New Posts New Posts RSS Feed: Mainsheet double bridle
  FAQ FAQ  Forum Search   Register Register  Login Login

Mainsheet double bridle
 Post Reply Post Reply Page  <1 3456>
Author
  Share Topic Share Topic  Topic Search Topic Search  Topic Options Topic Options
Guests View Drop Down
Guest Group
Guest Group
Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Topic: Mainsheet double bridle
    Posted: 06 Jan 17 at 2:50pm
P.S. if you want clear space to move freely under the boom, but don't want the issues of sheeting directly from the boom (moving part, still got a sheet floating mid air which can catch on crew), then you can have aft sheeting, run the sheeting inside the boom, down the mast, under the deck and coming up out of the floor. I've seen that on some yachts. 





Edited by mozzy - 06 Jan 17 at 2:56pm
Back to Top
PeterV View Drop Down
Posting king
Posting king
Avatar

Joined: 24 Feb 07
Location: United Kingdom
Online Status: Offline
Posts: 131 		Post Options Post Options   Quote PeterV Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 3:06pm
Sorry Mozzy, your calculation of 2.5:1 is wrong, and your example of the yacht arrangement demonstrates why, as you can arrange any number of blocks to change direction, including taking the sheet to the mast and back but it doesn't change the purchase at the end of the boom which is where the power is applied. It's very simple to demonstrate as the ratio (strictly the velocity ratio) is the amount of sheet pulled vs the amount the moving part moves. This is not affected by where the end of the sheet is arranged.
PeterV
Finn K197, Finn GBR564, GK29
Warsash
Back to Top
Guests View Drop Down
Guest Group
Guest Group
Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 3:19pm
Originally posted by PeterV

Sorry Mozzy, your calculation of 2.5:1 is wrong, and your example of the yacht arrangement demonstrates why, as you can arrange any number of blocks to change direction, including taking the sheet to the mast and back but it doesn't change the purchase at the end of the boom which is where the power is applied. It's very simple to demonstrate as the ratio (strictly the velocity ratio) is the amount of sheet pulled vs the amount the moving part moves. This is not affected by where the end of the sheet is arranged.

You can add any number of block and they don't add purchase as long as those blocks don't move in relation to each other as the boom moves. In the linked diagram as you sheet in, block C at the goose-neck doesn't move in relation to A on the boom or D on the floor. It is therefore a fixed block and adds no purchase.. so the system stays 2:1, despite all the extra blocks. 

When the rope comes off the boom at the mid point, that is a moving block (it get closer to the deck block as you sheet in). It's just that block in the middle of boom doesn't move as much the ones at the end, so has less impact on the velocity ratio. And like I said if you're taking the rope directly from midway on the boom, as you move around the ratio will change. 


 


Edited by mozzy - 06 Jan 17 at 3:46pm
Back to Top
Sam.Spoons View Drop Down
Really should get out more
Really should get out more


Joined: 07 Mar 12
Location: Manchester UK
Online Status: Offline
Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 3:49pm
I agree, the 'leverage' applied, assuming a take off exactly half way along the boom with the main 2:1 at then boom end will be half that of applying an extra purchase at the end (as on an old Ent or GP with a transom fitted traveler). The tail of the sheet coming off the boom is still contributing to pulling in the boom though. The conventional method of calculating mechanical advantage for a block and tackle is to calculate the movement in the load block(s) and the amount of rope pulled to achieve that movement, in the above case that would be exactly 2.5:1 (but, bear in mind that I'm disregarding the angles involved for simplicity and the mechanical advantage will change due to the angle of pull as the boom approaches centred).

Edited by Sam.Spoons - 06 Jan 17 at 3:50pm
Back to Top
Do Different View Drop Down
Really should get out more
Really should get out more


Joined: 26 Jan 12
Location: North
Online Status: Offline
Posts: 1312 		Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 4:31pm
Go on SS. Block (  Wink )  ratio according to my wife is the number of blocks divided by the number of blokes talking times the metres of really expensive dyneema 98 purchased. Another round anyone?  Beer
Back to Top
Sam.Spoons View Drop Down
Really should get out more
Really should get out more


Joined: 07 Mar 12
Location: Manchester UK
Online Status: Offline
Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 5:26pm
Geek Yup that works for me......
Back to Top
JimC View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 17 May 04
Location: United Kingdom
Online Status: Offline
Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 6:29pm
Actually chaps its all do do with how many angels are dancing on the pivot pin of the last block in the system...

As has been alluded to the raw purchase is a very blunt tool for comparing systems because the other factors like boom length and anchor position come into it so much. I suppose if there were ever a need to develop a standard method of calculating leverage one would have to measure the amount of sheet pulled in to move the boom from say 45 degrees to 35 degrees.

Fortunately practically speaking all we really need to do is compare similar systems, and purchase does the job...
Back to Top
Sam.Spoons View Drop Down
Really should get out more
Really should get out more


Joined: 07 Mar 12
Location: Manchester UK
Online Status: Offline
Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 7:04pm
The Blaze is 2:1 at the back plus the sheet from the boom mounted ratchet (giving a small extra purchase). It's noticeably lighter than it was with 4:1 centre sheeted, which is to be expected as that was equivlant to 2:1 at the boom end but with no additional purchase and twice as many blocks adding friction.

I have 6:1 on the Spice centre mainsheet, sheeted from about halfway along the boom. I end up pulling more string but can get the leech tensioned while out (often somewhat unsteadily) on the wire. It still feels heavier than the Blaze but, again, with a 12m+ FB mainsail and 6 blocks I guess that's also to be expected. I suppose I might try twin extensions, off the boom and a rear bridal there too one day (I don't suppose anybody will be protesting me as I hack around racing the Ents over the water)


Edited by Sam.Spoons - 06 Jan 17 at 7:05pm
Back to Top
Wiclif View Drop Down
Groupie
Groupie
Avatar

Joined: 04 Oct 05
Location: United Kingdom
Online Status: Offline
Posts: 82 		Post Options Post Options   Quote Wiclif Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 17 at 7:45pm
In my (much) younger days I used to sail a 505. I tried reducing the main sheet purchase to a 3:1, instead of the normal 4:1, on the basis that the Finn had the same mainsail size and it used a 3:1.

First time I tried it, it was blowing hard, and I could hardly hold onto the main sheet on a fairly broad reach.

Thinking about it on the return to shore I realised that the Finn rig was noticeably less powerful. It has just one person sitting out, whereas there were two of us. We also had another sail, and the jib was only half the size of the main and yet we also had a trapeze.   So loads more power for the same sail size
Back to Top
Do Different View Drop Down
Really should get out more
Really should get out more


Joined: 26 Jan 12
Location: North
Online Status: Offline
Posts: 1312 		Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jan 17 at 7:52am
Of course you all know this but sheet loads depend a lot on what you are expecting from the main.
I know of a couple of big two person trapeze boats that use a direct split tail main. In these cases the only job of the main is to open and close the sheeting angle, the leech tension is all kicker, in fact if you don't have enough kicker applied the main loads are pretty unmanageable. 
SS. You confirm the point by saying 2:1 boom end is much lighter than 4:1 mid boom. I suspect the boom end is applying less downward pressure than the mid boom setup.
Some classes like the last few inches of main sheet movement to open and close the leech and it is easier to play one control than trying to juggle the kicker or cunningham. (Spice?)
Back to Top
 Post Reply Post Reply Page  <1 3456>

Forum Jump Forum Permissions View Drop Down

Bulletin Board Software by Web Wiz Forums® version 9.665y
Copyright ©2001-2010 Web Wiz
Change your personal settings, or read our privacy policy