Replacing prop on 75 horse Mariner

[starkyb]

I am looking to replace the prop on my 17 foot aluminum Tracker V-bottom. Right now I am running a 13 1/2 X19 stainless prop. at WOT I am running at about 5500 rpm at 35 mph according to the gps. The recommended WOT range is 4750-5250 which. If I go to a 23 pitch would I get enough horsepower out of my motor and would my fuel economy improve?

 

[steelespike]

Re: Replacing prop on 75 horse Mariner

I think I would verify the tach before any changes.Guessing 2.33 gear ratio your slip is about 17% at 35 mph
Seems like a ss prop would do better.
Going to a 23 would knock about 800 off your rpm and kill your performance.I think it would also kill your mpg
because the motor would be struggling so.
A 75 will use "about" 7.5 gph at wot regardless if it a runabout or a house boat. The motor needs to rev freely within its rpm range.
If it turns out your actually at 5500 a 21" would put you at "about" 5100, a comfortable number
that may add some speed with a minimum loss of power.

 

[starkyb]

Re: Replacing prop on 75 horse Mariner

Ok. I was told by a prop salesman that for every inch of pitch you go up than you reduce the diameter by 1/4 of an inch. Is this correct?

 

[steelespike]

Re: Replacing prop on 75 horse Mariner

Ok. I was told by a prop salesman that for every inch of pitch you go up than you reduce the diameter by 1/4 of an inch. Is this correct?

Speaking very generally as pitch goes up diameter goes down. Not a hard and fast rule. Diameter has little affect on rpm.
If going g from a 3 blade to a 4 blade diameter will be less.One inch pitch up or down will net "about" 150-200 rpm change
So a 4" increase in pitch could net about a 800 rpm reduction.