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.
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