John, you might consider building up a set of tracks with HDPE/UHMW I rowed a Clackacraft last Friday that had such a system. Essentially it was four pieces of the plastic bolted to the bench seat. Stack the pieces such that the bottom sections allow about .125" on either side of the seat base. The next two strips are wide enough to hold the base in place as you slide the seat(s) from side to side. You may already know that you can cut/mill/machine HDPE/UHMW just like wood. An improvement over that system would be to cover the area where the seats slide with another piece of the material to make it easier to slide the seats and not wear the wooden seat bench.
If you build these "slots" the full width of the bench you can either use one seat centered or two spaced for two passengers. There may also be enough metal width in the seat bases to install a threaded knob that could be tightened down to hold the seats in place if they move to easily.
Hope this helps.
If you are going to be in Portland you can get them at Fisherman's Marine for $20.00 each. I just put some on a Fiberglass boat that I replaced all the wood pieces on. The boat I am currently building will have the slides as well but I will likely build my own. I attached a drawing of what I will be making out of UHMW.
Thank you Rick, and Mike: those are excellent suggestions. I was planning on running the track all the way across to allow one or two seats...and/or to simply adjust the balance. From what I've heard the UHMW is very strong stuff, i was originally skeptical. It seemed that if you were to lean back in the chair too much you may bust the chair out of the track. But seeing how UHMW is actually used and manufactured for this purpose i guess they will work. Now i have to find some... does anyone know of a source in Bend, OR? Or is there a website that has the best price on 1" strips?
Mike that is so great that you posted your own design for me to see and possibly use. This website is fantastic and everyone on it is so nice. Forums are commonplace on the web these days and I've never seen one where everyone is so positive and helpful.
Bend Plastic supply on American Lane has some UHMW and can order some, pricing is a little jacked up there though. Interstate plastics on the web has some pretty good pricing that gets much better the more you order. Not sure wich would be better for a small job.
I was just thinking, I need to order some UHMW from interstate, if you want I can ad some for you since we are both in Bend. Let me know what you want and I will l get a price for you. I will be ordering for seat slides, oar blocks, anchor bang plate and motor plates.