Yes, Gini, the diameter is 11." The 15" measurement is the vertical distance from the top of the ropehold to the bottom of the fender. In our opinion, the advantage of these round fenders over the tubular style as provided by Ranger is that they seem to roll more as the boat shifts forward or aft. Additionally, their diameter is greater than that of the tubular fenders (8") , thus providing greater distance between the boat and lock wall (or dock, etc).
Yes, we are increasing our number of round fenders from 2 to 4. The original 2 passed our "use test" last summer in the Pacific Northwest and this winter on the west coast of Florida. Our previous locking experience was on the Rideau Canal (and others) when we had a Monk 36 trawler. We learned that having several fenders deployed at varying heights was very effective at fending off. You never know what configuration you will encounter when locking through, so it is best to be prepared for everything! We intend to have ready for deployment 2 round fenders and 2 tubular fenders on each side. So, when the lock master directs us to a particular side of a lock, we'll be ready.
Regarding storage, when not in use we keep our aft fenders lashed to the cockpit railing, ready to toss overboard as needed. They are not in the way.
Let us know what you decide, and most importantly, after you have used them please let the Tugnut community know your opinion.
Regards, Gregg and Kathy
Santolina, R25SC