Improving low speed fuel efficiency

It’s well known that displacement hulls (as opposed to planing hulls), whether power or sail, more or less have a maximum speed, which is defined as “hull speed.” The traditional formula for calculating hull speed has always been 1.34 x (the square root) of the waterline length. Variations in hull design and the weight of a boat will change things somewhat, but fundamentally the formula holds. It’s a function of the waves that a boat makes as it moves through the water, and the resulting wave-making drag.
Most people also know that as hull speed is approached, wave-making drag starts to rise dramatically. I think few appreciate quite how much it rises. Take my Malo 46, Nada, with a waterline length of 38.25 feet. The nominal hull speed is around 8.3 knots. I have done modeling in conjunction with Hybrid Marine in the UK (, which demonstrates that it takes just 11 hp to drive this boat at 6 knots in calm water. This rises to 19 hp at 7 knots, 38 hp at 8 knots and 98 hp at 9 knots (the standard boat has a 110-hp Yanmar). In rough water, it takes 39 hp to do 6 knots, rising to 56 hp at 7 knots and 86 hp at 8 knots.
Boatbuilders these days like to get their boats up to hull speed in adverse conditions. As a result, the tendency over the years has been to fit ever larger engines. It’s not unusual to see hundreds of installed horsepower in a 40-plus-foot trawler yacht (for example, a 46-foot Grand Banks Heritage now has two 550-hp engines, while the 47-foot Eastbay, which is a planing boat, has two 700-hp engines). With all this power, at normal cruising speeds these engines are typically operating at a fraction of their rated output. In fact, in calm water a displacement boat is commonly operating on less than 30 percent of the engine’s rated power. A trawler yacht going down the Intracoastal Waterway (ICW), and observing the speed limits and no wake rules, might be operating on as little as 10 percent of rated power.
To find out how much power is being used at any given engine speed, we can consult a propellor curve. Propellor curves are calculated, rather than developed through real-life testing, and as such are approximations based on certain assumptions about the size and type of propellor being used, etc. The calculation also includes something known as the “propellor exponent.” This can be adjusted to accommodate the characteristics of the hull being driven (e.g., full displacement, semi-displacement, planing, etc.)
Regardless of how a propellor curve is developed, the power absorbed by a propellor as a function of its speed of rotation is different to the power that an engine can produce at those same speeds — i.e., the two curves are quite widely divergent. In practice, with a fixed-pitch propellor they can only be got to match at one speed. Propellors are generally sized such that this match occurs at maximum engine speed, or something close to it. This is because a match at any lower speed would result in the engine being overloaded at higher speeds, and a match at some projected higher speed would result in the engine never being fully loaded.

Assuming the propellor curve and power curve are matched at something close to full engine speed, the slower the engine is operated the more the curves diverge. What this means is that at slower engine speeds less and less of the available power from the engine is being used by the propellor, which is to say that the load on the engine, as a percentage of the available power, is less and less. A trawler yacht chugging down the ICW is not only using a very small percentage of the total overall rated power of the engine, but is also using a small percentage of the power available at the speed at which the engine is being operated. At cruise speeds, this power utilization is commonly below 30 percent of available power, and often below 20 percent of available power. If you’ve got two 700-hp engines in a 47-foot boat that is moving at, say, 6 knots, you’ll likely be using well under 10 percent of the available power.


Fuel (in)efficiency

In terms of fuel efficiency this low power utilization doesn’t matter — in theory. The engine’s fuel injection pump will simply cut back the rate of injection to whatever is necessary to produce the power being used by the propellor. In practice, it’s not quite this simple.
Various studies done over the years have demonstrated that as power utilization starts to fall below 50 percent of the available power, the fuel efficiency on diesel engines begins to decline. Once power utilization falls below 20 percent, fuel efficiency can decline dramatically to the point that the amount of fuel burned per horsepower hour of energy produced (i.e. the fuel burned per hp of energy used by the propellor) can easily be double, triple or even six times what it is at the most efficient fuel burn rates (which generally occur somewhere around two thirds of maximum speed).

This is one of the reasons why it is such a bad idea to run diesel engines lightly loaded for extended periods of time (such as when battery charging or refrigerating at anchor, or when using an AC generator for light loads). The excess fuel being burned results in a good deal of carbon formation that fouls piston rings, valves and exhaust systems, and can, in a worst-case situation, wreck an engine in less than 1,000 running hours.


Specific fuel consumption

Most engine manufacturers have curves that show the “specific fuel consumption” for the engine, which is to say the amount of fuel it takes to produce an hp/hour of energy at different engine speeds, but these curves are typically based on the assumption that the engine is fully loaded at these speeds. A few manufacturers show the specific fuel consumption on the propellor curve, which is a far better indicator of fuel burn rates in real life because it takes account of the fact that the engine is almost never fully loaded at any given speed. However, this kind of data is rare.
Instead, what is typically shown is the total fuel consumption for the propellor curve, in gallons per hour rather than the specific fuel consumption rate. The total fuel consumption must be divided by the hp used by the propellor at any given engine speed to arrive at the specific fuel consumption rate for that point on the propellor curve. This is what we need to compare relative efficiencies. If the specific fuel consumption rate can be calculated, it will often show deteriorating efficiency at slower propellor speeds.

In recent years the rather dismal fuel efficiency picture that this kind of investigation typically reveals has been changing. The driving force has come from ever-tougher emissions regulations both in the U.S. and in Europe, with the U.S. generally leading the field. These emissions regulations have forced engine manufacturers to make considerable improvements in the low speed, low load performance of their engines.


Electronic and fuel injection technologies

There’s no one technological advance that can be pointed to as underlying these improvements. Instead, we have a wide range of advances, all of which were on display at a recent Volvo Penta press conference in Sweden to celebrate this company’s centennial (it was founded in 1907). As such, Volvo Penta is a useful exemplar of new technology.

A key component has been the development of electronic control systems for engines. These underlie recent advances in fuel injection technology, notably common rail injection and electronic unit injection, both of which produce higher injection pressures than were found in the past, especially at slower engine speeds. This, in turn, results in more complete combustion, once again especially at slower engine speeds.
Common rail enables multiple injection pulses to occur on a single power stroke, with additional improvements in fuel utilization over conventional injection technology, while unit injection raises injection pressures even higher than those found in common rail systems and as such is used to project fuel to the far reaches of cylinders in larger volume engines. Both systems can integrate a wide array of sensors (pressure, temperature, oxygen, etc.) in a manner that further refines the injection process.
Volvo Penta’s D3, D4 and D6 engines are all good examples of latest generation common rail engines. The D9, introduced three years ago, and the D11 (11 liters cylinder volume) released at the latest press conference, are good examples of latest generation, larger engines (the D11 is 670 hp) using electronic unit injection.

Common rail injection (and the associated electronic engine controls) is now available in marine horsepower ratings down to around 100 hp. When we get below 100 horsepower, we find that existing mechanical fuel injection technologies will be able to meet emissions regulations for some years to come, so we are unlikely to see electronic marine engines, and common rail injection, in this horsepower range for at least the next several years.


Turbochargers and superchargers

For low emissions, injected fuel must be thoroughly mixed with an adequate supply of air. A turbocharger plays a critical role in pumping air into an engine and in maintaining an optimum air-to-fuel mix. Given that a turbocharger is driven by exhaust gases, and that the volume of these gases is proportional to engine speed and load, it’s been hard to get a turbocharger to operate to optimum effect at slow engine speeds and low loads. There are all kinds of interesting things happening to improve this performance.
The automotive world has been using variable geometry turbochargers for some time. These have electronically controlled blades. This enables the blade angle to be changed, thereby changing the geometry of the turbocharger to give maximum power in any situation. Volvo Penta’s D3 engine, which is adapted from the automotive world, has a variable geometry turbocharger.
Then there’s twin entry, “duopulse” turbocharging. In a standard engine all the exhaust valves discharge into a common exhaust manifold. The pressure pulses as each cylinder discharge can interfere with each other in a manner that inhibits turbocharger operation. With a twin entry turbocharger, the cylinders discharge into two separate manifolds that are carried up to, and into, the entry path into the turbocharger. The net effect is to use the exhaust pulses more efficiently, especially at slow engine speeds. Volvo Penta’s D9 and D11 engines use twin entry turbochargers.

If all else fails, you can always force combustion air into an engine at slow speeds and low loads using a mechanically or electrically driven supercharger of some sort. This approach is used on some larger engines, with the supercharger dropping out as the “boost” pressure from the turbocharger kicks in.


Other improvements

There are other ways of improving the gas flow through an engine. Most modern engines have four valves per cylinder (as opposed to two in the past) — two inlet and two exhaust. This introduces less friction. Similarly, a considerable amount of attention has been given to manifold design to improve airflow. (The twin entry turbocharger adds some friction with its additional manifolding, but this is more than offset by improvements in turbocharger performance.)

Unfortunately, the typical boatbuilder or boatowner concerned about noise now takes this highly optimized engine and puts it in a tight box that interferes with the gasflow! However, the impact of this is primarily seen at high engine speeds and loads rather than the low speeds and loads that are the focus of this article.
There have been all kinds of materials improvements over the years that have enabled more power to be got out of the same package, improving power-to-weight ratios dramatically. Volvo Penta’s new D11 block, for example, is cast from “compact graphite” (a form of graphite-reinforced cast iron), which is 50 percent stronger than conventional cast iron, permitting thinner castings.
Finally, whereas most industrial diesel engines are designed to run most efficiently at around two thirds of full speed, on the assumption that they will be run more-or-less continuously somewhere around this power output, a number of purpose-built marine engines are optimized for considerably slower speeds, on the assumption that full power will rarely be called for. This, too, improves low speed, low load performance and fuel efficiency. It does so at the expense of a small loss of efficiency at the higher end.
The net effect of all these incremental changes has been to considerably improve the low speed, low load efficiency of the latest generation of marine diesel engines. Whereas fuel efficiency on an older engine is likely to start to degrade when the load, as a percentage of the available power, dips below 50 percent, and almost always degrades rapidly when the load dips below 20 percent of the available power, many modern engines maintain almost a flat fuel efficiency curve down to around a 20-percent load. Below the 20-percent load level, fuel efficiency degrades, but not at anywhere the rate that it does with older technology.
For the owner of a displacement hull (power or sail), this means there is likely to be substantially improved fuel efficiency at cruising speeds and below (e.g. when maneuvering on and off a dock) even with the large engines installed in most boats today. There will also be less carbon fouling of the engine.

Of course, you’d likely get even better fuel economy with a smaller engine. This would only result in a small loss of top speed, most notably in rough water, but this minimal loss of speed seems to be a price that few are prepared to pay.


By Ocean Navigator