What's this thing for?

[Andrew Means]

Found this on my friend's Columbia 26. At first I thought they might be mast track plates to keep slugs in place but they're too small. They seem to fit better hanging on either side of the boom's track, and the webbing positions the block hanging below them in a way so one could rig up a main sheet. So is it some kind of auxiliary main sheet system? Why?

http://distilleryimage10.instagram.com/7a2b60cc0e2d11e180c9123138016265_7.jpg

edit: am I blind or did the option for adding a photo disappear?

 

[Tom Plummer]

Looks like part of a preventer, the part you have hooks onto the boom. The metal pieces fit in the sail track the block hangs below the boom. The other part block and the tackle, the part that is missing is attached to the toe rail when sailing down wind to prevent an accidental jibe.

 

[Morgan Stinemetz]

Preventer

Good call!

 

[Loren Beach]

Yup, it's a vang attachment.

I dragged your pic off to my desktop, renamed it, and used the 'Attachments' dialog box to upload it. It is about a four-click process, and is a tad more work than an earlier iteration of the site software, IMHO.
Just for fun, I cropped it and slightly reduced the rez so it takes up a little bit less room.

LB

 

[Vagabond39]

Some older boats like the Allberg Greenwich 24, and follow on Cape Dory 25 used boom reefing. The boom was pulled aft, and rotated to wrap the mainsail around it. This prevented having bails attached to the boom. Thus a temporary method of attaching a Vang or preventer was developed.

 

[Andrew Means]

Awesome, good to know, I'll tell my friend. His boat doesn't have boom furling, but he also doesn't have a vang, if I remember correctly...