Post below courtesy of Bob Sinter. Bob, as most of you know, is in charge of training, customer support, application engineering for Northern Lights.
"40 - 60% load is the sweet spot for almost any diesel engine. Sub 30% loads are almost always problematic, although some engines are more forgiving than others. 2 GPH pencils out to about 40 HP. You can get away with 30% load in any diesel for a while. But, it will virtually always lead to “wet stacking”, where exhaust byproducts condense in the water-cooled parts of the manifold, turbocharger (if you have one) and the exhaust system, regardless of whether it's wet or dry. Low loads also cause grossly high volumes of crankcase vapor blowby which overwhelms the vapor/mist coalescer systems and is aspirated into the air intake, creating smoke and soot. It also is the root cause of a lot of engine oil leaks.
In general terms, diesel engines tolerate low loads in this order, from most tolerance to least: 1.Naturally aspirated, non emission rated 2. Turbocharged, no after cooler 3. Turbocharged, jacket water (antifreeze) after cooler 4. Turbocharged, jacket water after cooler, emission certified 5. Turbocharged, seawater after cooled, non emission rated 6. Turbocharged, jacket water, second circuit keel cooled after cooler 7. Turbocharged, seawater after cooled, emission rated 8. High power density seawater after cooled (above 50 HP per liter)
With only a few exceptions, many passagemaking full displacement boats have at least twice as much power installed as they need. Experience has shown that is less than desirable. The trend toward more appropriate sizing is reflected in a number of new builds that are being specified with 4 cylinder engines where sixes were used previously and less installed HP overall. Robert Beebe’s Passagemaker firmly established the historical precedent in proper engine size for passage making yachts with its single 120 HP Ford-Lehman. That's the reality. Obviously, bigger, heavier boats require more power, but probably not as much as you think. In the absence of running at more appropriate loading, which is unfortunately incompatible with best cruising range, a daily WOT run is the second best thing - you may need to increase the time at or near WOT to give the engine a chance to heat up and clean itself out if you have wet stacking, smoke or oily looking drips near the turbo."