re: Physicists....help needed.

I'll take pics tonight and post em. I want to do the same thing, just lower it right in to the back of the truck. Just hope I have enough room where the garage door won't be in the way.

Short answer: You have 3 ropes providing vertical tension. The tension of the 3 ropes will add up to the weight of the canoe/yak.

That said, the biggest help you will see is in your lift weight. The amount of tension required on the tag rope to lift the load will be approximately one-third of the weight of the load (plus very small friction losses in the pulleys).

Having the one pulley/rope to the left and two to the right will possibly cause issues with the canoe/yak staying horizontal as you lift. The side closer to the rope will probably want to rise faster as each rope will see less load. I'd highly recommend adding a second pulley to the left side for ease of operation.

quote:

---

And then there is the friction. The smaller the pulleys the more the friction. So a 3:1 is not really a 3:1

---

Okay, 2.6:1. Without getting into the detailed design of the pulley, any estimation is kind of goofy on friction losses.

Just kidding.

A 70 lbs is easy. We had to lift two rescuers and one was over 250. The other was 200. We played with different systems and found the one that worked the best.

As drawn, only the right side will move until the yak is all the way up then the left side will go up once the right pulley tops out. If you notice in the video, the operator holds the right side down while cranking to get the left side to go up. Also, the video is heavily edited and we dont see one smooth cranking motion. Delete the pulley at the bottom of ropes 1/2 and just make two independent ropes to the same crank through single pulleys (birdnest trouble maybe?) If you need the mechanical advantage, make two independent systems like the one on the right.

ETA: See youtube comments by tuffnuff1 and wispershadow.

Well I'd post pics but it failed. Like some said it raised one side at first. I didnt have the benefit of a crank so trying to hold one side down and pull the rope was tough. I finally got it up but then with the pulleys and the slack i left in the rope it was hanging too low. Back to the drawing board.

I think it'll work. I didn't think a 70lb would get pulled unevenly, but I guess it doesn't matter how much it weighs.

OP, if you really want to be sure, instead of an eye bolt on the top left, use a double pulley, and make the whole thing symmetrical. Anchor the rope on the left side to a stud, and on the right to a crank of some kind.

That middle line should make sure that it doesn't need to be two separate sets, but that's not taking the load distribution or friction into account. If it were, say, a beam, the middle rope would get tight as soon as the right end raised higher than the left, and it would come up even. I think the reason the first drawing used two pulleys at the back is because 's are usually heavy at the seat.


It's gonna take some tinkering, but it'll put a smile on your face when that SOB lifts even for the first time.