Prop Ideas?

NOLA-boy

Seaman Apprentice
Joined
Apr 15, 2009
Messages
35
I have a 22ft 79' Starcraft Islander.
Weighs about 2,200# with fuel and me.
Mercruiser 470 with 1.84 MC-1 outdrive.
My recommended WOT is 4,200-4,600.
New tach (set for 4cyl)
Iphone GPS

I'm currently using a 3 blade 13 3/4 X 21 and can only achieve 4,300 at 35MPH(30 knots). According to the Mercury prop calculator, I'm getting about a 25% slip. The other day I had a load of 4 adults, 4 kids and 2 medium dogs and could only get 3,900 RPM and it was a dog out of the hole. I know I'm over propped and the diameter is too small.

I purchased a 4 blade 15 X 17 and had the same results. 4,300 at 35 MPH (30 Knots) The prop calculator shows a 7% slip.

Today I'm going back to the prop shop and get a 4 blade 15 X 15.

Am I on the right track?

I'd like a cruising speed of 30MPH (26 Knots). It looks like the 15' prop will put my top speed about there. (30/26)
I can currently cruise about 28MPH (24 Knots) at 3,600.
I guess I'm rambling and don't seem to have a real question. This is my 1st 4 cyl motor. I am accustomed to SBC performance. The 470 seems very fuel efficient though.
I'm not sure weather to go with a 3 or 4 blade either. The prop shop owner recommended the 4 blade because of my slippage. Thanks for any ideas.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: Prop Ideas?

Yes sir you are doing fine. The shallower pitch with the large diameter will enhance all the good things you are missing and not hurt you that much on the top end since you didn't talk about that. Besides, running the numbers through the calculator, it's pitch x rpms for mph so given the shallower pitch will up your rpms, your top end may not be affected all that much and I guarantee you this, your engine will love you for it. With less lugging of the engine, it should run more efficient, just like your car pulling a load up hill, vs flat and level, and your mpg should improve.

On the 470 I remember when they came out. This was a Mercruiser special design and appeared to have all the right things going for it. As it recall it had a closed cooling system which I really liked.

The prop shop owner got you in the right church, just the wrong pew....so to speak, but those things take trial and error. The shallower pitch will also help you to maintain your attitude with your large loads at cruising speeds. You won't be settling in the water and having to speed up then cut her back, etc. to maintain your cruise speed. Hole shot will be much better too especially with the jump seats that folks just love to sit in right next to the engine when you are trying to get the ARSE end up and on plane.

Mark
 

NOLA-boy

Seaman Apprentice
Joined
Apr 15, 2009
Messages
35
Re: Prop Ideas?

Thanks Mark. I am heading to the prop shop this morning I'll report back to y'all.
Thanks again
 
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