-- Last Updated: Jul-08-13 10:16 AM EST --
See my post above about the term "Light" as it applies to how shallow or deep the boat drafts in the water.
Like you say, it's hull design. Touring SinK's have a more rounded hull and are usually more narrow/longer. This distributes the weight (And displacement of water) along the length of the boat, as well as letting her draft lower in the water. This means the hull creates more drag along it's length, making it harder to turn, thus harder for wind to blow off course (Why many touring kayaks need rudders to help turn).
Sit-On-Tops are usually flatter on the bottom, and wider. The means they draft more shallow in the water. Most of their resistance is in the middle, where it's flatter. This means they are easier to blow laterally (Sideways) across the surface of the water.
Weight wise, you can take a touring SinK and a Rec SoT of the same physical weight and put them in the water empty. The SoT, because of it's hull design and how it displaces water is going to be "Ligher" in the water. It's a nautical term referring to how high a boat sits in the water (it's draft), and less about the physical weight of the boat.
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