Rigging Question -- Friction Device

[mrm]

Originally Posted by Dockhead

A gas shock won't work because they don't exist (AFAIK) with such a long stroke. The platform is large and heavy -- maybe 1.5 meters wide, 1.2 meters long, and weighing at least 50kg and probably more. Also, I cannot have anything which would rust here because everything will be splashed with sea water..

You don't need a long stroke, a car suspension strut should handle the forces on a shorter arm easily. See attached image for one idea.


Including the spring will give you weight compensation mentioned by others, so easier lifting.


The whole strut and arm could be hidden inside the hull, reducing corrosion risk.

[a64pilot]

Steel cables over pulleys led to either weights or spring inside of the boat, making the thing weightless?
Or maybe cables run to electric winches to raise and lower? Like a baby windlass?

[estarzinger]

Originally Posted by a64pilot

Steel cables (or dyneema) over pulleys led weights

I like the counterweight idea - perhaps put the weights in epoxied in PVC vertical tubes. You could make a pretty clear installation, without much risk of total failure.

Originally Posted by Dockhead

wrong-sized clutch

Ideally I think I would look for a clutch/jammer that had jaws you could make smooth (like filling the teeth with thickened epoxy) - as that would make the 'slow slip' easier to accomplish, smoother and less aggravating to the line.

Originally Posted by thinwater

I doubt he wants a lot of stretch; that would make for a wobbly platform. Some portion of nylon might help.

Yea, 'wobbly' is a potential issue with all the 'spring' related ideas. But nylon is dirt cheap and easy to play around with different elasticity coefficients to see if there is a 'setting' which provides shock absorption with enough stability. I think I personally would give it a try, if it is a total failure you have just wasted a few euros of rope, and you might find the perfect compromise setting.

[mrm]

Originally Posted by estarzinger

I like the counterweight idea - perhaps put the weights in epoxied in PVC vertical tubes. You could make a pretty clear installation, without much risk of total failure.

Renaming the vessel to 's/v Tubular Bells' might be in order, though ;-D


Seriously, counterweights on ropes work well on land, not much wave action there..

Yea, 'wobbly' is a potential issue with all the 'spring' related ideas. [...]

Only if overcompensated (i.e. made 'weightless') _and_ without any form of movement dampening (which was the original reason for this thread iirc). I still think the combination of gas/oil shock absorber (movement damper) with a spring is hard to beat in this application. Also, it seems this is how the production builders approach this problem, which on this forum may sound like a counterargument but.. not everything they do is wrong, they employ engineers too

[estarzinger]

Originally Posted by mrm

Renaming the vessel to 's/v Tubular Bells' might be in order, though ;-D


Seriously, counterweights on ropes work well on land, not much wave action there..

LOL - yea I hear you - but I suspect it would not be hard to fit the weights snuggly so they are not banging. Friction is not a 'problem' but rather almost a benefit in this application.




Only if overcompensated (i.e. made 'weightless') _and_ without any form of movement dampening (which was the original reason for this thread iirc). I still think the combination of gas/oil shock absorber (movement damper) with a spring is hard to beat in this application. Also, it seems this is how the production builders approach this problem, which on this forum may sound like a counterargument but.. not everything they do is wrong, they employ engineers too


Yea, I also hear you here - you could engineer a very elegant solution, but OP was looking for a 'dead simple' solution which did not require 'engineers'. For 'simple' it is hard to beat a rope

............

[Dockhead]

Originally Posted by estarzinger

............

Indeed. All of the ideas are great, but when they are much more complex and expensive than simply reinstating the old hydraulic system, I hesitate . ..

That being said, though, the lack of a counterweight is probably one reason why the old hydraulic system was unreliable.

Don't quite know what to do with this.

[JohnHutchins]

I'm having trouble visualizing what you're asking. And what do you mean by "hold it". Friction devices for climbing generally don't require you to actually hold the device in your hand as your hands are meant to be on the rope. They're attached either to you or a fixed point in the ground and you control friction with the angle of the rope or (with say a grigri which sounds like it offers what you want -- a failsafe against a sudden drop -- a lever that you operate with one hand while controlling the rope with the other). But it sounds like you've used these things and you know this, so I'm probably missing something. But what?

[Dockhead]

Originally Posted by JohnHutchins

I'm having trouble visualizing what you're asking. And what do you mean by "hold it". Friction devices for climbing generally don't require you to actually hold the device in your hand as your hands are meant to be on the rope. They're attached either to you or a fixed point in the ground and you control friction with the angle of the rope or (with say a grigri which sounds like it offers what you want -- a failsafe against a sudden drop -- a lever that you operate with one hand while controlling the rope with the other). But it sounds like you've used these things and you know this, so I'm probably missing something. But what?

I certainly know less than you think!

I have not used these things so can't quite imagine how I could use one in this situation. If you have some ideas, let me have them!

[Dockhead]

Originally Posted by mrm

You don't need a long stroke, a car suspension strut should handle the forces on a shorter arm easily. See attached image for one idea.


Including the spring will give you weight compensation mentioned by others, so easier lifting.


The whole strut and arm could be hidden inside the hull, reducing corrosion risk.

I got it; thanks. But I couldn't implement it that way because I would have to go through the transom at a level which would compromise watertight integrity. With ropes I can have all holes near rail level.

[Pauls]

For simplicity, how about attaching a couple lines running from the top of the transom down to the platform corners. Make them of small diameter nylon for lots of stretch. Adjust them so that when the platform is deployed they take most of the weight. If the platform is dropped they will act as springs and reduce the impact force significantly. Very light weight and dead simple.

[JohnHutchins]

Here's a grigri in use. Notice that the belayer has it clipped to his harness and is working the rope with his hands while the climber is on the way up. It could be clipped instead to any convenient fixed point on the boat, even as a temporary installation used only when you want to raise or lower the platform. When you're letting the platform down (the climber descending here) an internal cam in the grigri holds the line so it won't slip unless the lever is pulled and you can control with the lever how much slip is allowed. I'm not sure it's a better system then a well placed cleat that you take a turn around (and the cleat isn't really functionally different from a figure eight belaying device it just has more easily accessible surface area and so more easily accessible friction). But I think it could work unless I'm missing something.

The downside I see is that it adds some friction on take up as well so it would make raising the platform a bit harder but it is designed to provide a lot more friction going one way than the other while allowing the amount of friction to be controlled via a lever.

https://youtu.be/ePAc9xK9fac

[Dockhead]

Originally Posted by JohnHutchins

Here's a grigri in use. Notice that the belayer has it clipped to his harness and is working the rope with his hands while the climber is on the way up. It could be clipped instead to any convenient fixed point on the boat, even as a temporary installation used only when you want to raise or lower the platform. When you're letting the platform down (the climber descending here) an internal cam in the grigri holds the line so it won't slip unless the lever is pulled and you can control with the lever how much slip is allowed. I'm not sure it's a better system then a well placed cleat that you take a turn around (and the cleat isn't really functionally different from a figure eight belaying device it just has more easily accessible surface area and so more easily accessible friction). But I think it could work unless I'm missing something.

The downside I see is that it adds some friction on take up as well so it would make raising the platform a bit harder but it is designed to provide a lot more friction going one way than the other while allowing the amount of friction to be controlled via a lever.

https://youtu.be/ePAc9xK9fac

OK, I understand. But as I had thought, you need a hand to operate this. I need something which will add a constant amount of friction, mounted below deck and not requiring manipulation.

The cleat, as previously discussed in this thread, does not fulfill the requirement of protecting the platform from being dropped in case someone accidentally lets go the line.

[JohnHutchins]

Ok, no hands, except the crew will have hands on the line. Or won't? The classic solution would be to run the line through a set of pinch rollers that turn a governor. Theat could be an entirely hands free system for controlled lowering. It would probably even resolve the problem thinwater pointed out -- meaningful friction when the platform is mostly down may be enough to stop it moving before it gets mostly down. But it would be excessively complicated and not off the shelf. But maybe skipping the governor and just running the line through a sufficiently heavy set of pinch rollers would work. But still more complicated and less off the shelf than a figure eight

[conachair]

A picture is worth,,,,

Hard to visualize just how your lifting mechanism actually works, one rope coming up from the transom? Or from a gantry above?

[Dockhead]

Originally Posted by conachair

A picture is worth,,,,

Hard to visualize just how your lifting mechanism actually works, one rope coming up from the transom? Or from a gantry above?

Well, presently it works by a rope clipped near the outer edge, pulled up and let down by hand, with the hydraulic system disabled.

What I want is to drill a hole in the rail and another in the transom inside the cavity where the platform goes, put a through deck block in the transom and a through deck bush in the rail. Anchor one end of the line in the transom, lead it to a low friction eye at the edge of the platform, back to the through deck block and up through the rail.

Then I have double purchase and easy handling without a rope coming out from under the platform. Just need a bit of friction.

Also a way to get the platform out of the cavity, since I have reverse counter and the platform needs to be lifted before it goes over to swinging down. That's a different problem though.