December Technical Meeting

Practical Basic Boat Electrics

Our speaker for our last meeting for 2013 was member and committee man Rob Bingham who addressed us on ‘Practical Basic Boat Electrics’.  

Rob commenced by asking “Where shall we start?” – a signal that the subject is clearly wide in its scope and the results of ‘getting it wrong’ are substantial. 

So Rob started with an explanation of Ohms Law which is basic but very important theory that provides the fundamental guidance for amateurs. 

Ohms Law V  =  I  x  R and P  =  V  x  I  =  I2  x  R

 where V is the voltage in volts, P is the power consumed in watts,

I is the current in amps & R is the resistance of the conducting circuit in ohms. 

The most important outcome of Ohms Law in wiring our boats is to size the wire so that the resistance does not result in undue heat buildup in the wire for a fixed voltage (12 or 24 in our case).  Rob drew a parallel between what happens to the current in electrical circuits where wire size is too small and what happens to the water flow when you stand on the hose. In the electrical case, this will also be impacted by the voltage selected for the electrical system. A 24 volt system has the advantage of limiting the volt drop between the batteries and the powered device for a given wire size but may need a 24 to 12 volt converter as the availability of common pleasure craft marine equipment in 24 volt is somewhat limited. Alternatively, an inverter may be used to convert the 12 or 24 volt supply to 240v alternating current. In this case, a check on the output wave form of the inverter is recommended. A smooth wave form is required to operate 240 volt equipment satisfactorily.

 Rob then described the approach to boat wiring as summarized in the following key points;

  • develop an inventory of all the electrical equipment on the boat
  • list the rated power of each and calculate the current using Ohms Law – you know the voltage of the battery supply (V-volts) and the power from the appliance nameplate (P-watts), so calculate the current (I-amps).
  • add a 50% safety margin to the current
  • as a rule of thumb and to check the calculations, for current up to about 16 amps, the required wire cross section in square millimeters (mm2) will be about 10% of the current in amps.
  • for this purpose Rob provided members with a selection table of available wire cross sections
  • in selecting the correct wire, consideration also needs to be given to others factors which affect the condition of wiring over time – type of insulation and fatigue
  • all wiring in the marine environment should use tinned copper wire to avoid corrosion
  • the most appropriate insulation is PVC which will withstand up to 1200 C but silicon insulated wire may be used for higher temperature applications. Rob also made note of Kapton wire used in some aircraft (Editor note – a Google search to check the spelling of Kapton reveals that there are major issues in the aircraft industry regarding the use of Kapton)
  • wire should be selected in a range of colours to assist circuit identification or if similar colours are used, wiring can be labeled using Dymo labels or paper labels under clear heat shrink sleeves
  • wire is available from normal marine suppliers or specialist suppliers such as Jacar Electronics or Altronics
  • when running the wiring, it is important to avoid stress points that may promote fatigue and to ensure that wiring is not subject to damage due to rubbing on other surfaces. Wire can be run using spiral wrap plastic or similar split down the middle and supported with cable ties or by use of mini cable duct with adhesive backing.
  • once the wire is run, it is important to use the correct terminal for connection to switchboards and powered equipment. The most commonly used are the crimp type and the rollover type which are soldered and crimped (F Crimp type). Both these tend to allow moisture ingress. An alternative is to use heat shrink type where the adhesive seals the joints. In all cases, it is recommended that the correct crimping tools be used for the terminals selected. Rob provided a selection of his tools on the night for members to examine.


Rob’s presentation then moved on to other elements that need to be considered in designing and installing electrical systems in boats;

Circuit Protection

Installation of circuit breakers in preference to fuses is recommended. Rob noted that allowance needs to be made for short but high current draws at startup of electric motors. By way of example, a 20 watt motor may have a short term draw of up to 100 amps at startup. For this reason it is recommended that electric motor driven equipment be mounted as close as possible to the battery power source.

 Switches and Relays

High current devices (20 to 30 amp draws) cannot be switched via simple toggle type switches which will not accommodate these size currents. In these situations, there will be a need to use a relay device wherein a small current run via the toggle switch will activate a magnetic coil ‘relay’ that switches the larger current into circuit. 


It is important to select the correct type of battery for marine use – usually lead acid and can be of the maintenance free type which will withstand the pounding and vibration common in boats and are even tolerant of a sinking. The normal auto battery or ‘float discharge ‘ battery is generally not most suitable for marine applications as it is designed for quick starts which result in small discharges and then immediate recovery to full charge. The most suitable battery for marine applications is the deep cycle type which is designed to maintain close to full voltage for a drawdown of up to 80%. However, charging of batteries must be maintained as leaving batteries in a flat state is one of the major factors in shortening battery life. 

In selecting a battery size, reference needs to be made to the maximum current draw available from the battery and the length of time the battery will perform before needing a recharge. These parameters are specified by the CCA (Cold Cranking Amps) or the amount of current draw that a battery will deliver at -180 C for 30 seconds whilst maintaining a voltage of at least 7.2 volts (for a 12 volt battery); and the Ah (Amp hours) of the battery, being the number of hours the battery will deliver a 20 amp current before being fully discharged. 

Batteries can be charged using mains power, engine generator/alternators, aero (wind) generators or solar panels. Mains power chargers used to be of the transformer/rectifier type but have now been replaced by ‘smart chargers’ with built in microprocessors that monitor and adjust the charging process as the battery advances towards full charge. In this case, it is recommended that a thermistor protective attachment be plugged into the charger and onto the negative terminal of the battery to protect against overheating which may be caused by a defective battery cell. 


The preferred option for lighting on boats should be LED’s which consume only 25% of the power for the same light levels. These are available most economically via mail order (Rob noted the LED Shop in Queensland as one source of supply). Fluorescent fittings are also an option as they use only 50% of the current of a similar incandescent fitting. 

Protection of Equipment

For most boating applications there is a need for equipment to have a ‘protection of equipment against harmful ingress of water’ rating of IP_ 6 in accordance with Australian Standard AS 1939.

The ‘6’ denotes ‘protection against heavy seas or a strong jet of water from all practicable directions’. 

In closing, Rob recommended the purchase of a cheap multi meter, available for around $10, for inclusion in the boating toolkit.

 As anticipated, Rob’s sharing of his engineering knowledge and practical experience was once again very much enjoyed and appreciated by members who had plenty of questions on the night. Thanks Rob for a very enlightening presentation on practical basic boat electrics.