PS Advisor: Tangled Up in Turbochargers
Do turbo-charged diesels make sense in sailboats?
I have noticed that several new sailboats are being offered with turbocharged diesel engines. Seems to me these pose two problems: excess fuel consumption and excess heat. Do you have an opinion or technical advice on their use practicality?
I realize they are a quick and easy way to produce 75 horsepower from an engine that normally produces 55 horsepower. However, I had a colleague who re-powered his power boat with a turbo-diesel. He was told by the installer to ensure he was running the engine at a high enough RPM to engage the turbo-charger for at least 15 minutes each time he took the boat out. He said the problem was that the fuel consumption essentially doubled when the turbo was running and the engine ran hotter. While I will acknowledge that on a power boat, you can probably add several knots to the boats top speed with a turbo, I am not sure what advantage it would have on a displacement sailboat, where you are going to be limited by the boat’s hull speed.
Jeanneau Sun Odyssey 37
Chesapeake Bay, Va.
You raise some good questions regarding turbochargers. For answers, we turned to frequent PS consultants on such matters, Gordon Torresen of Torresen Marine and Doug Rose of Volvo Penta. Here is a capsule summary of what they told us.
First, horsepower selection, dry weight, and engine size are usually based on the boat designer’s specs. The design also dictates propeller design. Most sailboats have engines of less than 100 horsepower. If a naturally aspirated engine (i.e. not turbo-charged) fits these parameters, it is usually preferred. Primarily, this is because naturally aspirated diesels are simpler, less expensive, and arguably more reliable over the long haul. If no naturally aspirated engine fits the bill, then a turbocharger may be required.
In general, turbochargers allow an engine to produce more power than a naturally aspirated engine of the same displacement. The turbocharger forces more air into the cylinders than the engine can normally draw in without a turbocharger. This surplus of air allows more fuel to be injected and burned in a given time period, and this is the source of the extra power. This makes a turbocharged engine more efficient than an engine of the same displacement that is not turbocharged. For example, a 51-horsepower engine that weighs 498 pounds jumps to 88 horsepower with only a 46-pound increase in dry weight. The displacement of both engines is the same, and the dimensions are nearly identical.
The turbochargers on small diesels usually operate whenever the engine is running. The engine in your friend’s powerboat may be of higher horsepower and use wastegate technology, which allows the turbo to be operated at given RPM.
As you point out, increasing horsepower doesn’t mean a displacement hull will go faster. Likewise, fuel efficiency plummets as you approach hull speed. In terms of power and fuel efficiency, some of the biggest gains can be made in the water. Getting the right prop is often more effective than adding horsepower—and a whole lot cheaper. For more on propeller efficiency, Dave Gerr’s “Propeller Handbook” is a great place to start.