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Author Topic:   60-HP Motor Comparison

Fishmore posted 07-22-2010 03:07 AM ET (US)
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 [maximum engine speed] 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 the Propeller Calculator in the Reference section 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.
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 fishing and cruising?
jimh posted 07-22-2010 11:24 PM ET (US)
There has been prior discussion on this topic. See

Propeller Shaft Gear Ratio in Relation to Propeller Diameter, and Pitch
http://continuouswave.com/ubb/Forum4/HTML/007108.html
Effect of Gear Ratio On Selection of Outboard Motor
http://continuouswave.com/ubb/Forum4/HTML/005853.html
90-HP Motor Comparison
http://continuouswave.com/whaler/reference/compare90.html
In the last reference I cite you will find a method to allow you make comparisons of the propeller shaft torque produced by various arrangements of gears. This may be useful for your comparison of 60-HP motors.
There is also a follow-up discussion. See:
http://continuouswave.com/ubb/Forum4/HTML/005881.html
In all, you have several hundred comments which are directly related to your inquiry, and I think you will find most of them interesting and informative.
Fishmore posted 07-23-2010 04:18 AM ET (US)
Thanks Jim, excellent reading material.
So I guess the number of cylinders has nothing to do with gear ratio chosen. Perhaps the number of cylinders is chosen by the manufacturer due to other factors such as vibration or noise or whatever.
Gear ratio seems to be chosen by the manufacturer based on how they feel the engine will be used.
So basically a larger heavier boat needs to turn a propeller with more blade area then a lighter boat would need. So if the application calls for a larger diameter propeller then you need a higher numerical gear ratio. If the application does not require the larger propeller but it does require higher speeds then the motor needs a lower numerical gear ratio and a smaller diameter propeller. I guess that is why several manufacturers offer the same horsepower motor but with a different gear ratio and different lower unit.
Overall, after reading all this unless you can do a real world test you just don't really know which combination will work best for you. You just have to take your best guess and go for it. Your failure or success will depend on whether the motor you choose can get you on plane and still go fast enough for you. There just does not seem to be a magic formula to figure this out.
jimh posted 07-23-2010 09:22 AM ET (US)
Based on some of the theories presented in the prior discussions, we see that EVINRUDE is really much smarter than all the others because they used the highest (numeric) gear ratio. It was argued in the prior discussions that a higher (numeric) gear ratio would allow a slower propeller shaft speed, and this would permit a propeller of higher pitch to be used, which in turn would result in a higher pitch/diameter ratio, which in turn would result in higher propeller efficiency. Based on this argument, EVINRUDE has the best gear ratio among these motors, and MERCURY has the worst.
I have to say that this notion is based on an theory or principle asserted by someone other than me, and I am just applying that principle here, in a manner analogous to how it was applied in the prior discussions.
boatdryver posted 07-23-2010 10:17 AM ET (US)
If minimizing vibrations at or near idle rpm is important to you, I'd lean toward 3 or 4 cylinders.
When I last checked, Suzuki had the fewest dealers/factory trained techs in the SF Bay Area.
JimL

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Author	Topic: 60-HP Motor Comparison
Fishmore	posted 07-22-2010 03:07 AM ET (US) 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 [maximum engine speed] 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 the Propeller Calculator in the Reference section 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. 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 fishing and cruising?
jimh	posted 07-22-2010 11:24 PM ET (US) There has been prior discussion on this topic. See Propeller Shaft Gear Ratio in Relation to Propeller Diameter, and Pitch http://continuouswave.com/ubb/Forum4/HTML/007108.html Effect of Gear Ratio On Selection of Outboard Motor http://continuouswave.com/ubb/Forum4/HTML/005853.html 90-HP Motor Comparison http://continuouswave.com/whaler/reference/compare90.html In the last reference I cite you will find a method to allow you make comparisons of the propeller shaft torque produced by various arrangements of gears. This may be useful for your comparison of 60-HP motors. There is also a follow-up discussion. See: http://continuouswave.com/ubb/Forum4/HTML/005881.html In all, you have several hundred comments which are directly related to your inquiry, and I think you will find most of them interesting and informative.
Fishmore	posted 07-23-2010 04:18 AM ET (US) Thanks Jim, excellent reading material. So I guess the number of cylinders has nothing to do with gear ratio chosen. Perhaps the number of cylinders is chosen by the manufacturer due to other factors such as vibration or noise or whatever. Gear ratio seems to be chosen by the manufacturer based on how they feel the engine will be used. So basically a larger heavier boat needs to turn a propeller with more blade area then a lighter boat would need. So if the application calls for a larger diameter propeller then you need a higher numerical gear ratio. If the application does not require the larger propeller but it does require higher speeds then the motor needs a lower numerical gear ratio and a smaller diameter propeller. I guess that is why several manufacturers offer the same horsepower motor but with a different gear ratio and different lower unit. Overall, after reading all this unless you can do a real world test you just don't really know which combination will work best for you. You just have to take your best guess and go for it. Your failure or success will depend on whether the motor you choose can get you on plane and still go fast enough for you. There just does not seem to be a magic formula to figure this out.
jimh	posted 07-23-2010 09:22 AM ET (US) Based on some of the theories presented in the prior discussions, we see that EVINRUDE is really much smarter than all the others because they used the highest (numeric) gear ratio. It was argued in the prior discussions that a higher (numeric) gear ratio would allow a slower propeller shaft speed, and this would permit a propeller of higher pitch to be used, which in turn would result in a higher pitch/diameter ratio, which in turn would result in higher propeller efficiency. Based on this argument, EVINRUDE has the best gear ratio among these motors, and MERCURY has the worst. I have to say that this notion is based on an theory or principle asserted by someone other than me, and I am just applying that principle here, in a manner analogous to how it was applied in the prior discussions.
boatdryver	posted 07-23-2010 10:17 AM ET (US) If minimizing vibrations at or near idle rpm is important to you, I'd lean toward 3 or 4 cylinders. When I last checked, Suzuki had the fewest dealers/factory trained techs in the SF Bay Area. JimL