Question on which prop?

[jroberts650]

I have just purchased a 20' sweetwater pontoon with a 1987 mercury classic 50 (45 hp) outboard mounted to it. I have no tach installed on the boat but GPS says that i'm running 8-10 mph at half throttle and about 13-14 mph WOT with 4 ppl onboard. Prop looks a little rough but still has smooth edges on the blades. I cant read any numbers on the prop itself and dont really know how to determine the pitch or diameter. I would like to get about 13-14 mph at half or 3/4 throttle and 20 WOT. I like to stay around 1/2 to 3/4 to get the best fuel economy and dont really care about the WOT speed. Can anyone give me some suggestions on the correct size and pitch I should use?

 

[jroberts650]

Re: Question on which prop?

BTW, the prop I have now is 10.5" diameter, not sure what the pitch is or how to measure pitch. Just looking for 13-14 mph at 1/2-3/4 throttle for the best economy. I'm using 6 gal and hour at WOT and it seems a little excessive to me for only 14 mph.

 

[steelespike]

Re: Question on which prop?

Speed doesn't mean much without rpm.If we're going to do anything but guess we'll need a tach.
Your prop numbers should be under the prop nut. Maybe you can borrow a shop tach.
We'll need your prop size, wot rpm and speed by gps.
Your 20 mph top speed may be possible with a light load.with 4 and gear a good question.??
Also without a tach there is no way to tell how our selection is working.
If the motor is forced to run below or above its rated rpm by the prop it could mean an early exit.
Your motor will use about 5 gallons per hour in "perfect tune" whether its pushing a houseboat or a runabout.
You want to be sure the motor is tuned right and using 87 octane regular gas. see if you can find the prop size.
If I had to guess a 10" pitch is a good starting point.

 

[jroberts650]

Re: Question on which prop?

Under the nut it has 18 A, thats all it says so not sure if that helps.

 

[jroberts650]

Re: Question on which prop?

Number on prop is 48-73134-12