Hippocampus, with all due respect, coming up with meaningless metrics is easy. How 'bout "furlongs per fortnight"? But coming up with meaningful metrics by the ORIGINAL POSTER (OP) that suit HIS use model is the true issue in this thread.
I got a giggle out of the posters to this thread that immediately chimed in on THEIR choice of a boat that met the OP's metrics (a KK42). Early on in my ventures here in the PNW, I chartered one of those things. Compared to the overall cost of a week's charter, the cost of fuel was truly lost in the noise. So from that very limited and short sighted perspective of the true cost of boat ownership (reflected in the charter fee), the KK42 was "efficient". But my family and I were really disgusted in the overall experience, given the cramped living quarters, poor engine room access, and the absolutely atrocious roll, both underway, at anchor, and at the dock. It truly rolled our fillings out.
And "gyradius" as a consideration the choice of a coastal cruising powerboat? Seriously? OK, try this. Email Grand Banks Yachts and ask their naval architect what the gyradius is on a (for instance) 80's vintage GB42. I can feel their eyes rolling from here. This may be a pertinent design element when considering flume stabilization, or the latest Ocean Race race boat, but hardly for the perceived OP's use case. Ditto prismatic coefficient. The guy doesn't want to transport crude oil or other bulk cargo, but presumably travel easily and safely with friends and family. A skinny boat (presumably with higher prismatic coefficient than your typical GB) will be more "efficient". Well, fine. As long as you can live with the down sides, like excess LOA with attendant high moorage fees, living in a tube, and extremely limited design choices.
I don't believe throwing out for consideration such fourth-order design elements to an admitted new guy is doing him much service. And I stand on my statement that "efficiency" as a design element for recreational power boating is not only misused on this forum, but also disingenuous and a red herring for the uninformed.
Furthermore, it appears to me that the OP has fastened on a GB42, and is seeking affirmation from the forum. And his concern with the cost "...to fill the fuel tanks" indicates he has little understanding of the true cost(s) of boat ownership. If the cost of a tank of fuel is a concern for the OP, I hope he does significantly more research before he dives into this venture. I wish him the best in his endeavor.
Regards,
Pete