main sheet blocks

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I was looking at the recommended main sheet blocks on the Harken website illustration. There are 57mm blocks on the boom and 40mm blocks on the deck.  The smaller blocks have half of the breaking strength of the larger ones.  I have been considering using the 2152 (57mm Soft-Attach) in place of the illustrated 2149 (40mm soft-Attach) blocks.

I am wondering what the collective consciousness here might recommend, the good the bad and the alternatives.

Thanks in advance.


Many people seem to be using 57mm on the deck and a 57mm and a 40mm (40mm is aft) on aft end of the boom.  Those using braided mainsheets seem to use a 75mm block at the fwd end of the boom.

Geoff Sobering:
I've seen a number of people using all 57mm blocks, too.

Personally, I have a 57mm at the front of the boom, a 57mm for the first aft block, and 40mm blocks after that (all Harken Carbo AirBlocks).
The 40mm aft blocks help get the boom closer to the deck.
The 57mm block isn't a contributor to the deck/boom distance because it gets pulled to a 45 degree angle when you sheet in.

I use a braided mainsheet that runs through the 57mm blocks OK, but does have some resistance to going through the 40mm blocks. This is only an issue when you are coming into the pits and need to sheet out a lot (otherwise the only line running through any of the aft blocks in the 3/16" un-braided part of the sheet). I've gotten into the habit of reaching behind my head and pulling the sheet through the first 40mm block (on the deck) as I'm slowing down after a race (this is a Paul Goodwin technique).

Ken Smith:

Re: main sheet blocks
As a general rule, you are not capable of breaking any block.  You pull maybe 50 pounds, the sheet tension T is constant the length of the line, and at best the block sees twice the line tension pulling one way and the bail the load of 2 T pulling the other. Good luck finding a block rated less than 100 pounds.

I use two block sizes but have used three sizes in the past. Why? Firstly, because my tapered sheet shows the aft blocks a smaller line. Secondly because less line moves through aft most blocks.

What is gimportant is efficiency, not losing pull force to friction or losses making the line bend. The larger the block diameter, the less loss as the line passes through the block.  The line should not rub the sides. However, the larger the diameter, the higher your boom is when two-blocked.

My main sheet is a high strength core tapered from a poly outer cover and the part running through the blocks is very pliable. It just feels more free-running because of that. I think I can get away from larger blocks as a result.

So why different sized blocks?  Cost and height.  Ignore rounding errors And think of it this way:

The end bit of your sheet, near where it is nailed to the boom moves 1 times as far as the boom moves. The next bit between the aft deck block and the next boom block moves 2 times as far as the boom moves. Next vertical bit of sheet moves 3 times as far and the last vertical bit 4 times as far. The front boom block and the ratchet block are also seeing the 4 times motion. (How much sheet do you need to let the boom out 2.5 feet? 2.5 x 4 feet. 10 feet.). Starting at your feet, use bigger diameter blocks for the ratchet and front boom blocks and next boom block. Front deck block can be smaller and each after that could be smaller yet.

Bob Rast DN1313:
I have a braided sheet with small blocks, they have square metal piece on the front and back where line enters, I removed the metal  piece to enlarge opening for first block  also added a cone shape to end of sheet where braided part ends and goes to single line, I used a piece of carpet foam and wrapped with braided fishing line and added some epoxy to keep it from coming apart . Works fine this was a braided line from Ron Sherry, It is a lot easier to hold on to than single line  I can hold with 1 hand when block to block .

Bob Rast


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