Correct replacement prop.

[arvo11]

I have a 1989 Mercury Classic50 45hp 2stroke motor on my 24ft Tracker pontoon boat. I damaged the prop last trip out while trying to load the boat out in a high cross wind and shallow water. Gravel scratched it pretty good and wore the edges down. I would like to replace it. The number on the prop is 48 42738 A10 8P and it appears to be about 12" in diameter. This prop motor combo pushs the boat at 4500rpm wot at 12.5 gps mph on a calm day with three people on board. My question is: should I stick with the same specs or change. I don't know and haven't been able to find the correct wot rpm for this motor, but at 4500rpm the motor sounds happy although it takes a long time to get up there. Any thoughts or recommendations? Thanks.

 

[Silvertip]

Re: Correct replacement prop.

We need to know whether you have a big foot lower unit (2.34:1 gears) or the standard lower unit (1.83:1 gears). Big Foot will have Big Foot on the cowl. Big Foot motors will run a 13-1/4 to 14 inch prop at about 9 pitch. A 1.83 gear case is not an ideal unit for a pontoon. The prop is two small and would typically be a 10-1/2 diameter. 8 Pitch sounds about right but at 4500 rpm WOT something is wrong. WOT for that motor is in the 5500 rpm range so you are lugging it severely. Are you sure the tach is correct.

 

[arvo11]

Re: Correct replacement prop.

Thanks for the reply Silvertip. I just double checked the diameter of the prop and it measures 12 inchs exactly. The motor appears to have the standard lower unit, as the motor doesn't have the Big Foot logo anywhere. The only logo says "CLASSIC 50" on both sides of the lower cowling. The wrap around cowling just says MERCURY. I bought the boat with the motor already on it, so I don't know if the prop has been replaced. I guess it has if you say it should be 10 1/2 inchs in diameter. The motor starts and runs great, but doesn't have the rpm you say it should. I have ran it on a calm day with only myself in it and the best the motor turned then was 4800rpm at almost 14 gps mph. Do you think I should go to a smaller diameter prop, say a 10 1/2 or 11 inch. Or should I go to a smaller diameter prop and maybe change the pitch as well, maybe to a 7P? I have a larger motor in the works, but probably not for a few months. Btw, the tach seems ok, needle steady, no erratic bounce etc. Thanks for your advice.

 

[steelespike]

Re: Correct replacement prop.

Your rpm is low and slip checks high. A 7" might do better should get the rpm up and may get slip down.You might look for a 4 blade.

 

[arvo11]

Re: Correct replacement prop.

Thanks for your reply Steelespike. I just found the formula to figure slip and that is quite a bit. I will look for a 4 blade, 7pitch.