Need prop / tach help 1990 70 Rude

[Steve135]

Hi I am trying to raise the rpms on this boat with a 1990 70hp evunrude. The boat is a 20 ft flat bottom with a slight V at the nose. I know the motor is small! It had a 4 blade composite prop that was dammaged and thrown away before I saw the size it was. The boat was able to do 21 MPH with two adults onoard. It now has a 13 X 19 3 blade and the rpms are only 3200 at WOT I have both a 13 1/4 x 17 and a 12 1/4 x 15 to try. I am a bit confused on this end of the motor I know alot more about the powerhead! The small gearcase large gearcase thing confueses me also.<br />The 12 1/4 x 15 is off a 1980 60 hp which is a small gear case but fits the 1990 70 hp large gear case ??? I think if the formular is correct that I would gain 250 to 400 rpms per pitch change I need a 14 X 11 to be at 5000 rpms.<br />steve

 

[rickdb1boat]

Re: Need prop / tach help 1990 70 Rude

I can't tell you which prop pitch is best for your boat, but I have a pontoon boat and I swing a 14 X 11" prop on that and go 5 MPH faster than your flat bottom.(Same Motor) I can't imagine that your flat bottom boat would need that small pitch. Every 1" pitch you drop down does equal about 200 RPM gain. I'm just guessing here, but a 14 X 13-15" pitch sounds a bit closer to what you need. That's unless you really load the boat up heavily. Nevertheless, if your Tach is correct, 3200 is way low. At least 2000 low and I don't think a prop is going to cure that. Make sure your engine is running properly and use the larger diameter props on the 70 HP. Yours has a large gearcase with the ability to turn a larger diameter prop, which you should. Try the 17" and see what happens..

 

[Steve135]

Re: Need prop / tach help 1990 70 Rude

Hi Ok I ran the boat today and the tach runs up to about 3800 rpms and than drops to 2200 rpms when I advance the throttle any more. The motor clearly increases in rpms buy the sound and speed but the tach drop and stays at 2200 rpms till I retard the throttle. The battery is charging so the rectifier seems ok? The boat does 23 with the 13x19 prop I changed to the 13 3/4 x 17 and it clearly has a higher rpm at WOT but what I can't be sure because of the tach. Lost 1 MPH with it. the tach was NIB this season and hasn't work right yet.<br />steve

 

[steviecops]

Re: Need prop / tach help 1990 70 Rude

Hi<br />I fitted a new tach recently and it behaved just as you describe. Turned out to be the ground terminal inside the control box. The black lead that comes from the tach plug connector on the control box goes to a screw where it makes the ground. In mine, the screw was loose and the terminals very dirty. A clean up and re-tighten of the screw did the trick.<br /><br />Good luck<br />Steve