: Another thing I am wondering...

: Do I keep the full 3/8" or 1/2" diameter of the willows where they
: are mortised into the gunwales? My gunwales are only 3/4" thick, so
: that seems like it would not leave enough wood on either side of the
: mortise for strength. Should I shave down the ends of the willows to
: 1/4" and have more oblong mortises?

I don't know that I would go down to 1/4 inch. That seems a bit small. With the 1/2 inch diameter pieces you could flatten two sides just a bit--knocking off 1/16th on each side to bring it down to a rounded edged oblong roughly 3/8 by 1/2. Or, bring the entire end down to a round 3/8 piece.

There are devices called tenon cutters, which look like pencil sharpeners. They are used for shaping the ends of branches used in making log furniture. You chuck them into a hand drill, feed in the rough wood, and they'll chew a nice round end out of a strangely shaped tree limb. Picture is below from:
http://www.rbosworth.shoppingcartsplus.com/tenoncutters.html and the smallest one they offer is for making a part which is 3/4 inch in diameter. It is $350. Probably a bit big and pricey for your needs, but the picture looked good

However, Lee Valley has something for $35.50 in the right size (3/8) Check out:

http://www.leevalley.com/wood/page.aspx?c=2&p=44248&cat=1,180,42288,45539

A dowel or plug cutter, usually used on a drill press to make short plugs for filling in over recessed screws, is another tool that can be used to make those round tenons.

Finally, there are jigs you can make (or buy) which allow you to use a router with a beading bit. to reduce the size of the end of your rib stock. The basic trick here is to mount the router so the cutting edge of the bit is 3/16ths away from the center line of the hole you'll shove the stock into. Rotate the rib stock as you insert it, and the router will chew off anything that sticks out too far, leaving you with a round end which is 3/8 diameter. Personally, I'd be very careful about how I made something like this. Too many possibilities for safety problems.

However you turn down the ends, this way you can use a 3/8 drill bit to make your mortises. You'd just fit in the 3/8th ribs, and the turned down ends of the 1/2 inch ribs would fit into the same size hole. A 3/8th inch hole in a piece of 3/4-inch-thick stock should still leave it pretty strong.

Hope this helps

PGJ