I use a 3/8 poly line, just because I found it on sale, lol. I'd imagine that 5/16 will be plenty more than you need (same way I think). Put a loop in one end and the line can serve as the dinghy's anchor line as well (always like to have more than one use for everything). Those mushroom anchors ride the best. I store the outboard on the rail too, as mentioned above. Don't forget, in some states, at least down here, the dinghy has to be registered if you're going to use any type of propulsion, other than the oars. Local officers will seek out the dings and pull them over. Same rules apply to the little boat that applies to the big one (lighting, anchor, throw cushion, pfd's, etc.). I've had plenty of friends find this out the hard way.