what prop on 115hp mercury 2-stroke?

[Breedam]

Hello boaters,

I've got a 15ft glassfiber boat weighing in at about 450 points. It's equipped with a 4cil 2-stroke 115 mercury. It turns a 15p oem prop. Since the boat is quite light it makes to much rpm. Since im in Holland there's not that much choice for alu props. I can either buy a 3- or 4-bladed Solas Amita or a Michican Wheel Vortex. Shall I benefit form a 4-bladed prop or not? And what pitch size is about right? Is a 17p 4-bladed too much?

Marten

 

[steelespike]

A 4 blade does usually do some things better than a 3 bld. Should resist ventilation,have a stronger hole shot,stay on plane easier at reduced throttle
should seem smoother and may respond better around the dock.Top end will likely be a couple of mph slower.
It would really help to know your wot rpm and gps speed .What year is the Merc?

 

[Breedam]

Thanks for your reply, my topspeed is 43mph. I dont know the rpm but you can hear by the sound it's overrevving because it's making that sound just like a car if you change gear at the last moment at wot. Also the climb in rpm from half trottle to wot is to abrupt to think the engine has any hard times turning the prop.

thanks, marten

 

[steelespike]

You shouldn't make any prop changes without a tach.It's very important for the health of the motor that it rev freely within its rated wot rpm.
4750 to 5250.Without a tach we can't tell what your specific setup needs nor can we tell the results of the new prop.
You can get a hard wired tach or an induction tach will work.
Your right it is very likely over revving. You say the boat weighs 450 points.? Can you change that to pounds.
As a starting point I would suggest a 19"Solas Amita prop.It will surely require more but we need a tach to know how much.
Guessing some numbers to go with your 15" prop at 43 mph you may have all ready been operating at 7000 rpm.

 

[Breedam]

Will that be the amita-4 then?

 

[steelespike]

Your choice the 4 should deliver as I described. Should lower rpm slightly more than a 3 blade 19.