Blog No 5: Fuel Efficiency

Submitted by Russell Hunt on Mon, 05/09/2011 - 8:38am

 Buzzards Bay Blog No 5: Fuel Efficiency


By what measure is a boat considered fuel efficient?  Is 0.5 MPG efficient?  Well, it is if that boat’s a 70 foot sportfish traveling at 25 knots.  Is 5 mpg efficient?  It is not if it’s an 18 foot light tackle boat.  Fuel efficiency is of course a relative statement.  We must first look at the intended use.  Do I want to cruise offshore in potentially rough conditions, or will I cruise the coast via the ICW, always staying in protected waters?  To be able to handle rough water comfortably may require a heavier boat with a deeper v-hull to maintain comfort in a seaway, at the expense of fuel efficiency.  Do you prefer to cruise at slow speeds, or do you want to run fast?  Perhaps you would like the ability to cruise at a relaxed pace efficiently, and at times, run fast in order to outrun bad weather or just for time constraints. In the traditional mono-hull world, all of these choices demand that serious compromises be made.  No boat will not do it all…with a few exceptions.  It has been our position at Buzzards Bay Catamarans that a well designed, carefully built catamaran, utilizing advanced composite construction for light weight, and a high speed displacement hull shape for low drag, can in fact do it all with very little compromise. Our Buzzards Bay 34 delivers best in class interior, significantly higher levels of ride comfort, fast cruise speeds, and outstanding fuel efficiency.  If you’ve been following these blogs, you understand our advantages when it comes to sea-keeping ability, interior space, and have a good understanding of our speed performance.  For the Buzzards Bay Blog No 5, we would like to evaluate our fuel efficiency advantages against competitive boats over a wide speed spectrum.  

To accomplish this, we’ve assembled a broad mix of similar mission / size boats and we’ve broken them down into two groups, outboards and diesels. To make the results as meaningful as possible, we gathered single engine, twin engine, and pod drive competitors, so as to leave no questions unanswered.  

We’ve included some larger (by length) vessels, the MJM 40z, Grady White 36, Pursuit 375, and Back Cove 37 because these vessel match up to our Buzzards Bay 34 in actual size and space on board.  However, to be fair, we did include their smaller brothers (Grady White 33, Pursuit 345, MJM 34z) so there were no boats we were trying to “duck” in this comparison.    

For a little fun (and more than a little perspective) we also included a vessel that represents an extreme attempt at achieving fuel efficiency; the Greenline 33 Hybrid, which is a fully enclosed pilot house cruiser featuring a layout similar to the Buzzards Bay 34.  It is powered by an inboard diesel engine that is fitted with an integral generator / motor.  The operator has the ability to operate on battery power alone, diesel and battery, or just diesel, all with integral solar charging.  Its system is too complex to adequately describe here, but in nut shell, it is an attempt to bring automotive style plug in hybrid technology to boats.  Will the same fuel efficiency advantages that hybrid cars enjoy carry over to express cruisers?  We’ll see.  

Let’s have a look at the outboard powered boats first.  
  
(To see the graphs, see attachments below. )

Looking at the outboard fuel efficiency graph, you can see that the Buzzards Bay 34 enjoys a substantial fuel efficiency advantage over every other vessel here.  

At 15 MPH, the Buzzards Bay 34 is achieves 1.95MPG, which is 70% more fuel efficient than the next highest boat, the Glacier Bay 34 (1.15MPG).  Last is the Grady White 36 Express at .71 MPH, a full 174% less fuel efficient.  

At 20 MPH, the Buzzards Bay 34 is running 2.05 MPG, 85% more fuel efficient than the second highest vessel, Grady White 33 ( 1.11 MPH).  

Finally, at 30 MPH, the Buzzards Bay 34 is returning 1.38 MPG, with the next highest Grady White 33 at 1.23 MPG, a difference of 12%.  The Pursuit 345 is 22% less efficient at 30, and the others are all over 55% less efficient.  

At 30 MPH, you can see the deep-vee hull of the Grady 33 becoming a little happier, a little more efficient.  It has shed some of its wetted surface (lower drag), dropped its trim angle a little and is now in the groove.  The Buzzards Bay 34’s hull, being a displacement cat, does not receive the advantage of reduced wetted surface drag as speed continues.  In return, you get a hull that operates over a much wider range of speeds very efficiently compared to a planning mono-hull.  

Now let’s look at how different levels of fuel efficiency affect bottom line costs to operate.  (see outboard compare cost graph)

For this chart we calculated each vessels relative fuel cost per gallon, adjusted for their average fuel efficiency at the measured speeds (15, 20, 25, and 30 MPH).  This gives you a very true sense of the relationship between fuel efficiency and operating costs.  We set the Buzzards Bay 34 operating cost at current prices for a baseline ($4 per gallon, we’ll see what summer brings).  The next most efficient vessel on average is the Grady White 33, whose owner is paying $6.28 per gallon of gas in relation to the Buzzards Bay 34.  That’s 57% more expensive. The most expensive boat to operate is the Pursuit 375, with a relative fuel cost of $7.84 per gallon.  That’s a whopping 96% more expensive.  Rising fuel costs and uncertainty about the future seem to be something we’re all going to be living with for some time.  Wouldn’t you like to be paying $2, $3, and compared to some boats almost $4 less per gallon for gas?  When you’re running a Buzzards Bay 34, that’s just what you’re doing.    

Of course, fuel efficiency has an impact on range too.  The higher the fuel efficiency, the farther you can travel, or the less fuel you need to cover the same distance.  With the Buzzards Bay 34, we chose to use our fuel efficiency advantages to achieve a very competitive range while carrying far less fuel.  Less fuel means less weight to push, which means less horsepower is needed.  All but the Glacier Bay in the above comparison needed heavy Yamaha V8 350HP outboards to push them well, while the Buzzards Bay 34 is a real jack rabbit with lighter weight 6 cylinder outboards.   

Range at best cruise speed has the Grady White with a 387 mile range,  ahead of the Buzzards Bay 34 and virtually identical Pursuit 345 (350 miles and 346 miles respectively) by 10%.  The rest of the boats have around a 300 mile range.  The interesting thing here is actual fuel capacity; how much fuel it took to achieve that range.  The Buzzards Bay 34 has a total fuel capacity of 190 gallons while the Grady carries 350 gallons of gas, a full 84% more than our Buzzards Bay to travel just 37 more miles.  

Now on to diesel powered boats.  This gets interesting, because in this we group we have some boats that, from a design and build standpoint, really focused on fuel efficiency as their number one priority.  (See diesel fuel efficiency chart)

At 15MPH, the Buzzards Bay 34 is about 30% more efficient than the second highest MJM 34z.  The least efficient is Tiara’s Sovran 35, a full 158% difference.  This vessel, having pod drives, is lacking the downward angle thrust that is helpful to planing at slower speeds.  Pods are good for efficiency at higher speeds, but not always at slower speeds.  According to the test, the Tiara reaches a trim angle of 7 degrees at 17 MPH.  At this speed, the hull should be solidly on plane and leveled out to 3: 4 degrees.  This boat would not be too fun on the ICW where there is allot of slow to medium speed running, continually powering up on plane and slowing back down.  The lack of visibility would be potentially dangerous.  

At 20 MPH thru 30 MPH, there is not much difference in efficiency between the Buzzards Bay and MJM’s 34z, with both vessels far and away the most efficient among this group.  The MJM 34 is designed and built with an almost single minded mission to achieve maximum fuel efficiency.  Her narrow beam, minimal super structure, shallow v-bottom hull, and light weight construction compromise interior space, stability, and ride comfort in favor of getting the lowest fuel burn.  The fact that our significantly larger Buzzards Bay 34 can match her efficiency, and in some cases exceed it, while providing superior levels of interior space, ride comfort, and stability is significant.  

  Now have a look at the diesel compare costs graph.  This chart has the relative fuel cost per gallon of each vessel, adjusted for their average fuel efficiency at the measured speeds (15, 20, 25, and 30 MPH).  We set the Buzzards Bay 34 operating cost at current prices ($3.50 per gallon of diesel, we can dream, right?).  

The MJM 34 is about 13% more expensive to operate than the Buzzards Bay 34, with its owner paying a relative fuel cost penalty of  $0.44 per gallon over the Buzzards Bay 34, followed by the MJM 40z ( $5.22 per gallon), Back Cove 37 ($5.85 per gallon), Sabre 34 ($6.55 per gallon), and lastly, the Tiara 34 at a pricey $7.71 per gallon, a 120% operating cost increase over the Buzzards Bay 34.  

The MJM 40 has the most range at 554 miles, with the Buzzards Bay in second at 513 miles (on roughly half the fuel), followed by the Back Cove 37 with 454 miles, Sabre 34  (356 miles), MJM 34 (320 miles) and lastly the Tiara with 281 miles.  

Among purpose built vessels aimed squarely at fuel efficiency, pod powered, single and twin engine installations, the Buzzards Bay 34 is at or near the head of the class in every comparison.  With an optional fuel capacity increase of 30 gallons, there isn’t a category she would come second in.   

How does the diesel powered Buzzards Bay 34 compare to the most extreme vessel here, the Greenline 33 Hyrbid?  At 15 mph, the Buzzards Bay is 40% more fuel efficient than the Hybrid.  Unfortunately, the Hybrid is not able to achieve speeds to 20 mph, according to the test.  This performance may seem a little disappointing.  Still, it has the ability to operate very quietly on electric power alone, and for some, the onboard serenity that low noise levels produce is alluring enough on its own merit.  To these folks I say consider an outboard powered Buzzards Bay.  At 15 mph, the efficiency nearly matches that of the Hyrbid (2.05 vs 2.11) and at any speed above 7 mph, our Mercury Verado outboard powered Buzzards Bay is actually quieter because the Hybrid has to run its diesel.  In full electric mode the Hybrid is limited to a range of 20 miles at 6 mph.  I think there’s allot of compromise here for very little performance.  It’s neat and different, but are there real benefits to the owner?

There you have it.  Whether diesel or outboard powered the Buzzards Bay 34’s fuel efficiency is outstanding.  Factoring in her class leading size, ride comfort, and stability, and her dominating performance is even more impressive. This is what a good catamaran can deliver.  No gimmicks, no unproven technology, no compromises.  Just good catamaran design work (courtesy Chris White Designs) and light weight advanced composite construction techniques with quality craftsmanship.  

I hope you enjoyed this edition of the Buzzards Bay Blog.  To tell you the truth, we were a little surprised at how efficient the Buzzards Bay 34 is compared to the rest of the market.  Once we started to see all of the data compiled on so many vessels in one place, well…..we were pretty excited about the results.  If there are any questions regarding the data used or other vessel comparisons you would personally like to see, please let us know.  

Next months Buzzards Bay Blog No 6 will be covering shallow water operational design considerations.  Not as sexy a subject as fuel efficiency, but still very important.
-Thanks for reading.  

Sincerely,
Russell Hunt, President
Buzzards Bay Catamarans and
Multihull Development, Inc.
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Source Data Chart.JPG93.73 KB
outboard fuel efficiency.JPG43.21 KB
outboard compare cost.JPG45.4 KB
diesel fuel efficiency.JPG57.07 KB
diesel compare cost.JPG41.11 KB