In selecting a rating for a specific application, the most important consideration is the time that is spent at full throttle. These rating definitions identify the percent of time at full throttle. The definitions also identify the corresponding times below rated rpm.
A Continuous - The engine is operated at a rated load and at rated rpm up to 100 percent of the time without interruption or without load cycling. Typical use is 5000 to 8000 hours per year. Continuous operation is used for heavy-duty service in ocean-going displacement hull vessels such as freighters, tugs, bottom drag trawlers, and deep river towboats.
B Medium Duty - The engine may be operated at a load factor up to 80 percent and at rated load and at rated rpm for 80 percent of the duty cycle or for 10 hours out of every 12 hours. Typical use is 3000 to 5000 hours per year in displacement hull vessels such as mid-water trawlers, purse seiners, crew boats, supply boats, ferries, and towboats when locks, sandbars and curves dictate frequent slowing.
C Intermittent - The engine may be operated at a load factor up to 80 percent and at rated load and at rated rpm for 50 percent of the duty cycle or for six hours out of every 12 hours. Typical use is 2000 to 4000 hours per year. Typical use involves cyclical load and cyclical speed. Intermittent operation is used for planing hull vessels such as ferries, out and back offshore service boats, displacement hull yachts, short trip coastal freighters, and fishing boats that move at higher speeds.
D Patrol Craft - The engine may be operated at a load factor up to 50 percent and at rated load and at rated rpm for 16 percent of the duty cycle or for two hours out of every 12 hours. Typical use is 1000 to 3000 hours per year in vessels such as off-shore patrol boats, customs, police boats, some fishing boats, some fire boats, and bow and stern thrusters.
E High Performance - The engine may be operated at a load factor up to 30 percent and at rated load and at rated rpm for eight percent of the duty cycle or 1/2 hour out of every six hours. Typical use is 250 to 1000 hours per year in planing hull vessels such as pleasure craft, harbor patrol, harbor master, pilot boats, and some fishing boats.
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Operating engines above the rating definitions can result in shorter service life before overhaul. |
Typical operating parameters for each rating level are summarized in Table 1. Table 1 assumes the use of a fixed pitch propeller. If a variable pitch propeller is used, consult your Caterpillar dealer for marine engine performance and for information on reduced engine rpm. Optimum fuel consumption can be achieved by operating the engine at the Suggested Reduced rpm.
| 3116 & 3126 Marine Engine Ratings | |||
| Full Throttle | |||
| Rating Level | Time | Rated rpm | Suggested Reduced rpm
“Cruising Speed” |
| A | Up to 100 percent | 2400 | - |
| B | Up to 80 percent | 2400 | - |
| C | Up to 50 percent | 2400 | 2200 |
| D | Up to 16 percent | 2600 | 2200 |
| E | Up to 8 percent | 2800 | 2400 |
For most applications, the customer can provide profile information from similar vessels or from the actual vessel. If the information that is provided by the customer is not sufficient, instruments are available to more precisely define the operating profile. A 7D-1513 Tachograph Assembly can establish an operating profile by recording engine rpm versus time on a paper graph.