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, tugboats, 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. Medium duty operation is used for 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. With a cyclical load and with a cyclical speed, intermittent operation is used for planing hull vessels such as ferries, fishing boats that move at higher speeds, out and back offshore service boats, displacement hull yachts, and short trip coastal freighters.
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. For planing hull vessels such as off-shore patrol boats, customs, police, and some fire boats and fishing boats. The engine is also used for 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. For planing hull vessels such as pleasure craft, harbor patrol, harbor master, and some fishing vessels and pilot boats.
| NOTICE |
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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 propeller that has a fixed pitch. If a propeller that has a controllable pitch is used, consult your Caterpillar dealer for marine performance and for reduced engine rpm information. Optimum fuel consumption can be obtained by operating the engine at the Suggested Reduced rpm.
| 3406E Marine Engine Ratings | |||
| Full Throttle | |||
| Rating Level | Time | Rated rpm | Suggested Reduced rpm (Cruising Speed) |
| A | Up to 100 percent | N/A | - |
| B | Up to 80 percent | 1800 1350 1200 |
1700 1300 1150 |
| C | Up to 50 percent | 2100 1800 |
2000 1700 |
| D | Up to 16 percent | 2100 | 2000 |
| E | Up to 8 percent | 2100 1800 |
2000 1700 |
For most applications, the customer can provide profile information from similar vessels or from the actual vessel. If such information does not sufficiently indicate the operating profile, instruments are available to more precisely define engine operation. A 7D-1513 Tachograph can establish an operating profile. The Tachograph records engine rpm versus time on a paper graph.