Electrifying a classic motorboat
Q Under restoration is my classic 1946 Thames launch that I’d like to convert to electric propulsion.
Merlin III is a 12ft Kay Craft. Steering is from the brass steering wheel in the rear cockpit through wires wound over the steering drum then running on pulleys down both sides below the coaming to where the original outboard was attached at the rear.
There’s no sign of where a remote throttle may have been, so I assume it was on the outboard itself. From the driver’s seat it is possible reach back to adjust the outboard motor – I presume that’s why the boat was designed to have the driver in the rear cockpit.
Merlin is 12ft long by 4ft 6in wide, built of mahogany planking on steamed oak ribs, and I have calculated that it weighs about 200kg (0.2 tons). It is almost flat bottomed, and so was designed for use on rivers or lakes.
The speed limit on the Thames is 4mph, but the boat will also have to occasionally contend with the weir stream so an electric outboard will have to be able to cope with this.
There are lots of lovely old outboard launches on the Thames, whose owners would find any information very helpful. All the manufacturers’ advice seems to be for inflatables or fast power boats. Jon Wallsgrove, Sunbury on Thames
EMRYHS BARRELL REPLIES: Electric power would be very suitable for your boat, and there is now a wide range of options to choose from.
The simplest and cheapest are the so-called trolling motors. These were first developed in the USA in the 1930s, for fishing on lakes, hence the name. They were used as auxiliaries for boats with much larger petrol motors, and now sell in their tens of thousands. They are lightweight, and clamp to the transom or side of the boat,
They require a separate battery, usually lead acid, but also lithium for lightness. They’ll usually be 12V, but sometimes 24V. The motor will weigh around 10kg, but the battery, if it is lead acid, will be between 15-25kg – or half this if lithium. But the batteries can sit on the floor of the boat, away from the transom for trimming.
Electric motor power is measured in lb of thrust, and you’ll need a 55lb or 74lb for your boat. They have twist-grip throttles like any outboard, and can either have click-click throttles, with five forward speeds and three reverse, or infinitely variable electronic control. The former are cheaper, but only useful for half-hour trips as they use up the battery power very inefficiently.
For any serious use you need to buy the variable throttle versions. Price for the motor will be £200-300 for the click click, or £300£450 for the variable speed. Batteries will be around £100£150 for lead acid, £350-£400 for lithium.
If you’re going for lead acid, do not buy the so-called leisure batteries. They are useless for this purpose and will last only half a dozen trips. AGM or Gel are the better choice, with mobility scooter batteries providing a good option with a lot of choice. They’ll also allow you to break the weight down into smaller units for carrying.
Established motor names are Minn Kota and Motorguide, but there is a flood of far eastern copies.
Look at Sterling Power for lithium batteries, but you will need to buy a good quality charger, capable of charging lithium. Ctek chargers start at around £90 for 7A.
Stepping up to the next level the latest electric alternative outboard motors have integral lithium batteries which are mounted on the top of the motor, but can be removed to make them lighter to carry and mount.
Advantages are more range for less battery weight, no wires in the boat, and also more power and speed. The trolling motors will only give you a maximum of 4-4.5mph, whereas the integral battery motors will give you 5-6mph. Power is measured in kW, from 0.5-1.0kW for the integral battery models or 2-6kW if you use a separate battery pack.
Slightly confusingly these motors indicate their power in kW but then what they call a petrol power equivalent. In mechanical terms, a 1kW motor is 1.3hp, but the two main makes then equate this to a 3hp outboard on the grounds that they give you a similar thrust.
However you will be paying £1,600-£2,000 for motor, battery and charger.
The two main brands are Torqeedo and ePropulsion.
Remote steering will be less easy to achieve, with few of the smaller motors being set up for this. However the control head of the trolling motors can be removed and mounted remotely with extended cables, and you can easily rig up a steering arm if you’re handy with tools.
The ePropulsion EVO has a remote steering and throttle option, but at an extra cost.