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Mainsheet double bridle

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Sam.Spoons View Drop Down
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Topic: Mainsheet double bridle
    Posted: 06 Jan 17 at 10:05am
You still get the ability to control leech tension as the final purchase has much less affect that the 2:1 off the end of the boom and the direction of pull is never straight foreword. Good point about the tiller extension, this was my main concern with the Blaze but a small change in technique and it's fine. If I did take the final purchase to the floor L@ser style there would be nowhere for the extension to go (not a problem on a narrow boat/short extension but with racks/trap you need long extension).
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 10:18am
True... it obviously depends on how many purchases you have at the back, and to some extent how far along the boom your ratchet block is. But I remember it being noticeable on the 29er (only 2:1 at the stern). It made setting the strop length even more critical, and some times it was easier for the helm to take back the sheet and pull directly down to get the leech tension. . 

Anyway, my general point is that I don't think in many classes off the boom sheeting is the solution. And I don't think it's because they're Luddites that classes like Merins haven't tried it. 


Edited by mozzy - 06 Jan 17 at 11:52am
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 10:37am
A belated thanks Glitcher7 for posting that schematic, if I can't fix a temporary solution to my broken turret I shall give that a try.

I find all these methods interesting, none of them however beat just holding that boom in both hands and sheeting it in, I wish I could come up with something that gives a similar live feel in a boat, at the moment I'm still centre sheeted with a few purchases secured to the turret which I'm sure is why the top of the aluminium turret just ripped open last week.

I don't like sheeting off the rear of the boat, can't stand rope at the best of the times so having loads of it behind me is a no no, it's hopeless in light airs and a potential disaster in gybes if like me you're prone to not holding the wiggle stick thing properly I've lost count the number of times I've had it ripped from my grip as the rope passes across.

Interesting thread though keep em coming.

Edited by iGRF - 06 Jan 17 at 10:38am
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Sam.Spoons View Drop Down
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 11:48am
Yup, it's hard to beat a wishbone boom for feel :) Rear sheeting probably won't work on the Spice with it's 2m tiller extension (would have to go for dual extensions I guess), I have enough issues avoiding clipping the moving ballast upside the head as it is, not being able to pass it around the back in a transition would put the tin hat on it (the crew that is, avoids concussion when I'm the "dick on the stick".....). certainly with a long extension (i.e. big racks or a trapezing helm) boat you need to decide and if you want it off the floor then centre main or twin extensions is the only way. Tacking the long extension on the Blaze around the front needs care not to oversteer even without the mainsheet getting in the way (off the boom and well foreword). We had a discussion about tiller grip a little while ago and dagger/across the chest grip works here where it's definitely sub-optimal in the Spice (for concussed crew reasons amongst others).

Yes, a proper discussion beloved of proper sailing nerds this.....
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Post Options Post Options   Quote PeterV Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 12:11pm
There's a lot of misunderstanding of purchases on here! The ratio is counted at the moving part, which on a mainsheet is the boom. If the sheet starts here, goes to a block fixed to the boat and back around a single block on the boom the ratio is 3:1 (Laser arrangement). If you turn the same arrangement around so the sheet starts at the fixed block, goes around a block on the boom and then back around a fixed block there's only 2 parts on the moving boom and the ratio is 2:1 (Enterprise arrangement). With a bridle at the stern and a take off block further forward the forward block is only changing the position of the take off, the purchase is still working at the bridle position. Clearly if the purchase is moved forward (centre main) the mechanical lever is reduced and the loads will be higher, which is why a centre main usually has a higher ratio than a transom main, regardless of where the take of point is. So a Laser mainsheet is a 3:1 working at the end of the boom.
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 12:11pm
Originally posted by Rupert

Does the change of direction add a purchase? If I have a pulley in the ceiling to lift a weight off the floor, I'm still lifting the whole weight. Aren't I?

It's 1:1 pulling the load up.
2:1 pulling the ceiling down.
Actually 1+cos(angle of rope to ceiling) or something.....?
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Post Options Post Options   Quote Dave.B Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 12:23pm
Yes Mossy, no problem with sheeting angles. This is an oval section carbon boom which is really oversize for the boat but allowed the pivot block to be recessed up into the boom so the sheet can run inside and prevent hanging incidents.
With a gantry at the transom I can just get a long enough trapezing tiller extension to be passed thru forward laser style.
The wrap is enough for the ratchet. I thought it would take more work to develop trapeze helming techniques without a cleat, but it hasn't been too bad.
I'm sure I need to do more to tweak bridle lengths and kicker tensions, but without other Farr's for boat on boat comparison it's tough to work out
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 12:38pm
Originally posted by RS400atC

Originally posted by Rupert

Does the change of direction add a purchase? If I have a pulley in the ceiling to lift a weight off the floor, I'm still lifting the whole weight. Aren't I?

It's 1:1 pulling the load up.
2:1 pulling the ceiling down.
Actually 1+cos(angle of rope to ceiling) or something.....?

Yup, and as somebody said earlier if the block is on the weight and the rope fixed on the ceiling lifting up on the fall gives a 2:1 purchase, leading this through another fixed pully on the ceiling and pulling down still leaves you with 2:1 (but an easier job pulling down on the fall would make it seem easier I suspect).


Edited by Sam.Spoons - 06 Jan 17 at 12:40pm
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 2:12pm
Originally posted by Dave.B

Yes Mossy, no problem with sheeting angles. This is an oval section carbon boom which is really oversize for the boat but allowed the pivot block to be recessed up into the boom so the sheet can run inside and prevent hanging incidents.
With a gantry at the transom I can just get a long enough trapezing tiller extension to be passed thru forward laser style.
The wrap is enough for the ratchet. I thought it would take more work to develop trapeze helming techniques without a cleat, but it hasn't been too bad.
I'm sure I need to do more to tweak bridle lengths and kicker tensions, but without other Farr's for boat on boat comparison it's tough to work out

The wrap also gets less in light air when you are sat forwards, and more in a breeze when you move back.
So less drag from the ratchet in lighter air.
Some boats use a two block arrangement on the kite sheets to get this effect.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 2:19pm
Originally posted by PeterV

There's a lot of misunderstanding of purchases on here! The ratio is counted at the moving part, which on a mainsheet is the boom. If the sheet starts here, goes to a block fixed to the boat and back around a single block on the boom the ratio is 3:1 (Laser arrangement).

Erm... except that's not what the ratio refers to. The ratio is a movement ratio (velocity). I.e you pull 2m, it moves 1m. You still have to put the same energy in (by pulling more rope), but the loads are reduced. 

People often work out the ratio by adding up the lines coming out of the moving end... but that only works when you have one fixed point and one moving point, and you're working the rope from the fixed end...

So the laser main does not pull at a 3:1 purchase... It pulls at 2.X:1 because the boom is rotating about the mast, meaning the 'third' purchase off the boom doesn't move as much as the block on the boom end for each degree. If the block was half way along the boom, the maximum ratio the system could achieve would be 2.5:1. But that depends on the angle you're pulling from and whether that block on the boom is actually getting any closer to you as you pull. 

For instance, the crew on a 29er, standing at the mast pulling on the main sheet. As they pull, the block mid way on boom doesn't get any closer to them, so it's adding no effective purchase, and he's got a 2:1 system. Or... if the crew was on the leeward side of the boat, sheeting in, it would even be less than 2:1!

It's complicated still further in that booms don't just rotate in one direction (in and out), but up and down too. 




Edited by mozzy - 06 Jan 17 at 4:11pm
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