Some naval architects argue that the formula for calculating maximum hull speed needs to be updated, as a result of advancements made in the area of hull design and its lines, keel (centreboards/daggerboards) profiles changes and new shape of sails and materials they are made out of.
Examples:
Storm 22
Max hull speed 6.84kt was reached without problems in 3-4B wind in beam reaching and broad reaching. In around 5B wind, maintained speed was 7.4-7.5kt and 8.3kt with spinnaker.
Saturn 23GT
From calculations, maximum hull speed is 7.15kt. The boat commonly sails at 8kt and over 9kt with spinnaker.
Maxus 28
Maximum hull speed 7.8kt, maintained speed was 9.4-9.6kt, and 11.4-11.7 with the gennaker.
If a boat has the right SA/D and D/L ratios, it will start to plane and the THS equation is no longer relevant.
The equation that everyone knows, 1.34*LWL^0.5, is actually over 300 years old, having been first determined by Englishman Anthony Deane for the British Navy. It really can't be updated per se, as it is designed to calculate the speed which creates a bow wave with a period that matches the LWL of the boat in question. But, and this is key, it was designed for heavy displacement boats, like ships of the line. Works good there. But clearly it breaks in lots of cases, for instance on the Beneteau First 235. Dean says it should hit no more than 6.03 knots, and for sure the boat is capable of more than that.
Well about 15 years ago, a Naval Architect named David Gerr tried to figure out what was going on, and how to express it mathematically. For him it seemed that displacement as a function of length is the key.
The first part is rearranging the original equation like this:
1.34 =hulllspeed/(LWL^0.5)
This pulls that bow-wave length factor of 1.34 out, and expresses it for what Gerr thinks works better, a factor of displacement and LWL to calculate speed.
Mr Gerr then created a formula that actually takes LWL and displacement into account. It can be expressed in these three steps:
Step 1: D/L = displacement/2240)/(0.01 * LWL)^3 : Standard D/L ratio
Step 2: S/L = 8.26/(D/L)^0.311 : Here is the Magic
Step 3: Hullspeed = S/L * LWL^0.5 : HS calc with his factor in place, not the bow wave number
Throw in the First 235 numbers (I used 3500 for boat+crew displacement) and you get:
Boat 235
LWL 20.25 feet
Disp 3500 pounds
D/L ratio 188.17
S/L ratio 1.62
Hullspeed 7.29
Old Speed 6.03
7.29knots. That sound better!
PS for more info, see David Gerr: Nature of Boats, McGraw-Hill; Offshore, Dec. 94, pp 29-33
PPS If anyone is interested, I created a very simple XLS file to calculate these new speeds:
http://dl.dropbox.com/u/955501/Hullspeed.xlsx
Note - make sure to use an all-in weight, not the light-boat displacement given by the manufacturer. It really factors (like crew weight) into the calculations.
