COME ON GUYS THEY ARE BOATS.
Sail boats are too slow for me to fish very conveniently
Use your Fuel flow meter(or get one) and gps-speedo to optimize your trim ,loading and speed . put trim in center ,or most common position for first set of readings.
Make yourself a "fuel curve chart", take a piece of graph paper, put fuel flow on left side , engine speed on bottom, run boat in normal conditions, both dirrections.
start at idle , make a reading at every 100 rpm and note it, do this at until you have basic data, then pick revs you are actually going to use and adjust trim to
maximize economy at those points. you dont have to graph last 500rpm if you dont want.
A rule of thumb is for gasoline ,max fuel flow is near 1gph per 10 hp output.
if you are burning 15gph you are likely pushing out about 150, 20gph ~=200 etc.
2 drives increases base drag over 1. but bigger boats need the 2drives.
speeds below plane are fuel swillers ,except displacement hull speed= 4.5-6knots for most of us.
when boat hull gets up on plane mileage will likely increase "for a while" and then level for a bit, your ffm calculations will show this, then suddenly more revs wiill
use a lot more fuel and drop mileage steadily.
if your motors are in good tune, props right for setup, hull bottom clean and trim
in the mid range, most of us can cruise well in the 19-28 knot range, paying a price above or below that.
lots of factors will affect your economy , especially the need to slow for sea conditions, the desire to trim to improve ride and visibility, load, prop condition etc etc.
WE are making a lot of decisions that affect fuel use, steering a true course can help too on longer runs.
deeper Vs sometimes take a bit more to move, but give a smoother ride, flatter bottoms plane and skim easy but beat us to pulp.
small props that slip a lot may also cost a bit of fuel ,as big blade area and high pitch may strain engines- but get better economy.
so many trade offs.
For reference 231, single5.7 Carb ,bravo 2, 3blade, 138gal. +100#kicker 3 large men,100# ice,100# gear etc, 3batteries,tool box., bait tank water....
WE can get~ 4+mpg on kicker but Main= ~2.5-2.8 calm day ,light load (almost never) and just ~1.5 -2.0 nautical mpg when fueled up, gear and crew running into normal sea, trolling tuna and slogging back -
biiggest real factor is if we split the fuel 1-2-3 or 4ways.
Sample
72 gallons, $4.20/gal /#guys -- is about $300/150/100/75 fuel per angler.
For the guy with the 3250, IMHO you "need" those 7.4 b3s for big boat, thats 3tons per motor ,they can give you good engine life.
7.4 is ok iff you keep it below 3000-3200 rpm or where "secondaries" open if carb motors.
Yam OB's often have fuel flow data on Instrument readout, the rest may find a fuel flow meters info will pay for the meter or most of it. and totalizer info is a lot better than fuel gauges driven by float sensor.
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