Double ended vs transom for fuel efficiency ?

I don't think this is true in all conditions. Performance sailboats are different than powerboats in that they operate through a wide range of speeds. If you take two IC14's and cut one off at 10'6" the wavemaking resistance goes up considerably and pitch damping is decresed. Racing boat-for-boat on a triangular course I would expect that the shorter boat will win most in lighter air and the longer boat will win most in heavy air.

To return to the original question....to have a transom or a double-ender is a compromise......In designing any boat one can never take any feature in isolation, you'll be in trouble if you start making arbitrary decisions with no consideration of the rest of the boat. In today's world I don't think the practical "cost" of a double-ender is worth it. All the current double-enders I'm doing are style driven as I know I can create very efficient transom sterned boats. Once you make the decision to have a double-ender, you start weighing the costs and benefits.

My PL series all have transoms for numerous reasons, for pitch damping, to support a higher than "displacement" speed, to allow space and floatation for the far aft engine room, to increase deck and interior volume within a given length, and to allow easy and roomy water/dock access. The minimal drag increase is worth it in my view because the boat really spends 90% of her life not under way.......