Re: 1971 50 hp evinrude prop
thanks guys for the info. Let me give you all a little more details. My boat is a 1976 tidecraft fiberglass 15' bass rig. It's pretty light. I just bought this 1971 50 hp evinrude and it didn't come with a prop. That's my problem. I will be mainly just using the boat to fish. Probably won't be any pulling toys. I'm thinking any thing above 15 pitch might be too much for the motor but I don't know. Any suggestions would be appreciated.
I just pulled up the BAM prop slip calculator and ran a couple of numbers after I went over to the old-omc.de site and looked up the gear ratios and props for the 50 and 60 hp engines of that day.
The 50 runs 5500 +/- 500 rpm for the recommended WOT rpms. The gearbox is a 2.67:1 meaning the prop turns slower than the prop on the 60 for a given rpm. The no prop slip top speed is 29 (impossible, but a number for calculating) and assuming a reasonable 15% slip you can expect 25 mph for a number. 25 is a good clip and offers a smooth ride.....something those hulls weren't known for when later on they ran 70 + engines on them. BTDT
The 60 (which I know about) ran at 5000 +/- 500 with a faster turning prop with a 2.42 gearbox. The calculator gave exactly the same numbers for speed.
As I recall, back in those days, bass boats were narrow and long with pretty flat transom bottom areas and pretty much vertical sides. That means they would plane out easily. The increased cubes in the 60 just mean you have more hp to get you up and away faster and do a better job with more load on the boat. I still think that a gut feel for the right prop is as stated.
Props listed for both engines are A to Z but there is an OEM 13 1/2 x 15P available if I read the numbers correctly. Just scrolled to the top of the page, pulled up the iboats prop shopper, plugged in your numbers and came back with a dozen props to fit that engine.
The 50 has fewer cubes than the 60 but runs a slower prop at a higher rpm. To me that says that you should do just fine with the prop mentioned.
First prop listed is the Solas Amita 2411-135-15 at under $100 and has the rubber hub; meaning you don't have to fool with a prob hub kit that some of the newer designs call for. However, you said the engine came without a prop so the next question is: Do you have the prop mounting hardware? If not, on that sheet it shows the thrust washer and the prop nut kit. You will need both of these. If you have any socket wrenches that go up to 1" and a little over, you can install the nut without having to buy the wrench too.
The thrust washer inner diameter may be (probably is) tapered. If so, position the taper so that it fits the taper on the prop shaft prior to installing the prop. If you get it backwards the nut won't go on adequately. The nut kit takes care of the rear and torque would be in the order of 50 ft-lbs. If you don't have a torque wrench, put a 2x4 between the prop and the AV plate and with your hand about 12" from the nut pull her down moderately tight. How tight is that? More than snug and not gut wrenching tight either. Idea is to keep the prop from moving on the shaft.
Don't forget to grease the splines on the prop shaft prior to installing the prop. You'll thank your self later. A good grade of GL2 or Lubriplate brand white grease will do the trick. If in salt water, re application on an annual basis is recommended and the Lubriplate white was made for nasty salt water marine environments.
HTH,
Mark
