Re: prop advice needed
I use a prop calculator found on the net. There are 5 variables rpm,gear ratio,prop pitch, slip and speed. You need 4 of the 5 variables to figure the 5th.
Unfortunately slip is the tricky one.Theoretically your prop moves 19" forward in one revolution. Slip is measures the actual distance your prop moves forward in a liquid.
You can figure slip using the other 4 variables,but you have to guess slip to figure one of the other variables
A typical boat/motor combination will produce 4- 12% slip could be more or slightly less.
Your numbers guessing 10 percent slip produce negative slip,not possible.
A gps is usually accurate, Your slip could be lower but not enough to make 50.Perhaps the tach is low.
The gear ratio is from the merc site for a 115 4 stroke.
If I use 6400 rpm and 5% slip speed is only about 47.
Are you perhaps running in a strong current?