Please help for the right prop

[SEASWIRL1851CC]

A bought a 2002 4 stroke Yamaha 115 hp. It's on a 18.6 seaswirl 1851 cc 2650 lbs. On a full throttle it only max. to 25 mph. What prop should I get so it can deliver it's max. performance? I think the prop does it have right now is 13K, K I didn't see the pitch,(painted prop)-(white). Any help would be very much appreciated. Thanks

 

[v12mac]

Re: Please help for the right prop

The pros here are going to need to know how many RPM you are turning at WOT.<br /><br />Are you looking for top end? Hole shot? Do you ever ski or run with a heavy load?

 

[SEASWIRL1851CC]

Re: Please help for the right prop

Originally posted by v12mac:<br /> The pros here are going to need to know how many RPM you are turning at WOT.<br /><br />Are you looking for top end? Hole shot? Do you ever ski or run with a heavy load?

 

[SEASWIRL1851CC]

Re: Please help for the right prop

@ 25 mph the rpm is @ 5500. I just replaced my prop from 13 5/8 13 pitch to 13 5/8 15 pitch and it picked up 2 miles but the rpm droped to 4800. It that normal? I mainly use it for fishing, not skiing or any other activities. Any suggestions as to what prop should I purchase to be at 35 mph? My fishing boat is at 2650 lbs including the engine, with 65 gallons of fuel, 4 gallons of fresh water washdown with 2 anglers on board. Thank you for your help.

 

[Steamboat]

Re: Please help for the right prop

Something doesn't sound right about this. Just went to the Yamaha web site and looked at boat test with your engine and similar weight boats (angler and scout) and their running in the low 40's. Props seemed to be 13x17 or so but don't know if aluminum or SS and engine rpm's were higher (6100 rpm??). Are you trimming the engine up once on plane to get the hull out of the water? Did you buy the engine from a dealer? If so did they check it out for you? Take them for a test run and see what they say.

 

[LubeDude]

Re: Please help for the right prop

I agree with Steamboat, something isnt adding up.

 

[wtrojan]

Re: Please help for the right prop

To answer your rpm question, that’s absolutely normal to get the rpm change with a pitch change. Though in your case with two inches the rpm dropped by 700, and it is a lot. Usually that’s about 400rpm change with 2 inches of difference in the pitch.<br />You should get the pitch which allows you going at max rpm with regular boat weight, at WOT. You should not let her go more max rpm that allowed, and going WOT and getting less rpm than max is not healthy for the engine.<br />However, the performance will depend on the setup you are running and all this is assuming that you are trimming the engine properly and that everything else is ok, like the height of the engine on the transom. The height of the engine may be worth checking - as a general guide the anti-cavitation plate (the biggest plate on lower unit) should be in the range of one inch below to one inch above the keel, the latter being the good point to start the transom height tuning. The lower the engine on the transom, the higher the drag.