Building my own longtail this spring/summer and one thing I'm looking to change is an angle correction at the end of my dive shaft to get all my thrust going parallel to the motion of my boat (this gives me 10-15% more thrust). The only good way I can think to do it is to use a CV joint but what I don't know is how that will hold up in time?
I'll have a good boot around it and will double clamp it on each end but inevitably water and some sediment will get into the CV and wear, so I'll have to make sure I maintain it well.
I think it's worth the work if it gives me the push of a SD motor with the ability to dig like a long tail, but how do you guys think a CV will do under water in marsh conditions?

hope you weigh 500 lbs to hold your handle down on that motor.

have a conversation with an engineer before you waste your time with this. The loss in efficiency due to drag and friction from the joint is greater than any gains.

The way it's done now is virtually maintenance-free, why change that. You will never keep from damaging boots.

1st: The horizontal thrust will impart a moment about the motor mount which will cause the prop end to dive and you will likely be sitting on top of your tiller to keep it level. A SD and an outboard have transom stops that counter that thrust moment and translate it into forward thrust (the moment tries to lift the bow of the boat instead of the tiller)

2nd: The prop end is where most wear and tear will occur. The boot will be rotating through the mud/sand/gravel/veggies etc. and get trashed in no time. It will also create more drag because it is going to be larger than just the shaft/bushing/bearing arrangement.

3rd: How would you go about mounting it? Where would you get a suitable CV joint? CVs can't take thrust loading, they need to float so you would need to add a thrust bearing or bushing to your set up. I've been looking at ATV CVs and they all have stub axles on the outboard side and the inboard sides have too much plunge.

I bought a used Polaris CV driveshaft for my longshaft build. Instead of a U joint I will be using the CV joint. The metal is fully case hardened and was a B*tch to machine a bore and keyway for the motor. I annealed the stub axle as best I could without annealing the body and it was still a nightmare to machine.

If you know of a source for CVs that could mount directly to a 3/4" keyed shaft (like on a 6.5hp engine) please post.

Keep it simple and you will likely be better off.

If you have drawn it up, it would be great to see the bearing arrangemet.

Why not just use a second u joint? The seals on the cross bearing do a great job of keeping moister and crap out and keeping grease in, but I have no idea if they would withstand submersion. Here's a video I found of a rather crude looking build that is pretty much the design you've proposed. It might be worth trying to contact the guy to see if the basic concept even worked before investing too much time. I see no reason why a u-joint or a CV joint couldn't be enclosed in a sealed housing attached to the end of the drive tube, although I personally don't see the practicality.

use dual cardian joints if you gunho about trying it

maintenance on a U-joint is also much easier, just pump it up with grease at the end of the day. A CV would have to be pulled out, tore down and flushed.