It is very easy to design and build a one-off custom
boat that achieves 20 or 50 miles per gallon at
displacement speeds. There is absolutely nothing technological that stands in the way of such vessels. All that is required is to design for 10HP instead of 1000HP, and accept what fits.
We do this all the time: your house probably isn't what you want, its what you can afford. We'd all prefer flying around in private jets, but we put up with the torture of
commercial flights because that's what we can afford. If you want a
boat you can afford to drive anywhere when retired or on sabbatical, you just have to adjust your expectations.
As a thought
experiment: an 8 man rowing shell weighs 250 lbs for the boat, another 1800 lbs for the 8
men and the cox, so a little more than 2000 lbs going down the course. At about 60 feet long, the
hull speed is about 10.5 knots. Their
racing speed is about 14 knots, so well above
classic hull speed. The
power is about 2 HP total.
When I add up all the physical stuff I want, determine the widest
interior element (8 feet is more than enough for a walk around double
bed, or a longitudinally arranged
galley, or a
saloon with facing settees even with a dining table), and make it about 60 feet long, the weight study using foam
fiberglass construction is very, very easy and
cheap (far under $100K new, custom build, commissioned) to achieve a loaded weight of about 8000 lbs. The resultant boat should
cruise at 8 to 10 knots using about 8 HP, or about 20 MPG burning diesel, 10 MPG burning gas (outboard?). Halve the speed, quadruple the mileage.