mercruiser 5.7 with alpha one prop issue

[julliopat]

I all boater,,,
First the boat: doral concept 23 ft, powered by a mercruiser 5.7 with a alpha one. I bought that boat 2 week a go... run awesome but now I hit a rock and I need to change the prop.....
spec of the old prop:aluminum, 3 blade, 17 pitch, wot 4400 rpm at 42 mph.
The shop where I went told me I had the wrong pitch, so they sold me a 15 pitch, 4 blade aluminum prop.......but now the boat run 40 mph and go too high, 4800rpm!!!

what is the right prop for my boat if anybody know pls?
should I stay with a 3 blade or go witn a 4 blade?
should I go with a stainless or aluminum?

thanks Pat

 

[Mac525]

Re: mercruiser 5.7 with alpha one prop issue

We need the year of the motor and is it a mag or mpi 4800 may still be to low... what are you doing with this boat and are your numbers with it empty ??????

 

[Robbabob]

Re: mercruiser 5.7 with alpha one prop issue

Hey Pat,

Welcome aboard!

Your engine is rated for 4400 to 4800 RPM, so no worries about the new RPM. The 4-blade will give better hole shot but will also lose a little at the top end in speed, as you've pointed out. It depends on what you like to do MOST of the time; pull for fun sports (4-blade) or cruise and go fast (3-blade).

It seems odd your 250+ HP needs such a low pitch prop. Are you sure the motor is running perfect?

If you are going to run the same waters, where hitting rocks is common, then stay with the aluminum prop. SS will only cost you more to purchase and then repairs to the outdrive when you hit something hard.

Good luck!

 

[Tail_Gunner]

Re: mercruiser 5.7 with alpha one prop issue

I take it that is your boat...Nice boat..

When you did your test run.. was that with as many people you intend to take with you on a normal day or were you by yourself. Do you boat on a big body of water or a small pond....Do you ski and tube or pleasure boat....Do you intend to run at wide open throttle all day or just cruise..

Your boat and prop combo are in a good place now...4800 is not excessive rpm at all remeber you should spend very litle time there..Most boating occurs between 1000 and 3500 rpm....and 5-10 minute bursts at 4800 is not a bad thing

Setting up a prop is simple...One just makes sure the motor can turn the prop to the top end of the mfg ratings....from there it all falls into to place. 3 or 4 blade props just add a certian character to the boat.

 

[Mac525]

Re: mercruiser 5.7 with alpha one prop issue

OK Rob I'll bite ... how can you know what his motor specs are with out the year and which verson his 5.7 is there are a few to choose from. Worried you might have some mental powers your not sharing with the rest of us

 

[Robbabob]

Re: mercruiser 5.7 with alpha one prop issue

OK Rob I'll bite ... how can you know what his motor specs are with out the year and which verson his 5.7 is there are a few to choose from. Worried you might have some mental powers your not sharing with the rest of us

Okay Mac,

You're correct.. I actually made an assumption or two. Not smart when I'm not looking at the OP or talking with him. Looking up the 5.7 Mercruiser http://www.perfprotech.com/store/articles/mercruiser-v8-engine-specifications.aspx , I assumed the OP knew that 4400 was in his max range. Since 4800 is the minimum max they list, I didn't see a concern with over revving.

I'ts only magic when you don't share the details.

Hope this helps!