Rigging Question -- Friction Device

[Suijin]

Originally Posted by Dockhead

A horn cleat is fine of course - provided the rope is actually on it. But we don't kneel to lower the platform. I'm just looking for a bit of safety against breaking it.

I've never used a ratchet block - maybe that would do it? A descender wouldn't work, would it? I think you have to hold on to it, no? What I would like would be something which could be fixed under the rail (or possibly above) which would impart a constant amount of friction on the line so that it couldn't run out suddenly if someone let it go. Does such a thing exist?

If your goal is to prevent accidents then you need a solution that provides a constant braking action. A horn cleat is not going to do that for obvious reasons.

A ratchet block only works when under tension...and you need to ease the line over it. It would be OK for lowering, but not for preventing accidental drops.

A clutch would work but again if the line slipped from your hand while the clutch was opened it would provide little if any braking effect.

The figure eight/boom brake is I think your best bet as it does exactly what you want; allows the line to run free but not too fast. It's basically a friction device where a line under tension is braked but allowed to run. You can adjust the braking effect by how you configure the line. You'd probably want to run a few tests to see what shape, level of polish, etc. works best.

How heavy is this platform we're talking about?

[estarzinger]

hmmmm . . . . Seems to me a pretty simple solution - an 'oversized' rope clutch - would do exactly what you want.

A properly sized rope clutch grips the rope and does not allow it to slip (until you get above its working load). If you get one that is just bigger than proper size, it will allow the line to slip slowly under lower loads. So you just play around with rope diameter and rope cover texture and jaw size (many clutches can have different size jaws fitted) until you get a combination what slips the amount you want under the transom load. Would require some messing around and (in worst case) might require some small custom adjustments to the jaws (but probably you can get just right by playing with the rope - bulking it up or down just the right amount).

The nice thing about this solution is you can 'turn off' the friction completely when/if you want by opening the clutch.

You may also want a second 'properly sized' clutch in series with the above one, depending on how you plan to hold the line when the transom is correctly open.

Short of the above there are various things you could have custom fabricated which would do the friction/slip job, but I dont really see that any have an advantage over a carefully sized ready made clutch.

[TeddyDiver]

A door pump? That could be perfect.. Some parts would rust quite fast but maybe there's a way to protect it (like a rubber sleeve over the arm?)

Teddy

[estarzinger]

btw, this is an application where nylon (ge like 10mm climbing line) might make more sense than yachting poly.

Nylon would give you some significant built in shock absorption (assuming you had some length in play).

This thinking might also lead you to design your purchases with more legs than normal - which would build friction and elasticity.

[mrm]

How about a car shock absorber, permanently mounted to an arm welded to the pivot? It would allow for unrestricted slow motion, yet would dampen any sudden movements (as in uncontrolled drop) of the platform.


Edit: if it rusts, these are standard parts, affordable and widely available, easy to replace.

[thinwater]

Originally Posted by Dockhead

...which would impart a constant amount of friction on the line so that it couldn't run out suddenly if someone let it go. Does such a thing exist?

Not to my knowledge... which does not mean no. However, enough friction at the bottom of the travel would not allow it to drop at the top.

Perhaps you can see a ratchet block at a boat show or on a dinghy--very common on beach catamaran mainsheets (the last block before the cam cleat is a ratchet block). They work pretty darn well. The amount of friction is adjusted by rotating the cam cleat, which changes the amount of wrap.

Interesting puzzle.

[SaltyMetals]

Reading this thread i was about to suggest balancing the weight of the drop down with some hydraulic springs such are regularly used in cars for the rear opening hatch. I see MRM has just suggested the same thing. This would balance out the weight of yr drop down and would then not be needing a complicated handling system which possibly only you on your boat would fully understand.
Andrew

[farmall.dude]

What about a couple extension dampeners? Quick Google search brings up something like this: BANSBACH-EASYLIFT-Dampers. Looks like they have some pretty beefy ones (max weight 2200+ lbs!). There must be more than these out there too. Kind of like hydraulics without the complexity. Just an idea...

[CHRIS V CLEMENT]

Ratchet block is the way to go....try it....you will like it !!!

[uhu]

if you like "kiss"-solutions ...


from Boom brakes - dealing with boom brakes and the best gear on test

------------------------------------------------------------------
Capt. Don BoomBrake

At just $50 (c £40), this is the bargain of the bunch. A ‘climbers 8’ type, it is simple, just a polished stainless steel casting with no moving parts. In testing we did notice that line loads were higher for this type of brake and one does have to go forward to re-reeve the friction line as wind force varies. It did not disappoint, however, doing all that was required when enough tension was applied to the friction line.


Verdict

Awesome value! Not as easy to modulate the braking effect as the larger drum type brakes but for the money, hard to beat.

------------------------------------------------------------


worth a try :-)

[KiwiKen]

"I've tested that devise as a boom brake and was not very impressed. I've used figure 8s uncountable thousands of times climbing. It would not be my choice for this."

Don't you mean a Prussic Loop - The poor man's ascenders.

Prussic Knot,
Form a bight in the middle of a length of rope
wrap the two lines twice round the standing rope and pass them through the bight- easy to slide along the standing rope with one hand, but
lock on and won't slip under load - same principle as a rolling hitch.

[charliehows]

wichard boombrake gives a lot of variable control - I made a similiar device from scrap round s/s bar i had lying around, works very well indeed.

[zstine]

I have not read the whole thread.... There are shocks in all sizes. Small steering dampers from motorcycles could be use to dampen. But I would strongly suggest using some type of spring (like a pneumatic hood lift on a car, which are cheap) to both help lift the platform's weight and keep it from slamming. All heavy lift items generally use spring assist, from windows, car hoods, garage doors... Swim platforms

[thinwater]

Originally Posted by KiwiKen

"I've tested that devise as a boom brake and was not very impressed. I've used figure 8s uncountable thousands of times climbing. It would not be my choice for this."

Don't you mean a Prussic Loop - The poor man's ascenders.

Prussic Knot,
Form a bight in the middle of a length of rope
wrap the two lines twice round the standing rope and pass them through the bight- easy to slide along the standing rope with one hand, but
lock on and won't slip under load - same principle as a rolling hitch.

How on earth did you reason that I confused two completely unrelated bits of gear, and then decide what my mistake must have been? I've been climbing most weekends for 40 years, since when the only mechanical ascenders were Jumars.

Here is the article.

https://www.practical-sailor.com/iss...e_12238-1.html

A figure 8 can be used like this. There are several different ways to weave the line to adjust drag. But I believe a preventer and proper sheet and traveler management is a much better answer.


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

I really think the answer is a cleat, ratchet block, or winch, and pay attention. There are devices, such as an oversize clutch, that have some of this characteristic, but I think something needs to be invented.

Good luck!

[Dockhead]

Thanks for all the interesting tips and new ideas!! Wow! One reason why I love CF.

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.

But the idea of adding some spring tension is great! I hadn't thought of that. Maybe some kind of rotary spring/damper? Such a thing exist?

And as to friction -- thanks for the great ideas. I now visualize how a boom brake could work, or a wrong-sized clutch as suggested by Estarzinger (great idea!). Maybe a ratchet block could also work -- I could probably run the lines in a way which would assure tension against the block. I didn't know that they actually create friction rather than just stopping rotation -- I've never used one. Will play with one in the chandlery.