18nv,
If its an inflatable, why tie it on the outside? I would not recommend trying to carry anything inflatable on the outside of a plane, unless it is WELL braced to prevent it from changing into a huge parachute or air dam, more specifically.
Weight is not the issue at all when it comes to external loads. It is all about aerodynamics, and the aerodynamics of the load you are carrying.
Yes, I've carried canoes and kayaks on the outside of Cubs on EDO 2000's and Wips as well. No big deal, BUT be very careful of the legalities, which will vary, depending on where you operate. If you are going to be in Alaska, no problem, get in touch with the FAA and get an External Load authorization. If you're not in Alaska, good luck.
MTV