You estimate 40 mm extra draft when putting some 140 kg (double) - 75 kg (single)= 65 KG extra on it.

My estimate would be at least 50 mm, probably more. (When sailing on one hull, the other just lifted clear of the water)

Typically I use as "an rough extra draft" estimate the following formula :

(Length hull * width at widest point deck * 2/3 * 1/1000) * extra draft in mtr = kg's extra

or

5.49 * 0.30 * 2/3 * 1/1000 * X = 65 This leads to an increase in draft of 59 mm. 50 % more than your 40 mm estimate.

Even when using a shape factor of 3/4 (instead of 2/3) I arrive at 53 mm Increase in draft. Theoretically the shape factor of 3/4 is linked to a hull that is rectangular in crossection and that from the top looks like it is made up of 1 rectangle of halve the hulls length and one triangle of the halve the hulls width. It can easily be argued that this is the most extreme case with the lowest increase in draft. Real draft figures MUST be higher than any estimate this 3/4 approach produces.

P.S. The 2/3 approach compensates for the roundings of the hull from the 3/4 basic shape but still can be regarded as a underestimating approach.

Wouter