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

: What timing! Have I got a question for you.

: I'm trying to figure out what hardware to buy for my canoe seats. The builder
: recommends 3 3/4" spacers on the stern seat and 4 3/4 and 5" on
: the sliding bow seat. Add the width of the gunnels and the seat or support
: thwart, and you're looking at bolts of 6 and 7". In silicon bronze,
: that gets pretty expensive as I need four of each. The prices I've seen on
: the net so far run about $5 each in those lengths and 1/4" thickness.

: The builder says you can just buy extra 4" bolts and have them cut and
: welded together by a machinist shop to get the length you need. If I were
: to do that, is it ok to use the 3/16" bolts Northwoods Canoe sells or
: should I go with 1/4" bolts? I'm talking about silicon bronze. Is
: this something a machinist could do pretty easily? Or would it end up
: costing me as much as just buying the right lengths to begin with.

: Thanks,

: Patsy