One hour in 10 at Wide Open Throttle?

Some time back I got a question from an owner of a larger Bayliner concerned that he wasn’t running his engines hard enough and that, as a consequence, they may not last as long. The advice he’d been given was that diesel engines need to run wide open for at least one hour in 10.  In this case the comment was attributed to a professional service technician, but it’s not the first time I’ve heard it. I just shake my head when I hear these things.  That’s dangerous advice to be giving customers.  It’s 100% true that diesels hate running cold.  If the engine isn’t up to full operating temperature on each run, it is hard on them.  No debating that point.  But, wide open for 1 hour in 10 is a great way to get short life with the highoutput, recreationallyrated diesel engines typically found in planing powerboats such as the Bayliner in question.  Running low horsepower density, continuous duty rated engines at wide open is, of course, fine. But you’ll not find these engines in planing power boats.


Remember the height of the muscle care era of the 60’s and 70’s.  The highest HP Corvette of 1970 put out roughly 1 HP/CID (cubic inch displacement).  The B-series Cummins at 480HP is way beyond that 1 HP/CID mark – these are very high performance engines.  These are not the huge, low stress, continuous-duty diesels that developed the deserved reputation for running “forever”. Modern recreationally rated (non-continuous duty) diesels are high performance engines and need to be treated with considerable care. Specifically, running at WOT for anything other than short duration is asking a lot and, if maintenance and propping is not perfect, short life result.


Our engines haven’t ever run at 100% throttle for more than 30 to 60 seconds at a time.  I do this once every 6 months to check to see that they are operating correctly and can reach rated RPM +50 or more at full throttle in a fully loaded boat.  If you can’t do this, your engines are over-loaded (see: Diesel Engine Overload) or suffering from a mechanical problem that needs attention.  I’ve seen $50k destroyed in a few hundred hours via the combination of overload and running hard.  See the pictures below sent to me from someone who had just read the Diesel Engine Overload article saying “I only wish I knew earlier.”


It’s one the leading destroyers of recreational marine engines.  People buy a new boat and over time more and more “stuff” ends up on board and the bottom paint picks up minor growth.  More often than not, a year later the boat becomes over-propped from these factors and, as a consequence, the engines are overloaded.  Most owners think they can run at “200 RPM off the top”.  They do so without worry, but wonder why they are smoking badly and sooting the transom heavily.  If they are lucky, someone helps them.  If not, another pair of engines won’t likely reach 1,000 hours without major service.


It’s worth mentioning that just about every larger Bayliner (and Meridian) is propped near the limit for a lightly loaded boat.  If you have a Bayliner and haven’t taken 1” of pitch out from the factory configuration, you are probably over-propped.  Some, including ours, needed 2” of pitch removed to get rated RPM+50 at WOT with a fully laden boat. 


Back to the advice of running one hour in ten at wide open throttle.  You’ll hear folks warning you that you need to run 75% load or better, or that you need to run 1 hour in 10 at max.  The former is absolutely fine for a healthy engine, although unnecessary, and the later is a recipe for short engine life.   You absolutely do need to ensure that the engines reaches full operating temp on every run and that is the intent of the 75% rule.  By full operating temp, I don’t just mean that the coolant got to full temperature.   You need the oil hot as well and you won’t get this idling at the dock.  You can only get the oil hot when under load but, trust me, any of the larger Bayliners are under plenty of load well before 75% of WOT. 


We chose to cruise Dirona’s engines at 150HP which is only 55% of rated output (Cummins 270Bs) and we often operate them for weeks at a time never over 30 HP (7.5 kts) when exploring new areas. This means that for weeks at a time, they never go beyond 10% of rated load but, at this load both oil and water are get hot, which is the important factor.  You will hear terrible horror stories about how dangerous light load is to diesels but, as long as the engine is at full operating temperature and sees varying load conditions, this simply isn’t a problem.  Dirona’s engines have well over 3,600 hours on them and we load forward to thousands more.


If you want to play it safe, run conservatively as we do and get the 5,000++ hours you deserve.  There is no guarantee, a part failure can still get you but the odds are much better if you run conservatively.  If you really feel need to run close to the HP limit, get proper instrumentation, especially pyrometers, and keep a very close eye on the engine operating conditions and maintenance.  Under these high load conditions you have a much higher chance of early failure as there is no headroom at this load.  For example, check out this thermostat failure: Cummins 270B Thermostat Failure.  If we were running at very high loads when this happened, this small part failure could have overheated the engines perhaps before we noticed.  At high load, you need to have perfect maintenance, great instrumentation and be very vigilant to any changes in engine health. No matter what you  chose to do, make sure you can reach at least 50 RPM over rated (see the diesel overload article referenced above).  If you are overloaded, backing off a few hundred RPM won’t protect you from catastrophic failure. 


My view is that we need to prop correctly (no overload), get to full operating temp, run conservatively, and enjoy our engines for years.  Running high output recreational rated diesels wide open for 1 hour in 10 is just plain bad advice.







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