Information Sheet No 1

 ELECTRIFYING YOUR BOAT

 

1.  HULL

The hull design has a critical impact upon the success of any electric installation. The best hull is one that has a fine bow and smooth stern, preferably a transom that is not immersed. Long, thin hulls are more easily driven than short, wide ones and the smaller the immersed area, the lower the drag.

The engine hull and engine compartment also need to contain the electric motor, batteries and charger, so the appropriate space must be assessed. The battery compartment must be adequately ventilated (a powered extractor fan is generally required) and access is needed to top up the batteries. The distance between the various components must be as short as possible to reduce cable lengths.

Modern solid state chargers are quite compact, but they need ventilation for cooling and they can buzz when operating which may be intrusive if the charger is incorrectly sited.

2.  POWER REQUIRED

The power required in an electric boat should be kept as low as possible in order to minimise the weight and size of batteries which must be carried. Hence the importance of a low resistance hull and an efficient propulsion system.

A full description of the theory behind hull design for efficient operation can be found in the data sheet No 2 ‘Hull design for Electric Boats.’ In summary, when a hull is driven through the water, waves are created which cause resistance. For a given hull length there is a critical speed above which it is difficult to force the boat because of the rapidly increasing wave drag. This critical speed is dependent on the hull length and the shorter the hull, the lower the potential speed before resistance escalates. For a 16ft long hull the speed must be kept below 4 knots to contain the power required. For a 25ft hull the limiting speed rises to 5 knots.

Below the critical speed, the power required is determined more by the frictional resistance of the immersed surface of the hull. In these conditions the power has been found to be roughly proportional to the cube of the speed. This means that, on a hull length of 25ft, increasing the speed from 3 to 5 knots requires over 4 times as much power: put another way the range at 3 knots will be more than 4 times that at 5 knots.

The main factors to look at when deciding if a hull is suitable for electric propulsion are:-

·      Higher displacement gives higher wetted surface and requires more power.

·      A large diameter propeller turning slowly gives greater efficiency than a smaller, high reving on.

·      Long cable runs give voltage loss in power systems

·      Higher operating voltages give lower losses because lower current is required.

·      Operating conditions affect power requirements. Obviously motoring against strong tides needs more power and headwinds can considerably increase current consumption, particularly on craft with high topsides and cabins.

·      There must be sufficient reverse stopping power and for emergency situations (eg. Fast currents).

In electrical terms, power is usually specified is kilowatts not horse power. The  power required to drive a medium sized boat through the water at 4 knots is surprisingly low. Consider how easily even a large boat can be pushed through the water by hand and it is clear that the power needed is small. It is often less than 1kw, which is equal to 1.3 hp.

The power required to drive a boat can be determined by a tow test in which the boat is towed through the water with an instrument ( a spring balance) to measure the force at different speeds connected in the tow line. Calculations can then be made to convert the force into electrical power. There are always power losses in any drive system so if it is found that 1kw is needed to drive it through the water at a given speed, the actual power rating of the drive system must be greater than 1kw. A factor of 2 is often used in arriving at the final power specification.

3.  VOLTAGE

For safety, voltage should not exceed 50 volts, and 12, 24 ,36 or 48 volt systems are available. 36 and 48 volt systems are more efficient because transmission losses are lower. Batteries are available as 2 volt cells or 6 and 12 volt “mono-blocks” and the choice of battery is defined by the available storage space as well as the range desired.

The batteries will be charged using a mains charger, usually installed in the boat. Consequently, a 240 volt supply is needed to the boat: this supply must have suitable electrical safety systems or cut-outs.

4.  MOTOR

There are several types of motor suitable for installation in electric boats. All are DC motors and  operate at low voltages.

The simplest is a ‘Lynch’ motor which are permanent magnet motors delivering power up to 4Kw They have no field windings, are very compact and can easily be mounted in most engine compartments. They are often called ‘pancake’ motors because of their round, thin shape and can be mounted directly on the end of the propellor shaft.

There are several alternative DC motors with field windings which can provide higher power outputs. They are more cylindrical in shape and are foot mounted.

Plenty of natural fresh cooling air must be provided to reach the motor compartment. The electric motors can run hot and may have automatic overheating cut outs. (20% of the power drawn from the batteries goes to heating up the motor). If the fresh air is piped, it should be delivered below the motor air intake and the hot air allowed to escape through vents above the motor.

The output speed of low voltage electric motors is low so that the propellor shaft can be directly connected to the motor, with no reduction gear. At higher voltages, when reduction is required, a simple toothed belt drive can be used while some motors require a reduction gearbox. There is always a loss of efficiency in a gearbox system, so that power is wasted and range reduced.

Always fit an isolator switch directly between the battery and the motor.

5.  PROPELLER

Fit as large a diameter propeller as is practical in relation to the hull design. The tips of the blades must not get too close to the surface of the water or to the underside of the hull. The propeller and shaft are the main source of noise in an electric boat.

Seek advice as to the best propeller to suit the motor speed. Remember the need for reverse thrust.

6.  BATTERIES

There are three types of lead-acid batteries which are suitable for electric boats and you should buy the best you can afford to give long and trouble free service. It is unwise to skimp on battery quality because they are the power source you will need to depend on.

The best are  traction batteries. These are designed for slow, deep discharge rather than the short burst at the high power of the car starter battery.

They are available as 2volt cells or 6volt ‘monoblocks’. The 2 volt cells are the highest quality, having a 5 year guarantee, and should last at least ten years with very regular use. They have the added advantage of being available in a variety of different shapes and sizes to fit almost any available space.

The monoblocs, similar in size to a large car battery, usually come with a 2 year guarantee and are adequate for normal boat use.

12 volt, semi-traction, ’leisure’  batteries are considerably less expensive than traction batteries but they have a much shorter life. They are also considerably lighter than traction batteries, but this weight reduction comes from the fact that they have a smaller number of thinner lead plates, which is why they have a shorter life. However, they may be adequate for smaller boats with lower use.

Rough cost comparison:

6 off 2 volt cells –             180ah capacity - £280            £130/kwh

2off 6v monobloc –  180ah capacity - £200   £  90/kwh

12v leisure –             130ah capacity - £80     £  55/kwh

Clearly the leisure batteries are much cheaper but there is no doubt that the traction batteries will give much longer service.

The total battery weight is determined by the total capacity required. As a guide, battery weight for traction batteries is about 30kg per kwh and for leisure batteries is about 20kg per kwh.

Lead-acid batteries lose water during charging and use and must be topped up with distilled water from time to time. “Maintenance free” lead-acid batteries are also available which use a gel electrolyte and require no attention. However, they are expensive and can be damaged by incorrect charging procedures so care should be taken to ensure that the proper charger is used.

Batteries must be kept charged whenever possible and battery water level should be checked after charging. A battery will be seriously damaged if it is left partial discharged for several weeks.

Power batteries like to be deeply used down towards 80% discharge and then re-charged rather than being used for only a short time and recharged straight away.

More sophisticated batteries are being developed (eg. Nickel-iron) but they are too expensive to be considered.

7.  BATTERY CAPACITY

The choice of battery capacity will depend on the running time which you need between charges. Day boats which return to the same base every night need power for only one day’s cruising. Cruising boats need as much capacity as possible, so that they do not have to moor up at a charging point every night. But bear in mind that there is a limit to the magnitude of the current which a battery can accept during charging and therefore a limit to the capacity of the battery which can be recharged, say, during a 10 to 12 hour period overnight.

Capacity of a battery is usually defined in ampere-hours at a specified discharge rate. It is necessary to specify the discharge rate because useable capacity is increased if a battery is discharged slowly. The 5 hour rate is usually used as the benchmark: this is the ampere-hours delivered if the battery were completely discharged in 5 hours.

In practice, it is not possible to use the full specified capacity because a battery will be damaged if it is totally discharged. Traction batteries can be discharged to 80% of their rated capacity, while leisure batteries should only be used to 70% discharge.

A traction battery with a specified capacity of 120ah must only be used to deliver 25 amps for 4 hours to leave 20% - ie useable capacity 100ah.  In general however, a power battery will deliver more ampere-hours at lower discharge rates than at higher discharge rates. The example above would probably deliver 25% more capacity if discharged at only 10 amps, ie. it would deliver 10amps for 12 hours.

Required battery capacity for a given cruising time is determined by the following formula

                        Capacity        C = H x (W  x  1000 ) 

                                                                  V x 0.8

 Where C            =            Capacity in Ampere house (Ah)

H            =            Cruising time required in hours

                        W            =            Power required by the motor in kilowatts (kw)

                        V            =            Voltage of the motor

The factor of 0.8 is defined by the requirement to only discharge the batteries to 80% of their rated capacity.

 As an example, a boat requiring  an average of 1kw to drive it with batteries at 24 volt, the capacity required for 12 hours cruising would be:-

1 x  1000  x  12= 625 Ah

24     x  0.8

 As explained, power required to drive a boat is usually defined in kilowatts. It may therefore be necessary to consider the battery capacity in kilowatt-hours. This is simply calculated by multiplying the ampere-hour capacity by the system voltage and dividing by 1000. For example, with a 24 volt system and 625 ampere-hours,  the total capacity is 15kwhours: only 80% of this total is usable so the usable capacity is 12 kwh  (1 kw for 12 hours)

 CONTROLLERS

 Speed control is usually achieved using an electronic controller which  provides infinitely variable speed both forward and astern and precise speed setting. There are a number of suppliers of these units. Note they operate at quite high current values (up to 100amps) and so must be well ventilated.

A control lever will be required to actuate the controller. Several different styles are available, from a ‘joystick’ to a Morse type rotary control.

 9.  INSTRUMENTATION

 An instrument to monitor the condition of the batteries is very important in a cruising boat. A simple voltmeter and an ammeter, measuring battery voltage and discharging current, will provide sufficient information for the experienced electrician but a more sophisticated instrument will give much more information, including hours run and an estimate of remaining battery capacity.

A good electronic speedometer/log is helpful. GPS systems, including the ‘handheld’ units are now quite inexpensive (less than £100) and give accurate information on speed.

 10. CHARGERS

 Chargers must be chosen to match the battery capacity. The required output of a charger to recharge a battery in 10 - 12 hours can be obtained by dividing the capacity of the battery at the 5 hour rate by 8. Thus a set of batteries having a capacity of say, 175 Ah will require a charger with an output rating of 22 amps, whereas a battery set of 500 Ah will need an output rating of about 60 amps. If the charging time is reduced, the output rating must be increased.

Choose a charger with the ability to charge the whole battery pack in 12 hours. Check the 240 volt mains current draw for the battery charger. For most boats with recharge times of 12 hours or more it is unlikely to be greater than 13amps. However some larger boats may require a larger power supply from the mains.

Chargers need to be connected to the mains 240 volt supply through cables and connectors which are properly earthed and protected. Bear in mind that in practice the input amps will be larger than amps calculated from the charger output and fuses should be checked that they can carry the required current.

Locate the charger plug and socket connector in a place that is easy to reach and to plug and un-plug.

Batteries give off hydrogen when they are being charged and it is important to make sure that battery compartments are adequately vented to the outside air.

The hydrogen should be exhausted outside the boat by a fan.

Standards for cabling and connectors in the boat and to the shore are set by the Boat Safety Standards. Refer to the Boat Safety Scheme manual for details.

11. AUXILIARY ELECTRICAL LOADS

A DC/DC converter can be used to provide a 12 volt supply from the higher voltage battery bank to the 12v normal boat electrics. Some simple calculations should be done to assess the load but  the amount of power required for the ‘domestic’ electrics is usually quite small and can usually be ignored in the calculations of battery capacity.

12 .  CAPITAL COSTS

Rough guide to hardware costs for a typical electric installation:

            Motor                                                              £750-£850

            Control System                                             £1,000-£1150

            Batteries                                                        £90-£110 per kW hour

            Charger                                                         £600-£800

            Total for 15kwh                                           £3,700-£4,500

To this must be added the labour cost of removing the existing engine and installing the new system. This will vary depending on the complexity and could be between £1000 to £2000             .

13. OPERATING COSTS

Running costs are low. Cost of the electricity for a full recharge will probably be less than £1. The only other running cost is for distilled water.

Battery replacement will have to be considered during the life of a boat. Traction batteries, if cared for, should last at least 10 years when used under normal private boat operating conditions. Semi-traction batteries could last 4 or 5 years.

Electric motors and modern controllers are very reliable and have little to wear out and will give many years’ service with little need for maintenance but an annual service should always be carried out.  

‘Go Electric!’ is a project to support electric conversions on the Broads.

sponsored by

The Broads Society in co-operation with the Electric Boat Association.

April 2004.