Please help...1969 Mercury 650 65hp outboard prop question

[ElkLakePat]

Once again I am new so please forgive me for such a simple question but my prop is all banged up on my 1969 Mercury 650 65hp outboard engine and I want to replace it. It was like that when I purchased the boat. I have what I think is the mercury prop number of the bad prop and the numbers are 48-37698-13 ..yet I cant find that anywhere on the net. Maybe I have the number wrong? Id like to know how what the numbers mean and what I can replace it with. Any help would be greatly appreciated. Thanks in advanced.

 

[emckelvy]

Re: Please help...1969 Mercury 650 65hp outboard prop question

The "13" at the end of that number indicates the prop pitch, 13 inches in this case. About the right size for a 65hp to carry, and if the motor has been running well with this pitch that's what the replacement prop should be, too.

A typical size for this would be 13 X 13, the first # being the diameter of the prop, and the second # the pitch. Or, you might find a 13-1/4 x 13, even 13-1/2 x 13. If I recall, the largest diameter your gearcase will take is a 14".

In all of the above cases the 2nd # indicates the pitch, with the diameter varying a bit.

I don't expect that this is being used in a high-speed application, so any of those diameters would work well. The motor will "spin up" a bit faster with a smaller diameter prop, and I expect you'll find that the "13 x 13" is a pretty common size. The larger-diameter props would be suitable for pushing heavy loads and will actually be more efficient at doing so than a smaller-diameter prop.

Note that many of the newer replacement props have integral thrust hubs. Your propeller will likely have a removable hub that fits up against the beveled portion of the prop shaft, and also fits into a 'shoulder' in the prop. You may even have a removable thrust hub on the prop nut side of the old prop.

New props will most likely have a built-in thrust hub on the prop nut side. And the hub on the gearcase side of the prop will be built-in, with a flat surface.

You'll need a flat thrust washer which will bear against that beveled edge on the propshaft, and sit flat on the propeller's integral hub. These are commonly available and whoever you're buying a prop from should have one.

Hope that all makes sense!

You should even be able to look up propellers in the retail section of this site.

Here's a likely match I found at the Tacoma Propeller site after a Google search:

http://www.tacomapropeller.com/6433...-AL-Match-Propeller-Hub-kit-not-required.html

HTH and let us know how that works out for you.............ed

 

[old_mfg]

Re: Please help...1969 Mercury 650 65hp outboard prop question

Ed Mc, I used to see your name in the Trooper pages! Still got that thing?

ElkLake Pat,
Looking at my service manual, I'm thinking your original prop number was 48-37898 which was a 13x13 3 blade Al prop, typical as Ed mentioned.

I have an old 650 Merc myself and ran through this same mess the last few weeks in adapting to a new prop. Several visits to different shops (not many folks around now that a familiar with a 40 yr old motor!) figured out that I needed two parts to adapt a new prop. The Michigan Wheel I got specified a brass thrust washer by Quicksilver p/n 13191Q01 (was 13191A01). But this works for engines having a tapered prop shaft. Mine is square. Sounds like yours is too. If so, you also need a simple spacer, Quicksilver 23-86918T which was $40 for the silly thing. I'd give you the dims to make it, but its on the boat now (can take it off and measure it up if you need). The spacer goes on the prop shaft, then the thrust washer, then prop. Your "cone" shaped spacer ahead of the old prop is now obsolete and you also don't use the splined washer after the prop since the new prop has this feature built in.

My Merc is a '66 , 4-cyl engine. I have a 15-tooth spline, there are 13 tooth splines whether this changes things or not. I had an original 12x17 as 48-37902A2 and only turned 3700 RPM and about 25 mph. I probably need a 13 pitch to get better near my 4800-5200 WOT. I could not find a new 12" DIA prop spec'd for this 65 horse engine, so went with a 13.75x15 and wish I hadn't after running it the first time today - it takes all my throttle to get the boat on plane and to stay there at 23 mph. So interestingly my speed is only a bit slower with this big wheel, but no performance and about 3200 RPM now. So another $150 and maybe I'll try Michigan Wheel's 14x13 unless I can find a smaller DIA prop that fits with the smaller pitch I need.

I also understand I may need to try decarbing the engine, which a carboned-up engine can supposedly rob a bit of power and rpms.

Good luck.
Bryan

 

[emckelvy]

Re: Please help...1969 Mercury 650 65hp outboard prop question

Bryan, that's a ridiculous price for that spacer, ain't it! A simple piece of sufficient-strength pipe sized to fit snugly on the shaft would work as well. Your 65HP has the Very-Old square-style shaft shoulder, but around '67 or so Merc changed that to the tapered shaft.

The smaller motors definitely don't like to carry a large-diameter prop at speed, only good for low-pitch props pushing heavy loads. The smaller diameter lets the prop rev up a bit faster.

If you find a prop that gives you a good top end but is a bit slow out of the hole, you can add 'ventilation holes' to the prop body. This will allow exhaust gasses to spill over the prop blades at takeoff, providing a 'controlled ventilation' which lets the motor spin up to rpm faster. Works great on a ski boat.

You definitely don't want to overprop these motors, they have to spin up to the higher-side of their rpm-range in order to give good performance. I'd expect a 13X13 would be just about ideal for your motor.

Check the FAQ at the Repair and Maintenance forums' for decarbonizing info. Merc rings definitely don't like to be carboned-up and this could be deleterious to your motor's performance.

And yeah, I still have my trusty '90 Troop 3.4 LS, a great rig and they'll probably bury me in it!

Cheers...........ed

 

[old_mfg]

Re: Please help...1969 Mercury 650 65hp outboard prop question

Thanks for the info Ed. I believe I will try a decarb treatment to the engine and see what new life that brings to the performance with my old prop as a reference.

Further exploring my prop options, do you know whether the prop shaft of a 650 Merc will fit a prop from a smaller horse engine? I looked hard for props and do not see any in the original spec 12" diameter range that this motor had (12x17 was on it). Why is that?

Sort of already mentioned earlier, when I wanted to drop back to a 15 pitch, I ended up with a larger dia in doing so, and bought a 13.75x15. It is significantly worse on hole shot than the old 12x17 prop I have. Top end is almost as fast, but definitely lugging (3200 rpm versus 3700, either way I should be at 5000).

 

[hwsiii]

Re: Please help...1969 Mercury 650 65hp outboard prop question

Old, if you will start a new thread I will be glad to try and help you.



H