Originally Posted by sunchaser
Many of us have steered away from DDs for valid reasons. Namely, there are better options available once into newer boats. By the late 1990s DD's two stroke were replaced with Series 60s or other brands due to fuel costs and environmental reasons.
That said, if you are pursuing a pre 1990 vessel there are so many DDs around still that your vessel engine choices can be limited. Therefore follow Ski's and other's advice on mechanically checking out the 4-71s. To replace those engines with something newer would be way too costly.
Not quoting you to disagree specifically, just expanding on the idea.
Most of the "problems" associated with DDs happened in the 80s when there was a market for faster sportfish but not an engine that was ideal for the task. TIA Detroits putting out more than double the hp of their naturally aspirated cousins used a LOT of fuel and didn't last long.
The EPA effectively killed 2 strokes, and that included the Detroit 71 series.
For trawler uses, I'd put a natural Detroit running 1200rpm up against any 4 stroke out there, for both fuel economy and longevity, with no hesitation.
Go to full cruise, and absolutely Detroits get thirsty. At hull speed they are surprisingly economical. And every shade tree mechanic on the planet is familiar with them.
They were introduced in the late 1930s. They powered landing craft on D-Day. The US Navy uses the 71 series to this day in generator applications. A Detroit that's running before the atomic apocalypse will still be running afterwards. They are not the quietest or the cleanest of engines, but they are closer to bulletproof than anything developed before or since.
Also, its a common old fisherman's tale that nothing raises fish like Detroits. We've found it to be true...