What pitch and diameter?

Hello all,
I have a '74 Silverline 16 ft fiberglass boat. It is stern driven with a 120 hp mercruiser engine. I am looking to replace the existing prop. I have talked to a couple boat dealers and i am getting different recomendations from both. The existing prop is 16 pitch x 16 diameter. I dont know weather to replace with same size and pitch or if there would be a better prop for my boat. The WOT rpms are around 4200 if that helps. Top speed is 25 with current prop. Any help is much appreciated.

Re: What pitch and diameter?

Is the speed by gps?I believe 4,600 is the max rated rpm.Sometimes on a label on the motor. I can't speak specifically to that boat combination but typically that size/hp would run around 40 on a 19 or 21 pitch prop.Would run a 15 to 17 for skiing and such.While the 16" diameter might keep rpm down a little That pitch would normally overrev.I think something is up with your setup.You should chack that throttle opens all the way,check compression,is the bottom clean,prop and moter in excellent tune.I was surprized by the 16 X 16 prop but did find one in the prop section.Usually diameter isn't considered when picking a prop but I would lean toward a little less diameter.

Re: What pitch and diameter?

Thanks for the reply steelespike. Yes, that speed is GPS, although the current prop isnt in prestine condition.(has a few dings that have been filed). We primarily use the boat to troll with, but it would be nice to get to the fishing spot as fast as we can. Would you recomend smaller diameter with more pitch? Also I very rarely run it full open throttle.

thanks

Re: What pitch and diameter?

That's the std 4cyl chev.....4200 would be about all you want.
I run a 15X17 on my '70 120 and 17' 6" glastron. higher elevation of around 3400 where I use this one and about 4000rpm is top with normal load running "close" to 35mph. Good troll/hole-shot prop there, should be even better at 2500ft and below.
I've played the diameter game on this boat and ran a 15.5X15 trying to slow the troll a bit but it was the same as the 17....just too much blade area, and it was slower on top-end. 14.5X17 might surprise you with the correct design.

Re: What pitch and diameter?

Your 4200 WOT RPM is perfect for your 120. You don't want to spin it any higher. My 1981 120 has the Mercruiser factory sticker on it listing max RPM as 4200. The factory manual #2 (1974-1977 engines and drives) lists max RPM for your motor as 3900 - 4300 RPM.



My 15' trihull with the 120 barely breaks 30 MPH no matter what prop is on it. I've tried props ranging from 15" pitch where it turns 4300 RPM max to 21" pitch where it turns 3200 RPM max. They all run 31 or 32 MPH.

I'm guessing that your 16' is a little heavier than my 15'. I wouldn't be surprised if 25 MPH is the best you'll see.

Re: What pitch and diameter?

14 1/2"x17p should be the correct prop.

Re: What pitch and diameter?

Thank you all for the advice, I will try the 14.5 x 17.