Long shaft OB on a 10’ Zodiac

[Rol1964]

I have an opportunity to pick up a Zodiac 310 Cadet. I have a spare OB motor but it’s a long shaft.
What are the issues (if any) of running this long shaft on the dinghy? Thx.

[wholybee]

Porpoising , not getting on the plane.

[Jammer]

And structural damage to the transom (because the torsional loads are higher)

[thomm225]

It should work fine as long as you go slow and don't try to get up on plane....

[wingssail]

Originally Posted by Rol1964

I have an opportunity to pick up a Zodiac 310 Cadet. I have a spare OB motor but it’s a long shaft.
What are the issues (if any) of running this long shaft on the dinghy? Thx.

I personally think that you could use a long shaft on the cadet 310 however there will be some issues you would need to address:

• Engine torque bending the boat due to the propeller being farther down from the transom. The engine's lower unit acts a lever and the longer the lever the more bending force is applied to the transom. A partial solution to this is to raise the motor up on the transom a couple of inches. Use a piece of wood as a spacer. I'd recommend through bolting the motor bracket to the transom since shifting the motor up reduces the strength of the clamp.
  
• Lack of rigidity of the Zodiac Cadet. A tie in to the first point is that the Cadet is not a RIB (rigid Inflatable boat) and the back of the boat including the transom could tend to bend causing the thrust to be directed in a bad way. This may make planing more difficult.
  
• Cavitation. Since the cavitation plate on the motor will be below the level of the bottom of the hull it may allow more air to get down to the prop and cavitation may occur. On the other hand, a deeper propeller will be less likely to cavitate.
  
• Grounding. The longer shaft will touch bottom sooner so when you are in shallow water or approaching a beach you might be more likely to hit the ground with the propeller

Basically a long shaft motor will work better on a longer dingy, and one with a rigid bottom, but even then some of these issues will persist.

NOTE: many long shaft motors can be shortened by undoing a few bolts and changing a few parts in the lower unit. A motor mechanic could accomplish this.

[Pete7]

What is the engine?

That is quite a big dinghy for a 26ft yacht. Are you planning to keep it fully inflated at all times? About the best we can do is 2.7m, any more and we can't inflate it on the foredeck as the baby stay gets in the way.

Pete

[Rol1964]

Originally Posted by Pete7

What is the engine?

That is quite a big dinghy for a 26ft yacht. Are you planning to keep it fully inflated at all times? About the best we can do is 2.7m, any more and we can't inflate it on the foredeck as the baby stay gets in the way.

Pete

LoL!!! It’s not for the Pearson 26. That’s a funny mental image.

[BMD21]

I had a long shaft 10 HP Honda on a 10' Achilles. At low speeds the Honda threw water into the back of the Achilles and I had a heck of a time getting on a plane. So, I made an extension to raise the Honda up on the transom. Worked great. Never noticed any torque load.



Boris

[MicHughV]

I do believe it's possible to swap out the long shaft for a short shaft...I've done the exact opposite.

As posted above, the deeper depth of the prop will induce quite a bit more bending moment on the transom, which in turn is connected to the tubes. It might work for a while, but sooner or later you're gonna have problems.

I don't see much point in destroying a dink because you have the wrong shaft for it.
Rather spend the money and put the standard shaft on it. It's easy enough to do for your basic DIY-er.

[team karst]

Dont do it, it will be a flail. There are many times i wish my short shaft motor was even shorter on my 10’ avon.

[thomm225]

It will be fine. You just have to go slow.

It cannot throw water anywhere a foot or two under water just above idle and you can motor a long ways like that

[Rol1964]

Easy for me to do a shaft conversion if I had the drive shaft and the shortened lower end but both my motors are older so parts are either hard to find or ridiculously expensive… I think I’m going to buy the Cadet anyway. I’ll figure out the motor over the coming winter months. Thanks all for the comments.
Cheers, Rol1964

[Don C L]

Originally Posted by Jammer

And structural damage to the transom (because the torsional loads are higher)

Many years ago I tried it and it worked for a little while but eventually you are likely to damage the transom to tubes connection. I say don't do it.

[Tin Tin]

Back in 1998 we had a Zodiac 3.1m cadet, air floor, we used a 9.9HP long shaft 2 stroke Evinrude (that was the emergency pusher engine on the big boat), it ran fine at full plane, we towed kids in rubber rings etc - was not a problem. The Dinghy failed from sun damage long before any motor stress damage. This is just our story so no real statistics here.

[Rol1964]

Originally Posted by Tin Tin

Back in 1998 we had a Zodiac 3.1m cadet, air floor, we used a 9.9HP long shaft 2 stroke Evinrude (that was the emergency pusher engine on the big boat), it ran fine at full plane, we towed kids in rubber rings etc - was not a problem. The Dinghy failed from sun damage long before any motor stress damage. This is just our story so no real statistics here.

I have that same motor...Evinrude 9.9 long shaft. I don't expect any issues for the type of light use I have in mind. Cheers, Rol1964