I'm in somewhat of the same boat as Dan, considering the trailer hub. I was (and still am) concerned about the forces -- trailer hubs are designed for PRIMARILY radial forces with a considerable amount of axial forces during turns. Here's my thinking...
My motor can generate around 2000 pounds of axial thrust (100 HP). A 7,000 pound trailer hub is built for 3,500 pounds of radial thrust. Now, that's a gross miscalculation, I know. But it's a starting point. The hub can handle substantially higher loads, but those are the loads that the hub and bearings are rated for, assuming long term usage. Since the hub isn't built for axial loads, I went to dive into the bearings. Using the calculator at SKF Bearing Select I got this:
Dynamic Load Rating for each bearing in the system, converted from kN to lb f, is conservatively 20,000 lb f and 13,000 lb f (forward and reverse). Now, according to a calculation I found online Force = x*radial +y*axial where x and y are scaling factors. x is generally 1 and y is provided by the bearing manufacturer. As best I can tell, and there was a lot of confusion in my mind, for the bearings above the y is 1.8 (which implies that they hand radial forces better than axial forces). Now, another thing I read says that x can be .67 is the axial forces are much bigger than radial forces. So, that implies that for the the bigger bearing we have: 20,000 = 0.67*axial + 1.8*radial. Now, if I assume that I have already taken that into account with the .67 and 1.8, then I'm at about 1/3 as strong, and that makes sense. In that case, with limited radial forces, the bearing can run at normal usage with around 1,000 lb f and get it's full lifespan. The bearings themselves can take an order of magnitude more force for periods of time without issue (like when a loaded trailer goes over a speed bump), and I'll be running at or below 1000 pounds of force in normal operation. (Higher than that will take too much battery drain for my liking.)
Now, a proper bearing, built for axial thrust would, for less money, take 5-6 times as much axial force with ease. BUT, and here's my biggest concern, I don't know how to engineer one of those effectively. And my fear is that my jerry-rigged solution would be less effective in reality than it would be in theory. And a trailer hub, despite its many issues, is engineered by someone who has to make sure they will survive being used aggressively on dirt roads and the like. That's what's keeping me in the trailer hub space, at least for now.
Finally, slamming the boat into reverse is my #1 expected hard usage. If I put all 2,000 lb f that I can produce into the water, I'm over the normal use limits considerably, but nowhere near the 20-50,000 lb-f limits to deform the bearings or cause damage -- just experiencing excess wear.
Last thought. I'm only really pushing the boat at 300 lb-f so there's a pretty big margin there for general use, and I'm a very light user, so there's that. Still, everything on the boat is built to last and built at the top end (for 30 years ago). I hate to 'hack' something together, but I also want to be sure I'm using components that are tested.
That's my current thinking, Carsten (and Dan).
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