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millsrj 07-23-2012 08:34 AM

Boat Lift Modifications
 
Grab a coffee, this is a long story.

I bought a used boat lift about a month ago. It is a 3000 lb Dock-Rite vertical lift with a wheel winch. My shore drops off quickly to about 88" of water over about 9" of muck. I get someone to clean and re-weld all the welds, plus I get them to make 4 new legs 60" long. I figure it will be good to have about 14-16" of leg still inside the frame (the legs slide inside the frame like a sleeve) for added stability. The legs are 2"x1.5" T5 Aluminum tube (1/8" thick). I also got a 4500lb winch installed. My boat weighs 2050 lbs unloaded.
The lift is only 108" wide, and my boat is 96" wide. This new winch (when I got it home) sticks out 6.5" so I (stupidly) moved the bunks over to give clearance.

So I float the contraption out to my dock. I extend the legs 48" below the frame and it settles in nicely. I float the boat on and start to winch it up. Realize that the bunks are 5" off-center toward the outside here. The boat gets 6" above the water. The back-outside corner starts to sink and I figure it's due to muck, then the whole thing goes over. The back legs buckle and the boat slides off. Here are my questions for the group:

1. should I have done this in the first place. Is 48" of leg too much for 2"x1.5" aluminum tube
2. would T6 aluminum have made any difference?
3. was this all caused by the weight shift onto the back outside leg? if I did not move the bunks would this have happened?
4. would cross-bracing on the ends help? If so, should I cross brace the sides as well?
5. I could try to find thicker aluminum. Should I think about steel tubing instead?

thanks guys

Millsy

Barefooter92 07-23-2012 12:46 PM

4 ft of 1/8" tubing sounds light to me as you now know. Then have 1/4 of the boat weight at the end of 4' is like 5,000ft/lbs on that one tube. This was a recipe for disaster. Is the boat okay? Run it by an engineer next time or even a steel supplier. They have books for this. Raising a boat lift 4' is not the end of the world and can be done just not on 2x1.5 aluminum tube. If you dont have access to a free engineer buddy try your local college set it up as a classroom lab experience and see what they come up with.


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