having seen one on the merlin rocket page of the speed sails catalogue does anyone know what they're for?

There for mainsheet/kicker take offs on carbon booms when you dont want to be drilling to many holes. the strops spread the loads (hopefully) & therefore prevent to many breakages.

Strops are what crews throw when you shout at them.

that'll be why theres a market for them then

so what stops them sliding down. without drilling holes?

There is a bit of string tied from the outboard boom-end fitting, tied to the strop, also tied to the kicker-strop if you have one.

This means that the kicker pulls it inboard and the string holds it outboard and hey-presto, it doesn't move!

The superpars boom i had on Armed Forces had two stainless eyes on the top for the mainsheet strops. The kicker strops were tied to one of those.

On my new boat there is a thin piece of tufnol glued to the top of the (Still superpars) boom and with lots of holes drilled in it. The kicker strop is looped through these holes.

Drilling holes in the top of the boom to secure the strop is fine as the load bearing via the strop is in compression.  Otherwise, some form of eye arrangement underneath the booom to kicker et al would be in tension.  Not good for carbon.

I have a P&B sailcloth type thing.

I'm glad I shared that with you

Chipstow booms have their kicker strops glued to the upper surface of the boom - had it like that for 8 yrs and never moved a mm.

No holes, weighs nothing.

GGGGGGGG

Common GGG It must Weight something!!! all those milegrams add up.

I thought it was the holes that weighed nothing