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.