1975 50 HP Evinrude - Matching Prop To lower Unit

[JerEazy]

So i recently finished refurbishing a 1975 Evinrude 50 (13 Spline) - on a 16’ Cobia Bowrider of the same year. the gearbox keeps me limited on prop selection.

anyway - what came with it was an OMc 14/13 that hit 4900 RPM at WOT and 22 on GPS.

i decided to go 4 blade and drop 2-3 pitches and ended up with a Michigan Wheel 11.5x10. Figured the extra blade and the pitch difference should put me at 5500 ish. Which it did! But - where it’s weird is that the prop doesn’t want to “bite”. Then engine revs and forward motion begins. But very minimal speed increase until I get bow rise and you can literally feel the prop grab and begin pushing. But at WOT I’m getting 18.5 on GPS and it won’t stay on plan unless I keep it pegged.

Ive never dealt with a slip this severe and I can only assume it has to do with the decreased diameter or the dol-pHin installed on the LU.

so - step one is I’m going to take the original prop and have it refurbed and pitched down to 11.

but should I remove the fin? Reduce the drag? Maybe go to smart tabs?

interested in responses.

Thank you

 

[GA_Boater]

Remove what fin? The skeg?

 

[ahicks]

A 14/13 prop would be 14" in diameter, with 13" of pitch. Me thinks that might be a mistake. Diameter sounds way too big.

The fin would be gone immediately on any boat I own. They're famous for breaking off large chunks of cavitation plate.

If there's room to drop the engine below where it is now, that would happen, right after I got done removing the fin.

 

[jimmbo]

First of all, you are certain it is a 1975? 1975 was the first year for the small gearcase which used props with diameters 11 1/4" up to 13". From 1971 thru 1974 the 50 used the same props as the V4s 12 3/4" up to 14"
My OMC prop charts show that a 1975 50 should be able to use a 12 1/2 X 13 on a 16 ft. If it was an earlier 71 - 74, a 14"X 13" would be the choice.

Your engines WOT RPM ranage is 5000 - 6000.
Gear Ratio 1971 - 74 12:32; 1975 + 12:29

If you have a pre 1975 engine, and put a prop for a 1975 on it, the prop hub is smaller and lots of Exhaust gas will exit over the blades. That could be why you say you are getting lots of slippage until the boat gets onto plane, though I hardly call 18 mph planing.




As for the 'Whale Tail/Doel Fin" on the Anti-Ventilation Plate. Those things belong on the bottom of the lake/river/ocean.

 

[JerEazy]

The engine is a 50573B - so yes I can confirm it’s a 75 engine. Can I be positive it’s the original LU? No.

im not saying it’s the original prop - and I’m fairly certain how to read prop numbers. What I took off was a 14/13 that was in need of re-work. Being I wasn’t getting proper WOT RPM of at least 5000 I decided to re-prop with a 4 blade to lift the butt.

Yah - I wasn’t happy with 18 either. I’ll remove the fin tomorrow. Fill the holes or don’t bother?

anyway to confirm the LU specs? Measurement of the gear case?

the 11.5x10 fit perfectly. And got RPM within the sweet spot of spec. Just very unhappy with the performance.

 

[JerEazy]

reconfirmed this AM - 14x13 stamped on the old prop along with #391668 (which does come up as 14x13)

BUT when searching the Solas prop finder that lists as a 13 tooth spline Large Gearcase - and at Propshop inc has it as the 4-1/4 gear case (50 hp 2 cyl 1971-74 and 1977-79) compared to the 50 hp 2cyl from 1975 & newer which is listed as 3-3/8"

So maybe the LU has been swapped? Where would that measurement take place? diameter of the opening of the LU?

Fun stuff having the Frankenstein engine!

 

[jimmbo]

Model # plate could be from a 75, but you have something older. Does the Serial # on the plate match the Serial # thats on the metal disk(looks like an inverted frost plug) that's on the engine block?

Since the 14 X 13 fits, it appears you have the larger gear case and the props for the smaller case, while they fit the propshaft, have a smaller dia hub, which allows the exhaust to flow over the blades.

Grab a measuring device and measure the various diameters involved. Prop Hubs, gearcase opening

 

[JerEazy]

model # does match - i've done some work on the engine, and although i'm sure they interchange, the engine is a '75 model year by everything i can see.

But based on reviews of parts diagrams, and that I don't have any anodes on the LU like you find in the later models, if the gear case is the larger size - i think it would have to be a '71-74 - from what i see, after 75 they went to the smaller gear case on the 50hp (other LUs may match up that have the larger gear box - but my water pump and housing look and work like the 1975 50 HP)

i'm going to move this over to the J&E repair forum and see if i can get some feedback -

thank you.

 

[JerEazy]

Previous Owner Swapped Lower Unit?

i've got a 50573B (1975 50 HP) that i've been working on recently. Came with a 14x13 prop and pegged out at 22mph @ 4900 RPMs WOT after engine work, plugs, water pump, carb rebuilds etc. Everything worked well - but the prop was in bad condition and i need a few more RPMS to get into the happy zone so i figured i would do new. I wasn't too upset at the speed all in call. Its mostly a river and lake cruiser.

So i ended up with a 11.5x10 4 blade and hub kit. (i wanted to 4 blade for slow speed planing, acceleration, bite and more stern lift - plus i knew i wasn't getting much top speed out of a 50hp engine so might as well get the better ride) I was surprised the diameter was so much lower - but all the props that fit based on the model number were a bit smaller. So i really didn't think much of it. Installed just fine, no issues. Looked nice (but a bit small). Took it out for a test run, and obviously a bit weird.

I got to the correct RPMs, but the prop was slipping so bad you could actually feel it "bite" at about 10mph (besides that it was just engine sound and revolution.) and i topped out at 18 mph at WOT while occasionally feeling the prop grab again and lunge a little forward before letting go again. It was awkward to say the least.

I posted something on the prop forums (link below) thinking maybe it was the "dol-fin" installed onto the LU giving the prop some issues. Or maybe it was bad diameter - it was my first experience with this.

What was passed onto me there was that the 14x13 prop was standard on the larger gear case models (4-1/4 i believe). Not the smaller model of 75 and on (3-3/8?)

So can i please confirm that where i should measure is the exhaust opening? where the prop-shaft comes out. Inside diameter? outside diameter? And is there any other way to figure out what LU i have? I don't have any anodes on it. So i assume its an older molder. But there's so many different models, its tough to determine - and outside of just having the old prop re-worked, i would like to know what options are available for me.

Thanks in advance.

https://forums.iboats.com/forum/gen...opics/10731379-remove-fin-1975-50-hp-evinrude

 

[GA_Boater]

Combined 2 threads and moved the whole thing here into the J/E section.

If you want help, please keep questions in one thread. You're asking the same question(s) in two places and that never works to get consistent advice.

Thanks.

 

[JerEazy]

Thanks GA

The idea behind the new thread was it didn't feel like a prop thread anymore, more as a repair/service issue. But having it all in one place should help a lot.

Thanks again for your help.

 

[JerEazy]

Well let’s keep it interesting. LU diameter at the prop shaft measures 3.4 inches - assuming I’m off a little in where the measurement comes in, it’s the 75 lower unit.

i put the thrust washer back on of the 14x13 and it actually goes over the housing. Put the new 4 blade on and it actually enters the housing. Seems like it’s right.

engine is mounted as low as it can go and cav plate is about level with the bottom of the keel when trimmed down.

Open for thoughts.

PS - I took the fin/whale tail off

 

[JerEazy]

Last edit didn’t take. So update post:

Nice to have a fish camp near the house.

took for a test drive without the fin and the 4 blade installed. Smoother acceleration up without the “lunge” forward when the prop grabbed. Topped at 20mph on GPS @5600 with just me. Wife and daughter were with me yesterday at a combined 180lbs.

So so looks like I’ll just need to pitch up which means back to the 3-blade because the only higher pitch 4 blade is stainless - no need for that.

will update after next run. But I don’t think there’s any questions left on this thread.

sorry for a busy one!