November 6, 2011

Gas or diesel?

IS IT WORTH buying a new auxiliary engine for your good old boat? Or should you just put up with the constant need for spares (if you can get them) and ever-more-costly  repairs? And if you buy a new engine, should it be gasoline or diesel?
   Well, the average gasoline boat engine runs for 1,500 hours before needing a major overhaul. The average diesel engine runs for something approaching 5,000 hours under the same conditions — that us, roughly three times as long.
   Of course these are very general rules of thumb because the life of an engine depends on how it's used, abused, and maintained. But I should add that these estimates come from a man whose full-time job it has been for many years to persuade boaters to replace ailing used engines with new ones.
   Cynically, therefore, we may assume that his figures concerning the life of engines are a little conservative. According to the same man, the typical gasoline boat engine gets a "good" 1,000 hours of operation. During the next 500 hours, minor troubles become increasingly likely, turning into major troubles as the 1,500-hour mark approaches.
   It's interesting to note that an automobile engine runs an average of about 3,000 hours — about double the running time of of a gasoline boat engine — before requiring an overhaul at 100,000 miles. But most of the time, boat engines work harder than do car engines, and under worse conditions.
   A well-maintained gasoline boat engine run under the best conditions might indeed run for more than 1,500 hours without a major overhaul, but many will get fewer hours than that because of the atrocious conditions they suffer — salt air, damp bilges, intermittent operation, and all too often, pure neglect.
   Diesel engines are built more heavily, and to finer tolerances, than are gasoline engines. They thus accept more abuse and often deliver 8,000 hours of hard work in  fishing boats before requiring major surgery. At this rate, in theory, a well-maintained diesel auxiliary will last the life of the boat, because the average boat owner logs 200 engine hours a year.
  Unfortunately, in practice things are rather different. Engines like to run long and steadily. The shorter the running time between stops, and the longer the idle time between runs, the fewer the hours they deliver before needing to be carted off to the engine emergency room.

Today's Thought
Life too often presents us with a choice of evils, rather than of goods.
-- C. C. Colton, Lacon

“What jobs are hippies best fit for?”
“Holding on your leggies.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


Matt Marsh said...

Are you over-generalizing, John? ;)

I recall one mechanic a while ago, can't remember who, but he figured that engine life was pretty closely tied to total fuel burn. A diesel putting out 100 hp to drive a big slow genset might last 15,000 hours. That same block, but tuned and turbocharged to 300 hp for a speedboat, would be lucky to make it to 5,000 hours in one piece. Both will have burned about the same amount of fuel in their lifetime. The same applies to gas engines: drive a car gently and it'll last 300,000 km, but hold the happy pedal to the floor and you'll be in the shop for engine work at 100,000 km.

Salt air, damp bilges, pure neglect- no boat engine should have to endure that. Engine rooms should be clean, they should be dry, they should be well-ventilated with fresh air from high up, and with baffles or dorade boxes to keep water from coming along for the ride. And they should be easy enough to reach that the owner will actually check things like oil level and belt tension, instead of just thinking "aw, too hard to get to, it'll still work anyway".

I might also add that short, intermittent operation at underloaded or overloaded condition is more easily tolerated by gas engines than by diesels; the former just complains a bit while the latter starts tearing itself apart from the pain of being outside its comfort zone.

Jack of all Trades said...

No mention at all of electric! Shame shame shame, especially for a professional!

True, the cost for early adopters is high, but the relative lifetime cost is falling rapidly and will soon (3-5 years) be at parity with existing IC engines, sooner if hydrocarbon prices rise.

How many hours of use before an electric motor requires an overhaul? How difficult to find spare parts? How much less hassle?

John Vigor said...

Jack, what makes you think I'm a professional? I'm a professional writer who sails, not a professional sailor who writes. There's a difference.
As for electric motors, they are simply not practical as replacements for internal combustion engines st this time.
Of course everybody loves the idea of silent, clean, reliable, long-distance propulsion. In fact, as a newspaper columnist I once ran a spirited campaign against officials of a large city who were doing away with their electric trolley buses in favor of diesel buses. I lost, as usual.
I am not too scarred from that encounter, but as far as boats are concerned there is no alternative at this time that comes anywhere near the convenience or practicality of gas or diesel.

When container ships and airplanes start using electric motors I'll be convinced.

John V.

mgtdOcean said...

John, Don't know what cars your buying and how you maintained them but I can say that since the late 1980s my cars have all gone well over 250k miles without needing overhaul. Granted they have all been Toyota so that may be the issue, but I suspect even a GM will go well over 100K.

Next I'd say gas vs Diesel depends on how the boats are used. A weekend sailor that motors out of a slip then sails vs a "cruiser" who motors all the way down the ICW. In the first case a gas motor can last 10 years easily by which time there should be a much better motor available and then there is the opportunity costs for the money not spent on the diesel over those years.

Now that cruiser on ther other hand that drives up and down the ICW has a good case for a diesel.