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View Full Version : Why Don't Inboards Have Shear Pins??


Nvrgvup
09-06-2011, 12:28 PM
Wondering why inboards don't have some form of a overload clutch or shear pin to protect the boat and drive-line if you hit something. Maybe the torque and HP produced under normal operation would make these devices ineffective.

I have seen some of the horrible picture around here of busted up boats from hitting stuff and it scares me.

I have been considering replacing the metal key in my prop or hub with some kind of a plastic one to maybe provide some form of protection. However I am unsure if it would just break under load or not be any different than the current metal key if something was struck.

FourFourty
09-06-2011, 01:44 PM
Plastic key would not hold up. A brass one might work but you are only going to change the end condition of the damaged prop. You would end up with a destroyed prop with the normal key, or a slightly less destroyed prop with a softer key.....but, your prop would still be destroyed and the rest of your running gear would still get damaged the exact same.

phickman
09-06-2011, 01:48 PM
I have always been told that that is why they have brass props so it will bend and it can be re-machined. I have heard that if you have a stainless prop, it won't bend as easily and makes the transmission the weak point. I have only ran brass props and have never had any trouble. I am sure there is someone who knows if what I stated above is true about the stainless props and the transmission. I would like to know if that is true myself. I welcome the discussion.

Paul

JimN
09-06-2011, 02:13 PM
Wondering why inboards don't have some form of a overload clutch or shear pin to protect the boat and drive-line if you hit something. Maybe the torque and HP produced under normal operation would make these devices ineffective.

I have seen some of the horrible picture around here of busted up boats from hitting stuff and it scares me.

I have been considering replacing the metal key in my prop or hub with some kind of a plastic one to maybe provide some form of protection. However I am unsure if it would just break under load or not be any different than the current metal key if something was struck.

An outboard or I/O has several changes in direction for the drive shaft and the distances are more conducive to using a shear pin. On a direct drive, you can't put it on the drive shaft because the end with the prop will whip around and kill things. The transmission isn't a good option, either- it's an established design and it's not practical to re-design it/them. A plastic key could be used but it won't stand up to the shear forces with hard acceleration and minor hits. I could almost see trying a drilled-out metal key, though. The bad part about that is, it's hard enough to remove a prop that has been left on for a long time and after a key shears, it's a whole lot harder to do.

rd1900
09-06-2011, 02:44 PM
The key in the prop/shaft interface doesn't carry any real load. It's there for installation purposes, but all the force from the shaft to the prop is carried through the press fit between the prop and shaft. If it carried any load, it would be beat to death by the torsional forces in the shaft from the engine power pulses.

Also, our props are not brass. They are bronze, nickel and aluminum alloy, cheaper and easier to machine than stainless.

FourFourty
09-07-2011, 09:18 AM
Also, our props are not brass. They are bronze, nickel and aluminum alloy, cheaper and easier to machine than stainless.


You are kinda splitting hairs there....... Our props contain copper and zinc, so they contain brass..... They also contain tin, which means they contain bronze. They also contain aluminum, which means that you could say that they contain Nordic Gold (which is considered a brass alloy of Copper, zinc, aluminum, and tin). Nobody is really wrong about what they call them.

JimN
09-07-2011, 11:02 AM
You are kinda splitting hairs there....... Our props contain copper and zinc, so they contain brass..... They also contain tin, which means they contain bronze. They also contain aluminum, which means that you could say that they contain Nordic Gold (which is considered a brass alloy of Copper, zinc, aluminum, and tin). Nobody is really wrong about what they call them.

I'm not sure about the tin content but the material is called NiBrAl or Nibral and it's pretty easy to see what's in the mix.

Hoosier Bob
09-07-2011, 11:06 AM
Jet drive!:D

JimN
09-07-2011, 11:09 AM
The key in the prop/shaft interface doesn't carry any real load. It's there for installation purposes, but all the force from the shaft to the prop is carried through the press fit between the prop and shaft. If it carried any load, it would be beat to death by the torsional forces in the shaft from the engine power pulses.

Also, our props are not brass. They are bronze, nickel and aluminum alloy, cheaper and easier to machine than stainless.

Installing a prop would be easy enough without the key- a key is used to prevent turning and if the nut were to loosen at all, the prop would just sit there while the shaft turns, so it's definitely subjected to some serious loads. Several boats with a brass key came through the shops where I worked and the prop was a complete biatch to remove because of the distortion and slight rotation from hitting something small. Also, just because the shaft is stainless doesn't mean it can't be damaged when the prop hits something if the key is brass or bronze.

JimN
09-07-2011, 11:10 AM
Jet drive!:D

If I boated on a river, that's probably what I would have.

Hoosier Bob
09-07-2011, 11:27 AM
Dual drives should be able to keep the spray down. Add some fins and stabillity would be decent. I wonder if the pull would be soft though?:confused:

oldairboater
09-08-2011, 12:10 PM
Jet drives aren't as efficient in their design. Dabbled in jet boats with friends for awhile.

JimN
09-08-2011, 09:29 PM
Jet drives aren't as efficient in their design. Dabbled in jet boats with friends for awhile.

But they don't have a prop to destroy if the water has a lot of lumber floating in it.