Monday Minute - How Hard to Run Your Diesel

Monday Minute - How Hard to Run Your Diesel

Monday Minute lets me pass on tidbits from marine folks I have known over the years. Often a comment during a boat tour, or an off-handed remark as we walk through a facility, or a point made during a seminar, I find these words of wisdom in the pages of my PMM notebooks. While they are not usually meaty enough to fill an entire blog post or command an entire article, they are pearls of wisdom you will want to know.

I find all sorts of "knowledge" on the Internet, and it can be frustrating with engine-related forums on social media. The people asking the questions are often limited in experience and it is not surprising that the questions they pose can be pretty basic and appropriate answers would require more input. I'm not talking about single engine versus twin engine kind of talk, but rather: "We're looking at this used '80s boat on yachtworld or eBay and it has so-and-so engine(s). Are they good engines and is there anything we should know about?"

Within an hour there is all sorts of advice, some of it helpful, some not. Responding to such a specific question with the fact that you own a completely different boat with different engines and how well they work for you really doesn't help much, does it? So many factors come into play: total operating hours over how many years, condition of hoses, belts and other replaceable parts, ownership and maintenance history... to name a few.

At engine seminars at Trawler Fest and TrawlerPort, this came up pretty regularly. I think most engine manufacturers have serious diesel gurus who love to share their wealth of knowledge. And despite minor variances on technical specifications, the information they share is mostly the same.

Keep in mind that the following comments began in the early days before the different tiers of engine emissions changed the marine engine industry and necessitated electronic controls on all diesel engines. That was a game changer in many ways. It killed several popular engine models, and dictated the way surviving diesel engines handle operation over a wider range of performance. Rather than pollute the air with mechanical engines to allow them to reach operating temperatures, now engines self-adjust to keep from belching smoke on startup or building up carbon when running under light loads.

So for this Monday Minute, I will share the comments of Doug McElroy of Alaska Diesel (that dates us right there). Doug took part in a number of engine-related seminars over the years. When we were fortunate to host a seminar in Alaska Diesel's territory (Seattle, yes!) we were lucky to get the real deal experts of Doug and Greg Light and Bob Senter to spend an hour or so with us.

On the topic of engine requirements for a displacement cruising boat, Doug explained to the audience that the engine should be sized to push the hull at 90 percent of hull speed, using 75 percent of the maximum engine rpm. There is great info that can answer lots of questions.

The calculation for hull speed is a well known formula, easily calculated for monohull displacement hull shapes for sail and power. The formula is (HS = 1.34 x √LWL), which is hull speed (knots) equals 1.34 times the square root of the LWL. For a Krogen 48 with a LWL of 45'5", this equates to 9 knots as top hull speed.

So Doug is saying that for this Krogen 48, you should have a diesel engine that will push it at 8 knots (90%) at 75-percent maximum throttle. I believe the standard power is (or was) a John Deere six-cylinder diesel engine rated at 201hp.

He went on to say that while you are out cruising, owners should plan to normally run the engine above 50-percent load most of the time (based on fuel burn). Again, this can be calculated with the data in the engine documentation and fuel burn/performance curves. (Another engine expert smartly told me that if you run your diesel at 70 percent load all the time, it will run forever, as long as you give it clean fuel and plenty of fresh air.)

Before the days of the ECM and electronic emissions, variations of this advice were provided owners with semi-displacement boats. If you own a late-'90s Fleming 55, likely powered by a pair of turbocharged and aftercooled Cat 3208s for double-digit running speeds, you have a different scenario. Burr Yachts' technical wizard, Patrick Flaherty, recommends to raise the engine rpms at the end of the day to clean out any deposits that build up from running at slow engine speeds. You can see the results of this black build up on countless dark transoms as they pass by at slow speed, as in running up or down the ICW. He recommends that periodically one should run the engines up to normal speed to burn off any deposits from under-loaded 435hp diesels. Doing the Great Loop also does not allow running engines at the ideal speed/load. And taking a full displacement passagemaker across an ocean at slow speed to conserve fuel, it is important to remember to run the engine up to normal operating temperature and rpm once in awhile to make sure it is not creating issues that may affect performance later on, such as a clogged injector.

Doug McElroy also told us to have a similar mindset with regard to one's genset. You should keep a generator loaded to at least 30 percent most of the time, or you will have maintenance problems down the road. If you own an offshore passagemaker like the one I once crewed aboard, running the boat's generator to power ONLY the boat's navigation desktop computer is an incredibly bad idea. Both the delivery broker and I kept turning on air conditioning and other loads as this owner refused to see the long term health problems he was causing his ship's generator.

If you have a newer boat with electronic emissions controls in your engine room, your cruising world is less restricted if you want to chug along the Erie Canal at 6 mph with your fast cruiser. But if you are following her wake in an older boat with mechanical engines that don't enjoy electronic emissions benefits, you will need to blast it out once in awhile.

Have a great week, everyone!

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