Know-How

Fuel Consumption: Tug Vs. Cruiser

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Elmore was built in Astoria, Oregon, in 1890. She saw duty in the Alaska gold rush and was converted to a tugboat in 1902. More recently, she has been lovingly restored by Dee and Sara Meek. She is 78 feet on deck and weighs 150 tons. Her current engine, a massive Atlas Imperial, is rated at 110 hp at 325 rpm and weighs an incredible 10 tons. At 8 knots, fuel consumption is around 4gph. 

Now let’s fast-forward 120 years to my boat, Nada. She is 48 feet on deck and weighs 18 tons (one-eighth the weight of Elmore). The standard engine is a Yanmar diesel, also rated at 110 hp, but at 3,200 rpm, about 10 times as fast as the Atlas. The Yanmar weighs one quarter of a ton, or about one-fortieth the weight of the Atlas. At 8 knots, fuel consumption is also around 4gph.

How can this be? A dinosaur of an engine that is over 70 years old can push a much larger boat than ours, weighing eight times as much, at similar speeds with more-or-less the same fuel consumption! The answer has to do with propellers and shaft speeds.

It has long been known that when it comes to propellers in displacement hulls, size counts. The larger a propeller, the more efficient it will be. Elmore has a 58in or 62in (the engineer couldn’t remember which) diameter propeller, versus the 22in propeller on our boat. Therefore, Elmore’s enormous propeller is considerably more efficient than our small one.

Big propellers have to turn slowly. Elmore has a direct drive between the engine and shaft with a peak engine/shaft speed of 325 rpm and a “cruising” speed well below this. By contrast, Nada’s Yanmar has a 2.63:1 reduction gear, which drops the peak engine speed of 3,200 rpm down to a peak shaft speed of 1,216 rpm—about four times faster than that on the Elmore. This is a great video of Elmore maneuvering at engine speeds of about 160 rpm.

The prop aperture on our boat is plenty big enough to swing a wheel up to 26 inches in diameter and still have adequate tip clearance between the propeller b

lades and the underside of the hull. So the obvious question becomes: why not add a higher reduction gear to the engine, lower the propeller shaft speed and install a larger propeller? We could save fuel, achieve a higher top boat speed (because the propeller would convert engine power into thrust more efficiently than a smaller propeller) and we need suffer no performance penalty under sail so long as we are willing to buy a folding or feathering propeller. It seems like a no-brainer.

However, there are two problems. The first is that the boat may (or may not) become harder to maneuver in close quarters, although we could certainly go up a fair bit in propeller size and down in shaft speed, and still have perfectly adequate maneuvering capability. The bigger problem concerns the mass of the propeller and the effort, or torque, needed to get it turning. 

Mass—which is related to, but not the same as weight—is a measure of an object’s resistance to acceleration. The greater an object’s mass, the greater its resistance to acceleration. A big propeller will have more resistance than a small one. Torque is the force that overcomes this resistance to acceleration—in our case, it is the measure of the turning force being applied to the propeller shaft. 

The problem with internal combustion engines is that they develop no torque at zero rpm and only build torque with speed. At the idle speeds you see when a shaft is first engaged, a modern engine develops maybe a third of its full rated torque. If you put on too large a propeller, then when the engine is put in gear, the prop’s initial resistance will simply stall the engine. 

Things are even worse when switching between forward and reverse, because now the propeller is already turning, but in the wrong direction. The engine has to stop the spinning propeller and get it spinning in the other direction without stalling. More than one engine manufacturer has tried to improve efficiency by adding high reduction gears in order to decrease shaft speeds and drive large propellers, only to find the engines died as boats came alongside and shifted into reverse to stop. This can be quite embarrassing.

I can’t find the torque rating of the Atlas, but by my calculation its peak torque must be somewhere around 2,500 Newton-meters (1,850 lb. ft.), whereas for the Yanmar it is around 275 Nm (203 lb. ft.). This enormous difference is accounted for by such things as the huge diameter of the pistons in the Atlas (9in versus 3.3in), their long stroke (12in versus 3.5in) and the powerful lever arm this creates in exerting force on the crankshaft. With the 2.63:1 reduction gear, the torque at the Yanmar’s propeller shaft rises to around 715 Nm (530 lb. ft.), but this is still less than one third that of the Atlas.

Another issue that comes into play is the huge mass of the Atlas itself. Much of it is in the frame and engine construction, which is not relevant here, but a significant part is in the rotating machinery—the pistons, connecting rods and above all, a truly monstrous flywheel. Once these are spinning, they provide the force needed to drive through the inertia of Elmore’s massive propeller, even when she shifts from forward to reverse. The net result is propeller efficiencies we can only dream of with modern lightweight engines.

It is ironic that the incessant drive to decrease engine weight and increase their power-to-weight ratio has forced us to use increasingly inefficient propellers. Here we have a 120-year-old vessel that weighs vastly more than ours, with an engine that is many decades old, achieving fuel economy that is similar to ours. 

For all the amazing advances in technology that have occurred over the years, in some respects, it seems we have not come very far! 

Photo (top) by Jim Wells; Elmore engine photo by Dan Mattson; Nada photos by Nigel Calder

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