Display for Electric Propulsion System

As I design control and monitoring systems for a living, I decided to build a custom display for the electric propulsion system we are installing.  Eventually it might combine data from the battery management system (BMS), motor controller and solar charge regulator, but initially we can get most of the information we need from the BMS.

I bought an e-ink display module and programmed a Seeeduino (an Arduino clone with 3.3V power option) to drive it using the GxEPD library.   Getting suitable fonts and icons to fit in the available storage space on the chip took a bit of customisation.  I added a tiny 12V-to-5V PSU module and installed it all into the casing of an old Clipper compass display.

Fits perfectly into the case (with a small gap at the bottom)

The initial software is complete, though I have only tested it with simulated data because I haven’t powered the real BMS up yet.  The hardware worked in prototype but I need to solder it all together properly and add something to reduce the voltage on the incoming serial line from the BMS down to the 3.3V that the Seeeduino wants.

Battery Modification

The 16 LiFeYPO4 cells for the propulsion bank were actually bought as four 12V batteries rather than sixteen separate cells.  This reduces the amount of wiring required, and makes them easier to mount securely.

Unfortunately, the battery management system (BMS) requires a small wire to each individual cell to measure the voltage and balance the charge.  This meant I had to remove the glued-on lids from the batteries to access the individual cells.  A video on youtube gave me the confidence to attack my €2000-worth of batteries with a knife and hammer, and I managed to smash the lids off without too much damage (though the knife wasn’t much good afterwards).

The power connections on the top of each cell are made with M8 bolts, so I used a ring terminal to attach a monitoring lead to each cell.  As I wasn’t sure how the batteries were going to be mounted I didn’t want to have these new cables protruding from the sides, so I brought them out through a hole that I drilled in the centre of each battery’s lid.

These wires (obviously) aren’t fused so I’m being very careful about loose connections and chafe.  The bolts all have spring washers and are done up nice and tight.  I’ve added a thick piece of adhesive-lined heatshrink tubing around the cables as they exit the hole in the lid.

Once the lids were reinstalled I added a 6-way connector to each set of wires, and added mating connectors on the BMS harness.

Completed battery mod
Wiring harness for BMS

Later on I started removing the lids again to add a bead of silicon sealant around the join that I’d smashed open.  I was hoping to reduce the ingress of salty air and hence corrosion.  However, on the second battery the nut on the outside terminal cross-threaded really badly so I’ve given up on adding sealant now – I don’t want to damage any more.  The nuts are made from quite soft alloy, but to be fair I guess they are not really intended to be undone multiple times.  Luckily this doesn’t affect the inside thread which is the one which I need to connect the power cable; the damaged nut is really just holding the lid on.

Damaged thread on outside terminal

Electric Propulsion Part 2

A summary of the design:

The motor mount has arrived and is with the boatyard who are going to glass in some suitable engine mounts for it.  The batteries, BMS and charger have also arrived (great service from GWL Power in the Czech republic).  We are awaiting the rest of the kit from Lynch, which is due in a few weeks.

So far I have added the cell-balancing cables to the cells, built the wiring harnesses for the BMS and built most of the display (more details in future posts).

Electric Propulsion Part 1

We have removed the old broken diesel engine from our Contessa 26 and are replacing it with an electric motor.  This post discusses how we came to that decision.

We sailed all last year without a working engine, which was fine as we were based on a swinging mooring out on the river Orwell.  Picking up the buoy with shallow water, flukey wind and lots of other boats around was sometimes exciting, but we survived.

The main thing which held us back without an engine was being unable to manoeuvre in small spaces like marinas.  This also put us off visiting some places like the river Deben where there are (apparently) strong tides and narrow channels.

What Are Engines For?

Sailors basically use their yacht engines for four different things:

  1. Manoeuvring in and out of harbour.
  2. Going along when there is no wind.
  3. Motoring or motor-sailing when the wind is unfavourable.
  4. Generating electricity.

People’s definition of “in and out of harbour” varies wildly.  Some people seem to arrive off Harwich and think “well, that’s the real sailing done, lets just motor the last five tedious miles up the river Orwell,” whereas we sometimes spend an afternoon just sailing up and down those same miles!   For us, the final manoeuvring under power is only likely to be a few hundred yards in most places.

Being able to go a long way when there is no wind is occasionally useful.  We once motored all the way from Falmouth to the Scillies at the start of a long period of high pressure, which meant we got a great week of settled weather once we were there.  If we’d had no engine we would have missed out on this.

Even when we had a working engine, we didn’t tend to motor into wind much, partly because we like the sailing and partly because the engine was so feeble (even once we’d fixed the overheating problem) that sailing was faster and more comfortable than motoring.  Our boat is definitely from the days when yachts had an “auxiliary engine” rather than a motor which would power you through anything.

Charging batteries is a useful by-product of running a diesel engine.  Virtually no-one sails without a host of 12V powered equipment these days, including lots of safety-critical items like navigation lights and radios.

The Plan

We are installing an electric motor powered by a 4.2kWh bank of Lithium batteries.  Data on how far this will get us is hard to find, but it should give us at least an hour of motoring at a decent speed, which should be sufficient for manoeuvring in and out of most harbours.

For longer passages (like motoring to Scilly in no wind, or traversing the Kiel Canal) we will run a small petrol generator which will power a battery charger.  This should allow us to run almost indefinitely, although the noise will be annoying.  Depending on how much power is being drawn by the motor, we may be able to stop the generator periodically every few hours and motor on silently from the batteries for a while.

We will charge the batteries from:

  • Mains power in marinas.  We should be able to go from empty to full in about 3 or 4 hours.  Maybe we’ll start visiting marinas more often!
  • The petrol generator, while motoring along (or if we are away from everyone in a remote place, or in a really noisy port I guess).  Again, should charge the batteries completely in 3-4 hours unless we are motoring hard at the same time.
  • Solar panels.  When we are sailing for the weekend every couple of weeks this might be enough by itself.  If we are living on board and sailing every day, it will probably need supplementing.

Our 12V lights and instruments will be powered from a separate 12V battery, charged by the existing wind turbine.  We will also be able to switch over to power everything from the propulsion bank as a back-up.

Man Power?

Before committing to the electric route, I built a Yuloh (sampan-style oar) which pushed us along ok for short distances, but we only really used it once.  It was very long to store on deck, and the mounting pivot got in the way of the tiller.  We decided that it wasn’t very practical as a means of propulsion, particularly as we are planning to transit the 100km-long Kiel canal, so the yuloh sadly became firewood.


We were originally planning to use an outboard motor for the long passages rather than a petrol generator, but:

  • A decent long-shaft outboard costs nearly £1000, whereas a petrol generator costs about £250.
  • The outboard motor would have been very heavy to lift on and off the transom, dangerously-so in any sort of sea.  It would therefore probably have lived installed on the transom where it would stick out getting in the way and incurring extra berthing fees.
  • The outboard would have required a large bracket to cope with our very overhanging transom.   The push-pit was also in the way of any motor-tilting mechanism.
  • The outboard’s propellor would have been quite shallow so wouldn’t have worked very well in waves, and it wouldn’t push any water over the rudder like the built-in propellor does.
  • The petrol generator will be a lot more flexible -for example we could use it to recharge the batteries in a remote anchorage if required.
  • The noise of the petrol generator will probably not be much different to a petrol outboard.
  • We’ll arrive with our battery bank full rather than empty.


We’re really looking forward to motoring silently into harbours under electric power.  We’ve already enjoyed the amount of engine-related plumbing, exhaust piping, seacocks, grease and spares that we’ve been able to remove along with the engine.  I’m looking forward to having a clean engine bilge with no grease and no stink of diesel.

There isn’t much information out there about this type of project, and it is all fairly experimental so we don’t know how practical it is going to be.   It is hard to estimate how far the batteries will take us, and how annoying the petrol generator will be.  But we survived all last year with no motor at all, so hopefully it will be an improvement on that!