2 reasons off the top of my head: weight in armament. The Dame is caryinng more guns than the charlie may weigh!!!! lol (sarcastic) The overall weight of Dame should hinder it to being slower than charlie. plus... the mass part of your equation doesn't work out. First, the Dame would have a much deeper draft than the Charlie, but this cannot be represented because the floor or table is the waterline. this draft would slow it down considerably.
I think unfortunately an equation that uses mass or size of hull to calculate speed for lego ships can never be quite accurate. Though I am glad you tried, maybe i am wrong... but can you post some of the sizes and speeds of the real ships that you used so we can see?
Instead of using a linear formula I'm using an inverse tangent function where a ship has to be very very efficiently built to have a top speed of 20 knots or more.
Here's the formula:
atan is the inverse tangent in radians (atan is what needs to be typed to work in Excel).
L is the length of the ship in length (not including the bowsprit).
B is the beam or width of the lego ship (not including fancy details on the sides).
(doesn't matter what unit is used to measure length and beam, but they must be measured in the same units like studs, cm, in, etc.)
SA is the sail area of the ship in cm squared.
M is the mass of the lego ship in grams.
(any questions pm me or post on here :-) )
Here are the speeds (in knots) of the official released sets done so far:
(if not in bold, then i don't have exact numbers yet)
Row Boat w/ sail: 18.2 knots
Row Boat w/ 2 sails: 20.2 knots
Imper. Trad. Cutter: 14.9 knots
Emporer's Ship: 6.5 knots
Crossbone Clipper: 6.5 knots
Imperial Flagship: 16.2 knots
Armada Flagship: 11.5 knots
Carribean Clipper: 11.1 knots
Viking Long Boat: 16.6 knots
Black Seas Barracuda: 12.5 knots
Here are the speeds of my MOCs:
Chinese Junk: 14.6 knots
HMS Charlie: 12.6 knots
Xebec: 20.7 knots
Dame du Loupe: 17.0 knots
Will be editing more finalized details later.