Curtis
06-18-2001, 03:13 PM
My boat is slow in the water, I have a newer 17 foot walk thru with a freshly rebuilt Johnson 115, RPM's at WOT are 5050 tops. It is tough to get more than 36.5 MPH with a full load, including 3 deep cycle batteries, full tank of gas, two electrics, and two people. MT boat does not go much faster. Max HP rating for this boat is 115, motor has been dynoed and is putting out factory spec'd HP for that year(1996).
Does any one know the gear ratio of this unit so I can figure theoretical max speed. I have moved the unit up two holes and no change in top end or handling. Running a 14x17 prop,that may have been rebuilt.
Thanks
Curtis
Curtis,
I think that your problem is probably your propellor.
With a 17 pitch prop spinning at 5000 rpm, you are right on as far as speed is concerned.
With that engine and boat, you should be able to spin a 17 pitch prop at about 5900-6000 rpm.
I wonder if you might have a bad blade, bent or some other prop problem.
Keep switching props, until you can get your engine spinning in the mid 5000 - to 5800 rpm. That is the wot for that engine and should easily be able to spin that prop.
With that engine, you should be able to spin a 19 pitch prop that has a diameter of 13 1/2 inches - with decent blade area.
Get a couple try and buy props from your dealer, or some different used props that he might be selling.
I suspect that if you get the right prop - you will be moving out a bit better.
Depending on the weight of your rig, you might find that you can't get going a lot faster.
I assume that when you are up and on plane, you trim the motor back until you either start to porpoise, or until you start to get motor cavitation and then you trim it in just a bit.
I also am assuming that you have a decent weight distribution in the boat, so that with the right prop, you are able to get at least 2/3s of the boat out of the water. If you have so much weight in the front of the boat, or are unable to trim the motor up, and or if you don't have enough rake on your boat to lift the nose, and get the most of the boat out of the water, you are going to plow water, have lots of drag, and will limit your top speed.
If your rig doesn't weigh over about 1000 or 1100 lbs, you should be moving along in the low 40's with the correct prop, and if the motor is putting out rated hp, as you have indicated in your post.
Good luck
REW