Friday, November22, 2019

8:1 and 20:1 Cascaded Boom Vang

8:1 Cascade Vang System

Cheaper than using two fiddle blocks, lighter, and stronger. What is not to like? You just run the control line back to the cockpit where you put a cam cleat. You will likely need a turning block on deck but then you have the vang where you want it when it needs to be released quickly before you round up.

An intriguing variation is to use $5 repel rings from REI and Amsteel. It takes about 15 pounds pull to get 10 pounds on the other side of the ring. But that is giving you 25 pounds pull on the ring instead of 30 so the loss isn't as great as you might think. For every ring you use instead of a block, multiply the theoretical purchase by 80% to account for the frictional loss. This 8:1 system would be 6.5:1 with a ring instead of the top block. Even lighter, almost as much leverage, and very strong as those rings are rated at 5,000 pounds.

You could use 2 rings and have a 5:1 system, more than the typical 4:1 that so many people have, at a fraction of the cost. The remain two blocks are now only taking a fraction of the load so they can be very small and light.

If using all blocks, each block down the line needs to take 1/2 the load of the one above it, except both blocks with the red line have the same load. You cannot use rings for the red line as that can't be Amsteel because you can't lock it in a cleat and anything that locks in a cleat has just way too much friction through a ring.

As a final note, you can use two fiddle blocks and get a 16:1 if you want a lot of purchase. The upper two single blocks stay the same. For a 16:1 system, the math would be 2 x 2 x 4 = 16.


20:1 Cascade Vang System on Papoose

The vang system on Papoose is arranged to give even vang tension with boom position even though the primary padeye is aft of the pivot point of the boom. Before I did this system, the vang tension would increase by itself as the boom was let out. This was caused by the geometry of the layout with the pivot point of the boom forward of the padeye that was as far forward as it would go and still be on the cabin top. The solution is shown where there are two stand up blocks on the cabin top that are forward of the boom pivot point. The net is that the forward blocks exactly compensate for the aft padeye location so that there is no increase in vang tension as the boom is let out. The red line starts at one of the beckets of the two blocks on the boom, goes through these one of the stand up blocks, thought the first boom mounted single block, through the second stand up block, through the second boom mounted block, and terminates at a 4:1 pair of fiddle blocks. The red line system has 5 parts so the total purchase is 4 times 5 or 20:1.


16 More Vang Systems

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