Wednesday, March 18, 2015

Don't Underload Your Diesel Engine

I have been around boats with diesel engines for more than 40 years. In that time I have heard many times that we should not run a diesel engine under light loads for long periods because "it can glaze the cylinders". Another statement has been "diesel engines like to be loaded". Maybe you have also been told or read this but do you really understand what is going on with your diesel, why it is so important to run it with healthy loads and why you should not over-power your boat?
turbine wheel
A turbo charger turbine wheel fouled with soot and fuel, the result of chronic underloading. This gunge also fouls your upper cylinders, exhaust valves and exhaust system. From there it is washed out with the cooling water into the water on which you enjoy your boating. Photo courtesy of Steve D'Antonio.

I am a proponent of reasonable size motors in sailboats but often deal with owners who want to put much bigger motors in their boats than I recommend. My 36ft boat had 20hp, my 34 had 12hp and my 38 footer had 18hp. That 12hp could push my boat against a 40 knot wind on flat water. Sure, it was slow progress and the motor was working very hard but it could do it. In less extreme conditions the motor wasn't just ticking over to move her at reasonable speed. A 20hp motor would still be acceptable on that boat but anything bigger would be over-powering it.

Professional Boatbuilder magazine has an enlightening article on this subject on their website, written by their technical editor Steve D'Antonio. Steve also works with owners and builders through his own business, Steve D'Antonio Marine Consulting, Inc.
cross hatch
The grooves that retain oil in a cylinder wall, known as crosshatch, can be seen here. Frequent light load operation can wear away this pattern; known as cylinder glazing, it exacerbates blow-by and the issues that accompany it. Photo courtesy of Steve D'Antonio.
I don't want to repeat here what Steve writes about so clearly in his article, just to point out again that you do harm to your motor, to your bank account and to the environment by over-powering your boat, which inevitably results in you running your motor at speeds that will cause problems. Those problems won't only materialise "in the long run", they can start to appear when the motor has run for no more than a few thousand hours. You will be inviting self-inflicted pain and heartache on yourself and future owners of the boat.

Please read Steve's article. And for info on my designs, please visit

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