How do ratchet blocks work?

Moderator: GreenLake

How do ratchet blocks work?

Postby Alan » Fri Jan 04, 2013 11:24 pm

I've been reading the ratchet block threads with great interest. Reduce hand fatigue, improve safety, prevent embarrassment...yep, gotta have at least one of those.

Unfortunately, I haven't quite figured out exactly what it is they do. Let's say, for example, in a mainsheet: I *think* they allow you to leave the sheet uncleated most of the time, because they won't let the sheet run out all of a sudden unless there's a substantial load on it (like a gust of wind). You can let the sheet run out if you want to by "easing" it. Not quite sure what easing is, but I think it means pushing the sheet out of the block groove so it just slides around the sheave without turning it.

So far, maybe so good. Then come the variations: auto ratchets, manual ratchets, auto-manual ratchets, and switchable versions of these. Some Harkens allow ratchet tension to be adjusted; some Ronstans allow switching from manual to auto.

I think that on-off switchable manual ratchets act like a conventional block in the off position, that is, they're free-running all the time. I also think that in the on position they allow the sheet to run out if the load exceeds a set amount. I'm not quite sure how auto ratchets differ from this; do they allow the sheet to run out when the load exceeds a set amount, and do they somehow determine this load themselves?

If anyone can recommend a good book on the subject, I'll gladly read it and write a book report. :D
Alan
 
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Re: How do ratchet blocks work?

Postby GreenLake » Sat Jan 05, 2013 1:37 pm

So many questions.

A ratchet block turns in one direction and not the other. Special shaping of the "groove" in the block makes sure that the sheet doesn't slip easily, especially when under tension. To tighten the sheet, you pull as usual and the block rotates. To ease a sheet, you let go, also as usual. Because the block does not turn, there's substantial friction and the sheet will not pull out of the block unless you let go "a lot". For a normal block, the minute you slacken your pull on the sheet it would start to pull out, because the pull from the sail on the sheet would dominate. For a ratchet block, the pull from the sail would need to overcome your residual pull as well as the friction.

That's why it's easier to hold a sheet with a ratchet block than with a standard block. Because the effect is based on friction, it works best if the sheet makes a 180 degree turn around the block, but 90 degrees can give good results as well.

In light winds, you don't want the ratchet action, because the force on the sheet is minimal.

That's why there are various ways to adjust the ratchet action. I'm partial to the "auto-ratchet" feature, where the block works normally when there's little tension on it (light winds) and engages the ratchet the minute you have to pull hard to hold the sheet in place.

For my manisheet I am using the Ronstan 55 mm ratchet. It can be switched between autoratchet and manual. I've switched it to auto when I installed it, and never looked back. I did select a very "grippy" line for my mainsheet - that helped. The ratchet block is at a 90 degree turn at a mid boom location where it should be possible - in principle - to adjust the block's ratchet action, except that I've never seen a reason to do that.

I have another dinghy with Harken (?) 70 mm (?) ratchet built into the mainsheet swivel. That one has a simple on-off selector. I do not find that particular setup superior.

If you are leading your mainsheet around more than one block, you can use more than one ratchet block. Some people do. The effect would be to provide more friction, and therefore more holding power (you need less force to hold a line because the pulling force from the sail would first have to overcome both your pull and the total friction before the sheet starts to move). I have been happy with a single ratchet block.

My suggestion would be to start with a single block and see how that works for you.
~ green ~ lake ~ ~
GreenLake
 
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Re: How do ratchet blocks work?

Postby Alan » Sun Jan 06, 2013 2:48 pm

And so many answers. :) Thanks, GreenLake.

Your advice to try one block at first makes loads of sense, so I'm thinking of this one:

http://www.shopsoundboatworks.com/roorse55sifi.html

It will be a near-exact replacement for my existing boom block (by the way, I have a mid-boom mainsheet).
Alan
 
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Joined: Wed Jun 17, 2009 4:39 pm

Re: How do ratchet blocks work?

Postby GreenLake » Sun Jan 06, 2013 7:36 pm

i think that should be fine.

I switched to this line for my mainsheet http://www.fisheriessupply.com/fse-robline-racing-sheet and it works well with a Ronstan 55. (Can't remember the diameter, but probably the largest for which the Ronstan is rated)
~ green ~ lake ~ ~
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