A sailboat diesel engine starting system must be viewed as not simply a collection of series connected components, but as a total system. The typical starting marine diesel starting system comprises the following elements.
a. DC positive circuit, (which includes connections at the battery, the isolator or changeover switch, the solenoid connection, and solenoid contacts, the starter motor (which includes several components such as brushes, brush gear, commutator, bearings, windings).
b. The DC negative circuit, (which includes connections at the battery, engine block, the cable back to the battery, the engine block, and the meter shunt if fitted).
c. The engine control system (from panel and includes key switch, stop and start buttons, wiring harness, connectors, fuses etc).
d. The preheating system, (which includes the heating elements or glow plugs, and the connections, relays and connectors).
e. The engine starting battery.
The math of this analysis is that there are also a total of 14 connection points plus the solenoid coil, the starter motor, the battery, the key switch that can directly impact on the starting system and prevent the system functioning. Each point represents a single point failure with subsequent total system failure, with no apparent redundancy. If a person persists with turning over an engine that will not start they may also burn the starter motor out. There are other less common scenarios.
Starting Circuit. Relocate and connect the main negative cable as close as possible to the starter motor. This maintains 2 connections but takes the engine block out of the circuit, and generally reduces voltage drop in the circuit. These commonly vibrate loose and are rarely checked and tightened. In most cases a new negative cable will be required.
Separation of the start and charging systems eliminates the many problems of voltage surges and transients. There are now two redundant power systems, each one being capable of powering the vessel, and any single failure of one system will not affect the other. The alternator negatives provide some redundancy to the main starter negative. As critical equipment has been evaluated and appropriate maintenance strategies implemented, the average time between failures is also extended.
Spares. It is rare to see a boat with a spare starter motor, or alternator and these should be a prerequisite on an extended ocean voyage. Whilst some boats may carry spare bearings, diode plates and brush-gear the task does require skills and tools and it is quicker to change out the entire alternator. Invest in a spare starter motor and alternator, it is a small price to pay in the overall scheme of things. The starting system is more efficient with the negative at the starter reducing voltage drops, lowering current, reducing run time, and improving starting times, which also reduces stress on the motor. Coupled with carrying a spare starter motor/solenoid, there is a reasonable chance of being operational within an hour. Look after your sailboat diesel starting system, and also your boat charging system.