what prop will fit my motor??????

[matt88]

I have been shopping for a new to me cheap prop on ebay and I am not sure about which props will fit. The motor is a 1973 850 merc and according to iboats it has the 15 spline shaft. Will any 15 spline merc prop fit????

 

[Bifflefan]

Re: what prop will fit my motor??????

Any 15 spline for a merc with the correct hub size should fit.

Measure from the output shaft to the bottom of the ventilation plate, double that then take an inch off that number to get the max diameter.
Thats just my opinion on the max dia. you will have to look it up for sure.

 

[chum1]

Re: what prop will fit my motor??????

13 1/4 x 17p works good on mine, a 21p is way too much guaranteed

 

[matt88]

Re: what prop will fit my motor??????

Mine has a 21p on it now but it seems like it is over propped. Will probably have to get a tach on it first to be sure. With the damaged 21p thats on it now the best I have run is 33mph but thats on a small 15ft ski boat. According to the prop speed calc my boat should run 40-45. When I get it to top speed now I can actually back off about 1/4 throttle and the boat will still maintain 33mph on the gps. When I push it to wot the boat will lunge forward a little bit but the speed wont exceed 33mph so I am blaming my badly abused prop. After the initial test run I keep it under 30 to be on the safe side and to try to prevent blowing the darn thing up. Thanks

 

[matt88]

Re: what prop will fit my motor??????

guess my next question is does anyone have a good used 21p or up merc prop??

 

[chum1]

Re: what prop will fit my motor??????

just bought another 17p prop off of ebay $67.00 with shipping, i doubt you would do any damage by going to a lower pitch, the marina guy told me to use a 15-17 pitch on my 16 1/2' bayliner capri, the 17p works great and can pull me (190lbs) up with one ski, good luck

 

[matt88]

Re: what prop will fit my motor??????

Bought a timing light with a built in tach today. So hopefully I will know exactly where I am at very soon.

 

[Bifflefan]

Re: what prop will fit my motor??????

guess my next question is does anyone have a good used 21p or up merc prop??

The 2 posts right above this both state that they have or have had a 21 and are to much. Your looking for a 21p or bigger? Its your money I guess.

 

[matt88]

Re: what prop will fit my motor??????

There is also a post above that states that my boat has a 21p on it now and it is over propped. There is also a slight possibility that I might have a different hull than the other two posters and yes it is my money.

 

[matt88]

Re: what prop will fit my motor??????

Well, I bought a prop off ebay this morning and I hope this will work on my engine. I keep reading about different size gearcases in reference to props so???????? Im not sure if this one will work or not but its mine now. This is the info. that was in the ad:

This Michigan Wheel aluminum 13-3/8"diameter X 25 pitch propeller is New In The Box.

Fit Mercury 150-300 hp outboards and I/O's; Alpha 1, Bravo 1. 15 spline rubber hub.

I know some will say that this is way too much pitch but thats not the question here. The question is will this mount on my 1973 850 merc. As for the pitch discussion,I know a guy with a similar boat and the exact same engine and he claims to get 50-55mph so with a 25 pitch and a 2.3:1 gear ration this should put me in my factory recommended rpm range for that speed. Will it work?? I wont bet the farm on it but I am sure hoping that it will. If it dont then atleast I will know what I need at that point. Its hard to gauge an engine with a slipping prop.

 

[matt88]

Re: what prop will fit my motor??????

Been doing some research and I believe my engine has a 4.25 in gearcase and the prop I have ordered is for a 4.75in. gearcase. So At this point I feel like I have bought something that will not work,but I did find a small glimmer of hope.

http://www.maxrules.com/propindex.html

According to the information on this site which has original mercury prop numbers the numbers on a 13' bronze 21p prop for a 1973 850 has the same exact number as a 13' bronze 21p on a 1974 1500. Either way this looks like a great source of info. for someone trying to find the right pitch prop.