Now that the water-ski tower is in the boat, I've run through some stress analysis on what loads we can safely put on the pylon tubing and the boat transom.
The boat transom isn't a factor - the outboard is PUSHING on the transom, loading it in compression, much more than the tension a water-skier will pull.
Here you can run through my calculations for a 2" T4-Aluminum tube in bending stress. Using a safety factor of 3, the safe load (maximum) each tube can take is 415 lbs. So the tower should be OK up to 800 lbs of kids... I'd appreciate any feedback or double-check on my numbers!
So... what fun is math if you can't bend or break something, right?
We took some leftover tubing and put it up in the barn rafters... and loaded it up with some weight!
Here we tested it with a 140 lb kid. No bending or breaking yet - though the rafters creaked...
So we put ALOT of weight on... 140lbs + 90lbs + Dad (255lbs) = 500lbs of weight on the tubing, bouncing up and down.
The rafters were moaning and groaning! The tubing yielded slightly... less than 0.1 inches as far as we can measure. I think the bending of the tube was more because of the 'point loading' of the tube on the 2x6 in the rafter, though.
So, our numbers aren't too far off, and we are alittle more confident the tubing will hold on the tower. Now - on to the water!