Why is the limit of the speed of a sail boat dependent upon the length of the hull?

I don't know about waves. It is the same for a canoe or kayak or the length to width ratio.The longer the ''footprint'' on the water the more efficient it is(for a given weight). All boats displace water the more draft (boat under waterline) you have the more resistance you have.

the longer the canoe, the faster it will be. Canoes are displacement hulls; their maximum speed (displacement speed), which says nothing about the effort required to reach that speed, is determined by the formula; S (speed) equals 1.55 times the square root of the waterline length. Simple math reveals that an 18 1/2-foot canoe can be driven 6.7 miles per hour, while a 15-footer, can be driven 6.0 miles per hour.

Another example is skiing down a mountain with 2 foot skis then switching to 6 foot skiis. The 6 footers would go way faster.

It's not just sailboats, every boat has a "hull speed"

wikipedia

Hull speed, sometimes referred to as displacement speed, is a rule of thumb used to provide an approximate maximum efficient speed for a hull. It is only ever an approximation and only applies where the hull is a fairly traditional displacement design. It is usually described as a speed corresponding to a speed-length ratio of between 1.34 and 1.51 depending on which of the limited sources one refers to.

The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio and Froude number are considered more helpful. It is still used by amateurs in relation to traditional displacement hulls.

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I'm curious, being a sailor and an ex-would-be scientist, I have a hunch the wave generated is only evidence of the displacement. I might be wrong, but I think of it (the water) as any other fluid (air being another). Not to be confused with liquid. Like air, there's a terminal velocity, and whether it's motorized, or sail powered, doesn't matter, it's a matter of overcoming resistance.