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Diesel Engine Overload: Wide Open Throttle Measured by RPM
James:
I read your article and it made sense. [I think I've read most of your articles]. You mentioned in an earlier email that you run your Cummins 270Bs at 2250 RPM. 2250 is about 87% of WOT for the 270B rated @2600 rpm. D Pascoe [he has a great book out on mid size boats] feels that you should run 75% of maximum rpm 75% of the time. Of course he isn't continuously monitoring PSI and temperature like you are.
Response:
Actually, in heading out last night, it appears I
run 2200 rather than 2250. But with the poor accuracy of our
tachs, the RPM data is +/ 50 anyway.
I hear you on 2250 being 87% of WOT when measured by RPM. But it's not 87% load, although this is a very common confusion. They are actually running at ~60% of rated HP output. I'll explain the other factors and how to compute this number accurately.
Load on electronic engines is available as a standard part of good engine instrumentation. It's phenomenal in that at any cruising speed you can just look down and see XX% load and know exactly where things stand. On mechanical engines we don't have that level of detail so we approximate it with data we can get. Some folks use RPM but I'll explain in a bit why that really can't work. What does work is fuel burn. Modern, medium speed diesels but 18 to 20 gallons/hour/hp which is to say that if you are cruising along at a burn rate of 16 gallons per hour, your engines are producing very close to 304 hp or 152 hp each. This burn rate is very stable across all manufacturers. You can convince yourself of this by looking at the specs of Cat, Volvo, Yanmar, etc. and compute burn at HP. You'll see it is near constant.
Using my cruise speed, I know that our boat is burning roughly 17 gallons. Therefore each engine is producing 150 hp. Which is to say that they are 150/250 (270 metric HP is actually 250 HP so I use this number). This is 60% of rated output. If I added an inch of pitch on the props and kept RPM constant, the burn rate would increase. And, if I depitched, the burn rate would fall.
At 2200 RPM how much hp is being produced? With very low pitch, it'll be a very low HP required at these RPM. (In the extreme, put the transmission in neutral and 2200 RPM will require almost no HP and the burn rate will be down under 1 gal/hour). At high pitch, and this is the important point, at 2200 the engine could be producing rated HP or even somewhat more with mechanical engines. (Some electronic engines limit output and won't allow this condition). You could be cruising at 2200 feeling like you are taking excellent care of your engines and really running conservatively, but have an aggressive propeller pitch and be requiring more than rated HP at that RPM. You may notice the black smoke behind the boat but attribute it to being a "diesel" or not using the right fuel additive (a long topic in it's own right) and not understand it's the engines on their way to an early grave.
It's actually still a bit more complex than this, in that at 2200 RPM, even asking for full rated HP is too much ... you need to look at the engine manufacturer's output curve and see what is the max rated hp at 2200 rpm. On my engines it's about 175 hp at 2200 and it's imperative that you are below this number. Nonelectronic diesels will (reluctantly) produce more than rated HP at a given RPM and this is the death of many a marine diesel.
So, using the above data we know that while cruising I'm requiring 150 hp which is 60% of rated HP and, at that RPM, the engine is rated at 175. So I'm asking for 85% of what the engines can produce at that RPM (it's a fluke that we happen to be asking for 85% of HP at 85% of rated RPM  the same thing is true at higher and lower percentages of max RPM). I'm after two things: 1) running at less than rated HP at a given RPM, and 2) requiring MUCH less than rated HP. Generally, I think it's a bad idea to require more than 1/2 hp per cubic inch on a regular basis but that is just my conservative rule of thumb.
Back to looking at our cruising speed. I am requiring 60% of the engines rated HP. This means that the engines are running comfortably below their design point, but that's not enough. 60% is a great number, but you also need to ensure that you are asking for less than the rated HP at a given RPM. To show the importance of this, imagine requiring 150 hp at 1,000 RPM with my engines. This would be massively overloaded so we need to look at both: we're asking for 60% of the rated HP and we're 15% below the rated hp at the cruising RPM. If I remove another inch of pitch and I probably will, we'll continue to cruise at 60% of the rated HP but will do that at a higher RPM (less pitch). I would estimate that we would be at about 2300 and, at this speed, the rated output is roughly 200 hp so with that change we would be at 60% of rated HP 75% of HP at the cruise RPM. A happy place to be.
Keep an eye out for heavily loaded Bayliner 4788's running at speed, such as the one pictured below. You'll see what overpitched boats look like: black sootcovered transoms with engines running well beyond their design point. Note the clouds of black smoke visible 50 feet behind the boat.
 James
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