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How do I take care of my engine at low speeds?

2009 February 2
Dear Boat Doctor,
Several of my reference books state that running a diesel engine at the low loads or cool conditions typical in sailing will cause carbon build-up, that can ruin the engine. However, these books do not tell me how hard the engine should be run to prevent this problem, or what to do if I need to run at low load conditions for an extended time.

I find that unless I am running late for a scheduled drawbridge opening I usually operate my engine at 1,800 rpm or less. Usually the engine is run at low-load when moving the boat from the dock through restricted areas to open waters where the sails go up. When anchoring out, sometimes you need an hour of battery charge. Or say I am a couple of hours from home when the wind drops and I need to motorsail, I like to keep it under 1,800 rpm so we can talk over the engine (and to feel like I am still sailing.) I kind of like the soft, throaty purr of my diesel motoring along at a relaxed pace. But it is hard to enjoy thinking that this is making for gummy rings and valves. Can you put this into perspective?

Lynn Deedler
Sebastopol, California

Dear Lynn,
Like flossing and avoiding fatty foods, what you ought to do and what you actually do are often different things. It is true that running a diesel at low speeds, low loads, or not letting it warm up is not good for your engine. Low-speed running should be avoided but sometimes you just can't.
You need to run slow in a marina to be safe and legal, and sometimes you just need to charge. I can understand your reluctance to run fast when motoring through a calm but it probably is best to throttle up a bit. I don't know your boat or your situation, but the addition of a small solar panel may help reduce your charge times.

Engines run most happily and the longest at about three-quarters of the redline with a proper load on them, but this is just a rule of thumb. You need to take a look at how the engine drives the hull and the fuel economy.

A good place to start is to see if your engine is properly set up for your boat. In fairly flat water you should be able to achieve roughly redline engine rpm with a wide-open throttle. The redline rpm will likely be listed on the engine data plate somewhere on the engine, if not you may need to find the specs from a dealer or manufacturer. If the engine will not go that fast, the boat is over-propped (assuming the throttle linkage is correctly setup), meaning the prop is too large and is overtaxing the engine.

Assuming the boat is propped correctly, run it at three-quarter speed and see how it goes. Are there nasty vibrations? How is the speed and the noise level? How does the speed compare to a few hundred slower rpm? A sailboat hull will only go so fast, adding extra power just wastes fuel. A good thing to look at is the waterline at the transom; if you drive the boat too hard, the stern will squat down in the water a few inches.

I've told you what not to do and also told you there is very little you can do to avoid doing it, so what can you do to be kind to your engine? One of the best things is once a month or so pretend you are a powerboater, take your boat out and run the engine at three-quarter speed until it is good and hot. This will heat up the oil to evaporate out any contaminants and will clean out some gunk. It is also a good idea to use a good fuel additive. I like Diesel Kleen from Power Service (www.powerservice.com, 800-643-9089), which will help to keep the engine clean.