prop advice needed

cuzjuice

Cadet
Joined
Mar 28, 2013
Messages
10
I'm looking for an aftermarket prop. Currently running stock 19 pitch black max. Mercury 115 4s wot 6100@49-50 mph. Looking for a ss prop preferably a mercury marine. Any suggestions
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: prop advice needed

Assuming a 2.33 gear ratio and your rpm is correct you can't go 50 mph. Assuming 10% slip the speed would be closer to 42 mph.
A think your speedometer isn't accurate. A gps is better for speed.Some cells have a gps app.
The prop gurus may be along with more help.In the meantime read the stickys at the top of the prop forum and get us as much info as possible.Off the top of my head a 19" ss prop would be a starting point.
 

cuzjuice

Cadet
Joined
Mar 28, 2013
Messages
10
Re: prop advice needed

The speed is from a gps. How can you determine max speed from gear ratio and rpm? Just curious. Also the boat is a Stratos 1760 dv 17'6"x93 1/2" app. Weight 1500lbs.
 
Last edited:

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: prop advice needed

I use a prop calculator found on the net. There are 5 variables rpm,gear ratio,prop pitch, slip and speed. You need 4 of the 5 variables to figure the 5th.
Unfortunately slip is the tricky one.Theoretically your prop moves 19" forward in one revolution. Slip is measures the actual distance your prop moves forward in a liquid.
You can figure slip using the other 4 variables,but you have to guess slip to figure one of the other variables
A typical boat/motor combination will produce 4- 12% slip could be more or slightly less.
Your numbers guessing 10 percent slip produce negative slip,not possible.
A gps is usually accurate, Your slip could be lower but not enough to make 50.Perhaps the tach is low.
The gear ratio is from the merc site for a 115 4 stroke.
If I use 6400 rpm and 5% slip speed is only about 47.
Are you perhaps running in a strong current?
 

cuzjuice

Cadet
Joined
Mar 28, 2013
Messages
10
Re: prop advice needed

No current played a part. Regardless I am more than happy with how the boat gets around. My main concern is what will happen if I change to a stainless prop.
 
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