I agree with Ray about not putting the boats on top of the trailer. He's got a good point about the stresses involved. Besides the structural issue, there is also the issue of what you're going to do with them once you get to the campsite. You have to remove them from the trailer to put it up, and then you have to carry them to the water. If you do river trips with shuttles, you still need a way to transport the boats separately from the trailer. As another downer, if you have an airconditioner on top (probably not an issue with an '89) there really isn't enough room on either side of the A/C to put the boats unless they are extremely narrow racing boats.
I had a number of Coleman's before the TM, and I tried various ways to put the boats on top. There just wasn't a good way to do it for anything bigger than my little 13' solo canoe. Even then I needed a good set of tie-downs on the bow and stern in addition to 2 straps over the top of the canoe. The stresses are pretty high! We ended up putting the boats on our existing Yakima rack system on the tow vehicle. It isn't hard to put 2 boats on the extra long bar (I think it is 72"
. Note that the boats overhang the side of the vehicle rack on our Ford Expedition, but are just about even with the width of the mirrors. It is illegal in some states to have "stuff" hanging outside your vehicle width.
An "average" tandem canoe (Royalex) weighs 65-75 pounds. Kevlar might be as light as 35-40, aluminum as heavy as 85-100. A kayak can be anything from 30 (for a solo) to 75+ (for a tandem recreational kayak). That's a lot of weight for any roof-top system. Make sure you check the capacity of any rack you get, and make sure you use bow and stern lines in addition to the belly lines. My two tandem canoes (40 pounds and 65 pounds) come in under the 150 pound limit of my rack system, but many vehicles only have a 100 pound limit.
Boats that aren't tied down safely can come off. Think of the results if a 70 pound canoe is lost from a rack while driving on the interstate at 70 mph. The results will be a destroyed canoe and potentially a fatal accident if the boat hits another vehicle. This isn't something to mess around with. I know folks who have had this happen! Luckily none of the boats have hit another vehicle, but it isn't something to take a chance on.
Pam (who teaches canoeing and always shows her students how to tie down their boats safely)