[Fr. Frank]

Trust me. 35-40 knots in 6 -8 foot seas will be very uncomfortable after a while in anything less than 100 ft long.

A seeming eon ago I worked on an 87' aluminum gunboat built by Lantana Boat Yard for the Argentine Navy, that was converted to a party boat. (The Falkland Island bit with the Brits came up before it was delivered)

Twin 16 cylinder MTU diesels each producing 3200 hp through KaMeWa jets, 3000 gal fuel bunkerage, with a top speed of 52 kts at max gross weight of 92 tons. We could run full throttle in 5-7, but it would rattle your teeth when it got out of the water. In 6-8 ft I swear we occasionally got the entire boat in the air at 45 kts. On the other hand, backing off to 25-30 knots gave you only a gentle toss every now and then.

Remember the rule 9-5 for wave height to sea height, and 5-9 for sea height to wave height. In other words, in 10 foot seas, the waves are 18 feet from trough to crest, and when the waves are 9 feet trough to crest, the seas are 5 feet. That's because sea height is measured from sea level to crest, not from the trough.