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.
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.
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.
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.
It seems that one of the universal problems aboard small boats is not having enough room–not having enough storage space for clothes; tools; toys; spares; fuel; awnings; safety gear; etc… Immediately after finding a storage solution for a particular piece of gear, a new thing arrives that just has to be kept somewhere aboard and the challenge of finding a place to put it begins. I strive to keep my boat as simple as possible, but it still seems that there is never enough room aboard. When the space is shared, such as when guests come aboard, the problem compounds. The obvious solution to this problem is to have a bigger boat. A boat just a few feet longer than the current one sounds about right to a lot of people. This is such a common phenomenon that there is even a name for it: three-foot-itis. Three-foot-itis is when a boat owner decides that his or her current boat is inadequate, but that a boat three feet longer would be just the ticket. There is no other solution in the owner’s mind. A bigger boat must be found.
I succumbed to the above described boat-owner’s malady and have been working on Idle Queen for a while now. Idle Queen is a Dreadnought 32 built by the Dreadnought Boatworks of Carpentaria, California. She is three feet longer than Sirocco.
The space problem was just one part of what drove me to this boat, however. Cost was the other driving force. Everyone knows that bigger boats are more expensive, but in this case, I took a big step down. Sirocco, my last boat, is a beautifully finished boat. She has teak trim and bronze fittings and beautiful joinery. Idle Queen is home-finished with plywood and plexiglass and latex paint. I found her moldering in the back of a boatyard. She had been for sale so long that the sign had faded and broken. She is heavily-built and practical though. Because of her rough-and-ready working finish I won’t feel bad keeping her going with whatever I happen to scrounge or buy when things break. I don’t have to worry about a “yacht quality” finish. Strong and cheap will do it.
I will start getting some more pictures up in the near future and maybe some videos to document progress on Idle Queen‘s rehabilitation. My goal is to have a safe, strong, inexpensive voyaging yacht with this project.
Sirocco is out of her element for the time being so that I can get a few jobs done below the waterline. On the list was the cutless bearing for sure, but that sort of ballooned into a bigger project that involved removing the engine all because I couldn’t get a coupling to come free of the drive shaft! Well, there are many things that I can accomplish now that will only make Sirocco even better: repainting the engine and engine room; replacing the engine mounts; replacing the wiring on the engine; cleaning the keel tank below the engine; cleaning the bilge, etc. I will be busy for weeks to come!