Hi everyone. I just made a kayak hoist based on this design:
Also, similar to this one:
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,
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Are all your pulleys the same size?|
Posted by: clarion on Jun-20-13 1:18 PM (EST)
Posted by: gorphus on Jun-20-13 1:35 PM (EST)
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|
Posted by: guideboatguy on Jun-20-13 1:46 PM (EST)
That's to be expected.|
Posted by: guideboatguy on Jun-20-13 1:34 PM (EST)
This should help|
Posted by: guideboatguy on Jun-20-13 1:54 PM (EST)
The Bow is Heavier...|
Posted by: Kocho on Jun-20-13 2:55 PM (EST)
Oh, good thinking!|
Posted by: guideboatguy on Jun-20-13 6:36 PM (EST)
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!!
I've seen "boat disaster" videos that ..|
Posted by: Kocho on Jun-20-13 8:45 PM (EST)
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 ....
Basicaly two separate lines|
Posted by: grayhawk on Jun-20-13 10:59 PM (EST)
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.
What Grayhawk said...|
Posted by: carldelo on Jun-21-13 12:05 AM (EST)
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:
No need for apology|
Posted by: Guideboatguy on Jun-21-13 7:38 AM (EST)
yep, two independant compound pulleys |
Posted by: pilotwingz on Jun-21-13 2:01 PM (EST)
Pulley size actually doesn't matter|
Posted by: guideboatguy on Jun-21-13 2:29 PM (EST)
But a smaller pulley in the system|
Posted by: clarion on Jun-22-13 7:50 AM (EST)
Diameter not relevant|
Posted by: carldelo on Jun-22-13 11:30 AM (EST)
I'm not an engineer I only know by doing|
Posted by: clarion on Jun-23-13 8:35 AM (EST)
... 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 ..., |
Posted by: Guideboatguy on Jun-23-13 9:33 AM (EST)
Got it, good explanation|
Posted by: clarion on Jun-23-13 10:23 AM (EST)
That makes sense. Thanks.
Posted by: carldelo on Jun-23-13 12:39 PM (EST)
That was a good explanation by GBG. The fact that you use a two-rope system probably indicates that you have a good grasp of the situation, without necessarily being able to explain it.
caridelo , multiple pulleys ....|
Posted by: pilotwingz on Jun-24-13 6:58 AM (EST)
Posted by: carldelo on Jun-24-13 10:04 AM (EST)
You have it right re: compound pulleys - my mistake, and I'll fix my post above.
2 ropes is the way to go|
Posted by: gorphus on Jun-22-13 11:56 AM (EST)
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!
Pulleys (blocks on boats)|
Posted by: grayhawk on Jun-21-13 3:26 PM (EST)
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.