitch propeller or more cylinders and a smaller pitch propeller? Which combination of gear ratio and propeller will be the best for fishing and cruising?
