Rule in Question

VCR C.7.2 (a) (4)

Cleats for all sheets shall be fixed in the hull. 

Question

Can I use a swivel jammer as shown in the picture for my mainsheet? Alternatively, can I use a swivel cleat directly on the boom for the mainsheet?

Answer

No.

Any cleats for any sheets need to be fixed directly to the hull. 

By definition, the main thwart and side benches are part of the hull.

Any fixed wedges under the cleat to adjust the height and / or angle are legal, as long as the cleat is not moving.