Monday Minute - Thoughts on Sizing Engines and Generators

When I have conversations with industry experts we often drift off topic, a natural result of exploring the fringes of the main subject. In this case, I was working on a nontechnical piece as an introduction to turbochargers, but my Q&A with this tech at Northern Lights/Lugger drifted, as it often did, into another subject. I was happy to go there.

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We spoke of the importance of sizing the engine and generator to fit the application, in this case, a new boat. I asked if the company had a formula to identify the size engine best suited for a particular boat, taking into account whatever they felt were the important parameters to make the match. While we did go on to discuss turbochargers in detail, he offered these points on how to size a diesel engine and generator when building or repowering one's boat. Not enough for a full article, perhaps, but perfect for a Monday Minute. And for all those boat owners who want to learn the relationship between hull shape and power requirements, it might offer some helpful information, especially when sizing a generator:

If you are working with a displacement boat, the engine should be large enough to push the boat at 90 percent of hull speed, using 75 percent maximum engine rpm.

Plan to use the engine at more than 50 percent load most of the time. (The 50 percent load is based on fuel burn.)

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CPP. If you consider a Controllable Pitch Propeller (CPP) for your boat, match this to a Continuous Duty-rated engine, not one of the lesser, pleasure boat ratings.

Air. Some boats really benefit from an active ventilation system. Cool intake air is a good thing for maximum engine efficiency, and it is a worthwhile effort to accomplish this feature if it can be done without difficulty.

Fuel. Use a day tank if possible. The return line of the fuel delivery system should always go back to the day tank.

Coolant. Use a combination of distilled water and appropriate antifreeze (propylene glycol) in your fresh water cooling circuit. A low coolant alarm is a good thing, as is regular changing of the coolant in your system.

 

 A Northern Light generator with the side cover removed for maintenance and inspection. When located in an accessible location I find it a joy to maintain, keep it clean, and keep it running properly...as long as it is sized right for the boat.

A Northern Light generator with the side cover removed for maintenance and inspection. When located in an accessible location I find it a joy to maintain, keep it clean, and keep it running properly...as long as it is sized right for the boat.

Generators need to be loaded at least 30 percent most of the time. Do NOT size the generator for large, intermittent loads.

The best way to determine the correct size generator is to calculate the kW draw of all onboard devices, then multiply that draw calculation by 0.6. That is the size generator you need to install on your boat.

As I said, not enough for a full article, but useful and interesting information nonetheless. Have a great week.