In a time when the cost of “feeding” our diesel engines is higher than ever, it seems strange to talk about those engines “starving to death”. Yet that is exactly what is happening according to specialist marine engineer, Andy Marshall.

Marshall, who is the service manager at Westhaven-based General Marine Services, says a “surprisingly high” number of the diesel engines in New Zealand boats are literally dying a premature death. The reason? They are being starved. Not of fuel but of air.

“While the engines we are now installing are far bigger, more powerful and more sophisticated than they were a decade or so ago, the design of the engine rooms is pretty much the same,” he says.

“New diesel engines need far more air than their predecessors and the simple, sad and expensive fact is that they are not getting it. The result is engines that wear out before they should; engine rooms that run far too hot (and therefore damage other equipment such as generators and pumping systems), reduced fuel efficiency and black, oil-laden smoke that damages paintwork, timber and furnishings.”

Although the more air-hungry modern diesels are highlighting the problem now, it is actually not a new one. According to Marshall, older engines used to suffer, too; it was just less noticeable.

“Older, lower output engines generally operate at lower speeds,” he says. “They struggled on so nobody really paid much attention to ventilation. In fact, many older designs didn’t have ventilation systems at all, they simply relied on the cracks in the floorboards to let in enough air!”

However, even those lower output engines were actually starving in many cases; the problem was just harder to spot. These days, with modern turbocharged engines often running at up to 3600rpm, the problem can be far more serious.

“It also doesn’t help that the engine rooms haven’t got any bigger, even though the engines have,” says Marshall. This, he says, leads to a vicious spiral that often results in quite catastrophic and expensive failures.

“Basically, there is not enough air flow into the engine room to properly supply the engine and keep the engine room air temperature at an acceptable level. The engine intake therefore runs hotter than it should and the problems begin: the hotter-running engine heats the air in the engine room even more, the engine then breathes in this too-hot air, making it run outside the manufacturer’s specifications.”

Engines that are starved of cool air simply don’t last as long. They also use more fuel and often perform well below their best. Other equipment in a too-hot engine room is also placed under stress and is more likely to fail prematurely or struggle to perform at its best.

“I liken it to a turbo on an engine, except in reverse. Everyone knows that if you add air through a turbo, the engine performs better. Not supplying enough air just has the opposite effect; it’s like you are choking the engine.”

It is, says Marshall, a widespread problem, affecting both newly built vessels and older ones that have been repowered with modern engines. It occurs on small 9 or 10 metre launches, on larger motoryachts and superyachts and on commercial vessels. It is not restricted solely to motorboats, either.

“We see lots of problem with sail boats,” says Marshall. “The engines are often enclosed in tiny spaces with virtually no air flow. That’s bad enough if the engine is only used to get on and off the marina; when it’s on a yacht that motors around a lot it can be disastrous.”

While the problem appears widespread and the effects are often dire, the solution is relatively easy: get more cool air into the engine room and get more of that hot air out.

“Most people probably know that engines need air to breathe, they just don’t realise how much air they need.”

Like with most things mechanical, there is a formula which describes how much air a diesel engine needs to work efficiently: basically 2.5 cubic feet of air per minute per horsepower produced. In addition, sufficient air is also required to keep the engine room cool. This all adds up to a substantial amount, which is probably why it is so often under provided.

“In the old days, we could set up dorade boxes and they would funnel in enough air,” says Marshall. However, dorade boxes and funnels can no longer keep up with the demands of the air-hungry modern motors.

“In order to get the best out of these engines, we have to ensure substantial amounts of cool air are delivered to the right part of the engine room and, just as importantly, we need to remove as much of the hot air generated by the engine as possible.”

These days, marine engineers like Marshall and his team use special marine fans and blowers to get the air in and out of the engine room.

“We use high quality Italian-made Gianneschi units,” says Marshall. “The fans and blowers have to be able to run continuously for long periods of time. They have to be able to cope with salt-laden air, they have to be made of durable, marine grade materials and the brackets and housing have to be strong and able to handle the pounding they will get in a big sea.”

Marshall says that, from time to time, the General Marine team comes across installations where the owner or an inexperienced boatbuilder has used bathroom fittings, like extractors, in order to create airflow.

“The best place to site a fan or a blower is often not the most accessible part of the engine room,” he says. “You want to use a unit that is going to last, not one you’re going to have to squeeze in and replace all the time.”

Marshall says handy DIYers can install the blowers and fans themselves, especially on smaller boats, but he does recommend that they get advice first.

“It is important to ensure the air is going to the right place. It is also important to ensure it is not bringing in too much salt and moisture and that the correct air pressure is being maintained in the engine room.”

So, how does an owner know if their engine is lacking sufficient air? Marshall suggests owners ask the following questions: Does the engine produce black smoke at full power? Is the engine sluggish? Does the engine breathe heavily out of the crankcase breather?

“If the answer to any of these is yes there is potentially a serious problem with airflow in the engine room. This should be checked by an expert as soon as possible.”

View Gianneschi Blowers

This DC-powered centrifugal blower is perfectly positioned at the back of the engine room to remove used air.

This powerful AC axial-type blower is delivering cool inlet air right where it’s needed: to the front of the main engine.

This centrifugal blower is removing hot air from aft of the engine room and around the insulated exhaust.

This centrifugal blower creates air flow through lazzerette