I’m gratified that this subject has generated so much dialog and exchange of information and opinions. I have the greatest respect for Tony, I have corresponded with him on several occasions and I have been a commercial member of BD for many years. I respectfully disagree with some of his tips, including propeller nut installation order, it flies in the face of ABYC, USN, USCG and SAE guidelines (I write an entire column on this subject). I completely agree with his assessment of exhaust system issues being very problematic, (especially for heavily loaded engines). His website is a trove of valuable, practical information.
He acknowledges but downplays under loading issues, and I don't doubt what he says is accurate. His experience is with west coast vessels, mostly Cummins and a lot of commercial/fishing vessels, few of which putt about at low load, and because there are far fewer places to do this in that region. While on the east coast the intracoastal ww and many tributaries are low load havens.
I'm on different vessels every week with many different engines, Deere, Lugger, Cummins, Cat, MAN, MTU, 2 stroke DDs, Perkins, Westerbeke, Yanmar, Volvo, the gamut. All are recreational, most displacement some planing (and these run at displacement speeds most of the time), some dry stack, a few sail. Again, different than Tony’s experience and perhaps that of other cited diesel mechanics.
The effects I'm seeing, when I see them, the effects I write about and photograph, don't typically stop an engine per se, they are more insidious, they lead to rough running, smoking, poor fuel economy, fouled injectors and turbos and long term potential for oil starvation and thereby premature wear of lubricated parts, that are often not directly attributed to chronic under loading.
These aren't offset by the occasional high speed run, you can't turn the clock back on hundreds of hours of low load soot and varnish accumulation with one high speed run. I advocate a periodic heavier load for 10 mins out of every 4 hours. This is in no way harmful to any diesel engine unless it has other issues. BTW, Cummins and other manufacturers make continuous duty engines that are designed to run at 100% load 100% of the time, other ratings limit 100% load to 10 minutes, it varies from engine rating to engine rating.
You can cast about for opinions until you find the one that fits your view and experience, I don't fault anyone for doing this, it's reasonable. I'm simply reiterating that my observations, and the articles I write based on them, are based on my own first-hand experience, which I’ve only augmented in this case with input from engine manufacturers and volume re-builders.