Hi, you may love the fitness that rowing brings but I think you'll hate the frustration of going nowhere fast! Sibs just aren't made for rowing /paddling, and definitely not a 4.2 mtr SIB. Also you said wife, kids (presumably more than 1) and camping gear, that would be very, very snug in a 4.2 mtr SIB.
Have you looked at the Boatworld 435 kayak boat, that's about as near as you'll get to what you want, but I agree I don't think it's big enough for what you are suggesting.
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