There is many things to know about electric sailing and there are many questions as well. On this page we try to answer some of the most common questions we get about electric sailing.
Why do you use high voltages?
For safety. When more power is required for larger boats, either the current or the voltage can be increased. As a general rule of thumb, 200A is the maximum safe current to have continuously flowing through a system. Meaning that at 48V you can only go up to 10kW continuous, safely. 10kW is by far not enough for larger boats, thus the voltage must be increased accordingly.
If the current is too high for long periods of time, thicker cables and regular maintenance on all connection would be required due to the extra heat that is developed. If this is not done properly there is an increased fire risk.
Another reason for increasing the voltage instead of the current is to protect the system. For example, a fuse needs a certain amount of energy to blow fast and safe. The higher the current rating of the fuse, the more energy is needed. The energy to blow a fuse must come from the batteries. However, batteries also have a resistance, meaning they can only deliver so much current/energy. If a fuse is rated at 500A and blows safely and quickly at 1000A, then this is the current that must be drawn from the battery as well. If a higher voltage with a lower rated fuse is used, for example 200A. This fuse would require only 400A to blow.
Why do you use variable voltages?
This is to accommodate for the power requirements of a boat. We use standardized voltages as much as possible, being: 48V, 96V, 350V and 700V. It is possible for us to choose voltages in between these standards, but certain equipment is not always compatible in this case.
Is your system parallel or serial hybrid?
We use serial hybrid propulsion systems. The reason for this is that you are much more flexible in placing the components, the generator is not physically connected with the propeller shaft and can therefor be placed anywhere. Another reason is that the generator can run much more efficiently and for longer intervals, making it more reliable.
What is the maximum power you can deliver?
Our systems go up to 200kW.
What type of batteries do you use?
We use Lithium Iron Phosphate (LiFePo4 or LFP) batteries in our system. These are high performing and safe batteries to use. Unlike other Lithium batteries, LFP batteries cannot self ignite. Thus fire risk is minimized, we find this one of the most important factors on boats. LFP batteries can also be protected very effectively against misuse, increasing their lifespan and maintaining performance.
How long is the warranty on your system?
Are your systems suitable for existing boats?
Yes very much so. In fact, we specifically designed our systems with this use case in mind. We know there's a huge fleet of well built, existing boats which will be victim of the ever stricter emission rules and at some point must convert to electric propulsion. We offer the solution for these boats. There are also many boat owners who want to experience the tranquility of electric sailing, these people are at the right address as well.
Does STOK Electric cooperate with shipyards?
Absolutely. We prefer to work with shipyards since they have a lot of boat building experience and all the necessary equipment to install a new propulsion system. Also, many boat owners have a preferred shipyard which is close by, or just the place they want to go with their boat. We understand this and are therefor willing and able to work with any shipyard you prefer.
For the "do it yourself" owners out there, fear not, we're very much willing to work with you as well. We understand the joy of creating something yourself and want to be a part of that process.
What is the range of a STOK Electric system?
This depends very much on a lot of factors, like boat displacement, hull shape, shaft or saildrive, water and weather circumstances, etc.
From testing results we can reach up to 20 nautical miles with a 37kwh battery bank, motoring at 5 knots. The ship is a 57ft 25T displacement hull. We guarantee that the electric range is more than enough for day sailing and going in and out of harbor. The batteries can be recharged using renewables during the day as well.
If long range is required, the generator can be run for an hour or so to recharge the batteries. This way an enormous range is possible.
Can I use the generator already in my boat?
Yes this is possible. It must of course be considered if this generator is suitably large to function as a range extender. But we have products which can convert the power from the generator to the suitable voltage for charging the propulsion batteries.
Are hybrid systems more efficient?
In short, yes. A combustion engine powering a ship is not very efficient. Most combustion engines have a certain range and load at which they like to operate, when the motor is powering a propeller, the load and speed are constantly changing. This is not very efficient.
An electric motor has a very high efficiency at all loads and speeds, thus it is much more suited to power a propeller. If a generator is used for charging the batteries it can run at it's most efficient speed and load, meaning it consumes less fuel.
How are the batteries recharged?
The batteries which are used for the propulsion can be recharged in many different ways. We can use standard shore power equipment for charging the batteries, hydrogeneration, solar and wind are also an option to top the batteries up and compensate for the household usage. If a quick charge is required, the generator can provide plenty of power, or a fast charging shore power connection can be installed.
What is the lifetime of your batteries?
The batteries we use are rated for >3500 cycles. A cycle means completely discharging and charging the battery. These rated cycles are often measured under heavier loads than the battery normally experiences.
To give some examples, say we have a battery pack in our system which makes a cycle every day. This is 365 cycles a year. Divide this by 3500 and the lifetime will be at least 9,5 years. However, if you make one battery cycle a day with a boat you must be using it very, very often.
Making a single cycle a week on average is more realistic for most boat owners. In this case only 52 cycles a year are made, this increases the lifetime to at least 67 years. This is very long and the batteries will probably be replaced before the cycle count becomes an issue.
Therefor, we state that we expect the average lifetime of a battery to be around 15-25 years, and replace them at the same interval as you would a normal diesel motor.
What happens if the batteries are empty?
The batteries are protected from complete discharge, normally they will disconnect to protect themselves when the capacity is too low. If a generator is installed this will have started in time to charge the batteries back up again.
If there is a problem with the batteries, they can be physically disconnected from the system, the generator can be started and a safe place in a harbor can be reached to find the faulty battery.
How long does it take to recharge the batteries?
This very much depends on the charging method. When charging with the generator the batteries can often be charged in 1-2 hours. If a conventional shore charger is used it can take anywhere from 8-24 hours, depending on the charger capability, the shore connection and the battery capacity. If a specialized charger is used which can be powered from shore it can be charged in 4 hours for a typical battery system.
Can the battery bank be placed in different locations?
To a certain extend, yes. It is possible to divide the battery bank over two rooms for example, placed under two different beds. However the divided battery banks must still be relatively close to each other to limit the cable length between them.
Are there weight savings with an STOK Electric system?
In general, yes some weight can be saved. Batteries don't weigh much, for example a 37kWh battery bank weighs only 280kg. Add to this an electric motor and small generator, which would have about the same weight as a normal combustion engine but requiring a much smaller fuel tank due to the generator being used less and more efficiently. And the result is a lighter overall system.
What are the benefits of swapping to LFP batteries?
LFP, or LiFePo4/Lithium Iron Phosphate batteries are much lighter, stronger and longer lasting than conventional Lead Acid batteries. For example a 12V 105Ah LFP battery is as powerful as a 12V 220Ah Lead Acid battery, while weighing only 10kg instead of 65kg. And also being 23% the size of a Lead Acid battery.
LFP batteries are also protected from misuse allowing them to have a much better lifespan and also being very safe to use, safer than other Lithium battery chemistries.