In my mind, there is a common misconception about running your recreational diesel hard, that makes no sense at all. This myth lives on in various forums and dock meetings because of anecdotal information gathered from operators of commercial engines in the fishing industry. Marine diesels come in a lot of "different flavors". Even the same make and base model come in many versions that develop over time. Yanmar does not have good examples of what I mean, as their commercial and leisure marine engine are totally different animals. Nothing close about the recreational engine and commercial duty engines, so its hard to compare apples and apples. This article is written as a very simple common sense approach to this issue. Sure there are a lot of variables not mentioned here, but it's to keep things simple.
Since Yanmar does not use common cores for their recreational and commercial products, I have chosen to use Caterpiller as a better example. Take the Cat 3126 which competes with the Yanmar 6LY series to keep the conversation on approximately the same power ratings in leisure class. The versions start with a "A" rated model 3126B rated at 250 HP at 2400RPM, "E" rated engines at 300 hp, 350 hp, 385 hp, and go up to as high as 420 hp.
These engines all share similar blocks, cranks, displacement, etc... You could say the "A" rated engine is a de-rated recreational engine, or the others are boosted up versions of the commercial engine with by different tunings (injector pumps, turbos, etc.) I don't know which perspective is the right, but either way, different amounts of power, or load is expected from the same basic core. This is the simple part. The more power you extract per cubic inch of displacement, the greater the loads on the engine. The engine is designed to do a certain amount of work, in a certain time, and do it very well. This is the basis for horsepower ratings. So if we are to extract more horsepower from these common cores, how do we keep these high horsepower versions from self destructing? Enter "engine use ratings." These ratings are a guide to understanding the intended use of the engines. Yanmar has similar ratings for their commercial products but it is more complicated to compare their different engine. Caterpiller rating work much better for this purpose. Here are the Caterpillar engine use ratings right from Caterpillar's website:
Caterpillar Marine Engine Ratings Guidelines
Ratings should be applied on the basis of vessel operation, and on the type boat or hull design. Vessel descriptions such as workboat, pilot boat, or ferry boat; and hull design such as displacement or planing hull do not define the operating conditions of the vessel or the power demands on the marine engine. More than one rating could apply to these descriptions, depending on how the vessel is operated. Listed below are descriptions of ratings A, B, C, D and E followed by an overview chart.
Unrestricted Continuous - A Ratings
For vessels operating at rated load and rated speed up to 100% of the time without interruption or load cycling (80% to 100% load factor).
Typical applications could include but are not limited to vessels such as freighters, tugboats, bottom drag trawlers, or deep river tugboats. Typical operation ranges from 5000 to 8000 hours per year.
Heavy Duty Marine Engine - B Ratings
For vessels operating at rated load and rated speed up to 80% of the time with some load cycling (40% to 80% load factor).
Typical applications could include but are not limited to vessels such as mid-water trawlers, purse seiner, crew and supply boats, ferries, or towboats. Typical operation ranges from 3000 to 5000 hours per year.
Maximum Continuous - C Ratings
For vessels operating at rated load and rated speed up to 50% of the time with cyclical load and speed (20% to 80% load factor).
Typical applications could include but are not limited to vessels such as ferries, harbor tugs, fishing boats, offshore service boats, displacement hull yachts, or short trip coastal freighters. Typical operation ranges from 2000 to 4000 hours per year.>
Intermittent Duty - D Ratings
For vessels operating at rated load and rated speed up to 16% of the time (up to 50% load factor).
Typical applications could include but are not limited to vessels such as offshore patrol boats, customs boats, police boats, some fishing, fireboats, or harbor tugs. Typical operation ranges from 1000 to 3000 hours per year.
High Performance - E Ratings
For vessels operating at rated load and rated speed up to 8% of the time (up to 30% load factor).
Typical applications could include but are not limited to vessels such as pleasure craft, harbor patrol boats, harbor master boats, some fishing or patrol boats. Typical operation ranges from 250 to 1000 hours per year.
For our use, the pleasure boat ratings are the "E" ratings above. The load factor in mechanical engines can be derived by this formula: Actual Fuel Rate / Maximum fuel rate X 100 %. A 30 percent load factor is very light. Pushing "trawler" semi displacement hulls at fast speeds, can easily exceed the 30% load factor for recreational "E" rated engines. Do that for long periods of time, say 4-8 hours a day (the rated time for 30% load is 8% or 2 hours in a 24 hour period) and you are operating well outside of the design parameters for that engine.
If you plotted this stuff out and tried to make sense of it, you would find that these high performance versions are to be operated in a very limited way. If you ran that 385 hp version like the "A" rated version, say 80% of 250 hp which might be around 2100 rpm, I don't doubt that the thing will run happily all day long just like it's cousin. But if you ran that engine 80% of 385 hp all day long, day in and day out, it will be very short lived. So when the local fisherman says that 3126 Cat diesel likes to run hard, think about that in terms of his de-rated engine that is made to do that all day because of the reduced power expected from that engine.
A killer of marine diesels is the melting of the piston tops from running at great loads for extended periods. Tremendous heat is generated in the combustion chamber as boost and fuel combine to melt the pistons, not to mention the adverse effect on valves. The horsepower rating and engine ratings work together to provide for a serviceable life for a given engine model that is used in a variety of services.
With Yanmar, the difference is even greater. Our pleasure engines are short stroke, fast spinning engines, and the commercial jobs are long stroke, slower turning engines that are horsepower rated at 25-30 percent of the leisure class engine for similar displacement. The leisure class ratings are very limiting to keep the engine within the design parameters and give long engine life.
So this is how I look at it. My Yanmar 370 has a continuous rating of 290 hp at 3100 rpm. I think that's great for selling boats at the dealer, but I don't think that means you can really expect to use that horsepower for any extended period of time. When I purchased my Mainship, I was concerned about this idea of having to run my diesel at 80% power to get long life from it, so I talked to Mack Boring about this. They told me if I ran the boat at 7-8 knots, that would be sufficient to keep the engine properly loaded, and keep it at proper operating temperature, for a very long engine life.
I run my Yanmar at 1800-2000 when ever it will be a long run, which turns out to be around 7-8 knots. If I run higher than that, it is for short runs, an hour or less and never run it WOT for more that 5 minutes per hour. I have run the boat like this for the past 11 years and never had a problem. I faithfully maintain the engine and expect that engine to out live me.