Correct prop for Mercruiser 5.7L

[sjohnson44]

I just this year got my boat restored and was able to take it out during the summer. The motor is a rebuilt 5.7L, boat is a 20' capri. Alpha I Gen II 1.47:1
The hole shot is sluggish and if I go too far with the throttle its harder to keep control. More or less it will start shimmying side to side. I question the prop being correct.

When I bought the boat it came with an aluminum 14.25x21P prop. The boat came with a 14x19 from the factory according to the original manuals. SO obviously someone changed it along the way, but was it for the better?

Does the 14.25x21 prop throw any red flags for anyone else?

Id like to upgrade to a ss prop, would this help my situation? Or should I go back with the 14x19?

I have been looking at a Laser II 14x19.

 

[kmarine]

Re: Correct prop for Mercruiser 5.7L

You need to compare speed to engine rpm and try a few props before making a final decision. especially now because you have the availibility to use 4 and 5 blade props to determine you best performance curve. I recomend finding a dealer or prop shop with demo props to try out first.

 

[Maclin]

Re: Correct prop for Mercruiser 5.7L

The shimmying side to side is a big red flag. Of the props you mentioned, either one would be a good place to start for a baseline. But make sure the hull and the outdrive are OK, and also that the engine is in top tune.

 

[steelespike]

Re: Correct prop for Mercruiser 5.7L

What is the present gps speed and rpm? Are you running the 21" prop?
Does the boat get up on the water and run on the rear 1/3 to 1/4 of the boat.
Does the boat respond to up trim or does it lay down like its glued to the water.
Clarify shimmying side to side. Is it rocking side to side (chinewalking) or what?

 

[theBrownskull]

Re: Correct prop for Mercruiser 5.7L

Try to clarify what steelespike said because it will help. I have the same gear ratio with my 5.0 mpi and it came with a factory aluminum 19 pitch. I went to stainless with a 17 pitch.

 

[sjohnson44]

Re: Correct prop for Mercruiser 5.7L

The shimmying comes from the back end, as if the outdrive was loose swaying side to side. The engine is rebuilt and as tuned up as it should be.

I dont know the speed because the speedo was not hooked up, the rpms would be a guess of around 4000 at 3/4 throttle. I have not had it at WOT yet because of the shimmying. Not to mention the boat will beat you to death on anything over a slight chop.

The hull is in decent shape for its age. I have fixed some chunks that had been chewed out of it with MarineTex. Its not perfectly smooth tho, if that makes a difference.

It does respond to trim height, with it down its glued to the water and up the bow is up.

I ended up buying a 14x19 laser II, I was going to put it on today but its different than my current aluminum one. The Laser II is a LH and I thought that was "standard"? So would I have had the wrong prop all along? Some sites say RH is standard and some say LH is standard.

 

[jestor68]

Re: Correct prop for Mercruiser 5.7L

"Standard" rotation is right hand. Most Mercury SS props come both ways for those with twin motors needing counter rotating props.

Suggest you check the steering connections(behind engine) for any looseness/play.