With the recent release of the final report of Nigel Calder's HyMAR project, marine internal combustion / electric hybrid powertrains are making waves once again- and not necessarily in a good way, as Calder's team found that the benefits of the expensive hybrids were limited to a relatively narrow, low-speed operating regime. We've known for quite a while that the major advantages of gas/electric and diesel/electric hybrid cars- namely, instant start/stop and regenerative braking- don't apply to boats. We've also known for quite a while that hybrid systems only make economic sense aboard ship if the house loads are comparable to, or larger than, the power required for main propulsion.

Tech addicts need not fear, though: There are many other fuel-saving and pollution-reducing technologies waiting in the wings, and hybrids aren't quite out of the running yet.

Hybrid Electric

Wait, didn't we just reject this?

Well, yes, on economic and efficiency grounds. It's not out of the running, though, because hybrid tech offers two big advantages:

• Dead-silent, exhaust-free running at very low speeds
• Gobs of torque (and therefore thrust) at slow speeds for docking manoeuvres

On those grounds alone, a sufficiently wealthy boater can justify a hybrid drivetrain, even though it won't provide any cost or fuel savings over a conventional engine. And as electric car technology improves, we'll see corresponding improvements in the cost and range of all-electric drive systems, and in the efficiency and cost of hybrids.

How it works: An electric motor is coupled to the propeller shaft, and a powerful generator is coupled to the engine. The connection between the two may be entirely electric (series hybrid) or, more likely, mechanical with the option to run on either system alone (parallel hybrid). A large, high-voltage battery pack buffers the electrical side of the powertrain and stores enough energy for short, slow trips without the engine.

Already seen in: Cruise ships (series hybrid), Steyr Motors integrated diesel-electric packages (parallel hybrid), fishing boat trolling motors (usually electric alone, but they're a series hybrid system if you add a small generator to charge the battery), Torqeedo dinghy outboards (which are pure battery-electric, charged from shore power or from the mother ship's generator), Elco motor launches.

Coming to: High-end cruisers and charter yachts where dead silent gunkholing and excellent dockside handling are higher priorities than cost.

Cylinder Deactivation

How it works: When the engine is lightly loaded, its computer shuts off several of the cylinders. The remaining cylinders, now more heavily loaded, are therefore forced into a more efficient region of the fuel map. In other words, four cylinders running at 30% of full power will use less fuel than eight cylinders at 15% of maximum.

Already seen in: Pickup trucks and large SUVs, which require plenty of reserve power for towing or passing and so end up loafing along at 10% throttle most of the time.

Coming to: Powerboats that use light truck gasoline engines. The old General Motors V6 and V8 blocks that currently dominate the mid-size powerboat market won't be around forever, and their replacements in the road-going sector already have this technology.

Direct Injection

How it works: Instead of mixing fuel and air on the intake side of the engine, the fuel is injected directly into the combustion chamber. This allows much more control over how the fuel mixes with the air and how the flame front propagates. By varying the timing and quantity of the fuel spray, the engine computer can make use of lean or stratified combustion modes to reduce fuel consumption at part throttle.

Already seen in: Most modern diesels and two-stroke outboards already use direct injection.

Coming to: 4-stroke gas engines, particularly those that spend a lot of time at part throttle, are starting to get direct injection in the car market- that technology will find its way to boats soon. Diesels and two-stroke outboards are getting increasingly sophisticated injection systems and computers, with the ability to fire multiple shots of fuel at different times during a single power stroke.

Catalytic Converters

How it works: Exhaust gas is passed through a honeycomb of exotic metals at high temperature, which catalyze reactions that convert unburned fuel, nitrogen compounds and carbon monoxide into less harmful gases. Catalyzed combustion oxidizes unburned hydrocarbons and carbon monoxide to yield water and carbon dioxide, while smog-forming nitrogen oxides are reduced to nitrogen and oxygen. For this to work, the composition of the incoming exhaust must be within a very narrow range corrsponding to nearly stoichiometric combustion; exhaust oxygen sensors and computer-controlled fuel injection are practically essential for keeping the engine within the operating range where catalysts can work well.

Already seen in: Virtually all gas-powered cars of the last few decades. They're also starting to show up on some diesel cars, although the composition of diesel exhaust makes it difficult to get catalytic converters to work reliably.

Coming to: Like it or not, we'll be seeing cat-cons on boats eventually. There is a (possibly valid) argument that since many boats route their exhaust underwater, they don't contribute to smog as much as cars do, and there is a (very valid) argument that fitting catalysts to marine engines- diesels in particular- will be a difficult and expensive task. Those are technical issues, though, and given enough money, technical issues can usually be overcome if regulations force the matter.

Variable Valves

How it works: Any particular choice of intake and exhaust valve timing, lift and duration will be ideal for just one speed. Slow the engine down and you'll get smoother running with a shorter, lower intake valve motion; hit the throttle and you'll want those valves open wider and longer to move more air faster. With a conventional camshaft, you're locked in to one set of timings; variable valve systems allow the timing, and possibly the lift, of the valves to be varied according to the current speed and load.

Today's variable valve timing and lift systems are mechanical, often with computer control. Most work by adjusting the phase of the camshaft- rotating it slightly ahead of, or slightly behind, its usual position- but some systems can switch between different cam profiles, and at least one is able to throttle the engine by varying the valve lift. Systems that do away with the camshaft entirely, opening each valve with an electromagnetic actuator, are in development but are not yet cost effective for production engines.

Already seen in: Every major carmaker has a variable valve system, although some of them save the technology for their "halo" cars. You might already have it in a newer, larger outboard.

Coming To: Standard, mechanical variable valve systems are becoming more popular in the car sector, and will trickle down to boats in the next generation of marinized 'crate' engines. Fully computer-controlled electromagnetic valves are a bit farther off, but keep an eye out in five or ten years.

Mixed Cycles

How it works: There are more variations on this theme than you can shake a boathook at. The basic idea is to change the thermodynamic cycle of the engine- instead of the Otto (standard gas engine) or Diesel cycles, an engine designer will combine some parts of each (and some features that appear in neither) to favour fuel efficiency, or power output, or exhaust emissions. Exotic cycles combined with direct injection and variable valve timing can yield engines that switch from a diesel-like compression ignition cycle to a spark-ignition cycle on the fly, allowing the engine computer to effectively change the type of engine according to the current conditions.

Already seen in: Research labs, mainly, but some funky cycles are starting to show up in the car sector.

Coming to: Boats with highly variable load conditions, i.e. sportboats and weekend power cruisers, will benefit the most from this technology. Those who rarely run outside a narrow, well-defined power band are likely still better off with standard diesels.