performance issues

[freebird63]

I have a 1984 evinrude 70hp on a 1989 truline sportcat fiberglass vee hull, the prop is an aluminum 12 3/4 x 15, it will only run at 28mph per gps at 5000-5200 rpm, ran half a can of seafoam thru it, champion plugs, the motor is mounted on the third hole down on the transom, I thought maybe the motor was setting too low in the water. The guy I bought the boat from replaced the floor and put all new foam under the floor. Its a smooth running boat as far as rough water goes. But I feel this boat should be able to run at least 40mph. Any ideas why it does not pull 40mph????? I am getting discouraged. A boat mechanic suggested putting a SS prop on it. The rpm's are suppose to be 5000 to 6000, wouldn't a bigger SS prop bring down my rpm's???? also the boat guy said that for every hole i moved my motor up I would gain 3mph. Please someone give me some suggestions before I go out and sell this boat.

 

[Solittle]

Re: performance issues

You will get max performance by raising the max rpms at WOT to the max rated by the engine manufacturer - for most OMCs this is around 5,500 rpms. at 5,000-5,200 rpms you are lugging the engine. Shoot for 5,500 or a tad more than that by raising your engine and testing. Start with the cavitation plate just above theprop even with or 1/2-1" above the bottom of the hull. Move up a hole at a time untill you get the max rpm. Be carefull not to get so high as to suck air into the pick-up for the water pump. It costs nothing to do this. If you are still not satisfied then start looking at props.

 

[freebird63]

Re: performance issues

I can only raise the engine up one more hole. So if I were to look at SS props, would I want one bigger then the aluminum prop that is on the motor now???? current prop is a 12 3/4 x 15 aluminum. I read somewhere that the recommended SS prop was a 13 1/4 x 21, wouldn't that bring my rpm's down?????

 

[kenmyfam]

Re: performance issues

Photo would be nice of your motor on the boat. You do not mention anything about the weight of your boat either and consider everything you have withv you, people, gear, gas etc. Do you have power trim ?? and if not have you tried adjusting that ??? where is the cavitation plate currently in relation to the bottom of the hull ??. How is any weight you are carrying distributed ??
Set up is a huge factor of boat and motor performance. I would start experimenting with the things that do not cost anything to try and go from there. depending on your total load weoght you may be close to your max speed right now. I do not know what hull you have either so even hull design can have an effect on performance.
Good Luck and please keep us all posted.

 

[freebird63]

Re: performance issues

I will post some pictures of the boat when I get home from work tonight. Its a vee hull fiberglass boat. Am not sure about how much it weighs. It has an 19 gallon tank under the center console, one battery in the back of the boat. 28mph is with me and one other person. I know the boat is not lite, am not sure how I could go about weighing it??? The gas tank has about 10 gallons of gas in it. The last two times I put gas in it, I used 89 octane. Its a really big boat for only being 15 feet long, the beam is like six and a half feet.

 

[ezeke]

Re: performance issues

I would try Solittles's advice before anything else and raise the engine a notch. Also experiment with the trim while on plane.

If you were to go from aluminum to stainless steel with the same dimensions as your current prop, you will probably pick up 200 RPM. All other things being equal, any increase in pitch or diameter should lower your RPM.

At 5200 RPM your optimum speed would be 30.5 MPH, so you are getting about as much as you should with that propeller until you increase the RPM. At 5800, you could potentially reach 34 MPH with your existing setup.

 

[tashasdaddy]

Re: performance issues

stay with an aluminim prop the SS is harder for the motor to turn. your AV plate should be 1-2 inches above the keel. you may have to change pitch on the prop.

 

[freebird63]

Re: performance issues

So there is no way that this boat with this motor would be able to hit 40mph??? One of the reasons I bought this boat and motor was the hope that it would go 40mph

 

[freebird63]

Re: performance issues



The first picture is with the outboard on the third hole down and the second picture is with the outboard on the last hole, can't raise it up anymore, she is maxed out.

 

[freebird63]

Re: performance issues

Yes the outboard has trim & tilt and have messed around with it while running, does not change the speed any. I raise it up til it sounds like its starting to suck air.