Friday, February24, 2017

Unique Mainsheet System Analyzed

From Boat at Oslo Show as seen on Sailing Anarchy

It isn't often you see a completely new way to rig a mainsheet. I saw this posting on Sailing Anarchy and though I would share an analysis of what they are doing and why. Here is a picture of the boat

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The basics of the mainsheet system is that it is a 4:1 course and a 8:1 fine (not rigged on this boat). The interesting thing is that it is controlled mid ship but the rigging is end boom.

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Here is a closer look at the overall system. Note that there are three blocks that tend to cover up the traveler block. They are apparently part of the backstay adjuster.
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Here the control lines are brought forward. Both the backstay control and the traveler control are here. Nothing complicated about this.
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Here is the boom end block. This is one of the more interesting aspects. It is a triple block and has two lines going forward. These lines are on the ends which keeps this block stable. Normally, when a block has lines going both forward and down, it wants to rotate or capsize. But with forces from both ends, I don't think it is going to do that. These lines go down to a double block on the traveler.
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This is the most confusing of the pictures because the backstay blocks and the traveler block are on top of each other. First the traveler block in the background. There are four lines going down from the boom end block and back up to the end of the boom. That makes the traveler block a double. A little odd how that is rigged as three lines come from the aft of the boom end block and only one comes from the front. Some lines might cross if they are not careful on how it is rigged.
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Here is a sketch of how the mainsheet might be rigged from the boom end to the traeler. No lines rub so it is clean. I ran the two outside lines down straight to the double block and the center line crossed over to the other side along with the single line coming down from the center forward position in the boom end block.
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The blocks on the deck deal with the backstay. The last backstay block is a double and all four lines go down to the deck's three single blocks. Not really the topic here but the purchase is 4 x 2 x 2 = 16:1 on the backstay.
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The mid boom mounted block brings the line down to the deck mid ship. It is interesting that it is a fiddle and not a single with a becket. Fortuniately I saw a video of this boat in action and it gest even more interesting when the rigging is complete. Both lines from the fiddle go down to the deck but only one goes to a block and cleat for the course action. The other goes to a single block fine tune so you have a 8:1 fine and a 4:1 course.
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Here is the fine adjustment. You can see that the fine is double ended with one end going to each side of the boat. The course is on the swivel cam block.
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Finally we have the swivel block and cam for the mainsheet discussed above. You can see the blocks and cleats for the fine adjustment although the line is not rigged in this shot.
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Here is a shot of the boat sailing from a different angle so you can see the separation between the traveler and the backstay adjustment. This boat has a little different backstay purchase.

Another work about purchase. I said that main course system was 4:1 but it is a little more than that. With the two line going down to the deck from mid boom, that is another 2/2 purchase so the system is actually 5:1 and 10:1 with the fine. But only 4 lines go to the traveler so the traveler efficiency is diminished effectively shortening it by 20% depending. That is a tradeoff of a system like this. The traveler becomes less efficient because it isn't moving all of the lines.

In summary, this is a very interesting and creative setup. I found it interesting to analyze and hope you enjoyed the write up.

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