Help me hang my canoe

June 1, 2009 6:45 AM

I built a pulley system to hang my canoe from my garage ceiling. It’s awesome, you should see it. Unfortunately, there’s a slight problem.

Here’s a simple diagram. Sorry if it’s confusing. It’s not to scale, obviously.

It all works very well, allowing one person to raise and lower our 60 lb, 17 foot boat. Once raised, it hangs nicely from the ceiling, allowing the garage door to open beneath it.

The problem is, when you pull down on the rope to raise it (far right of diagram), the bow of the boat rises before the stern. You constantly have to pull down on the bow to equalize the two sides and level out the boat as you raise it. This works, but it’s kind of a pain in the ass.

Is there any way to rig it up so that both sides rise equally, while still maintaining the mechanical advantage of the pulleys? I’m not an engineer and I’ve forgotten most of my 8th grade physical science class.

posted by bondcliff to home & garden (14 comments total)

Move the anchor point of the bow toward the center. And/or the anchor point of the stern further astern.
posted by gjc at 7:03 AM on June 1

Put another pulley attached to the ceiling where the "tie-off" is now, string the rope through it. Then you can tie both ends of the rope together (probably using a slightly longer rope so it will reach)- pull both ends of the rope at once, and voila, your problem should be solved.
posted by dunkadunc at 7:03 AM on June 1

Try getting rid of the C and D1 pulleys. Have the rope go from tie off > A > B > D > E > you. That (assuming that the weight of the canoe is balanced between the A and B pulleys -- you may need to shift them forwards or back) should lift the canoe as a unit, rather than lifting it by one end.
posted by Forktine at 7:45 AM on June 1

I agree with Forktine - try bypassing those extra pulleys.
posted by orme at 7:56 AM on June 1

Also, maybe try rigging it like one of those bike pulley lifts?

Looks similar, but not exactly, so maybe there's something special about the way they do it.
posted by orme at 8:00 AM on June 1

Overengineered for the win! I'd move the C pulley to center mass, eliminate d1 and possibly d2, running straight from B to E. (pretty much what Forktine said)

We have an 18 foot aluminum grumman, weighs a hellovalot more than 60lbs (me=jealous), and I'm certain I could hoist that w/o any problems.

The other option I see would be to double-pulley from both ends (at the ceiling) with the middle-rope running the length of the keel, your pulldown directly astern or at the bow. I'd also be inclined to put the boat pulleys on S hooks and just snap those under the bow/stern end pieces---but it looks like you have a system for that already. That would give you a helluvalot of mechanical advantage, but it would also have you pulling for days to get the boat the ceiling.
posted by TomMelee at 8:05 AM on June 1

Another possibility is that one of the pulleys on one of the "legs" is binding so that the other one is getting a disproportionate level of force.

Another solution would be to wire it up so that there is a single point of attachment to the boat. This would be optimal if your attachment points in the boat do not synch up with its center of gravity. Two wires from the boat to a tie-bar, parallel with the boat. Then, attach the pulley system to the tie-bar wherever the center of gravity is. This way, any pulling force onto the boat has to raise both sides equally. Then, I'd run the line up to a pulley in the ceiling, and then use whatever system of pulleys that are necessary for force multiplication across the ceiling, and then down again to the pull point.
posted by gjc at 8:09 AM on June 1

Forktine, I originally had it set up that way, however I didn’t like the way it raised. The rope stretching between A to B had a lot of force on it and it prevented it from being raised very high. It was much more difficult to raise and it had the same problem of the bow rising before the stern.

The weight of the boat is evenly distributed at both ends.

Orme, that system it pretty much exactly what I’ve built, with the addition of the E pulley. I’m not sure why, or even if, that one works evenly.

One of the problems I have is the boat has to raise as high as possible so the garage door can open below it.
posted by bondcliff at 8:13 AM on June 1

Your problem is that you have two pulleys on your load one of which is getting pulled twice as much as the other. IE: A is only getting pulled on one unit for each unit you pull the rope and B is getting pulled two units. The bike lift has the same problem.

Swap D and E and tie off the left hand side of B to the roof. String your rope Roof -> A -> C -> double pulley at E; Add a second rope going Roof -> B -> Single D -> Double E. Pulling on the two ropes at the same time will then raise your boat evenly while still preserving your 2:1 mechanical advantage. Once the boat is parallel with the ceiling tie the ends of the rope together to keep front and back in sync.
posted by Mitheral at 9:23 AM on June 1 [3 favorites has favorites]

I've got three of those bike lifts in my garage, and if you pull the rope very quickly the end of the bike closest to the pull rope will rise more quickly than the other end [as Mitheral notes], but once you stop it levels off by itself rapidly. Failure to level off generally means the rope has jumped the pulley, which is my one complaint with the thing.

If you pull at a nice even rate you hardly notice the "rocking".

I suspect the difference is something to do with either the smoothness of the pulleys [maybe yours don't move as freely as the ones in my garage? Admittedly, that seems really unlikely, as the pulleys on this bike lift are nothing special.] or the side-by-side double-pulley makes a big difference compared to your over-under double-pulley.

If it were me, I guess I'd try the side-by-side double pulley first to confirm/rule-out.
posted by chazlarson at 9:56 AM on June 1

What if you set up two independent pully systems, one for each end of the boat and then tie the two ends of the rope together at the point that you pull?
posted by talkingmuffin at 11:44 AM on June 1

I made you a picture of my idea.
posted by talkingmuffin at 12:03 PM on June 1

I'm thinking better quality pulleys with a lower coefficient of static friction - Then what you've got set up will self-level, like chazlarson notes.

(You're probably using sleeve-bearing pulleys, and a larger ball-bearing pulley would probably be a significant improvement.)
posted by Orb2069 at 1:07 PM on June 1

Your problem is that you have two pulleys on your load one of which is getting pulled twice as much as the other.

Reading this was one of those "light bulb going off" moments. I feel silly for not realizing that.

The problem is, doing it that way (the way talkingmuffin pictured) doesn't give the same mechanical advantage and between rope stretch and friction it was rather difficult to hoist. I think I'll buy a couple small blocks and tackles (bloxen and tackles?) to make it a bit easier to haul up.

Also, thanks to attempting to "fix" it after work when I'm tired, my next askme question will be "how do you repair a cracked hull after you let your canoe drop six feet onto your garage floor." followed by "how do you forgive yourself after doing something really, really dumb."

Thanks all!
posted by bondcliff at 6:18 PM on June 1