Question for choosing a new prop?

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?

Re: Question for choosing a new prop?

A 20 ft with a 50 hp should be in the 15 to 18 mph range, but you have to know what the moptor RPM is before messing with prop changes or you run the risk of over reving the motor and blowing it up. You can pick up a clamp on style tach for 20.oo from several sorces.

Re: Question for choosing a new prop?

I took off the prop that i have now and measure it, the diameter is 10.5" but not sure of the pitch or how to measure. I will do a tach later on when i get the time but right now I'm just looking to increase the speed 5-6 mph so I can have a cruising speed around 13-14 mph at 1/2-3/4 throttle so I won't have to go WOT and keep the fuel economy. I'm burning 6 gal. and hour at WOT, seems a little excessive fuel consumption for only 13-14 mph.

Re: Question for choosing a new prop?

6 gallons a hour is real excessive. Right off hand I would guess your way under propped and the motor rpm is way high. I will try to explain this as plain as I can. A outboard motor's RPM is controlled by the prop as much or more than the throttle control. You can not guess you have to know or you will blow the motor. Its as simple as that. To low of a pitch prop may very well be exceeding the max RPM at half throttle.

Its your choice, spend 20.00 bucks and get a clamp on tack, or blow a motor and spend 2000.00 for a motor your choice.

You should be running a 9, 11, or a 13 pitch prop on that size motor and it should be stamped into one of the blades. Max speed with light load is going to be around 18 MPH.

Depending on the motor and condition you should be burning no more than 2 to 3 gallons a hour at WOT

Re: Question for choosing a new prop?

under the nut it has 18 A, thats all i see. Not sure if that helps or not.

Re: Question for choosing a new prop?

Not so much to me, but I know very little about merc prop numbers.

Re: Question for choosing a new prop?

Found a number, a bunch of gunk over it. 48-73134-12 Will I need a tach to be precise on another size prop or can I get close enough without one?

Re: Question for choosing a new prop?

That is the merciury part number for the prop. You apparently don't realize the importance of knowing engine rpm regardless whether you need a prop or not. YOu cannot simply slap a prop on the boat and expect 5 -6 mph increase. If that were the case, why not slap a 19P prop on and see what happens. I can tell you your speed would "decrease" not "increase" Your engine has a given amount of power. It takes power to twist the prop. Too much pitch and you lug the engine and it goes "poof". Too little pitch and it overrevs and goes "poof". The ideal prop is one that allows the engine to operate at or very near the top fo the engine manufacturers recommended wot rpm band. For your engine that is probably about 5500 rpm. You are being asked what your engine rpm is with your boat and the load you normally carry. Since you have no idea what that number is you cannot compare it to what the recommended rpm is. Therefore we cannot begin to help you. For all you know you already have the best prop. You indicated you don't care what WOT speed is yet you want to inicrease speed by 5 - 6 mph. If a prop is wrong at WOT it is wrong across the entire engine rpm band. So I'll join the others and say - "get a tach and then report back.

Re: Question for choosing a new prop?

I want to increase the WOT speed by 5-6 mph, yes but only becuase I plan to cruise at 3/4 throttle for fuel economy and would like to have a little higher cruising speed. I know I need to put on a tach and I plan on doing that before I purchase a prop of any kind. Reading some of these other threads I have found out how important a tach is. I will report back after the install and hopefully you can help me with the correct prop, Thanks.

Re: Question for choosing a new prop?

On my orginal merc prop you have to take the prop off the motor, there will be the size and pitch stamped on the inside rim of the prop facing the motor

Re: Question for choosing a new prop?

From another site this is what prop you have now:

"I have for sale an aluminum mercury prop,the prop it self is in good shape It is just the paint that is rough,there are no dings on the prop.The prop is a 10.25 Dia.x 12 pitch,13 spline.It will fit 40-70 hp mercs or mariners. The # stamped on the hub is 48-73134-12 I am asking $35.00 for it."