About Boat Starter Motors

Part 2 of this 3 Part Series on Boat Starter Motors reliability and installation.

The operation of boat diesel starter motors and the performance depends on many of the circuit elements. A starting system must be viewed as not simply a collection of series connected components, but as a system of several parts.

The typical boat starter motors comprises the DC positive circuit, which includes the battery connections, the isolator or changeover switch, the solenoid connection and solenoid contacts, the starter motor and the various components such as brushes, brush gear, commutator, bearings and the  windings.

The DC negative circuit, which also includes the battery connections, the engine block which is often part of the return path, the cable back to the battery, and the meter shunt if fitted. Last is the engine control system from the panel which includes the key switch, stop and start buttons, wiring harness, connectors and fuses. 

All parts offer common points that prevent starter operation, and bad connections are the most common cause of failure. The most common faults are high resistances and excess voltage drops in both the positive and negative circuits. Battery changeover switches are also a major culprit here and I have frequently encountered this, and it pays to have high quality switches. The main start cables should be rated to have minimal voltage drop at full rated current.

In addition the cables should be kept as short as possible and as large as possible to also minimize losses and maximize power availability to the starter. As an indication of start current levels, the instantaneous stalled condition short circuit currents can be up to 3500 amps in large engine starters before the load drops to a few hundred amps.

Even small values of resistance and voltage drop can have significant effects and reduce or prevent starting. 

Installation Boat Starter Motors

Boat starter motors installation is generally part of the marine engine manufacturers specification and boat user factors are limited to a few factors only. The first is being mechanically secure, and the second is that the attached cables are of the correct rating, and that the terminal nuts are properly torqued up so that they do not work loose. 

Things that are within a persons ability to control are the connection of the negative cable which should be attached as close as practicable to the starter. In most cases it is fastened at the closest engine point, which generally makes a poor contact and inserts resistance into the circuit, as the engine block becomes part of the circuit.

Starter motor design is generally robust as it must withstand the shocks of meshing, engine vibration, salt and moisture laden air, water, oil, temperature extremes, and high levels of overload. 

Boat Starter Motors

Boat starter motors on boats are by default located low down towards engine bilges, subject to leakages from seawater cooling systems, seawater injection points into exhaust elbows, as well as the unexpected high bilge level. The position generally is out of sight and therefore general inspections are often cursory and inadequate.

Check your engine and check the starter motor, if its leaking at the top end then it's probably leaking over the starter also. Corrosion can be rapid on a starter motor. Another area to check is whether you can physically remove the starter, in many installations this is extraordinarily difficult, with tool access to remove holding bolts nearly impossible. One thing a marine engine manufacturer really considers is access to starters, or not.

A little practice can pay dividends when things go wrong. In addition remove each bolt and apply some thread lubricant such as copper-slip or similar, so that bolts don’t seize in. More about the boat starter motors and also about boat diesel engines.