Dressy nuts? Interesting image that brings to mind. :-)

So you're saying nuts and washers on both ends rather than a carriage bolt top and a nut and washer on the bottom?

: Great question! Unfortunately, I don't know the answer, only a guess.
: My first guess is that the cost savings of buying the shorter bolts would be
: more than offset by the cost of labor to join them. Not only that, but now
: you have welded metal in a marine environment, which can cause problems.

: Here is an idea: try to source threaded rod in the material of your choosing,
: and some of those dressy nuts. With a little work with a hacksaw and a
: file, you can have 8 custom lengths with which to attach your seats.

: Hope this helps,

: DAve