I am building a kayak hoisting system for my garage, and was originally going to use a standard two-rope system to keep the kayak level when hoisting, but I came across this single rope schematic, and was curious if it would actually lift the kayak level or not. My spidey sense says "no", but wanted to confirm:
7Do you care whether it is level during hoisting, or just that it eventually becomes level?– Jimmy Fix-itAug 30, 2020 at 14:47
1Duplicate/related: diy.stackexchange.com/questions/95636/…– MakyenAug 30, 2020 at 18:08
@Makyen - Great find!– IronEagleAug 31, 2020 at 1:29
personally I'd just use a step stool and a shelf to store the kayak.– JasenAug 31, 2020 at 7:05
I have a commercial 2-rope system and find it handy to be able to easily adjust the levelness independently, and keeping it level during hoisting is not difficult at all, assuming that was one of your concerns. I also investigated building my own, but found it was more cost-effective to purchase an off-the-shelf system (as a bonus it has an integrated rope brake that automatically engages if the tension on the free end of the rope to raise/lower is released)– Colin YoungAug 31, 2020 at 14:04
No, it will not, at least if the load is balanced as shown. The leftmost part of the kayak / canoe, having a snatch block on it, will experience twice the lifting force as the front of the kayak. If the kayak was moved towards the left, it could potentially balance out, but you will likely need to do some manual adjustment as the kayak is lifted.
1That's what I figured. I will stick to a simple two rope system then. Thanks! Aug 30, 2020 at 1:32
1yeah, use two ropes like on a venetian blind.– JasenAug 30, 2020 at 1:43
7Fitting a block at the right hand end at(3) and tailing off the line to a fixed point at (4) would give the same effort on both ends. It may not balance perfectly but might be worth a try. Aug 30, 2020 at 13:30
Wont work, the left side of the canoe will rise in preference to the right side. Even if the right side is heavier, mismatches will tend to arise.
Keep both the ceiling pulleys, and add an extra pulley to the ceiling pulley on the left. Use two ropes (one short, one long) that go from the canoe.
When the canoe is in the lowered position, connect the two ropes at the left of the "double pulley" so the short rope ends there. If done thoughtfully this will stop the canoe lowering further so it can be unloaded one end at a time.
The remaining long rope goes to the wall attachment - one rope solution.
2You could double the purchase by starting both lines at the ceiling, then down to blocks attached to the slings, then up to the blocks on the ceiling before running to the cleat. Aug 30, 2020 at 11:14
You could make the one rope arrangement symmetrical by fastening the far end of the rope (right in the diagram) to the ceiling and from there going down to and around a pulley at the canoe. Then both ends of the canoe would be supported by two sections of rope.
But this would be unstable since the relative heights of the two ends of the canoe would not be controlled.
You must use two ropes pulled together.
2Rube Goldberg suggestion to make your symmetrical arrangement stable The lower left pulley should have double the diameter than the lower right.:Now rotate both lower pulleys so their axes align and connect them with a shaft (maybe with a sliding square section to adjust the distance). The load will stay level, until any imbalance overcomes the friction between rope and pulley.– Bobby JAug 30, 2020 at 9:49
@Bobby J: In the simplest analysis of these ropes and pulleys problems I think one assumes that the pulleys are frictionless and massless. The pulleys have neither static friction nor dynamic resistance (zero moment of inertia) so the tension in any single rope arrangement is everywhere the same (whether symmetrical or asymmetrical). I think this means the diameter of the pulleys is irrelevant. The pulleys could be of different diameters and this would not affect the analysis. Aug 30, 2020 at 14:43
@Bobby J OK I see you propose connecting the pulleys so that they do not turn freely but are synchronized as by a toothed belt like an auto timing belt. This is an ingenious solution in which the diameter of the pulleys would matter, but might not work because of reaction forces on the pulleys. I personally cannot figure that one. Aug 30, 2020 at 14:52
As mentioned by other answers, the original arrangement has two problems:
- The double rope on left side lifts with twice as much force as the one on the right.
- There is no force to keep the sides balanced, so any slight imbalance would grow without limit.
Here is a single rope arrangement that solves those problems:
Doubling the rope along the right side pulley makes the force equal.
Spreading out the ceiling attachment points provides negative feedback: if one side is higher, the angle of the ropes on that side is shallower and the lifting force is smaller. Thus the high side will rise more slowly, regaining horizontal orientation. It is important that the spacing of ceiling attachment points is equal on both sides.
The spacing is a trade-off: wider spacing gives more stability, but it also increases the force required to pull the canoe up.
It may still be simpler to just use two ropes.
5I see what you're doing to try to control pitch, but it has a couple of problems: first you need a lot of spread to have those balancing forces be meaningful (and getting the outer pulleys that spread in a typical garage may not be possible), and second, what increases balancing force also makes lifting force (as well as side force on end tie-off and outer pulley) exponentially increase as you get the kayak near the top. It won't be possible to lift it to design height. Aug 30, 2020 at 18:08
2As with all single rope solutions, this could work in an ideal situation (up to some height; see previous comment by Harper), but usually ends up unbalanced in the real world, due to friction and an assumption that the load is perfectly balanced. This type of solution will tend to result in one side rising more than the other (usually the side closest to where the rope is being pulled), and be frustrating to correct.– MakyenAug 30, 2020 at 18:13
1I've got a bike lift that does exactly this. The bike goes up mostly level, and if it gets too far out of level you give it a shove with one hand. When the bike (kayak) gets up the top it will run out of travel on one side, which will level it up nicely. Given most kayaks bulge in the middle, it would take some serious angle for it to drop. OP should clip the mooring line to the lifting line anyway, as a second backup but its really gilding the lily.– CriggieAug 31, 2020 at 7:56
This can work but it does depend on not much friction in the system Aug 31, 2020 at 15:36
Won't work. There is always some friction. Two separate ropes is the only way.– BohemianAug 31, 2020 at 22:39
The pulleys grant too much freedom of motion
You can tell the original arrangement is dopey because it absolutely fails to control pitch. If you were to lift halfway and tie off the rope, you could just grab the forward end of the kayak and raise and lower it (and the opposite end would move the other direction.
jpa's solution has the same problem. Little pitch control, at least not compared to the other forces it creates.
Polypipe's solution is correct, essentially the "two rope" solution but ganged to one final lifting rope.
Reducing lifting force
what OP and jpa are both trying to do is reduce lifting force by using pulleys as a force multiplier. In both applications it happens at the cost of pitch control.
The better way to do this, if it's even desired/needed, is to put the pulley advantage on Polypipe's "one rope". That's going to be at least one more pulley obviously.