Prop replacement on 75 horse Mariner

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starkyb

Petty Officer 1st Class
Joined
Apr 21, 2007
Messages
217
My Tracker currently runs at 35 mph at 5500 rpms. According to the service manual my recomended wot rpms should be 4800-5250 rpms. The current prop is a stainless 17" prop. Should I move up to a 19 or 21 inch prop to bring my rpms down. I know that in doing so I would lose some hole shot but I would sacrifice that to get a higher top end. What I am wondering is would I gain top end speed even though my rpms would drop with the prop change?
 
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steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Prop replacement on 75 horse Mariner

Is your speed by gps?Some cells have a gps app.
Your tach could be reading high.
A standard 17" prop does calculate to almost exactly 35 @ 5500 rpm.
But your ss prop probably has some cup and calculates like a 18" pitch, this produces about 5200 rpm.
 
Bifflefan

Bifflefan

Commander
Joined
May 27, 2009
Messages
2,933
Re: Prop replacement on 75 horse Mariner

starkyb said:
My Tracker currently runs at 35 mph at 5500 rpms. According to the service manual my recomended wot rpms should be 4800-5250 rpms.
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That handle in your right hand, if you pull it back just a bit you can get into the 48-52 range. There is no rule that you must slam it to the dash. If you had 2 people on the boat when you tested it, then put 4 people on the boat you will not get the same results. with 4 you probably would have to run wide open.

Should I move up to a 19 or 21 inch prop to bring my rpms down.
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No. Use less throttle when you have a lighter load and more when you have a heavy load. If you go to more pitch, when you have a heavy load the boat may mot even plane out.

What I am wondering is would I gain top end speed
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Most likely not. You may even lose some top speed.
 
Texasmark

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,558
Re: Prop replacement on 75 horse Mariner

Those numbers don't bother me. Have been running Mercs over the upper limit at least since '89. Below the lower limit especially on a 2 cycle engine is quite another story. If you like the performance of your rig as it stands, I'd leave it be.

Mark
 
jestor68

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: Prop replacement on 75 horse Mariner

starkyb said:
My Tracker currently runs at 35 mph at 5500 rpms. What I am wondering is would I gain top end speed even though my rpms would drop with the prop change?
Click to expand...

The simple answer is yes; you will gain a little speed, even though the WOT rpm is less.

How much you will gain varies with the application. You must compare the anticipated gain with the expense of another SS prop to determine if it's worth it TO YOU to change.
 
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