We are now the owners of a power catamaran, one that has no engines. It is therefore necessary to choose the new electric propulsion system and the batteries. We have just signed the purchase of a system of two 50 kW engines under 96V and 72 kWh of batteries. Before sharing the technical details of the system we have selected, I will explain how we have researched and reached this decision: how much power is necessary for our needs and then which suppliers were considered and consulted.

Propulsion Power Required for our Catamaran

Original Jag 530 Propulsion System

Dean Catamarans had powered the Jag 530 with two Vetus DTA 67 engines of 286 HP each. These engines allowed a cruising speed of 15 knots and a top speed of 20 knots. Since we are switching to electric propulsion, we will use kW as the unit of power and not in HP. The original engines therefore had a power rating of 210 kW each.

A quick estimate of the solar energy that we can expect to produce with the available surface gives us around 15 kWp. kWp means “kilowatt peak”, or the energy that the solar panels can produce under an ideal solar situation. Realistically, we can expect to generate 5 to 6 times this “peak rating” per day, or between 75 kWh and 90 kWh for a perfectly sunny summer day. It is therefore obvious that it would be completely useless to install a 400 kW propulsion, because the energy generated in one day would be consumed in less than 15 minutes. In short, relying on the old engine power rating is not the right approach.

Our Expectation of the New Propulsion System

We must turn the problem around and decide what cruising speed we consider and what top speed we wish to achieve. A cruising speed of 4 to 5 knots seems adequate because we really want to avoid having to use any energy other than solar and we are determined to live slowly, in harmony with our environment. Most sailing boats generally sail at speeds of 4 to 6 knots. To give a concrete example, let’s take the distance from Nice to Calvi for a crossing to Corsica. It takes about 22 hours to cover the 100 nautical miles in a straight line at this average speed, or a little less than a day and a night, which is usual for sailors. For the choice of top speed, being close to 10 knots would be desirable, either to be able to reach shelter quickly if the weather turns bad, or to be able to move against a strong current in a pass, or other difficult situation. In any case, we must be aware that the top speed will not be sustainable for long. Maybe 30 minutes to 60 minutes maximum.

Once we have defined our desired cruise and top speeds, we can use formulas for displacement hulls (our catamaran will never be able to reach planning with her 20 tons). Two formulas are useful for this: Keith’s formula and Crouch’s formula. Keith’s formula takes into account the weight and the length at the waterline of the boat, while Crouch only uses the weight. They give results with significant differences, so for our estimates, we decide to take the average between the two.

There are other formulas very close to Keith’s: the Wyman’s and Geer’s formulas. They all have the same form, but with different coefficients.

This average between the Keith’s and Crouch’s formulas gives us that to move at 4 knots, we would need 10 kW, at 5 knots 18 kW and at 10 knots 100 kW.

These estimates lead us to decide that 2 motors capable of delivering 50 kW peak each would be sufficient.

Recommendations from Yachting Experts

Another way to get good advice is to ask boat building professionals what power they recommend, keeping in mind that we want to move slowly, like a sailboat and not like a fast motorboat.

Our boatyard recommends a motorization of at least 5 HP per ton of boat. They use this rule to size a diesel engine to a sailboat. This is 3.8 kW per ton, so 76 kW for our 20 ton catamaran.

While searching for electric motor vendors, we found that Fischer Panda recommends 2.5 kW per ton, or 50 kW for our 20 ton catamaran.

Another data source, on the website Multihull.de suggests 1 kW per 300 kg, or 3.33 kW per ton, or 66.6 kW for our catamaran.

All these data sources confirm that our calculations to have 100 kW peak with our 2 engines make sense.

Validation by Comparison to Motors Installed on Already Built Electric Catamarans

Yet another way to validate our calculations is to compare our results with what electric boat builders have done already. I have collected all the information I could find on electric and solar powered boats in a document for the past couple of years and I can filter for catamarans of a weight close to our Jag 530 and see what electric motor power they have installed.

I summarize in the table below the interesting information for this post:

Seeing these numbers reassures us about our decision to re-power with 2x 50 kW, in line with what has been done before, especially for catamarans built up to a year ago. In 2020, new models have started to use more powerful engines, but most are hybrid electric catamarans with large diesel generators onboard to provide the energy necessary to power the motors.

On this speed/power curve of the new Silent Yacht Silent 55 Cruiser, we notice that the power range from 100 kW to 300 kW pushes the catamaran from 10 to 17 knots.

Electrical Voltage of the Propulsion System

Now that we have defined the desired power for our propulsion system, we must decide another important parameter: the supply voltage that we will use for propulsion.

The Dean Catamarans Jag 530, like most boats of this size, had battery banks for the 12V circuit for the navigation electronics and safety equipment and a large 24V bank for everything else. To get 100 kW from a 24V bank, it would be necessary to draw a current of more than 4000A. This is technically impossible.

12V and 24V do not present a risk of electrocution for humans, even in wet environments. 48V is considered the high limit of non-hazardous voltage. We must be cautious with electric systems above 48V, for the installation, operation and maintenance of the electrical system.

The 4 voltages commonly used by propulsion motor and controller manufacturers are the following, followed by the current required for 100kW at this voltage:

• 48V, at 2000A
• 96V, at 1000A
• 144V, at 700A
• 400V (from 360V to 420V), at 250A

The lower the current, the smaller and lighter the electrical cables are, but we increase the danger due the high voltage required.

Another factor comes into play: the choice of the voltage of the propulsion system dictates the choice of the voltage of the battery bank. With high currents, we cannot rely on DC-DC conversion for 100kW of power (for example to step up the voltage from 48V to 400V for the motors). A DC-DC conversion loses about 5% of the energy in heat and these converters are prohibitively expensive.

We will therefore need to find a several electronic components that can operate at the voltage of the high voltage battery bank:

• the battery chargers from the solar panel output (MPPT chargers)
• the inverters to supply the 220VAC onboard
• the DC output of the backup diesel generator
• the chargers for the 12V and 24V battery banks

At 48V, the offer is plentiful. Above that voltage, components are more difficult to find. A quick search shows that there are some MPPT chargers for 96V batteries, as well as inverters. At 144V it is more difficult and at 400V+, it is extremely complex to find anything, because it is well above the voltage of the do-it-yourself projects.

In conclusion, there are two voltages that are suitable for our project: a 48V propulsion (ideal) and a 96V propulsion (acceptable, with the advantage of currents reduced by half in the cables compared to 48V).

Motor Rotor Rotation Speed

The rotation speed of a propeller of a sailing boat is generally between 850 and 1650 rotations per minute (RPM). We have the space to install propellers up to 1 meter in diameter under our catamaran so it would be rather interesting to have a low rotation speed with large propellers. This would minimize noise and vibrations, avoid the risk of cavitation, etc.

Diesel engines use a gearbox to reduce the rotation speed of the propeller shaft to around 1000 revolutions per minute (RPM). For our project, the objective is to avoid a mechanical speed reduction gearbox, which would add additional cost, weight, noise, maintenance, etc. We are therefore preferably looking for a supplier who has electric motors we could put straight on the propeller shaft, with maximum power at a speed close to 1000 RPM.

Marine Electric Propulsion Suppliers Considered for our Project

Fischer Panda

Model EasyBox HV | Power 50 kW | Voltage 360V | Direct transmission Yes

Pros:

• German supplier renowned in the maritime world for its diesel generators
• Engine running at 1200 RPM with a torque of 398 Nm

Cons:

• Voltage 360V too high
• Only 20 kW in the low voltage products at 48V

OceanVolt

Model ACX30 | Power 30 kW | Voltage 48V | Direct Transmission Yes

Pros:

• A very detailed commercial proposal, with speed-power curves taking into account the wind, distance estimates with different battery capacities, etc.
• A renowned brand dedicated to electric and hybrid marine propulsion
• Motor cooling with “Cooling Fin”, i.e. no need for seawater heat exchanger

Cons:

• An extremely expensive commercial proposition, out of our budget (more than 30k per motor, without battery)
• Only 30 kW by putting 3 ACX motors in parallel on the same shaft, which partly explains the cost (3 motors, 3 motor controllers, etc.)

Molabo

Model ISCAD V50 | Power 50 kW | Voltage 48 V | Direct Transmission No

Pros:

• Early stage German startup
• Good communication with their co-founder and their technical-commercial team
• Very detailed technical information provided under MNDA which gives confidence in their system
• Very light engine (less than 50 kg)
• Commercial proposal (with Chinese Linkage Power batteries) that fits the budget

Cons:

• Still at the testing prototype stage, so availability date at the end of the first quarter likely to slip
• Engine with maximum performance at 4350 rpm, which therefore requires a gearbox

Aquawatt

Model Gear motors for extra heavy duty | Power 50 kW | Voltag 144 V | Direct Transmission No

Pros:

• Complete and well organized catalog, covering motors, batteries and other accessories
• Small structure, based in Austria

Cons:

• Small structure, answered that they will not have time for our project (probably motor not yet available)
• Solution with a gearbox

Tema Marine

Model Electric Propulsion System | Power 23 kW | Voltage 96V | Direct Transmission Yes

Pros:

• Manufacturer with good interesting installations of electric or hybrid propulsion systems
• Air cooled motors, with fan directly on the rear rotor of the engine (no risk of cooling system failure)

Cons:

• Only 23 kW at 96V and high voltage from 30 kW

Krautler

Model WA WAz 30 AC | Power 30 kW | Voltage 144V | Direct Transmission No

Pros:

• Complete and well organized catalog covering the motors, batteries and other accessories
• An Austrian industrial company with significant marine electric propulsion installations (including an electric catamaran on the Spree in Berlin)

Cons:

• Solution with a gearbox
• Only 30 kW

Bell Marine

Model DriveMaster 45W | Power 45 kW | Voltage 144V | Direct Transmission Yes

Pros:

• Dutch supplier renowned in the maritime world
• Catalog covering all our needs, even propellers, solar chargers, etc.

Cons:

• Difficult communication, because we have to go through their dealer in Spain for any commercial proposal and the dealer does not know the products well
• Price from the dealer is too expensive, then we get a 10% price reduction just by asking… not a good experience and it breaks trust - our contacts are only salesmen, will the complete system be properly designed?

RiPower

Model 50IB | Power 50 kW | Voltage 96V | Direct Transmission Yes

Pros:

• German entrepreneur who can supply all the components of the propulsion system, even the propellers
• Cheaper than the large companies
• Trust more easily placed in a entrepreneur (like me) who masters his subject from A to Z, than in a company that is too big and rely on resellers
• Enthusiastic in our email exchanges, we can see that the owner is passionate and shows it on his websites and his YouTube channel

Cons:

• All projects already delivered were smaller than our 20-ton catamaran
• Communication in English difficult; use of Google Translate by email

E-Tech Electric Drive

Model 35 WGH | Power 35 kW | Voltage 144V | Direct Transmission Yes

Pros:

• Motor cooling by “keel cooling”, so no seawater exchanger
• 315 Nm at 1600 RPM

Cons:

• 3 months lead time for 35 kW engines
• High price for only 35 kW

Electric Yacht

Model QuietTorque 30 LC | Power 30 kW | Voltage 48 V | Direct Transmission Yes

Pros:

• Manufacturer with a good reputation
• In 48V

Cons:

• Supplier based in the United States; complex logistics (import, transport)
• System with 2 bulky motors and with mechanics between the motors and the shaft
• Only 30 kW

Aquamot

Model MA300 | Power 30 kW | Voltage 96 V | Direct Transmission Yes

Cons:

• Only 30 kW

Elco

Model EP-100 | Power 42 kW | Voltage 144 V | Direct Transmission Yes

Pros:

• One of the suppliers with the longest experience in electric marine propulsion

Cons:

• Supplier in the United States; complex logistics (import, transport)
• Very heavy engine (over 300 kg per engine)

The System Ordered for the Refit of the Jag 530

The selection of our electric propulsion supplier has evolved a lot over the last 2 months. I was very interested in working with Molabo, not only to help an early stage startup, but for the qualities of their engine too. Unfortunately, our last discussions showed that it is necessary to use a gearbox due to the high RPMs of the motor. The additional cost is acceptable, at about 600 Euros per motor, but the goal is to have the motor on the propeller shaft, so I abandoned their proposal. Their solution is very interesting for fast and light boats, not necessarily for our slow and heavy boat.

I then decided to work with Bell Marine. Their product is solid and although their 45 kW engine works at 144V, they can provide MPPT chargers from the solar panels for the 144V batteries (AERL Coolmax MPPT chargers) and offer a 144V inverter too. Buying a complete system from them solves my biggest problem with 144V. However, after a first contact with Bell Marine in the Netherlands, they redirected me to their dealer in Spain, because they do not sell directly to customers. The few contacts with the reseller were not fruitful. No problem sending multi-page PDFs with images and pricing, but no technical expertise on the requested products and a slow response in getting each answer. My confidence in having a technical partner able to ensure that all the components will be well chosen, well configured, has disappeared.

I finally went back to my list of possible suppliers and I contacted a German entrepreneur, integrator of electric propulsion systems for boats and land vehicles, RiPower GmbH. The first attempt at a telephone call was not successful, because the owner does not speak English and my German is too limited to discuss the project. We therefore agreed to communicate by emails in English and he will use Google Translate. After detailing the project, providing specifications and photos of the catamaran, RiPower prepared a commercial proposal including the 50 kW 96V motors, batteries, throttles, a 220V charger, propellers including the calculations, cables harness. An additional benefit is that the RiPower workshop is 1h30 drive from our home in Berlin and we will be able to go onsite if needed.

The order has been placed with RiPower GmbH. We can’t wait for our motors and batteries to arrive in Alicante!