The biggest difference is availability.
With an electric motor instead of a diesel in your boat you can achieve the same/similar performance in power/speed. At the same time you can get better reliability and lower maintenance.
However, the biggest difference is that you have to manage availability.
With a diesel availability is taken for granted. We assume that if you have fuel the engine is available:
- whenever you want it
- however long you want it
- at whatever speed you want it
these are not the same with an electric motor as everyone of them depends on your management, preparation and planning around availability of the battery bank.
Note that here I’m talking about electric motors that use battery banks that are charged primarily from renewable energy. If you have a generator (usually diesel in this case) that can generate the same power as the motor uses then you are in the same situation as with a diesel engine.
What affects availability?
There are three key variables that affect how much electric motor availability you have at any particular time. All these need to be managed and none of them can be changed instantly at the point of need.
Hence, using an electric motor requires a mindset, especially changes in how you manage the boat and the plans you make.
The three variables are:
- Technical specifications (particularly of battery bank and renewable energy generation)
- What you have been doing up to this point (ie what state is your battery bank in at this moment)
- What you are planning to do (and what contingency plans you have made)
What makes things easier?
Spending more on the technical specification increases your capacity and reduces the amount of management and planning you need to do. For example a bigger battery bank or a larger solar panel array both mean you will have more capacity available and so less need to manage the capacity and the plans become easier to make.
Choosing a catamaran makes things easier as there is always going to be more space for solar panels.
Cruising less often or less intensively makes things easier as you have longer between passages to charge your batteries.
Cruising is warmer climates makes things easier as there will be more sun to get more power out of your solar panels (at the extreme, solar panels are going to be of zero use in a polar winter with 24 hours of darkness).
Longer passages (as long as there isn’t a tight schedule) makes things easier as you can sail for days while charging the battery bank ready for the next landfall.
Better sailing performance with special focus on light winds and going to windward (when people are most likely to motor or motor-sail)
What makes things more difficult
Tight and fixed deadlines.
Wanting hands off systems where you pay someone to do the maintenance and then have instant and complete availability 24/7.
Always available “luxury” (air conditioning, hot water, heating, large freezers and fridges, electric autopilot, electric winches)
Complicated coastal waters (tidal inlets, long and narrow harbour entrances, big marinas, headlands with big tidal streams, lots of traffic)
Diesel inboard engines have managed to create a reputation of always being available and for most weekend sailors this has been the reality. The engines get professionally serviced each year, they are not run many hours, people avoid bad weather and they keep their boats where high quality fuel, parts and expertise are all available. As you go further and for longer, especially to remote places, this changes a bit with care needed over fuel quality and parts availability being more challenging as well as having to be more self sufficient in maintenance skills. So management of availability with diesel engines is only an issue for more adventurous, intensive cruisers.
On the other hand Electric motors require a far more hands-on management process for all cruisers as the battery capacity is far less than a fuel tank and the rate of charge from renewable sources far slower. This means planning ahead and that has a much bigger impact on those who have not had to do this least with diesel engines.
If I motor-sail now to speed the passage, will I have enough battery left to motor into the harbour or up the river? That means thinking about tides/currents and the weather (how much solar power will I generate during the passage).
It means thinking about what might be needed in the next few days. Suppose the wind gets up from a different direction, do I have enough battery left to motor away from the anchorage if it becomes unprotected tomorrow.
It means being very aware of both consumption and generation. If I run the watermaker then when am I going to recharge the batteries, might it reduce the ability to get in or out of a harbour.
It will mean changing passage planning. Probably needing to be more flexible. If the wind drops you might not be able to motor fast enough for long enough to make a tidal gate.
Many sailors using harbours such as Chichester, have a working assumption of motoring from deep in the harbour to the open sea whatever the tide is doing. That might fully use your battery capacity with an electric motor.
If you are cruising outside the tropics in the winter your solar generation might only be 10% of what you would get in the Bahamas. The total power you can use over a period of time will be dramatically reduced so you will need to pay far more attention maximising the generation eg adjusting the angle of your panels (actively pointing them at the sun to increase their effectiveness), to keeping them clean and free of shade.
In some ways this is going back to ways of the past when yacht engines were unreliable and not powerful enough to push you against an unfavourable tide so that you didn’t factor motoring and consistent/predictable passage times into your plans.
What we are doing to handle this?
On the technical side
- Efficiency is key everywhere. A brushless electric motor is better than one with brushes. Switch to low power everything eg LED, self contained solar powered wind sensors, wind vane self steering.
- Lithium batteries. At the moment Lithium-ion phosphate (LiFePo4) technology has the edge with higher capacity, faster discharge and charge rates, able to be more fully discharged without damage.
- Simple and basic. We are starting with no fridge or freezer in the UK. No electric winches, no electric toilets.
- Improve the sailing performance as much as possible. So we are replacing a back of mast furling system with slab reefing on a longer boom with a larger sail area supported by battens. We are looking to changing from a single point mainsheet to a track. We are cutting lots of weight from the interior and systems.
- Adjustable solar panels that allow extra capacity be deployed at anchor and in calm conditions.
- We are keeping separate banks of batteries for house and motor while having the ability to transfer energy from either bank to the other
- We have invested in a significantly oversized anchor to reduce the chances of having to evacuate anchorages.
We are using Victron monitoring tools (battery bank monitor, mppt controllers) that give us the most data possible on past and present energy generation and consumption.
We are following a revised version of the tradition that the engine starter battery should be kept separate from the house bank. This requires larger battery capacity overall and some extra components, however, we believe that this complexity does give us the reassurance of being able to protect our motor availability. At the same time, when in a protected anchorage we can choose to charge the house bank from the motor bank if you have a few cloudy days and are using a lot of power (maybe for clothes washing, cooking, or hot water). On the other hand when leaving an anchorage we can charge the motor bank (albeit not very fast) from the house bank although with the understanding that might mean we need tot turn off the house items such as electric autopilot, or fridge & freezer or no induction cooking. So our version has has some separation but also allows us to run DC to DC chargers at anytime to be able to “steal” power from one battery bank to charge the other.
More than anything else having an electric motor with a fossil fuel free goal means having realistic expectations. We expect to
- sail on passages, using the electric motor for marinas, rivers and tight harbours
- have more variable passage times as we won’t be using the motor to keep consistent average speeds
- work more closely with the tides rather than be able to motor against them
- work hard at maximising solar generation by using additional panels that have to be moved around, by tilting panels to their most efficient angles
- work hard at minimising consumption of all appliances
- by motoring more slowly to significantly increase range
I’d started this several times as a post about the disadvantages of electric motors, but I’d struggled with it. That is because, unless you have an unlimited budget, the issues are all about attitudes and expectations.
If you believe in reducing fossil fuel use then the differences are entirely manageable.
If you are not concerned about fossil fuels and the climate emergency then the inconvenience of managing availability is going to appear a deal breaker.
The technology is changing fast and modern boat design trends (such wider beam carried aft, fewer ketch rigs) make it simpler to fit larger solar capacity. It is likely that over the next few years there will be further gains in battery capacity. Monitoring and Management will be more sophisticated and automatic. So gradually the need to manage availability will diminish, especially for weekend sailors as a battery bank that is fully charged at your home marina will cope with a weekend of sailing and motoring.