Making Electric Motor plans

So, if we do decide to fit an electric motor what are we looking at?

We follow five sailing channels on Youtube who have fitted electric motors:

Our plans are different, in part because we are in the UK and so pricing and availability is different. In the UK there are a small number of suppliers who will fit complete systems but they are outside our price range (about the same cost as the boat). Unfortunately we have not found UK suppliers of sailing boat specific kits.

So we are looking at buying parts and installing it ourselves. However, we are also not wanting to be too adventurous, so we are looking at components that have been used for marine applications by others.

This is where we are at so far:

How powerful should the electric motor be?

Calculating what size of engine to get is quite complex. The nature of a boat moving through water is that the power required to move faster rapidly increases with speed, until the point where adding more power does not give more speed (probably around 7 knots/13kph for Vida). So there is no point in buying a really powerful motor.

On the other hand, when you are motoring into a strong headwind more power is needed for the same speed. While we intend to be cautious and avoid dangerous situations where we can, however, we recognise that the time when a motor increases your safety most, is when you have to motor away from a shore in a strong headwind (and if that is somewhere with a restricted channel or an adverse tide then sailing will be most difficult). At these times more power is needed for the same speed.

In the end a sensible rule of thumb seems to be to replace a 39hp diesel with about the same hp in an electric motor (although the two hp figures are not directly comparable for reasons I still don’t understand).

Fortunately, at slower speeds (using less power – so that the batteries last longer), a 40hp engine at 25% power isn’t very different in efficiency than a 20hp engine at 50% power (and will be less likely to overheat).

So we are looking at a 40hp motor, unfortunately that restricts the options in the UK as the most widely available motors are less powerful than this (or are designed for speedboats rather than heavy cruising boats).


Most 40hp DC electric motors run at 72 or 96 volts. A few run a 48 volts. Higher voltages mean that at the same power the wires can be thinner as the current is reduced. Electric motors draw a lot of power and so with lower voltages the current can be very high.

However, there are two disadvantages of higher voltages. Firstly, they get more dangerous. Secondly, they need more batteries. You get high voltages by connecting batteries in series. So four 12volt batteries in series gives 48colts. You need 6 batteries for 72volts and 8 for 96volts. That gets expensive! If you decide you want to increase your range you can add extra batteries in parallel to your battery bank, but if your bank is 96volts you need to buy 8 batteries at a time.

So we have decided to look for 48 volt motors and start with a relatively small bank of batteries that we can add to in the future.


Unlike Kikka and Dan on Sailing UMA we have decided to go for a brushless motor. More expensive, hard to find secondhand but safer as they don’t have the same risk of sparks (some authorities and insurance companies won’t insure boats with motors with brushes, especially if they have gas on board due to the risk of explosions).


Some electric motors are water cooled (typically using fresh water with a sealed system and a radiator). Others are air cooled, some with integrated fans.

Our preference is for air cooled for simplicity, we will have to make sure we provide an adequate supply of dry air to the engine room.

Motor of choice

So we are looking for a 40hp, 48volt, air cooled, brushless motor.

The US supplier used by Beau and Brandy is Thunderstruck-ev and they have a HPEVS AC35 motor kit. I’ve have found a UK supplier of these HPEVS AC motors. So yesterday I was able to pop in and visit Falcon Electric who are focused on electric cars. That takes us a few steps forward.

So we are looking at an HPEVS AC51 motor package from Falcon Electric which gives 40hp at 48volts. It comes with a lot of the stuff we need (Controller, Wiring harness and display).

Other stuff

Of course we don’t just need the motor, there are lots of other bits too.


We hope to be able to reuse the propeller and the propshaft (after the existing coupling has been cut-off).

We will need to replace the cutlass bearing (this is the bearing through which the propeller shaft enters the boat, it is typically water cooled by the sea). Once the existing engine is out this should be straightforward.

We are looking at replacing the original stuffing box (this is what stops water from entering the boat through the propeller shaft opening). There are much more modern, drip free, reduced maintenance options now available.

We will need a thrust bearing as electric motors are not designed to resist the push of the propeller.

We will need a way of connecting the motor drive shaft to the propeller shaft. This will probably need to be a belt with pulleys as the motor has a maximum speed of 10,000 rpm and the propeller is much less (probably in the region of 3,000). This is something we need to calculate as the propeller speed needs to be matched with the propeller itself and the boat hull shape/speed. In the US Thunderstruck-ev sell these, I’ve not found anything but suppliers of parts here.

The engine will need a mounting (at one end part of the gear reduction). Fortunately, there is a stainless steel fabricator at Beaumaris we should be able to use. With the gear reduction we should be able to position the motor reasonably high so we can be confident it won’t get flooded even if the boat took on a lot of water. We will sort out some form of cover to direct cooling air and make sure that air is as dry as possible (and it might be nice if we can choose to divert the warmed air outside in warm climates and inside in cooler places).


The dimensions of the electric motor are far smaller than the current diesel so we will be able to fit the battery bank in the same space (giving us loads of extra space where the fuel tanks are at the moment). We will start with a small battery bank, probably four 12V 100AH Lithium-ion Phosphate. When we see how that works for range we can decide how much more to add.

We need to plan exactly how we will handle charging. Probably the simplest (but not most efficient) will be to install a DC to Dc charger so we can keep all our renewable energy charging our house battery bank and then charge the engine bank from that. Otherwise the voltage switching and balancing gets a bit complicated.

Controls and monitoring

We want to end up with a simple, single lever control that controls both direction and speed (eg push forward to go forward with speed controlled by how far you push, pull backwards for reverse). We will need to sort out displays of battery and range that we can see outside. Obviously, these are slightly different for boats than cars (speed is an order of magnitude different for a start).


This is going to need more thought. There are lots of dependencies to work out. For example, we are going to need the yard to lift the old engine off the boat at least (we probably need to get it out from under the cockpit floor which is under the wheelhouse roof ourselves). However, as the yard is very full that will need to wait until some other boats have been launched in the spring. At least this will allow us to workout all the details before rushing into it.

Other advantages

  • We don’t have to learn how to maintain diesel engines 🙂
  • We should be able to sell the existing engine for more now than in a few years time.
  • We gain lots of space in the cockpit locker
  • We gain storage space in the corridor to our aft cabin.
  • We don’t have to replace the water coolant seacock, we can get rid of it.
  • Access to the cockpit drain seacocks and bilge will be much easier
  • The boat will be lighter
  • We should reduce the amount of maintenance we need to do in the future
  • Fossil fuel free 🙂
  • Much quieter when motoring
  • No Diesel smell (brilliant for helping reduce seasickness)
  • Zero fuel cost and much more independence from harbours when cruising


  • Cost (rough budget about £12,000)
  • Reduced range (depends on the size of the battery bank but certainly only a few minutes at full power initially)
  • Renewable generation limits. How fast can we charge everything? We are going to need to cover the boat with solar and move them around to maximise power generation.
  • Restricted cruising grounds by availability of enough sun to charge everything – we might need to go south for the winter 😉
  • With electric cooking, electric dinghy motor and this electric motor we will need to carry petrol generator in case we can’t keep up through renewable generation.

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