A lot of boats in the 30-40' range have a 6:1 mainsheet system, which has often been upgraded from a 4:1. Sadly, even though system power has been increased, some of this gain frequently is lost because the system is incorrectly reeved. This means that the falls of rope in the system will rub against each other when the rope is pulled, markedly increasing friction.
So, how should a 6:1 be reeved? There is a trick. Start with the blocks positioned at 90 degrees to each other:
Once you have positioned the blocks correctly, it is actually quite hard to get it wrong. First, attach the sheet to the becket on the lower block and pass it through the top sheave on the top block.
The rope then leads down to the bottom block and passes through the right sheave (looking down from the upper blocks). Pass the line over the top and exit underneath.
Keep following the pictures. Next, pass the line through the middle sheave in the opposite direction of your first pass.
Next, pass the line over the left side sheave on the bottom block in the same direction as you did on the right sheave.
When you thread the fifth (middle, top) sheave, it is vital that the rope go down to the last sheave (the middle one on the bottom block) without rubbing against any of the other falls of the tackle.
Once the system is threaded correctly, you can cleat the rope and put some tension in the system. This causes the two blocks to sit at approximately 45 degrees to each other, but the ropes will sit squarely in the sheaves and the falls will not touch.
Consider using the back of the cleat as a becket.
This has the advantage of ensuring the bottom block sits up with the cleat nicely presented. If you decide to fit a fine-tune system, the second purchase will sit outside the main system, making it much easier to use.
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