Marine engines have a tough life, they are expected to run under load all of the time. Unlike your automobile, your boat is always "pulling", as if it is in a perpetual state of "going up hill." You can easily overload a marine diesel engine, so we have included this Tech Tip to help you understand what's involved in checking for an overload condition and making a correction.
What factors cause load on the engine?
What is an overloaded engine?
Why can't I just run the engines slower?
What are the consequences of continued operation with an overloaded engine?
How do I properly check for an overload condition?
How do you correct this trouble?
Q. What factors cause load on the engine?
A. Gearbox Reduction: This is usually a fixed ratio except in two speed gearboxes. Your engine load is affected by your gearbox ratio. Hydrodynamic Drag: This depends on your type of hull, the cleanliness of the boat bottom, the type of underwater gear, the boat's loaded weight, and sea conditions. Propeller (Proper) Loading: You can overload a marine diesel engine by installing a propeller which is either too large, has too much pitch, or both.
Q. What is an overloaded engine?
A. An overloaded engine is an engine which cannot reach its rated RPM under full load conditions. An overloaded engine is not to be confused with an engine that cannot reach or maintain its full load rated speed due to mechanical fault, bad fuel, or hull maintenance issue.
Q. Why can't I just run the engines slower?
A. The engine is still overloaded, it is just running at a slower speed.
Q What are the consequences of continued operation with an overloaded engine?
A. Some examples of what may occur due to overloading a marine diesel engine include: Excessive Black Smoke; Slow Acceleration; Excessive Fuel Consumption; Overheating; Reduced Engine Service Life and Component Failure Throughout the Exhaust Gas Flow Path (e.g., valves, pistons, rings, liners, turbochargers, and exhaust manifolds). Generally, the higher the performance levels of the engine, the greater potential for trouble!
Q. How do I properly check for an overload condition?
A. The boat needs to be run at full speed to be sure that it is running past its full load speed rating by a minimum of 2% over or a maximum of 3% over its full load rated speed.
For example, on a 2300 RPM rated engine, the equation would be 2300 X 2% + rated speed = 2346 for minimum full load rated speed 2300 X 3% + rated speed = 2369 for maximum full load rated speed
Note: Other engines with full load speed ratings can use the same equation. For example, on a 3300 RPM rated engine, the minimum full load rated speed would be 3366 RPMs and the maximum full load rated speed would be 3399 RPMs.
Q. How do you correct this trouble?
A. Consult with a marine engine service center (such as Scott Marine Power) to make certain the engine(s) are in proper running order. This means making sure the cooling system is working well, the fuel and filters are clean, checking to make sure there is good air supply, getting propellers/underwater gear cleaned and having your boat bottom cleaned.
Consult with a propeller shop for a recommendation based upon your observed full load engine speed. Have the propeller shop modify your propellers so your engine(s) can achieve the ideal engine speed and proper load.