Vang Preventer: Advise where to position

[StuM]

Originally Posted by svHyLyte

in the event the yacht rolls down and one pokes the "prevented" boom in the water, the preventer carries load caused by the flow of water across the boom/foot of the sail.

A couple of posters talking about the boom in the water appear to have missed that the OP has a 40ft beam catamaran.

[UNCIVILIZED]

Originally Posted by StuM

A couple of posters talking about the boom in the water appear to have missed that the OP has a 40ft beam catamaran.

On this you're entirely correct. And such wasn't lost on me. Though I thought that the manner in which I wrote what I did showed as much. But maybe not?

Either way, other readers of this thread may have monohulls, & thus need to know the dangers of certain Vang, & Preventer configurations. Including the need for a panic button on a hydraulic vang, to ease it Right Now, fully, for when the boom's dragging.
Along with a pressue valve built into the system which controls a hydraulic vang. That is purpose designed to automatically blow the vang on it's own if the system's pressure reaches a certain point. With that point being set to correspond with a boom being dragged through the water.

Also, if the vang is controlled by a line that's connected to a purchase system in a mechanical vang. Then the control line needs to be laid out or stored in such a manner that it's super easy to release, along with being free to run. Should said line need to be released due to an emergency with the vang. Such as if the boom's being immersed at speed.

And for some, all of the above is old news. However for the majority of sailors, it's not something which they're aware of, nor have they experienced such circumstances. But it's something which it behoves them to learn... The easy way. Meaning via someone else's mistakes.

[Dockhead]

Originally Posted by svHyLyte

Ideally, a "Preventer" (to prevent an uncontrolled jybe) should run from as near the outboard end of the boom as possible so that, in the event the yacht rolls down and one pokes the "prevented" boom in the water, the preventer carries load caused by the flow of water across the boom/foot of the sail. A mid-boom connection allows the load to bend the boom about the point of connection, which it is not designed to do, and has caused more than a few broken booms. The preventer should pass through a turning block as far forward of the mast as practical on a given boat to keep the angle between the boom and the preventer as great as possible to minimize induced compression in the boom.

The function of the vang is to minimize vertical lift in the boom to control that shape of the leach of the sail. A double vang led to the rails rather than the mast base can accomplish that but cannot effectively function as a preventer without risk to the boom in the manner mentioned above.

FWIW...

This.


Combining vang and preventer simplifies the rig but at the expense of not having either a decent vang nor a decent preventer.

The vang is a primary sail control -- the only control over leech tension whenever the boom is beyond the end of the traveler. You can't shape the mainsail worth anything without one. Don't compromise it by trying to make it do other jobs. A solid gas strut vang ("rod kicker" etc.) is the best type (short of a full hydraulic one etc.) because it holds up the boom and balances it. Will keep it from falling on you if the topping lift is accidentally released. Worth its weight in gold.

As several here have said -- preventer should be rigged from the end of the boom to as far forward as you can get it -- generally, the bow cleats. The more angled it is in any plane, the more the force on it and its anchor points will be multiplied.

[svHyLyte]

Originally Posted by StuM

A couple of posters talking about the boom in the water appear to have missed that the OP has a 40ft beam catamaran.

Stu--

You are correct. Note however that the info only appeared in the OP's sidelight and not in the body of his/her text. Moreover, his/her illustrations were of a mono leading one to assume... Had he/she said words to the effect "I am sailing an xyz with a 40' beam...". Regardless, however, my comments concerning end boom preventers holds true in any case. That said, different ships, different long splices, eh?

[rocco lombardi]

Rigid vang for vanging. "alic cole" preventer for preventing. Cole preventer works as follows; block @ stemhead aft, continuous line running back port and starboard (spliced snap shackles on end are nice) to dedicated cleats on cockpit comings, and padeye @ boom end aft. Clip on end-ease boom out- cleat other side, before gybe, center boom-unclip-clip other side...Easy, safe, and way cool! THAT'S FOR FREE YA'LL

[thinwater]

Looks like a double mainsheet to me. Much simpler. Not my favorite, but not a new idea.

[pbmaise]

Update:

1. Thank you to all for input.

2. I have two machine shops remanufacturing parts to attach mast to rotating ball and ball to mast step. I feel this is in part needed since I have redesigned the rigging and it is fairly low tension. Removing rigging tension reduced compression force on the mast foot.

(My extreme beam is what led to the switch to lower tension rigging. I am employing a spring system that automatically takes up slack on lee.)

3. The machine shop also is making two new chain plates which are being attached to central hull. I have elected to position in line with the mast. The reason is primarily due to the fact it is strongest point being closest to main cross beam box.

4. The chainplates will serve two pairs of lines. One pair is for mast rotation. One pair is for light downwind sailing.

The biggest risk I foresee comes during light downwind sailing. During this type of sailing the boom will be farthest from the centerline and potentially experience the greatest amount of speed when coming across during an uncontrolled event.

Two lines will be similar setup as shown in first drawing with controls led back to cockpit winches for quick release without going outside.

5. When on the wind, or during heavy off wind sailing, the boom position will be mostly about +- 25 degrees from the center line. This narrow range can be controlled from the end of the boom where I can employ two major 3/4" padeyes. This setup will be similar to the one shown in the photo posted by Thinwater.

6. Regarding snapping the boom. Yes a concern. Anything can break. However, likely boom failures have primarily been for hollow aluminum masts carrying a lot of sail. I am specifically down sizing sails here and this boom is nearly solid Philippine mahogany made with West System epoxy and wrapped in fiberglass. The spar was originally intended as the mast on a different large trimaran.

Note: This will be my initial configuration and I will make changes after testing and seeing if crew can be trained. That includes me.