Originally Posted by psneeld
8-10 knots on a semi displacement
hull is gut wrenching...I'm guessing it was in the range of 15 gallons per houror about 0.75 NMPG.
At 20 knots it was 20 gallons an hour for 1.0 NMPG which was great for that class boat (mid sized express / sportfish).
If I wanted to cruise
her...I would shut one engine
down and run the other at about 1000 rpm
and she would burn about 2.5 - 3 gal per hour at 5-6 knots or 2.0 NMPG. So you see...slowing down helps but only to a degree...hull shape and cubic inches are important.
I'm hoping my 40 foot 1988 Albin gets around 4 NMPG at 5-6 knots. She only gets 2.3 NMPG at 7.5 knots. That's a trawler with a single
Boats in the 35-45 range are gonna hate anything from 8-15 knots and then they better be planing...just a function of size, hp and hull design. The vessels that may do 8-10 knots efficiently wmight be a few power cats.
Huh? I would recheck your numbers. I have a 43' 34,000lb planing hull with twin Turbocharged CAT 3208 TA engines (375 HP ea) which is definitely not your average displacement hull single, in fact I would not dream of calling it fuel efficient. BUT, the numbers you are quoting don't match ours. Without boring you with the entire chart, hear are some fuel consumption
numbers for our boat. We ran the boat on two runs, one Northbound and one Southbound recently over a 100 mile course to calculate fuel burn and ran the specified RPM for a specific period of time from a measured tank and we also captured and measured the return fuel. The results were averaged and temperature adjusted to yield our consumption
chart. I have a high degree of confidence in these results.
Running on both engines:
1,500 rpm = 8 gph @10.1 knots 1.26 nmpg
1,400 rpm = 7 gph @ 9.2 knots 1.31 nmpg
1,300 rpm = 5 gph @ 8.3 knots 1.66 nmpg
1,200 rpm = 4 gph @ 7.4 knots 1.88 nmpg
1,100 rpm = 3 gph @ 6.6 knots 2.2 nmpg
Again this is a relatively heavy planing hull boat. A semi displacement (semi planing?) boat should be much better. We did not test with a single engine, but I guess is may be less fuel consumed. We will check that next month.
Two caveats: On the weekend we ran the boat we ran just offshore
in maybe 2' of chop, pretty smooth. Heavy seas or winds would affect our numbers correspondingly of course.
The speed numbers are from our GPS' (two units) but the rpms are from the boat tachs which are not calibrated and the true rpms probably vary from the gauges, but the gph & nmpg are unaffected by this.
There are plenty of boats out there, don't get discouraged.