Generally speaking and all else being equal, hull speed = square root of WL length X 1.34.

30 foot WLL boat = 7.33 knots

60 foot WLL boat = 10.38 knots, or 41% faster. Note the 60 ft boat won't be twice as fast because the formula is based on a squaring function.

In practice, the WLL will not be the same one 30 ft boat to another because of hull shape. As one boat heels -maybe even to nearly the same degree of heel as the other boat- the shape of the hull will change the effective WLL somewhat, as will changes in its overall wetted surface contribute to more or less drag that the other boat may experience.

Once u begin to reach that 'real hull speed' -which will differ boat-to-boat even if the WLL is the same between boats- it takes very large increases in power (sail or motor) to drive beyond that 'real' hull speed. Indeed, once you get to about 90% of that theoretical hull speed, that last 10% begins to require more than 10% more power, hence, 90% of theoretical hull speed is about what can be practically achieved wih reasonable power, be it wind or mechanical.

Case in point, the DDG my son was in for nine years:

- two turbines, full speed, one shaft trailing: 24 kts

- four turbines, full speed, both props powered: 35+ knots, not 48 knots

Twice the power, 10 to 12 knots more, not 2X more knots.