what prop do i need

[fastrthnstock]

i have a 79 merc 115 on a 78 trimate 2 my top speed is 42 mph at 4k rpm i was told my boat should turn about 5k or so rpm ... does my mph vs. my rpms sound right? id like to go a good bit faster if possible, not to sure if it makes a difference or not but my motor has no jackplate...

 

[Mark]

Your speed is about right for that setup. There is likely more but need to know what pitch prop you are running now. Pull it off and have a look...it should be stamped somewhere. Ideally you should be turning that motor 5000-5500 RPM.

 

[fastrthnstock]

i pulled the prop it said ss 13x19... i also drained the oil on the foot and only 6 oz. or so come out, so i put new oil in could that have had anything to do with my low rpm problem?

 

[Mark]

I would say your tach is off. With that prop and speed you are probably turning closer to 5000 RPM. If you were spinning only 4000 you would only be running 32 MPH.
Probably not a lot you can do to wake that rig up. A 1979 115 will be putting out less than 100 HP at the propshaft. Your next move is likely more horse power. The gear oil is not slowing you down.

 

[fastrthnstock]

Thanks, i thought that may be the case just wasnt sure... i was reading a few other post on here and some people with the same boat and motor are able to reach 50 to 55 mph with a 13 1/4 x 21 prop... does that sound right to you?

 

[Mark]

I doubt it. You will probably go slower as I think it will be too much prop for that motor. That motor is happier when it is running maximum rpm. I also don't believe guys are getting 55 mph with inline 6 115's.

 

[fastrthnstock]

Thank you again... i guess 42 mph is good enough for me...

 

[ssv1761982]

Don't lug a 2 stroke

You need to get the tach checked or try/borrow a different one. You really need to know what rpm you are turning. IF indeed it is only running 4k, you can hurt the motor.