When considering fuel consumption and all the tradeoffs one can make to improve it, I think it's import to look at a ranking of the factors that impact efficiency.  When looking at it, keep in mind that efficiency is measured in MPG or some other form of distance traveled per unit of fuel.  Looking at GPH (which seems to be all that most boaters want to talk about) really tells you nothing about efficiency without also saying how fast you are going, and at that point you might as well just do the MPG calculation and be done with it.

Now on to the ranking of things that impact MPG.

#1 and #2:  By far, the two most significant factors in MPG are your boat's displacement and how fast you are going.  You only get to impact your displace once when you select the boat you buy.  After that it's an immovable constant.  Speed you get to impact anytime you want.  Slow down and you'll get better MPG.  Speed up and it will get worse.  I'd argue that for any given boat, speed is really the only thing that matters.

Things that make only a small, often negligible difference:

• Engine size:  Engine size impacts how fast you CAN go, but it has little impact on MPG at any given speed.  For example, identical boats with a 60HP engine and one with a 100HP engine for all intents and purposes will consume the same fuel at any given speed (assuming both can attain that speed).  If there is a big difference in engine size, say 60HP vs 500HP the differences will be more noticeable, but within any reasonable range they will be the same.

• Single vs twins: Again, within reasonable ranges, fuel consumption will be much the same with twins versus a single engine.  So a single 500HP will consume essentially the same as twin 250HP.  Not exactly the same, but close enough to not matter.

• Variations in hull shape and configuration:  Comparing like kinds of boats, differences in efficiencies will be relatively small.

• Sweet spots in engine RPM.  Yes, engines have an efficiency sweet spot, but seeking that out is insignificant relative to speed selection.

• New common rail engines vs older mechanical injected engines.  There will a be a slight improvement in MPG, but the benefits are really more around quiet operation, no smoke, no smell, easy starting, etc.

I don't mean to say the above don't matter at all - they do - and it's good people like to optimize them.  My point is to keep them in perspective relative to the big factors, speed and displacement.  A slight change in speed will make a difference in MPG that will dwarf the impact of any of these less significant factors.

Another myth is that there is an efficiency sweet spot just below hull speed.  I've never seen anything that shows this to be true.  Reality is that the faster you go the worse your MPG will get.  There's no inflection point - it's all downhill.

However, there is an interesting phenomenon once a boat goes onto a plane, assuming it's capable of doing that.  The MPG flattens out and remains nearly constant regardless of speed. I first noticed this reading performance reports in magazines, and have verified it on my own boat.  From about 12kts up to 20kts (top speed), the boat gets the same MPG.

So when comparing MPG numbers, it's important to also compare the boat's displacement and speed to be sure you have an apples-to-apples comparison.  I remember reading an article in a popular boating magazine on the virtues of Pod drives.  They compared the MPG of two boats of similar length, one with a pod drive and one with straight shaft props.  The Pod drive was some 20% better MPG and the article attributed this to the Pod drive.  However, when you looked at the displacement of the boats, the difference in MPG was 100% attributable to the displacement, not the Pod drives.

Peter,

You have said in a succinct way what I have been saying for many years.  I
always hear boaters talk about gph figures and then query them as to the
other parameters only to find out that they don't really understand what is
actually going into efficiency calculations.  Everyone seems to get hung up
on the 80% of WOT (or some other figure) as the efficiency point and will
try to run that with the result that they are far less efficient because of
the speed & hull form issues.  Usually these are newish boaters that have
read or heard something about the issue and fail to apply logic to the
statement or do any research on their own in order to fully understand.
Usually I turn them on to Trawler and Trawlering so they can read and learn
from all the discussions.

Thanks for articulating this in such a concise manner.

Paul Brannon

-----Original Message-----
From: Peter Hayden
Sent: Tuesday, November 22, 2011 6:53 PM
To: trawlers@lists.trawlering.com
Subject: Re: T&T: Diesel fuel consumption

When considering fuel consumption and all the tradeoffs one can make to
improve it, I think it's import to look at a ranking of the factors that
impact efficiency.  When looking at it, keep in mind that efficiency is
measured in MPG or some other form of distance traveled per unit of fuel.
Looking at GPH (which seems to be all that most boaters want to talk about)
really tells you nothing about efficiency without also saying how fast you
are going, and at that point you might as well just do the MPG calculation
and be done with it.

Now on to the ranking of things that impact MPG.

#1 and #2:  By far, the two most significant factors in MPG are your boat's
displacement and how fast you are going.  You only get to impact your
displace once when you select the boat you buy.  After that it's an
immovable constant.  Speed you get to impact anytime you want.  Slow down
and you'll get better MPG.  Speed up and it will get worse.  I'd argue that
for any given boat, speed is really the only thing that matters.

Things that make only a small, often negligible difference:

• Engine size:  Engine size impacts how fast you CAN go, but it has little
  impact on MPG at any given speed.  For example, identical boats with a 60HP
  engine and one with a 100HP engine for all intents and purposes will consume
  the same fuel at any given speed (assuming both can attain that speed).  If
  there is a big difference in engine size, say 60HP vs 500HP the differences
  will be more noticeable, but within any reasonable range they will be the
  same.

• Single vs twins: Again, within reasonable ranges, fuel consumption will be
  much the same with twins versus a single engine.  So a single 500HP will
  consume essentially the same as twin 250HP.  Not exactly the same, but close
  enough to not matter.

• Variations in hull shape and configuration:  Comparing like kinds of
  boats, differences in efficiencies will be relatively small.

• Sweet spots in engine RPM.  Yes, engines have an efficiency sweet spot,
  but seeking that out is insignificant relative to speed selection.

• New common rail engines vs older mechanical injected engines.  There will
  a be a slight improvement in MPG, but the benefits are really more around
  quiet operation, no smoke, no smell, easy starting, etc.

I don't mean to say the above don't matter at all - they do - and it's good
people like to optimize them.  My point is to keep them in perspective
relative to the big factors, speed and displacement.  A slight change in
speed will make a difference in MPG that will dwarf the impact of any of
these less significant factors.

Another myth is that there is an efficiency sweet spot just below hull
speed.  I've never seen anything that shows this to be true.  Reality is
that the faster you go the worse your MPG will get.  There's no inflection
point - it's all downhill.

However, there is an interesting phenomenon once a boat goes onto a plane,
assuming it's capable of doing that.  The MPG flattens out and remains
nearly constant regardless of speed. I first noticed this reading
performance reports in magazines, and have verified it on my own boat.  From
about 12kts up to 20kts (top speed), the boat gets the same MPG.

So when comparing MPG numbers, it's important to also compare the boat's
displacement and speed to be sure you have an apples-to-apples comparison.
I remember reading an article in a popular boating magazine on the virtues
of Pod drives.  They compared the MPG of two boats of similar length, one
with a pod drive and one with straight shaft props.  The Pod drive was some
20% better MPG and the article attributed this to the Pod drive.  However,
when you looked at the displacement of the boats, the difference in MPG was
100% attributable to the displacement, not the Pod drives.

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Snip...#1 and #2:  By far, the two most significant factors in MPG are your boat's displacement and how fast you are going. 
 
Reply...I think the hull SHAPE plays a much larger part in this equation than just a pure displacement number. My 27,000 sailboat shaped hull is far more efficient to drive than a comparable displacement trawler.
 
Snip...However, there is an interesting phenomenon once a boat goes onto a plane, assuming it's capable of doing that.  The MPG flattens out and remains nearly constant regardless of speed. I first noticed this reading performance reports in magazines, and have verified it on my own boat.  From about 12kts up to 20kts (top speed), the boat gets the same MPG.
 
Reply...My personal fuel burn observation on my 28' fishing boat do not match this comment. My sweet spot is 3800 rpm which gives 23.5 kts and right at 13.5 gph per engine. At WOT, this boat gets to abt 32 kts with 24gph per engine or about a 30% decrease in fuel economy.
 
Joel Wilkins
m/s Miss Magoo - Columbia 45
m/v Liberty Call - Trophy 2802
Treasure Island, FL