Bill,
Timely question.
First, you cannot legally carry external loads on aircraft in the Lower 48 States, unless its a Beaver or an Otter. I'm not sure this applies to Experimental (amateur built) aircraft, but I believe it does.
The deHavilland airplanes have canoe racks and kayak racks, as well as lumber racks, approved on their type certificates. Therefore, it is legal to carry external loads on those aircraft.
The Alaska Region of FAA has issued (a long time ago, but it's been recently updated) a policy on external load operations in Alaska. It is a good, logical policy, which probably explains why it hasn't been adopted nationally. That said, there is talk of doing just that.
Commercial operators CANNOT carry canoes unless they are operating one of the deHavilland aircraft mentioned, EVEN in Alaska. The airplane has to go into the restricted category, which is prohibited under virtually all 135 certificates.
Now, as to the size of canoe that actually works, assuming you are in Alaska, it depends on the type of aircraft. On a Cub type airplane, I really like to keep them short. I've carried 15 foot canoes on a Cub, but it ain't real pretty. The boat sticks way too far forward for comfort (upsets longitudinal stability) and the stern is back there playing with water rudders.
A 12 or 13 foot boat works just fine on a Cub. Two of them is a bad idea, unless you are REALLY light in weight. Two canoes with a GW load can give you religion.
Kayaks fly better than canoes, and I wouldn't be too afraid of two kayaks on a Cub. Again, try one first, see how it works, then try two at VERY light weight, see how that works, and then put a little load at a time in it.
Believe me, just a little extra weight with an external load can really get your attention, so go stepwise.
And, if you're in the lower 48, don't let the FAA catch you.
If you're in Alaska, get a copy of the external load policy from a FSDO, follow it and get a restricted category A/W certificate, and follow the instructions in the policy. It's good advice, generally.
I use ratcheting cargo straps to secure loads. I don't like rope, cause it's too hard to get really tight, and too hard and slow to untie. Get the cargo straps that have welded rings and hooks, and a good strong strap. They will last forever if you take care of them.
Oh, by the way, your insurance will be non-existent if anything happens with an external load, since you've changed the airworthiness category of the airplane. Unless, of course you can talk them into a rider on your policy
. Yeah, like that'll happen.
MTV