how to determine prop

badbowtie

Chief Petty Officer
Joined
May 18, 2007
Messages
634
I have a 1985 25' cruiser I tried today for the first time new to me boat with a alpha drive and I have no idea what prop but it revs way higher than it needs to be. I tried to figure out what prop it has and cannot find any numbers anywhere to tell me what kind, I even removed the nut and on that flat surface it only says michigan then it also says 5480 and also says 021, on the side of the hub where the blades are there is a bunch of different casting numbers stamped around it.
It also looks to have a hub that is not replaceable and a raised hub like this prop on ebay.
http://www.ebay.com/itm/MICHIGAN-WHEEL-031035-MERCURY-EQUIVALENT-16-X-15-ALUMINUM-RECONDITIONED-PROP-/150740889378?pt=Boat_Parts_Accessories_Gear&vxp=mtr&hash=item2318db6f22
 

bnicov

Petty Officer 1st Class
Joined
May 25, 2009
Messages
348
Re: how to determine prop

Try a local prop shop to see if they can identify the prop you currently have. If you are revving to high, you need a prop with a higher pitch, ie you currently have a 19pitch, you'd need either a 21pitch or 23pitch. Let them know what the boat is doing and they can recommend something for you.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: how to determine prop

How do you know its revving too high?
If you tell us your rpm and speed,the motor size/horsepower
we can probably tell if your setup is working and a ball park prop size.
 

badbowtie

Chief Petty Officer
Joined
May 18, 2007
Messages
634
Re: how to determine prop

At 5,000 rpm I still have throttle left and I was only running 24mph gps At 4,000 it is doing 17mph this is a 85 century mirada 255 cruiser with a alpha drive and a 350. 1.50
 

topglock

Seaman Apprentice
Joined
Oct 29, 2011
Messages
30
Re: how to determine prop

Sounds like you may have some slipping going on, or way to little prop...
 

steelespike

Supreme Mariner
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Apr 26, 2002
Messages
19,069
Re: how to determine prop

Guessing a 17" prop 5000 rpm would produce about 47 mph not likely out of a 25 ft cruiser.
I feel its the tach reading high. Find out the prop pitch and we'll know.
 

badbowtie

Chief Petty Officer
Joined
May 18, 2007
Messages
634
Re: how to determine prop

Okay well I talked to the big prop place local that is all they do and gave them a bunch of numbers from it and they are thinking I have a 16" 11P which would explain all the take off and high reving.
 

badbowtie

Chief Petty Officer
Joined
May 18, 2007
Messages
634
Re: how to determine prop

I just spoke with michigan prop and they did say I have a 16" 11p so I have to decide to buy a 16" x 14P or a 16" x 16P
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: how to determine prop

If your able to exchange props go with the 16.
Otherwise the 14" is a better bet because you can continue to use it if it is a little light just watch the rpm.
 

badbowtie

Chief Petty Officer
Joined
May 18, 2007
Messages
634
Re: how to determine prop

Yea the place I have bought from in the past will not let me return I may call a couple other places tomorrow and see what they say. I have also bought props on ebay but not finding any good deals on 16" props. I also was wondering since the boat is so big and heavy how 4 blade props work verse 3 blade never have had one.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: how to determine prop

A 4 blade. should help improve hole shot though going to a 16 from 11 it may be slightly slower out of the hole.
But it should stay on plane well and respond better around the dock.Of course top end should be better.
 

badbowtie

Chief Petty Officer
Joined
May 18, 2007
Messages
634
Re: how to determine prop

If I go with a 4 blade I would have to go with a 15" 16p or is this going to have to much slip dropping down to a 15" or is 16p going to be to much I hate to spend 140.00 and it be a paper weight. I also found a used 15.25 x 15p three blade with new hub for right about 100.00 unless they would give me some off trading my 11p in. When I called michigan wheel they just said I should stay with a 16" for the extra to help with prop slip but is the 3/4 of inch really even make a difference.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: how to determine prop

A 4 blade has about 30% more blade area and the diameter is less than a 3 blade because of the extra blade increasing the drag.
One reason your present prop is a 16" diameter is because it is such a low pitch.If you check the prop listings you will note that generally as pitch goes down diameter goes up.
Your pitch was so low that we don't have real numbers for your base line performance.You weren't able run at wot and may not have been planing.Michigan wheel calculated what pitch you needed. You might ask them what they think of a 4 blade for your setup.
They may have some numbers to suggest.
I just noted that almost every 16" pitch is in a 16" diameter. 17" are all under 16"
15" was a mix.
that's just prop design.Usually diameter is a secondary consideration not considered except in
certain specific applications like heavy hard to push boats.
I have a feeling the 11" was on there so the po didn't have to give up the original prop.
 
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