light boat heavy boat same prop??

[barbwire44]

Ok, stupid question without a tach, but for some reason or other my tach is not working, anyway. Have a 100 hp evinrude v4, was originally on a 16' sea nymph aluminum fishing boat, ya... scary fast!! the prop is a stainless 13 X 19, I have since taken the motor and added it to my 97 sunbird spirit 170 fiberglass boat which seems much heavier and larger all the way around, like I said my tach quit workin, but this set up on the aluminum boat would run lets just say high high 60's mph, now with it on my sunbird I can only get 34 mph and a slow holeshot???? I am guessing this thing is over propped, but how much I don't know, any suggestions on how far to drop the pitch, wanna use as a family boat water sports, tubeing skiing etc...
Thanks
all ya any props better than others or should I say any props to stay away from???
thanks again

 

[MikDee]

Re: light boat heavy boat same prop??

Actually, you're not far off, either a 13X17", or 13X15", should work for you, with the 100hp motor. You really need a Tach to find out for sure. Don't be too concerned with diameter, it gets a little bigger as the pitch goes down, and vice versa.

 

[Silvertip]

Re: light boat heavy boat same prop??

Something is wrong with the installation or your numbers. You don't lose nearly half the speed by going one foot longer, even if one boat is glass and the other was aluminum. I would start by getting the tach working, then make sure the throttle is opening all the way. Remove the intake cover and with the throttle lever full forward, the throttle plates should be perfectly horizontal in the carb bores. Without a tach, you are shooting in the dark regarding prop selection. I run a 19P on a 17 foot walleye boat with a 75 HP engine and have no trouble reaching 40 MPH. I also question the high 60 mph figure as with a 19P prop, a 2.0 gear ratio, and using 12% slip the engine would have to turn 7500 rpm at 60 MPH (hardly high 60's).

 

[jammer]

Re: light boat heavy boat same prop??

Yes first get the tach working. Since it doesn't work the not working tach can mean something else is wrong.

 

[barbwire44]

Re: light boat heavy boat same prop??

I don't know why the tach doesn't work, my first thought was that the boat had a yamaha 115 four stroke on it originally and the tach was for it, and that was why it doesn't work with the evinrude 2 stroker??? and there is no dial on the back to switch positions like I have read on this forum some tachs have. I did buy a 13-3/4 X 15 prop and ran it sunday and it was much better out of the whole, actually felt alot stronger, but the top speed remains the same of 34 mph??? my speedo actually reads 31mph, but my buddy in his nautique ran beside me said his two speedos were between 33 and 34 mph. I did buy another engine last week and it is a 115 with power tilt and trim, maybe with it I can hit the 40 mph mark, but I really don't plan on putting it on for a while. Maybe the boat just doesn't cut the water good?? it is a 1997 sunbird spirit 170..