Looking for advice on Prop for 120 inboard

[rparker75]

I am new here so excuse me for being a little slack. I have a 18.5 Rinker 1982 with a 120hp inboard 4 cyl. The foot is a 400 OMC stendrive and has a
13.5X15 prop on it now. The boat does not want to plane easy with gear in it or pulling tubes. Should I change prop diameter(smaller) or should I change pitch? Not sure if my gauges are right, but shows the boat at around 5000 rpm WOT speed show about 30.

 

[SparkieBoat]

Re: Looking for advice on Prop for 120 inboard

If your RPM gauge is right you should not go down in pitch, (if it is not you need to fix it) most inboards should be pitched to hit 4500-4700 RPMs at WOT, yours could be different so you should find out what the recommended RPMs are for your motor., but what you need for better performance is more HP..a 120 on a 18.5 boat is a bit under powered. But you can ad trim tabs and it will help you get up on a plane quicker, which could help you out some. but no prop will add HP. you also should have your motor tuned up to make sure you are getting the most from it. you could get more speed with a 17 pitch prop (probably) but you would lose some hole shot, but if you add trim tabs it may work out, also you may want to get a vented prop to help with hole shot.

 

[rparker75]

Re: Looking for advice on Prop for 120 inboard

I appreciate that I was also told by someone that a 4 blade prop might help also. Is this true?

 

[steelespike]

Re: Looking for advice on Prop for 120 inboard

Be a little more specific about weight and how long to plane.

Yes a 4 blade will help though I think you have an underlying problem.
Please read the stickies at the top of the page and get us as much info as possible.
Be sure the motor is in excellent tune,Good compression,Throttle opening all thge way.
Do you trim down for hole shot?Do you trey to balance the weight so not to much in the stern? a 15" p[rop is pretty far down the list toward power props. with a 24" being the hghest pitch more for speed. 13,11,10 and 9 below.

 

[SparkieBoat]

Re: Looking for advice on Prop for 120 inboard

a 4 blade 15 P prop would slow your RPMs down some which would be ok, but should help your hole shot some. again I stress to make sure your motor is in good running condition.