What pitch prop for 1990 90hp evinrude

Hey guys.

I have a 1990 hp evinrude on a 16.5ft dual console aluminum boat. It has a 13 3/4 x 15 pitch prop on it and will run about 5500 rpm and make about 35 miles an hour. It has a ton of holeshot, but was thinking I would get more speed out of it. My buddy has a 16ft boat with a 75 and can hit 41 or so. I was curious if a 17 pitch prop would maybe be something to try? Would I see better performance out of something different?? His is a 19 pitch..

I am also considering putting on a sting ray hydrofoil to help eliminate porpoising when barely trimming the motor. I have to drive with the motor all the way down to achieve 25-30 mph. Would that be a good help for that??

Thanks

[yarcraft91]

Assuming the numbers you give are accurate, you have the right prop on that boat as things currently are. According to my literature, 5500 is max rpm for that motor.

35 mph does seem slow for that rig and you report porpoising when you trim out. The motor may be mounted too low. If so, raising it should help with porpoising and will increase rpm, letting you run a higher pitch prop. Which holes in the motor mount are the bolts in?

Thanks Yarcraft.. I will have to check when I get home and get back to you. It does seem like it sits pretty low in the water..maybe bringing it up would help. I can't trim up at all without getting it to start bouncing.

[yarcraft91]

Ideally, take someone boating. Have the guest drive the boat, get it on plane, then you look over the transom and see if the anti-cavitation plate is submerged or above the water (getting splashed is OK). It should be just above the water when on plane. Raise the motor if it is not. When I bought my current 16' boat, the motor was mounted as low as possible, creating excess drag and porpoising. After I raised the motor, the boat ran faster, porpoising was under control and threw a lot less spray.

[propmann]

Originally Posted by Mr.BRN

Thanks Yarcraft.. I will have to check when I get home and get back to you. It does seem like it sits pretty low in the water..maybe bringing it up would help. I can't trim up at all without getting it to start bouncing.

Most likly your current prop wil not allow the motor to be raised with out blowing out all the time. YOu may want to look into a stainless prop so you can raise your motor.

[REW]

Brn,
Run a compression test on your engine.
I wonder if your engine is a bit down on power.

Are you running your engine at a location that is high above sea level like Co?

Normally, this engine should be able to spin a 15 pitch prop at about 6500-7000 rpm.

Right now, if you are only turning 5500 rpm, you won't be able to spin a prop with a larger pitch, unless there is something wrong with your current prop.

It is possible that you may have a prop that is mismarked, or a prop that is defective in some way.

You could try a new 17 pitch prop on your motor, and with only one person in the boat, take it for a run and see what your engine rpm is as well as your GPS speed.

If you have access to a dealer that has a test prop for your motor, you could have the dealer test the hp out of the engine. A test wheel is a wheel that is put on the engine that has a calibrated load matched to the engine. Depending on the rpm obtained when the engine is put through its test paces; the hp can be calculated that is being produced from the engine.

If you really are only turning a true 5500 rpm on a 15 pitch prop at near sea level conditions, you are likely putting out something less than 90 hp.

Good luck
REW

[Phil T]

REW may have something with his suggestion for checking the engine compression. My 1997 90hp Honda turns a 19" pitch prop at that rpm on a 17 1/2' boat.

Hey guys. I did recently check the compression on it. I'll have to look for sure but if I remember right they were 110, 110, 108, 105. I mentioned it to the mechanic and he said the VRO evinrudes were lower on compression anyways to start with. I'll have to check, maybe my engine is mounted down too low.. The cavitation plate should be even with the bottom of the boat about???? Thanks all for the suggestions.

Okay, I checked the mounting holes. It is mounted on the lowest hole..... furthest down possible. I did measure with a tape and the bottom of the anti-cavitation plate is exactly the same as the bottom of the hull. I had it worked on and spark plugs replaced recently. He did a compression test before putting in some new gaskets and plug and they were at 115, 110,110, and 105. After he was done he tested again and they were at 110, 110, 108 and 105. The prop shows that it is a 13 3/4 x 15. Do I have to run it wide open on a lake and check the anti-cavitation plate and see if its in or out?? Like I said, I can't even trim it up at all...i have to take off all the way down, and can trim it just a hair before it starts bouncing, any more and it will start making goofy noises(cavitation). Any ideas?

[yarcraft91]

When I got my Alumacraft 16.5' boat, the motor was mounted as low as possible. Sounds like yours has trim and porpoising problems like mine had. On plane, my boat threw an amazing amount of spray, porpoised with any trimming out and didn't perform well at all. The anti-cavitation plate was buried in the water and that was causing the spray, plus lots of extra drag. After I raised the motor, the excessive spray went away, the porpoising was greatly reduced and boat performance was greatly improved.