It probably depends on the boat - playboats may squeeze in. I have an older 8'4" surf (somewhere between a play boat and a traditional whitewater, think I snagged it in '99) kayak and am pretty sure it won't squeeze in, though there's a chance it could if I flip it and put the tail over the passenger seat into the dash area... not too inclined to try (I haven't been able to use it in years.
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Do you need to travel on the highway much? I'd be more likely to throw a Yakima on there for the occasions I needed it. The boat's not heavy, is reasonably good aerodynamically ... should be OK. I did a 17' sea kayak, this boat, and a bike on a 28.5" (IIRC) bar spread years ago, about a 400 mile trek at highway speed. Was on a Neon.