Centre Mainsheet

  • 69572
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08 Apr 2010 22:31 #20633 by 69572
Centre Mainsheet was created by 69572
I'm in the process of converting my boat to a centre mainsheet, using bits of kit from the garage (and the odd visit to ebay!). I noticed the centre mainsheet kit that Trident sell contains a wooden bracket for attaching to the centreboard case - has anyone got any images of this? I was thinking of making a loop of (about) 4mm line all the way round my case, with an eye tied at the rear, into which I could shackle my ratchet block. Without using the Trident fitting, is there a better way to achieve an anchor point for the bottom block? Obviously without something solid to mount the block to, I won't be able to use any sort of stand up spring, and will have to rely on the tension in the sheet coming down from the boom to hold the block up. As the sheet will be having to travel an extra metre or so to the forward block, and down to the centerboard case, what's the recommended length (and rope) for the new sheet? Looking at what's listed on Trident, it seems Jimmy Green have similar stuff listed as Marlow Excel Fusion - see http://www.jimmygreen.co.uk/marlow-excel-fusion.html

Also, is there anything in the rules that permits / bans a mainsheet cleat? My boat was built with a bullseye fairlead running to a cam cleat mounted to the vertical face of the rear bouyancy tank, which I've never really used due to the extra friction thru the bullseye (may be something to do with the current mainsheet). Although my son is just coming 5, and has been for a few sails, when I'm racing I'll probably be singlehanded, so would benefit from the ability to jam the mainsheet during spinnaker moments.

Thanks

John

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14 Apr 2010 08:14 #20639 by Martin Egan
Replied by Martin Egan on topic Centre Mainsheet
Hi John,

I've not got an image of the Trident kit, but I understand from another Mirror owner at my club that there is a knee that goes down the back of the daggerboard case with two plywood pieces that clamp either side of the case.

You can lash a block to the case in the manner you describe. I've done this as a quick and easy way of converting boats. Snag is that the rachet block flops around when there is no mainsheet tension.

If you are not planning on using a mainsheet cleat, you can fit a triangular block glued to the underside of the thwart and to the back edge of the case. This needs to be about 100mm long by 50mm high (check this is enough for the eye you are going to fit) and about 20mm thick. This size will work if you daggerboard knees finish level with the back of the daggerboard case. If they extend further aft than that, you may need a shorter block that fits between them. I'll e-mail a photo.

If you want to use a mainsheet cleat (yes, they are now legal) you need something that forms a horizontal platform. I just have a block glued under the thwart and a little knee down the back of the case to support it. I'll e-mail a photo of this

The length of sheet depends on how much purchase you want to have. To keep costs down you can re-use your exiting mainsheet. You will need to tie a piece of line (3mm, 4mm) with a loop in the middle of it to the two mainsheet attachement points on the transom. This is called a hawse. If you tie the mainsheet to this, then up to a block near the end of the boom, along to one near the middle of the boom and then down to your block on the back of the case, you should find your old mainsheet is long enough. This arrangement give a purchase of about 1.5:1 which is a bit less than the original aft mainsheet arrangement which is 2:1.

If you want more purchase, you can put a block on the hawse, then start the mainsheet on the boom (ideally have a becket on the block near the end of the boom and tie it to that), then down through the block on the hawse, then back up to the block on the boom, along the boom to the block near the middle of the boom and down. This gives a purchase of 2.5:1 so you need a longer sheet, but you may find your old mainsheet is still long enough. If not, you need to buy a longer one. I calculated that 8m should be plenty, but you might get away with 7m, depends how long a tail you like to have with the sail right out. I use 8mm 16 plait matt, but then I'm a bit "old school" regarding this. If you like the feel of a polypropylene blended cover, I'm sure the Fusion will be fine.

One drawback with centremain is lack of space for the crew between kicker and mainsheet. You can increase this by:
1) having the block in the centre of the boom a bit (100 - 150mm) further aft, so the sheet does not go vertiacally down, but at an angle.
2) move the kicking strap fixing block forward by 50mm or so (there is no measurement restricition on this anymore).

Don't forget to have some sort of constraining loop or sleeve on the boom to stop the mainsheet dangling down and strangling the helmsman. You could re-use that bullseye off the aft tank for this.

I also fit a gybing strop to the boom with the mainsheet running through a ring in the end of it.

MCA Webmaster 2011 - 2018

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14 Apr 2010 17:12 #20640 by Nigel Thomas
Replied by Nigel Thomas on topic Centre Mainsheet
Hi - here's a quick view of how i attached the block directly to the thwart. Although this was on a Trident grp boat, a similar principle could be used on a wooden boat. It uses :
- Holt 3 hole swivel base HA4222
- Harken U adapter H093
- Elastomer plastic spring R1814
- ratchet block.
There was no issue with crew space, nor did the crew end up sitting on it, nor tearing any clothing. It works well.
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15 Apr 2010 23:58 #20642 by bobt
Replied by bobt on topic Centre Mainsheet
That phot looks like a lot of load onto flexible plastic. My Trident has a glass fibre strap with an eye moulded into the inside of the hull, aft of the centre board slot, with an small fairlead at the thwart. A length of line is then lead from the hull mounted eye through the fair lead and a spring to the ratchet. This would be dificult on a wooden boat and therfore something similar to the kit mentioned earlier would be needed, the loads need spreading. both these methods are, both expensive and permenant fixtures so I can see the advantage of trying the loop around the centre board case, as not all of us see centre main on a mirror as an advantage. This at least this gives you a chance to try it at a relitivley low cost, but I think I would fit the fair lead, packed as neessersary to the aft edge of the centre board case with a spring above to support the ratchet.

Bob.T (single handers rep)

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16 Apr 2010 22:30 #20643 by 69572
Replied by 69572 on topic Centre Mainsheet
All

Thanks for your comments and suggestions, especially Martin for some off-forum advice and photos of the 'shelf' he's built under his thwart. Without manufacturing something from wood (I gave it up in about P3, many years ago), the idea of the fairlead and spring sounds interesting - if I'm imagining this correctly, without the benefit of a hull mounted eye, I would still use my centreboard loop, but pass it 90 degrees up thru a fairlead on the back of the case, forming a small loop, which should pass thru a stand up spring and the shackle on the ratchet block? (Sorry about the 'thru's, too much working for a US company!)

As I discovererd last weekend, without any load on the block, the loop does tend to slide down the centreboard, causing the block to bash the thwart. I suppose a benefit of the fairlead and spring block would be a reduction in the play of this loop - if the idea works well, would there be sufficient strength in the centreboard case to fit a length of (say) aluminium strip, about 150mm by 15mm by 2mm (what I have in the garage) to each side of the aft of the case, creating a tang that I could pass a piece of line from one side, up thru the fairlead, spring and shackle, and down again, back to the other tang ?

Thanks

John

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19 Apr 2010 22:31 #20645 by bobt
Replied by bobt on topic Centre Mainsheet
That all sounds feasible John, hope it all comes together.

Bob.T

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