Chamberlin 47 Power Cat “Foreign Affair” Design concepts by Robin Chamberlin

UNIQUE AND EVOLUTIONARY

It's a funny thing; fish aren't flat panels and squared off shapes or constructions. I wonder why? Maybe the creation and evolution of fish has produced the best possible body form for hydrodynamic efficiency.

The under-water fairbody (ie hull form) of Foreign Affair in many ways mimic that of fish. A form that cuts through the water and allows the water to exit from the boat, coming together smoothly as with fish tails.

Long fine hulls act like shock absorbers allowing the vessel to cut through seas with buoyancy taking up slowly. The high, set back, bridgedeck means virtually no impacts in the most vulnerable area of catamarans.

The displacement hull is not trying to develop any lift, as in a planning hull. All shapes will develop some lift at speed. Even a marble will skip (ie plane on the interface) if delivered at the right angle and velocity!

But in developing surfaces to provide lift through the water, resistances are increased. This is why a 40’ planning hull at say 10 to 16 knots is hopelessly inefficient, dragging a huge wake, burning up a huge amounts of energy. The energy (fuel) required to overcome this resistance dictates bigger horse power and more weight.

And here is the double whammy - the range of the vessel is then limited by the weight of fuel capable of being lifted onto the plane.

With long fine displacement hulls, increases in displacement means only an increase in wetted surface as the major resistance factor. The problem is fine hulls necessitate big variances in sinkage or immersion if big payloads (long distance fuel, water and stores) are to be carried.

This design gets around this problem and maintains efficiency from lightship to fully laden in part by rotating fore and aft trim around the propeller area. From 6500kg to 10 500kg displacement, transom immersion changes by only 120mm, while the bows trim down by 350 mm. This means a clean foil form fairbody is operating throughout her displacement range.

Tankage is in the hulls and spread between 8 tanks; 2 fuel, 2 water in each hull, which allows a degree of fore and aft trim control while keeping a low and central weight distribution.

Foreign Affair’s performance figures show the difference between lightship and maximum load boat speed is only around 1 to 1.5 knots throughout the cruising speed range, with a top end speed variation of 1.5 knots ie 21.5 knots lightship and 20 knots laden.

This is testimony to her unique hull form when you consider that is more than four tonnes payload difference! At under 8 knots it is hard to even differentiate the performance.