The carb swap discussed above went well and this originally 5 HP motor has now been converted to a 6HP Tohatsu MFS 6A2S. We took our 20 foot Grumann cargo canoe, loaded fairly heavy with two guys (equalling 500 lbs total — fairly heavy boat for this small motor) out on a local lake for speed trials. We found that the top speed is now increased from 8 to 10 mph, using the stock 7.9” inch diameter X 7.9” pitch prop.
At full throttle with this prop, the RPMs are approx. 4750 rpm. Since the spec’d max rpms for the 6 HP motor are 5000 to 6000 rpm, I am thinking that this prop is too coarse a pitch, and that I should try the 7.9” X 7.0 inch pitch prop, to get the rpm’s up into the 5000 to 6000rpm range, and so tweak the top speed up maybe a mile or two. But I do not have access to technical data for this motor, or relevant experience really know
The reason this is critical is that our fishing and hunting occurring here in Alaska is on the fairly fast flowing (about 5mph) Tanana River. So, when going up-river, the 10 mph speed that we have now achieved is half used up by the River’s 5mph speed - meaning that our actual speed up the River is only about 5mph! So, you can see that a 1 or 2 mph increase cuts our transit time and round trip fuel burn greatly.
A couple of factors to consider are 1) We can’t just use a larger motor on this boat because of canoe weight limitations, and 2) this is a strong motor and we are running synthetic oil for extra protection.
Can anyone comment, either from experience and/or access to technical data (for instance a power curve)?
Thank you.