boarder290
Cadet
- Joined
- Apr 29, 2011
- Messages
- 8
Hey guys, I have been reading a lot on the forums and have a specific question that i cant find an answer to. I bought my first boat 2 months ago. It is a 19.5 foot mirro craft, looks to be late 60's, early 70's with a simple plywood floor and plastic seats. It is a shallow v and came with a 1961 40 hp Johnson. I took that motor off and put a 1978 70hp Johnson on which my grandparents gave me off their pontoon. I went through and cleaned the carbs really well and decarbed the motor recently.
I bought a service manual and it said the recommended rps were 4500 to 5500. Currently the prop is a 13.25 17" and will turn at 4900 rpm's with one other person and a small amount of gear, and be doing about 33 mph according to gps. I also have the anti cavitation plate about an inch above the bottom of the hull. From reading other posts it seems like 4900 was acceptable back in 1978 but with modern fuel its better for the motor to be around 5500 to 6000. I also do some tubing with the boat and with 4 people on the boat and one on the tube the rpm's are 4200 which i believe is lugging the motor. My main concern is longevity of the motor. I was thinking of a 13.75 15" to bring the rpm's up some. I was hoping to also be able to use the same prop for crusing and tubing if possible. I'm looking to stay with an aluminum prop (being a poor college student and all), but i'm willing to spend the little extra to buy a higher quality aluminum one if its worth it.
Id appreciate any information regarding brands to look at or changes in my setup or a correction on anything I am thinking above that would be appreciated. Thanks in advance.
I bought a service manual and it said the recommended rps were 4500 to 5500. Currently the prop is a 13.25 17" and will turn at 4900 rpm's with one other person and a small amount of gear, and be doing about 33 mph according to gps. I also have the anti cavitation plate about an inch above the bottom of the hull. From reading other posts it seems like 4900 was acceptable back in 1978 but with modern fuel its better for the motor to be around 5500 to 6000. I also do some tubing with the boat and with 4 people on the boat and one on the tube the rpm's are 4200 which i believe is lugging the motor. My main concern is longevity of the motor. I was thinking of a 13.75 15" to bring the rpm's up some. I was hoping to also be able to use the same prop for crusing and tubing if possible. I'm looking to stay with an aluminum prop (being a poor college student and all), but i'm willing to spend the little extra to buy a higher quality aluminum one if its worth it.
Id appreciate any information regarding brands to look at or changes in my setup or a correction on anything I am thinking above that would be appreciated. Thanks in advance.