Why would anyone build tandem axle cart? The wheels that you normally use to build carts are rated for more than 300 lbs, and a C-tug can typically haul 2 kayaks, so long as they're not 2 Hobie Pro Anglers or Native Titans. It's not because I'm Vector from Despicable Me saying, "Have you ever seen a tandem axle kayak cart. No you haven't, I have the only one."
For me it was because after years of faithful service, my original kayak card, the one with the blue pool noodles, was literally falling apart under the weight of my pedal kayak, the Old Town Predator PDL pictured. The cart did fine with a WS Ride 135 and the Jackson Coosa HD, but it began creaking when I put the heavier kayak with all it's gear on it. Finally, toward the end of last summer, it started fracturing the 'bunk' PVC members one by one. I was able to chisel out the old ones and make repairs, but after 3 consecutive trips of dumping the kayak off the cart in the parking lot, I knew I was going to have to find a remedy.
I thought about the C-tug - a bunch of people have them and like them. Supposedly after you get the tie-down routed, it's smooth sailing from there. They fold up and you can put them in the front hatch like I do with my old cart. I've never seen one break. I tried one out, and the biggest issue for me was getting it under the Predator. It was easier than the one I had, which was designed for you to lift the stern of the kayak up and then slide the kart under. The Predator has a tendency to want to turn over when you lift it, more so than the other kayaks for which I'd used this kayak. The C-tug took quite a few tries, even with it's kickstand, to get everything lined up well enough that you could get it cinched down. I also thought about the Boonedox Landing Gear. Those would be a great system for most similar kayaks, but with the deck/gunwale design of this boat, people that have done it have had to improvise quite a bit. The gear tend to end up much closer to the transom than you'd want for balance purposes.
So I decided on this design. I figured that the tandem axles would give the stability to the cart that made positioning under the kayak easier. In the limited testing I've done so far at my house, that seems to be the case. Once you figure out where to set the cart, and the direction to point it, you can just use 2 hands and lift by the stern handle and sit it on the cart with relatively little fuss. No need to kick up a kickstand. So far no creaking or complaining. It doesn't want to turn sharply, which is predictable for anyone that's ever pulled a triaxle trailer with their truck. You have to lift on the bow when you make turns, but otherwise everything works well on hard surfaces.
I'd change a couple of things. I made the frame the width I did before cutting the round stock into the axles. I was waiting to borrow a horizontal bandsaw to cut the round stock, and in the end just cut the 4' piece in half with a hacksaw (it turned out to be way easier than I suspected). So I would like to have made the cart a couple of inches wider, and I think it would benefit from being a couple of inches wider. The axles sticking out sort of bother me. The short unsupported 'bunk' members could have just used caps to hold on the pool noodle.
If anyone is interested, I'm happy to give you a source list, cut plan etc. by private message.