1971 pin drive with a spun hub?

1965seaaira

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Joined
Aug 10, 2020
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6
Hey all, brand new boat owner here. Got a 1971 40hp Johnson 40EL71B that i got revving up great, but once i hit half throttle and above, it almost feels like it's braking and revving/slipping. From what i've read everywhere that's a spun hub- but this prop seems to be a pin drive (or shear pin). Is there actually a hub in here too? Looks too tight for that- but maybe it's more on the other side.

Also, if that's not the cause, do you suspect that my anit-ventilation plate and prop is too high for the boat which is causing the slippage/ventilation?

Thanks!
 

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Texasmark

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Dec 20, 2005
Messages
14,544
Agree. Those lower units, looking at pictures in sales catalogs from engine mfgrs. of the day, show the AV plate an inch or so below the line of the bottom of the boat....engine like you have it, yardstick extended from the bottom and crossing the LU, above the AV plate, not below it as would happen in your case. Question is, how is it that it's as high as it is? Sitting on a shim...piece of added wood between the clamp bracket and transom? If you had a fast boat with rated max HP and were running a SS performance prop, then that height could/would work.

On the rubber slip hub, check closely the material where the "drive pin" is inserted (we called them shear pins too because they sheared at the worst time....like windward side of Rip Rap...where the fish feed.....when you need to move away). You probably will find the part where the pin mounts to be brass and we know the prop material is aluminum. That's your clue. The two materials are mated with the rubber, shock absorbing hub.
 

1965seaaira

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Aug 10, 2020
Messages
6
Thanks guys. Yeah got a new prop on today to check the hub. Still not planing out so gonna try some manual trim adjustments. In response to shaft length and mounting question the motor is actually a long shaft with nothing on the transom to elevate it higher. Perhaps I need to lower the transom mount a few inches since there is a false horizontal shelf (With drain holes) 6 inches down from the top of transom.
 

1965seaaira

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Joined
Aug 10, 2020
Messages
6
I should say tho that the new prop helped a lot. Just think it can do better still. Here is a pic of the transom I’m considering cutting into to lower the long shaft motor mount. Other option would be a trim adapter thing but previous owner got a janky steering setup. Trying not to redo all of that right now but maybe I have to.
 

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Texasmark

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Looks like a 15" mid section on a 20" transom. With the splash well where it is, cutting the transom down will cause you 3 problems: It may be part of bracing for the transom to support the weight of the engine and added prop thrust torque. The splash well is designed to catch water over the transom caused by dumping the throttle too fast (usually) with a heavy load in the stern (rear of the boat) or following seas coming over the transom.

Cutting the transom down will mean cutting into the splash well. You will have more vulnerability to over the transom water and your splash well will be of little if any use. Additionally, when you tilt the engine up, and you will have to do that from time to time if it doesn't do it for you....hitting a submerged object, the engine/cables will jam against what's left and become damaged.

Lots of engines of that vintage around.....might just find yourself a 20" replacement.....would be your best investment. Swapping the midsection/LU is not that big of a deal I'd think.....drive shaft and shift shaft uncoupling and stabbing, plus mounting bolts to powerhead are all you need to do if you get a LU with a clamp bracket attached......seller may even swap you for a small retainer.
 
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