Prop advise

[buckinrut25]

I recently got a used boat for for fishing local lakes. The issue i'm having is the prop that is on the boat is damaged beyond use. I think the one that is on it is a little low on the pitch. I do not want to order one to find out I need to replace it as soon as I get WOT RPM and Speed. I am hoping to get advise or a good educated guess. my main concern is mid range power and fuel economy. 1976 mercury 500 50hp currently running a broken 48-50244-11 on a 76 skeeter metro k3 15 ft Tri haul with a 56" beam weiging in at 1700lb fully loaded including fuel. The best I can tell the prop is 13 spline 3/4 shaft 10.5". Thanks for any help.

 

[buckinrut25]

Just an update. After doing a little research I found that the idea RPM range is 5000-5500 and the gear ratio is 183. I'm not sure if this helps but I figured it could not hurt. Any advise is appreciated and thanks for looking.

 

[steelespike]

Your number ending in 11" is 11" pitch.thats a good guess.
When guessing on prop selection I like to go light.That way if it's not right you will still have strong performance and just need to watch the rpm.
If your selection is too high you may have poor performance,may lug and you shouldn't use it excessively.
I'm thinking the 11" is right.

 

[buckinrut25]

My local boat guy has a used 12 pitch that he is going to let me use to get some real world numbers so i'm not guessing. I'll check back in when I have some usable numbers to go off of and hopefull get the best prop for it. Thanks for the help.

 

[buckinrut25]

Ok, I went out last night lightly loaded full of fuel. I was about 300 lb lighter than what the normal load will be. With the borrowed 12p AL prop I got 31 MPH average over 4 runs and peaked at 5800. Speed was GPS and RPM was from a TinyTach. That is about 300 RPM over my max and 500 over my target RPM. The boat had a good hole shot and planed quickly around 4 seconds. I would like reccomendations on a good prop for this. I run mostly in calm lakes and rivers and I do run in water as shallow as 2ft. The guy I borowed the prop from said the 12p Spitfire might be a good fit. What do you think? Thank you for any help.

 

[steelespike]

31 at 5800 12" is about 13% slip a figure I usually use when I have to guess slip.
The Spitfire is said to deliver 3 blade speed with four blade performance and no loss of rpm.
​ So it seems you need to increase pitch to get your rpm target.
The spitfire should help retain your hole shot.