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

I've been told that it has to do with the length of the wave that the boat generates while moving through the water, but I don't understand why that's a limiting factor. I'm referring to boats that displace water as they move, and not those that ride on top of the water such as hydrofoils or catmarans.

Update:

Please don't quote wikopedia....or any other source that doesn't EXPLAIN why or how the hull length controls the maximum speed. What's the physics involved!

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• Loomon
Lv 4

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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