If I can add to all the above great info. From experiance, you want your chain to flow as cleanly and smoothly as possible. If you don't and you get a bunch up, it is entirely possible to have a wrap around or some form of a jam with the gipsy. The amount of force the gipsy can apply is freightning and WILL cause damage. Trust me, I have been there and had that happen. And it always happens at the worst possible time. I have since re-engineered the chain system and it works a treat.
One idea to make the chain quiter and slip easier is to line the inside of your pipe with a teflon plastic material. Although this will mean you need a larger diameter pipe. Thye next best thing is to use Bronze. Bearing bronze would be easy to get and is good at corrosion
resisitance and easy to machine. Impossible to bend, so it would be a straight pipe. Bronze is "slippery" to steel
, so the chain would slide. Do not use Galv pipe, it will wear and rust quickly. SST would be your cheapest easiest to shape choice offering the greatest resistance to corrosion
As to the chain hitting the side of the Ply boat, I would coat the sides in pleanty of Epoxy
resin and then do a final coat using Epoxy
Carborundom (spell?). This produces a very wear resistance surface. Or if you want to get a little techy, use a layer of Kevlar matt. This has excellent wear resistant ability.