need purchase advice

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 ! :-)

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.