I`m not sure you can apply Froude`s law to a small light displacement catamaran - we may sail displacement boats that might not plane, but it`s not sensible to apply a law that was written based on observations of heavy dispacement boats.
Further to this, if you take the length to width ratio of the average beach cat with a rounded hull-shape ie symetric hulls, you will find a ratio of about 12,5 : 1. If you take this and multiply it by say 2metres you get 25metres - A keelboat of 25metre
length with a 2metre beam would have to lug a large lump of lead around to keep the mast pointing skyward, or would need a few hundred crew out on trapeze ! That`s why cats are faster than mono`s. So next time you sail faster than Froude`s law
says you should be able to, just smile ! Life`s a bit faster on a cat.