Boat Beam Width: Sea vs. Lake - Understanding Hull Speed

Question
Why for rowing boats  is a 6 foot beam recommended for the sea and 4 foot 6 inches beam recommended for lake.

Answer
I have not heard this recommendation before but your top speed with out plaining will be between 1.28 and 1.35 times the square route of your water length.  Because a wider beam is more stable it has an advatage on ruff water.  On smooth lake water a narrower beam will help move your hull speed towards 1.35.