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Rigging question.

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423zero View Drop Down
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Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Topic: Rigging question.
    Posted: 27 Jan 19 at 12:07pm
I intend to change from centre main to centre/aft main, the reason, boom is very long and mainsheet block is to far forward, causing issues in strong wind.
I have blocks to do 'Laser' style system, but don't like the idea of miles of mainsheet round my feet, would like mainsheet to go straight from a single block on traveller to single block on boom and then to run through existing mainsheet blocks, hopefully this will make for a shorter mainsheet.

Existing mainsheet system.

https://photos.app.goo.gl/v7VJVU6sSocn4fmUA

New system, if it will work.

https://photos.app.goo.gl/FSVXcux4tQUtqpWa9



Edited by 423zero - 27 Jan 19 at 2:11pm
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JimC View Drop Down
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 1:40pm
I can't get to those URLs. Check security?

If you end up with the same load on the mainsheet you'll have the same amount of rope round your feet, it makes no difference whether its 4:1 to the middle of the boom or 2:1 to the end of the boom.
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Post Options Post Options   Quote jeffers Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 1:43pm
Getting http 404 errors for both of those links.

Can you use a rear bridle/split tail type mainsheet? Then you just need a block on the boom. I seem to recall this is what the Blaze has now over the true centre main setup (which has reduced sheets loads a fair bit as well).

Paul
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423zero View Drop Down
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Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 2:12pm
Links now working.

Jeffers,
Not sure, but I think you need traveller to prevent boom being pulled to centre of boat, boom should only be pulled to corner of boat.


Edited by 423zero - 27 Jan 19 at 2:45pm
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 2:18pm
What's the boat? That will work, I guess but on my vintage moth I had a block on the bridle for 2:1 at the stern, and then off the boom at the kicker anchorage, which would be way simpler.

If its a Minisprint that ought to be more than enough purchase and much less friction.

Edited by JimC - 27 Jan 19 at 2:20pm
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Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 2:39pm
Minisprint. 
Existing system works well until wind is strong, then you are really hauling at mainsheet and having to use camcleat in between pulls. 
what ratio is it ? I think it is 3to1 ?

Have tried off the boom can't get comfortable with it, rear main I have same issues, I have practiced and practiced with rear sheeting, just can't wear it.
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 3:02pm
To compare purchases on mainsheets its often easier to just mock it up with string, because the boom take off position has such a major effect. Just rig it up with the current system, and your various proposed new ones with strings and shackles for pulleys, and measure the amount of string between full out and full in.

The current setup isn't one I'd like very much, because the last foot or so is at a very unfair angle. It is a 4:1, but because the take off is so very far forward its probably equivalent to about 1.5:1 at the stern.

I'd definitely have a fixed stern bridle (but then I've had that on all my boats for the last 30 years!). The minisail sail is really rather small, and you might even get away without a purchase at the stern, but I'd start with sheet from the boom, down to block fixed on the bridle, then along to to where you have the mainsheet now, but no purchase there. You will find it very easy to oversheet like that, and you'd need to use solely the kicker to control leech tension - not such a good idea if you have an original flexy minisail boom!

Edited by JimC - 27 Jan 19 at 3:05pm
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 3:28pm
The Blaze system is basically similar to a Laser but with the mainsheet straight of a ratchet on the boom (attached to the kicker take off). Works an absolute treat with significantly lower sheet loads than the centre mainsheet. Putting the ratchet on the turret would not work well in the Blaze with the racks requiring a 1.5m+ tiller extension (and the need to stay forward when tacking in the light). With a centre main you pass the extension around the back, with off the boom, it goes around the front, if you did it Laser style you'd need twin extensions.

Are you trying to reduce sheet loads when it's windy or get rid of a pile of mainsheet in the bottom of the boat? FWIW the off the boom system on the Blaze needs about 1.5m longer mainsheet to make up for the bit between the ratchet and block at the back end of the boom. Having fewer blocks in the system probably helps reduce friction a bit.

Oh, and as JC says, on a unarig you definitely need a traveller of some kind or you won't be able to get leech tension without overshooting. If there is a jib then centring the boom is usual so twin tails or a tower works but a unarig usually needs the boom over the quarter. Also as Jim says I'd go 2:1 at the back end of the boom and no purchases up front just Laser style from the boom block to a ratchet/cleat on the floor.


Edited by Sam.Spoons - 27 Jan 19 at 3:38pm
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
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Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 4:20pm
Sam,
Want to reduce sheet loads, 'Laser' system would add loads of mainsheet, using the system I have described you will just add the length along the boom and the length from traveller to boom when you are running, 'Laser' system would add double the length from traveller to boom when running.
Boom is 65mm diameter so quite a lot stiffer than original.
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 19 at 7:35pm
As Jim says you need twice as many purchases to get the same advantage with a centre mainsheet, so the mainsheet used is 4 times the distance between the blocks when running and a bit between the ratchet/cleat and your hand. The upshot is that you end up with more or less the same amount of mainsheet in the bottom when sheeted in upwind (well mine did in the Blaze).

It's relatively simple physics, if you want to lower sheet loads you need a greater mechanical advantage, you can gain that with more purchases in the centre or by increasing leverage by having them at the end of the boom. Either way you are pulling more rope which ends up in the bottom upwind....... 

I'd mock it up with an old halyard or whatever you have that's long enough, thread it the various different ways you're considering and measure how much rope moves through the final block between running and close hauled. That will tell you the ratio of each arrangement and how much rope will be underfoot. The mechanical advantage will give a good indication of how much less effort sheeting in will take but bear in mind there is a fair bit of friction and fewer and/or larger diameter blocks will reduce that.

I have replaced the 38mm blocks on my Spice with 57mm last year but haven't managed to sail it since. It has a 12m fully battened mainsail and sheet loads with the original 4:1 were so high I converted to a 6:1 but I'm guessing the extra friction was doing me no favours as it didn't help as much as I expected. I'm hoping that the bigger blocks will reduce the friction enough that I can go back to 4:1 (or engineer a 5:1 setup).


Edited by Sam.Spoons - 27 Jan 19 at 7:37pm
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
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