On motoring efficiency

Cavendysh and Sirocco
Cavendysh and Sirocco facing each other. Sirocco weighs three times as much, but motors in a flat calm at almost the same economy as the smaller Contessa 26.

Many sailors pride themselves on how little they can use their engines, but any boat equipped with an auxiliary will end up motoring some percentage of the time.  Being able to motor somewhere is a big part of the reason for having the engine, right?  My last three boats have each come equipped with auxiliary engines, and I have used them at times to travel a good many miles on rivers, canals, and in confined channels.  Sometimes that meant motoring all day, and I would begin to wonder just what I was getting out of each gallon of fuel consumed.  With fuel prices currently over $4 per gallon this subject is on my mind frequently, especially now that I have moved to a much larger boat.  So, I pulled out my logbooks and did a little figuring.

Here is Cavendysh on stands next to Sirocco.  From this angle it is a bit more apparent how much heftier Sirocco is.
Here is Cavendysh on stands next to Sirocco. From this angle it is a bit more apparent how much heftier Sirocco is.

The three boats that I am comparing are a Contessa 26, which weighs about 5400 lbs; a William Atkin “Ben Bow”, weighing about 17,000 lbs; and a Dreadnaught 32, weighing in at 20,000+ lbs.  They are all powered by inboard diesels.

One of the things that I had enjoyed most about my trim little Contessa 26, Cavendysh, was that she was very efficient both under both sail and power.  I figured that would be the case when I bought her.  Efficiency was one of the reasons that I chose the design.  For power, she carried a little one-cylinder Bukh diesel engine that made all of 9-hp.  The propeller was a 2-blade fixed model.  I could motor all day at 4 knots with that little one-lunger banging away and only burn a couple of gallons of diesel fuel.  Cavendysh returned about 20 nautical miles per gallon on calm water at that speed.  Pushing the throttle forward farther resulted in a big drop in economy.  She returned about 10-12 miles per gallon at 5 knots.  I motored quite a lot when I had the mast down heading south from the Great Lakes.  My efficiency through the water over 150 hours of motoring worked out to about 20 nautical miles per gallon of diesel.  That number reflects varied conditions–sometimes I was motoring into wind and chop, and sometimes I had the wind behind me (when the mast was down), but overall, that is pretty stellar economy.  The little Bukh could run for ages on just cupfuls of fuel.

A view of Sirocco with her freshly refinished bottom.  I still have to peel the tape off...  Sirocco has a full keel with a deep forefoot.
A view of Sirocco with a freshly refinished bottom.  Sirocco has a full keel with a deep forefoot.
Looking at Sirocco head-on while she hangs in the slings.  Her fine underwater lines are apparent from this angle.  The very bottom of her keel is narrower than that of the Contessa 26, which contributes to her relative efficiency.
Looking at Sirocco head-on while she hangs in the slings. Her fine underwater lines are apparent from this angle. The very bottom of her keel is narrower than that of the Contessa 26, which contributes some to her efficiency.

When I moved to Sirocco, the “Ben Bow” designed by William Atkin, I was prepared to spend a lot more for fuel.  The new boat was almost three times the displacement of the Contessa, but also longer on the waterline (28′ versus 21′) and with a much fuller keel.  The big full  keel meant a lot more wetted surface, which results in more drag as the boat has to overcome more friction with the water.  The engine in Sirocco was a big jump up too–a 28-hp three cylinder Beta diesel driving a three-blade fixed prop.  However, I was surprised to find that my mileage over the last 150 hours only dropped to 15.2 nautical miles per gallon.  That was running mostly between 4 and 5 knots, and mostly in calm water, although at least 10% of that was motoring into headwinds, which consumes a lot more fuel.  Sirocco was also capable of motoring at over 6 knots, but would burn 2/3 of a gallon per hour at that speed (9 miles/gallon).  Motoring at 4 knots in a glassy calm I could still get 20 miles per gallon out of Sirocco–about the same fuel economy as the Contessa, but much more efficient because Sirocco weighed three times as much.  I was more comfortable on the larger boat, as well, and the engine was much quieter.

This is a good view of Idle Queen to show off her long, shallow keel.
This is a good view of Idle Queen to show off her long, shallow keel.

My present boat, Idle Queen, at 20,000 lbs, is definitely the biggest and heaviest of any boat that I have owned.  She is also the most lightly powered.  Her inboard engine is a 15-hp, two-cylinder Beta Marine engine that drives a 3-blade fixed propeller.  That gives her 1.5 hp per short ton (2,000 lbs) of displacement–relatively less than half as much power as either of my last two boats, which were both very close to 4 hp per  short ton.  Would this drastically different setup change the economy of motoring in a calm?  Well, I don’t have as much data yet as I do for the other boats, but it seems that at 4 knots in a glassy calm Idle Queen gets very close to 20 nautical miles/gallon!  At 5.5 knots on flat calm water, Idle Queen seems to burn around .5 gallons per hour, giving an economy of 11 miles/gallon.  I need more data to be sure that figure is accurate, as it was taken over relatively few hours, but I have enough data to be close.  

I am surprised that all of the numbers are so similar across such different boats and engines.  I really thought that the Contessa 26 would come out far ahead of the bigger boats, but that was only true when motoring into wind and waves–hence the better overall economy of the Contessa.  Even then it was not as great a difference as I had expected.  The numbers show that moving displacement hulls at relatively low speeds in a flat calm is really quite energy efficient.  There are many factors at play here, but I think that the biggest surprise is that I can move a 20,000 lb boat in calm conditions at about the same cost per mile as a 5,400 lb one as long as I keep the speed to about 4 knots.

The next biggest surprise is that the larger wetted surface of Sirocco and Idle Queen really didn’t translate into a much greater cost per mile under power.  Modern boats are made light and with small keels and rudders to improve their efficiency by reducing wetted surface.  At least as far as economy under power is concerned, the difference is not as big as I had thought.  Granted, even the Contessa had a relatively full keel, but she had less than half the underwater area of the other boats (measured by how much bottom paint it took to paint her).

When the wind and waves come up, the smaller boat requires much less energy to keep her moving, and then her fuel economy is much better than the bigger boats.  Motoring into wind and sea is frustratingly slow as well as expensive.  Idle Queen’s fuel economy quickly drops to about 10 nautical miles/gallon with only a 12 knot headwind and small chop.  Motoring into 15-20 knots and a bit of sea will take her right down to 2 miles/gallon or so.  I have only tried this for a short while, so my figures might be off a bit, but you get the idea. 

My estimated long-term figures are based on actual volume of fuel added to the tank versus engine hours and average conditions.  Still-water economy was measured in calm water (no current) by GPS, on glassy calm sections of canal, and the fuel volume was taken by sounding the tank.

A head-on view of Idle Queen while she hangs in the slings.  Her very shallow keel is evident, as well as her almost barrel-shaped (round) underbody.  This semicircular section, along with her shallow keel helps keep wetted-surface low.
A head-on view of Idle Queen while she hangs in the slings. Her very shallow keel is evident, as well as her almost barrel-shaped (round) underbody. This semicircular section, along with her shallow keel helps keep wetted-surface low.

 

 

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