Our Whaler is 11'2" and with the 4 stroke 20 hp Mercury engine weighs in at 659#. Our crane is rated for 800# and we store it on the top. We did not need to reinforce the top. I should point out that we needed to install a new motor and wiring for the crane as the old wiring was too light and burnt out the old motor. Additionally we purchased chocks specifically made for the Whaler. Be advised that the older Whalers weigh significantly less than the newer ones so you will want to contact Whaler for the weight of the one you would like to buy.
Our Whaler is brand new, cost less than a new RIB such as a 12' AB etc. And, it is interesting to note that a 12' RIB does not weigh much less, if at all, than the Whaler. The Whaler is a heck of a boat.
Magic, 1996 Grand Banks Europa
Westport, CT and Stuart, FL