Hi everyone. I just made a kayak hoist based on this design:
http://www.ncangler.com/forums/threads/33861-DIY-Kayak-Hoist
Also, similar to this one:
http://www.grayhawk.zoomshare.com/1.shtml/My%20Pulley%20Systyem
The rope is tied off in an anchor at the bow end, goes down to a pulley at the kayak, up to a pulley on the ceiling, over to a pulley by the stern section, down to a pulley at the kayak, and back up to a pulley to the free end of the rope for tie off.
When I pull the rope, the stern side comes up and the bow side drops. All the pulleys are free spinning, rope guides through them properly, etc. Anyone have any ideas why this might be happening? Thanks,
Are all your pulleys the same size?
– Last Updated: Jun-20-13 1:31 PM EST –
Do you have the same number of pulleys on each side?
That’s to be expected.
– Last Updated: Jun-20-13 1:42 PM EST –
I can't see the details in that photo you posted links to, but with the arrangement you described, the two ends are free to move independently, exactly as they would if the two ends were simply supported by a single rope that was looped over the top of a single pulley. Explaining why that is so would take me an hour, but if you could actually compare models of the two methods you'd see that this is true.
If all pulley wheels had exactly the same amount of turning friction, you might be able to get the two ends to raise and lower together, but I doubt it. I think the only practical way to keep both ends at the same elevation while raising or lowering is to have a partner tilt the boat while you operate the rope, or you could stop periodically during operation and adjust it yourself (but fear not - see more details below).
Off the top of my head, I haven't thought of a way to control this issue with additional pulleys, but perhaps it can be done. The good news is that you can pull the rope and let one end go up until it stops, and THEN the other end will do the same. By the same token, if one end drops to the ground while the other rises, that's no problem either. That end will stop dropping at that point and the ropes will continue to run their route while lifting the other end. In other words, everything works out by the time you are done raising or done lowering. It's only the during the process of lifting or lowering that the boat will be uneven. It WILL become level at the top or bottom.
The differential of a car works the same way but with gears instead of pulleys (but both are WHEELS). The only thing that keeps both driving wheels turning at the same rate during straight-ahead driving is that they are on the same ground. Put the wheels on two different free-rolling treadmills and it would be completely impossible to make them turn the same speed, and when stationary, you could spin one of them forward and the other would compensate by spinning backward, just like the two ends of your boat can be made to teeter-totter in place.
pulley layout
There are 5 pulleys in total. 2 on the front half, 3 on the back half. Maybe I should make the pulley at the rear an eye bolt instead?
That won’t change anything
– Last Updated: Jun-20-13 1:48 PM EST –
That extra pulley on one end could be eliminated if you pulled UP on the free end, but the overall mechanism will remain the same. So it also doesn't matter if you use something other than a pulley for that purpose.
This should help
– Last Updated: Jun-20-13 2:32 PM EST –
I didn't want to bury this within my other post where it might get lost. It looks to me that the pulley on the end of the boat closest to the free end of the rope is not turning easily enough. If that one sticks it will prevent the free-running of rope and as it rises it will LOWER the other end of the boat. Oil it up or replace it with a pulley that turns easier. That SHOULD solve the problem of the opposite end moving down while the end closest to you rises. If that doesn't fix the problem, you are back to the "car differential" situation described above, where in the absence of something physically forcing the two ends to remain at the same elevation, it just won't happen. Again, just keep pulling until it levels off at the ceiling, or keep lowering until it levels off at the floor. OR, keep adjusting it as you go. It should be easy to tilt the whole boat while you operate the hoist simply by grabbing the end nearest to you and forcing it up or down.
Here's an easy way to demonstrate to yourself that the pulley connected to the boat, closest to the free end of the rope, is not turning. Cinch the two adjacent sections of rope together where they exit that pulley, or have an assistant grip them with his hand while you operate the hoist. It will do exactly what you are describing, and you will SEE that the pulley isn't turning.
The Bow is Heavier…
– Last Updated: Jun-20-13 2:56 PM EST –
The easiest would be to adjust the trim of the kayak, although, depending on how stiff the pullies are, it might not help, as the bow pulley will always get less pull than the stern pulley (there is friction in 3 pulleys before the rope gets to the front).
You might want to pull the rope from both front and rear at the same time and have the center of the rope fixed. This way you control the angle. Same # of pulleys...
Or if your pulleys can swivel, tie-off both rope ends and pull down from the middle between the 2 center pulleys. Again, you control which side pulls how much.
Oh, good thinking!
Yes, lifting the “far” end of the boat simply can’t be done without outside help or until the “near end” reaches the end of its range of travel, because more pulleys are turning when that end moves. The system takes the easy way out. Good thinking!!
It’s still not really a problem - just a bit of an inconvenience - as already explained, but your perspective now makes it clear that simply trying to reduce pulley friction won’t make it work any better than it does now.
I’ve seen “boat disaster” videos that …
Destroy boats by not having them balanced on the two straps - boat tilts down, boats slides out of the pulleys, boat falls off to its demise 
And those were BIG power boats …
I initially did a pulley system for my kayaks, but quickly removed it - too cumbersome to use and more hassle than help. My garage is low so I just hang the straps from the ceiling. I slide one end in the loop of one strap, then walk towards the other end supporting the boat in the air and then loop the other one. Or just drive under the boat with my car with rack on and lower or raise the boat - I got about 10" clearance and that is the easiest way.
Basicaly two separate lines
Bow line is tied off at the top, down through a single block, up to a single top block and then runs toward the rear.
Stern line is tied to the top, down around a single block, back up around a double block. The other side of the double block guides the bow line back.
You end up with two lines in your hand to lift the boat level or you can pull either one to angle the boat.
You can use a one piece line if you bring the bow line back and then kind of reverse thread the stern lift but you still end up with two lines in your lifting hand.
Hard to explain but mine is still working after all these years.
What Grayhawk said…
The way to guarantee equal lift is to have independent lines for bow and stern. This isn’t a great photo, but shows another version of grayhawk’s hoist:
http://i97.photobucket.com/albums/l234/carldelo/Kayak%20storage/Storage009.jpg
There are two lines, each anchored at the top, down to a pulley at the boat cradle, up to a second pulley (hung from the same anchor) and off to the haul point. The lines are knotted together where you haul them to guarantee equal lift. Actually they’re knotted to a heavy duty steel ring that hooks to a cleat when the boat is at its proper height. This setup gives a 2:1 mechanical advantage.
Forget all the stuff above about friction and the number of pulleys; it may be true but is not reliable enough to base a design upon (sorry GBG).
No need for apology
– Last Updated: Jun-21-13 7:39 AM EST –
We agree totally. All I said is that the hoist as originally built can function IF you keep the boat level by hand or allow it to level itself at the full-up or full-down positions. I myself wouldn't want that system since nothing can be done to it "as is" to solve the teeter-totter problem (the number of pulleys operating on each end pretty-much guarantees which way the teeter-totter will tip when you try to lift the thing, that's all, but it's still "out of control" no matter what). The system you and Grayhawk describe is far better, and a practical solution.
yep, two independant compound pulleys
– Last Updated: Jun-21-13 2:03 PM EST –
...... of equal size (ratio) , sounds correct to me .
I've not had a need to lift a canoe/kayak to a ceiling myself , but that's an good way to do it all right .
So you guys , using the compound system ... how's the pull weight feel , pretty easy ??
Pretty certain the O.P.'s original design would work fine also if he could figure out the wheel ratios so the bow and stern would lift equally . Smaller and larger pulley wheels .
Pulley size actually doesn’t matter
– Last Updated: Jun-21-13 2:31 PM EST –
For any single pulley, the length of "take up" of rope on one side is always exactly equal to the distance of "pull" on the other side. The pulley is just a wheel, and rope passes over its surface exactly the same as the pavement passes by the surface of the wheels of your car. There's no "gearing up or down" via that interface. This should be easy to visualize since people often use eye-bolts and carabiners instead of pulleys and they work exactly the same, but with more friction (people even rig up compound pulley systems with multiple carabiners for mechanical advantage, and you get the same mechanical advantage as you would with pulleys, not counting loss due to the extra friction).
It's the number of pulleys you put into the system (and therefore the number of lines going directly between the lifting point and the load) that determines mechanical advantage, not the sizes of the pulley wheels.
In the case of this particular hoist, the portion of the rope that lifts the boat is free to slide back and forth through the system independently of the part that's outside the system (the part that the operator pulls on), and that's why the boat won't stay level. Put a hand on one end of the boat to steady things and you can keep it level, but it would be better if that weren't necessary. Using the two-rope method described later in this thread, you end up with the same setup that's used to raise and lower Venetian blinds. To get Venetian blinds to "tilt" requires that you pull the two lines unequally.
Pulleys (blocks on boats)
With a single pulley system it doubles your purchase or half the weight. The more pulleys you use the more purchase you have but also more line to pull.
On sailboats you may have 3 or 4 pulleys (blocks) at each end of the mainsheet.
But a smaller pulley in the system
– Last Updated: Jun-22-13 9:35 AM EST –
... will pull harder than the larger ones. I don't know if it's technically friction or a smaller lever, but on a single pull system a smaller pulley in the mix will make it harder to balance the two ends while going up
Diameter not relevant
– Last Updated: Jun-24-13 10:05 AM EST –
GBG is correct, pulley diameter is irrelevant to the kinematics of a pulley system. If the motion of a system requires friction at the hub to generate the desired motion, that indicates improper design.
Friction at the rim is needed to keep the rope and pulley moving at the same speed. To remove it from the analysis, the pulley system can be modeled (mentally) as toothed belts turning gears.
For the system described by the OP, a cog-belt system with frictionless hubs would still be unstable, so it is under-designed, i.e. improperly constrained.
Per pilotwingz' comment, the pulleys in mine and grayhawks designs are NOT compound pulleys, but single pulleys mounted in sequence. A compound pulley is a single wheel with multiple grooves - not needed in this design.
EDIT - this last statement is incorrect - a compound pulley is a multiple pulley system. See pilotwingz' explanation below.
2 ropes is the way to go
Thanks for all the input, folks. I pondered the issue for a while and came to the conclusion that a 2-rope system is a better method (hat tip guideboatguy). I’ll see if that works, though I need to get my pulleys to swivel and/or get better pulleys. The rope keeps coming off the wheel, ceasing any progress!
Thanks again.
I’m not an engineer I only know by doing
… and by observation. When I was a kid pulling bricks and cement up 35’ of scaffolding using a bucket and pulley, I was glad the pulley wheel was 12". Something tells me if the pulley wheel was 2" the load was going to be a lot harder to lift. Maybe it was all in my head.
Not all in your head …,
– Last Updated: Jun-23-13 9:40 AM EST –
... but not a factor that's properly applicable to this situation either. If the hub creates a lot of friction (like with a poor-quality pulley), a larger wheel will make that friction less apparent to the operator when using the pulley to do work. That's for the same reason you can open a door by twisting a doorknob but you can't if the knob is removed and you try to twist the bare shaft with your fingers (bigger wheel creates greater torque, and torsional resistance is less apparent). Also, if a large-diameter rope is needed to accomplish the job, forcing it to kink tightly around a too-small pulley wheel adds a bit of resistance that you probably will feel when pushing your own limits.
Former p-netter DuluthMoose built a fancy z-drag system using very tiny pulleys, but they were mucho-expensive pulleys made for the tree-trimming business. Each one had a tiny, high-quality ball bearing inside the hub, and they were exceptionally smooth running in spite of being very small relative to the work load they were designed for. I'd put one of them up against a hardware-store pulley ten-times its size as far as ease of action goes (just a guess of course, but trust me, it was pretty dramatic how smoothly and effortlessly they operated under heavy load (we had the chance to try the system out on a badly pinned boat!)).
In the case of this hoist as originally built, one COULD attempt to balance the friction among the pulleys so that the boat would stay level while being lifted, but you'd never get it exactly right, as it would be like trying to balance a lever on a pin point, and even changing to a different rope would alter things enough to ruin your efforts (back to that situation of different ropes bending around the wheel with differing amounts of resistance). Further, even if by some miracle you got it right, the friction that would "balance out" while lifting would be totally wrong while lowering. You can see this on any basic pulley hoist where there's friction, especially with cheap pulleys and there's a lot of friction, in that the rope on the side of the pulley closer to the operator has much more tension on it while raising the load than while lowering the load, but the tension on the rope that carries the load is the same in both situations. This indicates greater friction while lifting than when lowering. The less friction in your pulley wheels and due to "flexing" of the rope, the less you'd see this effect, but you could never eliminate it, and thus you could never make all friction forces balance out for both lifting and lowering.
Bottom line, I have no doubt you noticed an advantage to using a bigger pulley when lifting bundles of bricks, but for fixing this boat hoist it just makes more sense to put the ropes in control of keeping the boat level rather than hope to control the situation via internal resistance to operation. That's what Carlelo was saying when he said relying on friction as part of the design means it's a bad design.