Preventer Sails and Rigging- Standing and Running Forums

Great!  Then maybe you can explain it to me, LOL !

Here is my interpretation which may not be right and if it's wrong maybe I'll delete it 😛

I guess they made two identical, short "boom messenger lines" out of amsteel blue SK75 (or similar line).  Each end of the messenger lines have a snap shackle. The lines are attached to the boom to be ready to use.  One line on each side of the boom running along the boom (port side & starboard side).  Attach one end of the line at the boom gooseneck, and attach the other end of the line at mid-boom.  Then, run two permanent foreguys along each beam: one end attached to a strong point at the bow (through a block if necessary to run back to the cockpit along the beam outside of all hardware), and the other end attached to what(??) in the cockpit. 

A simple diagram would be worth a thousand words at this point.

Set up the preventer before the gybe by going to the mast and clipping the messenger line's gooseneck-end snap shackle to the permanent foreguy.  At this time the foreguy is slack.  Then gybe.  Then tighten the foreguy(??) to set the preventer.   When rigged, the foreguy runs tight along the beam, the messenger line is clipped around the foreguy (but free to run bow-to-stern??), and the other end of the messenger line is attached to the mid-boom. 

Remove the preventer before purposeful gybe by going to the mast and unclipping the mid-boom end of the messenger line, so the messenger line's snap shackle on the foreguy is free to fly back to the cockpit where the messenger line can be unclipped from the foreguy.   Then the messenger line is completely free. Then re-rig the messenger line on the boom to be ready to go again.

It seems the downside of this setup is that it requires going to the mast to set and to reset the preventer for each gybe.  The upside is that the attachment point is mid-boom so it can be reached easily (compared to end-boom preventer which might require leaning out over the beam to un-rig).

Here is what a local sailing guru had to say about the original description, when I sent it to him:

If you're concerned about the boom coming toward the centerline only, this would do the job and it would have an optimal lead angle for preventing the boom from moving in.  The thing it does not do, as a more traditional preventer would do is to work as a vang at the same time in that it will tighten the leech and minimize twist.  If you have a vang in addition to the preventer – you're covered.

I guess the other thing is that if you are flying a spinnaker at the same time you'd have a weather foreguy for the spinnaker pole and a leeward one for the boom.  Okay, now when you jibe you'd have to switch them.  No biggy, especially when cruising and you jibe weekly vs. moment by moment.