raveneye wrote:
Here's the problem:

I have a potential job where I'll need to spend about 6 hrs/day at a
speed of about 1 m/sec = about 2 knots (doing marine benthos mapping on
calm days). But field sites will often be several miles from boat
ramps. As far as boat length goes, around 5 meters is probably the
minimum size for the work.

If I bought a planing boat, I could get to a field site fairly quickly,
but then a planing boat is not going to be a good choice for spending
all day at such a low speed. A displacement hull it seems would be
ideal for the work, but might take forever to get there. An
inflatable that two people could carry is another possibility -- you
could put in anywhere, and it would be capable of higher speeds. But
again a flat-bottom seems not the best choice for long hours at low
speed.

As far as fuel efficiency goes, my understanding is a planing boat
would be lower than a displacement boat at any speed (assuming the
displacement boat is below its maximum for its size), but I don't know
where the inflatable would fit in. Would a 5-meter inflatable have a
higher fuel efficiency at 1 m/sec than a 5-meter displacement hull,
because the inflatable is lighter, which makes up for the less
efficient design?

Any answers/suggestions welcome ! :-)

You can have difficulty slowing a lot of boats down to 2-knots and
maintaining any sort of steerage. For that reason, I'd suggest a
displacement boat with keel. The first little bit of a breeze won't
send you skittering off across the surface like a bug. If the wind
comes up during the day, you would enjoy being better prepared sitting
through the resulting waves. It's hard to know what you mean by
"several miles" but even a six knot boat can cover a nautical mile in
10 minutes. You might want to run on a very small trolling motor when
surveying.

As far as fuel economy goes, you would have to go out of your way to
use much fuel at all
at 2 knots in any 18-foot boat. I'd suggest making comfort while aboard
a much higher priority than speed (unless your work is 10's of miles
from your launch ramp) or fuel economy.