The most common question we get asked at Eco Boats is from boat owners who are trying to find out if it would make sense to replace the petrol or diesel motor in their boat with an electric motor.
The answer to the question as to whether an electric motor is right for your boat is different for each boat and each owner, how the boat is used and what the performance and range expectations are. You could write a book if you wanted to cover every aspect of the decision-making process that precedes converting your boat to electric. But, without going into too many specifics, it is easy to outline a few recurring themes that apply to everyone. We’ve outlined the most common decisive factors for you in this article.
Hull types, Speed and Range
Generally, speed is the biggest limiting factor when talking about electric propulsion for boats. To push a boat through the water fast (at say 10 knots) or to bring a hull up to planing speed (at say 15 to 20 knots), requires exponentially more power than to go slowly at 4 or 5 knots. Any small to medium size boat (say up to 10 m long / 5 ton weight) can be quite easily brought up to a speed of 4 or 5 knots with relatively little energy required to keep up that speed. This is called ‘displacement speed’. Many boats are designed to only go at slow speeds (think old wooden boats, most sailboats, old ferries, tugboats, rowboats, most dinghies and tenders). We call this type of boat hull a ‘displacement hull’.
Speedboats have a different hull shape that is designed to (at higher speeds) lift themselves up onto their own bow wave and rise up out of the water to skim across the surface. These are called ‘planing hulls’ and they can travel at both ‘displacement speed’ (when going slow) and at ‘planing speed’ (when going fast). Think water-skiing, fast tinnies, and all modern speedboats. However, to get the boat up to the much faster planing speed, exponentially more energy is needed. If you can make a boat go 5 knots with a 10HP motor, you do not simply need 3 times that (30HP) to go 15 knots, more likely you will need a 90HP motor or even more to reach that speed. That is why you often see small speedboats with enormous outboards of 200HP or more on the back, all this energy is needed to maintain this high speed.
In combination with speed (when boating electric) on-board energy storage is the next biggest limiting factor, because even the best and most expensive batteries available today contain relatively very little energy compared to fossil fuels such as petrol & diesel. For this very reason it’s technically difficult and very expensive to have an electric speedboat. Yes, there are electric speedboats out there and sometimes it’s the only way to go. For example, on some freshwater lakes in Europe it’s simply prohibited to use petrol engines, so if you are a millionaire living on one of those lakes and like to go water-skiing then by all means you could get yourself a high-speed electric boat. However, the cost of systems and batteries will be astronomical, you will need access to shore power to re-charge and the range / running time will still only be a fraction (think 60 minutes at top speed max) of what a comparable petrol-powered speedboat can offer. For this reason here in Australia we could say that an electric speedboat is not ‘economically viable’ (in simple words, too expensive and too many limitations).
For that reason here at Eco Boats we focus on the type of boat hulls where it DOES make sense to use electric propulsion: ‘displacement hulls’. For the remainder of this article we will only be talking about applications for displacement boats that travel slowly and forget about electric speedboats.
Now that we have learnt that it takes exponentially more power to push a boat through the water it will come as no surprise that the same applies to slower traveling speeds.
To explain this in a bit more detail we work out a basic example of the below mid-size, traditional sailing boat that is fitted with an 8kW electric inboard motor:
Boat Design: International ‘metre class yacht
Length of waterline: 9 m / 29 feet
Weight (displacement): 8,000 kg
In the table below we see the amount of energy that the electric motor consumes from the batteries measured in kilowatts (kW) and the speed the boat travels at in knots. It only takes 1,000 watts (1kW) to let this 8,000 kg boat travel at a speed of 2.6 knots (almost 5km/hr). 1,000 watts equates to roughly the same energy used as 10 bright domestic light-bulbs, so it’s not bad that you can push such a big, heavy boat at 5 km/hr with the equivalent energy of 10 light-bulbs.
Efficient Cruising Speed at 4.2 Knots
Top speed at 6.7 Knots
But something interesting happens when we want to go a little faster.
If we now push the throttle forward and use 4,000 watts (equivalent to 40 lightbulbs) wouldn’t it be nice if the boat was going 4 times as fast? Unfortunately, this is not the case, we are now using 4,000 watts but are only going 5.7 knots (approx. 2 times faster) so there’s a lot of energy being used that does not translate to higher speed – this energy is lost in overcoming friction with the water and creating ripples and small waves.
If we now keep pushing the throttle forward we see this effect become even more dramatic, as we increase the energy consumption from 4,000 to 6,000 watts the speed we gain is only 0.8 knot extra (from 5.7 to 6.5 knots). If we keep pushing the throttle forward all the way to top speed it gets even worse as now we’re using 8,000 watts (equivalent to 80 light-bulbs) and the boat is maxing out at 6.7 knots (only 0.2 knots extra).
If we put this information in a graph (see below) we can clearly see the exponential relationship between ‘energy consumption’ VS ‘speed’.
The graph makes it easy to see that the boat of our example has a ‘sweet spot’ between 4 and 5 knots of speed. Going anything faster than that we see the blue line starts to flatten out around 6 knots and the boat simply doesn’t go much faster; it has almost reached it’s ‘hull speed’.
To travel at maximum speed (‘hull speed’) means the boat requires so much more energy that it’s not sustainable for long periods in the case of electric boats as we would struggle to fit enough battery capacity on-board. Even for petrol and diesel powered boats it’s not sustainable to go at top speed all the time, that is why commonly when talking about boat speed, we talk about ‘cruising speed’ and ‘top speed’.
The workings of this example are applicable to each and every ‘displacement hull’ regardless of it’s size and weight, the graph will have pretty much the same shape. It will even be the same regardless of what type of motor the boat has – it’s got nothing to do with the boat having an electric motor, it’s the same for boats with a diesel motor or petrol motor; to make the boat go a little bit faster, the hull requires exponentially more power. This is because the boat is trying to come up to it’s own bow-wave, but because the hull shape doesn’t allow for it to come up to the ‘plane’ (see about planing craft above) it will perpetually travel at an angle (bow up/stern down) and just create more and more waves as it goes faster. Even if you would put an extremely powerful engine in a displacement hull, it will not go faster, it will just make bigger waves. A good example of these are tug-boats seen in working ports. With their enormous engines and relatively short length they create massive bow & stern waves, even at relatively low speeds.
There is a direct relation between the length of the boat and it’s hull speed, this has been known for centuries and it’s fundamental to all calculations around ship design, boat speed and range.
Naval architects have come up with a basic rule to calculate the relation between the length of the boat and the expected maximum speed it will travel at;
Hull speed is expressed as 1.34 x the square root of the boat’s waterline length. The sailboat of our example has a waterline length of 29 foot, so it should be able to sail 1.34 x 5.38 = approximately 7.2 knots.
Now that we know that the speed the boat is traveling at has a direct relation to the energy it consumes we can understand that the range the battery bank can offer is also directly related to the boat speed. Our example boat is fitted with a 48V battery bank that offers 15kW/hr of usable energy. The slower the boat travels, the longer the range, in the above example if traveling at 2.6 knots the motor would be able to run for 15 hours, covering a range of 39 nautical miles (nm). If traveling at full speed, the motor would only be able to run for less than 2 hours, covering 2 x 6.7= 13.4 nm – only one third of the range at the slower speed. The rule of thumb with displacement boats is that they travel most efficiently at speeds up to 60% of the boat’s theoretical hull speed, for our example boat that would be 0.6 x 7.2 = 4.3 knots. At that speed the battery bank offers a decent range of about 7 hours running time or 30 nm. This is in line with the cruising speed we can derive from the curve in the graph above.
Many books have been written about what we’re outlining above in much more detail and it’s well worth just Googling some of the terms used such as ‘displacement hull’, ‘cruising speed’ and ‘hull speed’ to learn more about what these mean for your boat and how it performs.
The above explains in a nut-shell a) why electric propulsion is generally not suitable for speedboats and b) why speed is directly related to the range available from the batteries as going faster means you will run the batteries empty much, much quicker.
Batteries for electric boat motors
Now that we have a better understanding of how much energy our boat needs to make it travel at a certain speed we can look at the ways of storing the necessary energy on-board.
Here is why this too is different for each boat and each owner;
You can have 2 identical boats but one owner might only go out for 3 hours at a time, pottering around in sheltered waters at low speed, whilst the other owner might want to take his/her boat offshore and travel for 8 hours at close to maximum speed.
One owner might keep his/her boat at a marina or private jetty with access to shore-power to re-charge the batteries, whilst the other owner keeps their boat on a swing-mooring and will need to rely on solar or wind-energy to re-charge the batteries.
There are so many variables on this that it’s impossible to have a one-size fits all approach when it comes to quoting an electric drive system for a boat. Therefore, at Eco Boats we like people to complete the Questionnaire-electric-propulsion so we get an understanding of the boat owners’ type of use, speed and range requirements and budget so that we can put a tailored quote together that matches those needs. If your requirements are unrealistic we will tell you what is achievable or when it might not be a good idea at all to go down the path of electric propulsion. In the end we strive for happy clients and will tell you what you can expect upfront to avoid disappointment.
Generally there are 2 popular options to store the energy for an electric boat; with ‘Lead-acid’ batteries or ‘Lithium’ batteries.
Again, there are many books written about the topic of batteries and it would go too far to even try and explain all the intricate differences between all the various types of batteries. However, there are basically 2 options when it comes down to batteries, each with their own pro’s & con’s;
Lead-acid batteries have been around for over 100 years and are simple, reliable devices that can provide reasonably deep discharges, are relatively cheap and virtually maintenance free. However they are big and heavy compared to Lithium batteries of the same capacity and they do not like to be left sitting empty after use, which will drastically cut short their life-span. Good quality deep-cycle Lead-acid batteries, if treated well, can offer up to 600 or 700 charge-and-discharge cycles and will generally die of ageing after 6 to 8 years regardless of the number of cycles they’ve done.
Lithium batteries are technologically advanced, often micro-computer controlled devices that can provide extremely deep discharges, are exponentially more expensive than Lead-acid batteries and can in some instances require quite advanced monitoring/maintenance skills. However their size and weight is generally less than half of a Lead-acid battery of comparable capacity and they don’t mind sitting almost empty after use with no ill effects on their overall life-span. Good quality Lithium batteries can offer up to 3,000 charge-and-discharge cycles and will generally die of ageing after 8 to 15 years regardless of the number of cycles they’ve done. The ultimate life of Lithium batteries is still not well known because it’s a relatively new technology with few Lithium batteries in the field that have been installed more than ten years ago.
Deciding what battery type would be best for your boat, again depends on the type of application, usage, budget and other factors. For example to replace a small 3HP petrol outboard on a dinghy, it would not be feasible to have a big, heavy bank of Lead-acid batteries onboard, so luckily there are practical and affordable products such as the Torqeedo Travel 1003 that come with their own light-weight click-on-top Lithium battery. The battery has it’s own in-built smart monitoring and charging system making it extremely user-friendly. It’s also designed for use in the marine environment so the whole unit is waterproof and it can be simply re-charged at home or on-board the mother ship
Now if we have a 4 ton traditional wooden sailing boat that is kept at a marina and is used for day-sailing on sheltered waters and we are replacing its diesel engine with a Bellmarine DriveMaster Ultimate it’s probably more sensible to use Lead-acid batteries. The weight of the new set-up with small and light electric inboard and medium size Lead-acid bank is about the same as that of the heavy old diesel engine, it’s fuel tank and lead ballast from the bow that was used to counterbalance the weight of the motor aft. For this type of boat weight is not really an issue and because it can be plugged into shore power the lead-acid batteries should last many years.
For a lightweight racing yacht or multi-hull where space & weight is a crucial factor one might be more inclined to install a large Lithium battery bank (if the budget allows for it).
One decisive difference between Lead-acid and Lithium batteries is the overall number of cycles the batteries will offer.
For a recreational boat owner who goes out twice a month that would equate to less than 200 trips over and 8-year period. Well within the natural lifespan of both Lead-acid and Lithium batteries. For a commercial operator running a charter business 7 days a week they will have clocked up enough trips to wear out a Lead-acid battery bank in less than 2 years. The commercial operator will have to replace the Lead-acid batteries perhaps up to 4 times in an 8 year period, so in this example it will actually be cheaper to use Lithium batteries because they offer so many more cycles over a similar natural lifespan. Add to that the additional benefits of Lithium such as their smaller size & light weight, extremely deep discharge ability, etc so there are certain applications where it’s much more sensible to go with Lithium batteries.
Charging from shore power:
Now when it comes to re-charging the batteries onboard, there is again no one-size-fits all approach. When the boat is kept in a marina with access to shore power, or kept on a trailer where it can be charged at home, that would be the quickest, cheapest and most reliable way to charge the batteries after each use. The Australian Government has a clean-energy offset scheme that allows you to add clean energy to the grid to the amount used to re-charge your boat, making it truly carbon-neutral. More info at: www.greenpower.gov.au
Charging from solar power:
If the boat is kept on a swing-mooring it’s a little more challenging to re-charge the batteries. When working with Lead-acid batteries they should not be discharged anywhere below 50% of their nominated capacity to prevent wearing them out too quickly (remember that Lead-acid batteries don’t mind a deep discharge, but letting them sit empty even for short periods of time will drastically shorten their lifespan). When working with Lithium this is less of an issue and one could say that Lithium batteries are more suitable for situations when solely relying on solar or wind-generated power to re-charge the batteries after use. With lead-acid batteries some energy is lost in the charging process, around 15 to 30% depending on battery type, and with lithium batteries this is negligible, and this difference can be significant when you have a limited recharging source such as solar or wind.
Regeneration when under sail:
For sailboats that travel long distances at relatively high speeds (like cruising multi-hulls) the batteries can be re-charged by means of the propeller free-spinning when the boat is under sail. The electric motor then acts as a powerful alternator that feeds energy back into the batteries. This only works at higher speeds (consistently 6 knots or more during long ocean passages) and cannot be relied upon by weekend sailors.
When there’s no access to shore power or insufficient sun a small generator can be brought on-board or permanently fitted to provide an additional means of re-charging the batteries. If the generator is running in conjunction with the electric motor this is called a ‘serial-hybrid drive’. This should be used only as a means to extend the range and is not suitable to gain higher speeds. Some boats have a large diesel engine as main propulsion system, with a small electric motor coupled onto the propeller shaft so the craft can run electric at low speeds and when maneuvering inside a marina. This a called a ‘parallel hybrid drive’. Particularly for larger boats hybrid solutions tend to be much more expensive and complicated than a ‘full electric drive’ or ‘conventional diesel drive’ and therefore not viable for most recreational boaters.
Generally for recreational boats, the conversion of combustion engine to electric drive system should not be done from a cost-saving perspective. Whilst it’s true that you won’t have to purchase diesel or petrol anymore and electric motors are virtually maintenance free the initial investment will be similar or higher than that for a comparable combustion engine. So for most recreational users the reasons to go electric are different; such as no noise, no fumes, no oil leaks in the bilge, no more flammable petrol on-board, low-maintenance and a generally much cleaner and more liveable interior inside the boat. Sailors get the same quiet experience with the electric motor as when under sail and off course there is zero pollution into the waterway and if re-charged with solar or green energy zero pollution into the airways too.
For commercial operators the reasons can be all of the above, but in addition to that we often find that converting to electric can realise a cost saving as the cost for fuel and maintenance for commercial craft can be substantial and is on-going. As an example one of our commercial customers, operating a 24/7 patrol boat, used to run the vessels on petrol motors and the cost per day for fuel was $50 to $60. Now with electric propulsion the cost of electricity is about $2.60 per day. This is a saving of around $20,000 per year and gives a very quick payback on the extra capital cost of Lithium batteries. Some commercial users have to adhere to strict guidelines around the use of flammable liquids in their area of operation and for nature / wildlife watching tours the benefit of a quiet drive-system is obvious.
Maintenance & technical aspects
Electric motors themselves have only few moving parts and are generally virtually maintenance free. Most electric motors do not require oil-changes, no lubrication or drive-belt changes. Some of the more sophisticated products that rely on Lithium battery technology or have in-built electronics can be subject to electrical faults / errors in a similar way to for example computers and other modern appliances. For users in remote areas this can sometimes pose a problem as spare parts or service partners are not as readily available yet as for diesel and petrol motors. However, with proper planning and a choice of the correct set-up that is well-matched to the requirements and environment the boat will be operated in, this should not be a problem.
Making the right choice
After reading this article you should now have a better understanding why there’s no simple one-size-fits every boat approach when it comes to choosing the right electric propulsion system for your boat.
Here at Eco Boats we have over 10 years experience with electric boats of all shapes and sizes so we can advise you what will and will not work. If electric propulsion is not going to match your performance and range expectations we will tell you upfront to avoid disappointment. If your boat is suitable we will discuss with you how you use your boat, what options you have for re-charging the batteries and what your budget allows for. We will then provide you with a detailed quote outlining all components.
We don’t carry out installations but can provide you or your shipwright of choice with advice and guidelines on how to install the system. Because electric motors are so much smaller, lighter and less complicated than combustion engines both DIY boaties and professional shipwrights and boat builders will find installing an electric motor generally easy & straightforward. Contact us today or fill out the below questionnaire to find out if electric propulsion is something that suits you.