I must post a bit of correction to Ann's post above (sorry, Ann).
First, while weasel piss is great stuff, it isn't appropriate for this problem, nor is any other penetrating oil. Just doesn't work
in stainless/aluminium seizures which are mechanical in nature.
Second, since the items that are frozen in place are U-bolts, attacking with an impact driver isn't appropriate either. One could be of use to remove the nuts on the U-bolts, but I don't think they are the problem... it is getting the bolts themselves out of the alloy casting that is the issue.
IMO, the only useful approach will be to heat the hell out of them and then drive them out while everything is still smoking hot. It may take repeated efforts, but if you get the casting hot enough it should eventually come good. If possible you should support the casting while beating on the bolts, for there is a danger
of breaking off the tang that the bolts go through (I'm guessing here that the bolts are in the horns sticking forward for the spinnaker
Good luck, mate... this could be a tough one!