driveshaft/coupler

Brian,
Getting fancy with a stainless coupling? 303 is common and is the choice for machining. Don't forget you want the bore of the coupling to be a interference shrink fit. .000" to -.001"

Are you sure you don't want to go with the double taper? They are real nice!!

Brian,
Sorry about that! If you went shopping for a new shaft, you'll find references to alloys with words like "aqua" this and that!! They are specialty alloys that really were developed for salt water use. I feel they are way overkill for your use plus more expensive. I've made shafts for two of my own boats and went with 302 - 303. The 303 is just easier to machine. Both have a very good tensile strength and of course plenty of corrosion resistance. The key to making the shaft will be the straightness tolerance. It should not run out past .002" per foot. With the "aqua's" shafting material, it's real straight!!!

Pete, I looked around work today and think I've deciced to make a new coupler 2" longer than the original and using my existing driveshaft, I have the right size piece of 303 sitting here already and this will also allow 2" of adjustment for and aft for the driveshaft. Do you see any problems with this?

no problem, will be wire edming the internals

Brian,
As soon as Tim mentioned the keyway issue, I thought about you EDMing it but then notice you're a step ahead of me!! I really don't see a problem making the coupling hub longer. However, it's not really going to very easy to adjust due to the interference shrink to fit to the shaft! Plus you'll want to accurately dimple the shaft for the set screws.

Brian,
If I'm getting this correct, you were able to get paper into the coupling bore along with the shaft so you had roughly .002" clearance? Fretting?

Lecture? No!!! The tight shrink fits really came into play when HP's and RPM's started to climb. Back even in the 60's no one ever worried about it as long as the shaft went on the coupling relatively easy. Then we started to see lots of fretting, set screws loosening and even loosing prop shafts out the couplings. This was especially true when alignments weren't done. Now, (small lecture!!) with your high HP's and wanting to put some aggresive proping on the boat, I highly recomment the shrink fit. The only problem is they can be a PITA to get on - hench the double taper!!!