12mm dyneema has a much greater breaking strain than the 7500kg you have suggested...it's somewhere in the region of 16,400kg.
When manufacturers provide dyneema sailing rope
, it's often covered in polyester or a similar material to provide abrasion and UV resistance and to make it easier to use on a winch
. The cover doesn't really offer any additional strength but it is factored when they measure the diameter. Therefore, a 12mm 'spectra rope' with a protective cover might actually be 7mm Dyneema SK75 Single
Braid, or similar, which has a breaking strain of 7500kg.
So, to answer your question, 12mm dyneema will be plenty big enough, in fact it will likely be big enough to lift
out of the water
. 12mm double braid, (with a 7mm dyneema core) will also be plenty big enough. If you look at the specs on the blocks and traveller which all come into play when reefing, they will be nowhere as strong as the 7500kg dyneema.
I hope this helps.