5 step Electrical BLDC sailboat motor program

BarcelonaSail’s electrical BLDC sailboat motor program in 5 steps.

The criteria we listed in the prior blog is the starting point for our 5 step electrical BLDC sailboat motor program. With this electrical BLDC sailboat motor program we translate our criteria into technical requirements. Summarizing our priorities/criteria :

1.) 20 kn miles autonomy

2.) same boat speed as current diesel engine

3.) easy access of components for quick replacement

4.) standard product components

A high priority is the autonomy of 20 nautical miles. Because this depends mainly on the seize of the battery pack we decided to put that priority aside for now. First we start with the power needs.

Step 1.) What is the power and torque of the current diesel engine at the propeller?

Most diesel engine boat motors come from other applications such as trucks, tractors and lawnmowers. Our Nanni diesel 2.50He is a rebuilt lawnmower.  The power and torque indicated for diesel engines is always at the crankshaft, and is very different from the power at the propeller!  The graph below shows, in dark blue, the power at the crankshaft and, in light blue, the calculated power at the propeller. (information from Nanni Diesel)

Barcelonasail standard rpm settings are around 2600 RPM. From the above graph we learned  that we need between 4hp and 7hp at the propeller.  Literature studies, online articles, https://lynchmotors.co.uk/technical-reports/boats/electric-drives_boats.html  and existing electrical engine sailboat solutions, all confirm that a 5kW electrical BLDC engine is sufficient.

The torque at the propeller depends on the gear reduction. Torque at the crankshaft on the excising diesel engine is 30Nm and a standard reduction of 2.6 implies that the torque at the propeller is around 60 – 70 Nm. This is without assuming losses. The losses are roughly 50% and comes close to 30Nm at the propeller. We received information from engine makers that 30Nm at the propeller is sufficient for boats up to 4500kg.

Step 2.) Select electrical motor + controller

With this information we start selecting an electrical BLDC motor, during which we learned a lot.

1.) The efficiency of electrical BLDC sailboat engines depends on the torque that is required. See the graph below. As the torque increases the efficiency drops.

2.) BLDC motors can deliver a high torque and power at a low RPM (1000). Such an electrical motor is bigger and more expensive than an electrical motor that produces torque and power at high RPM, 35.000 for example.  High RPM motors are found in the aviation industry, they are very light, small and cheap. Optimal propeller RPM is below 1000, this means a gear reduction of 35 in case of high RPM motors, creating losses.

3.) Electrical Motors tend to operate at with maximum efficiency (90%) near the rated power. For example a 5kW BLDC motor works most efficient at a 4.5 kW input power setting. We will not require that much power most of the time. At cruising speeds we suspect to consume 2kW input power, see graph below. For BarcelonaSail high efficiency at 50% of the rated power is more interesting than high efficiency at the rated power.

4.) BLDC electrical motors between 3kW and 5kW cost between 150 euro and 1000 euro.

We are currently still researching electric BLDC motors for our sailboat motor program but we have defined the following:

The electrical motor must have a high efficiency 90% at 2kW input power, have good torque at low RPM, and must be economically priced.

A BLDC electrical motor requires a separate motor controller which should be supplied by same brand as the  electrical motor supplier brand.

Step 3.) Select power pack – batteries

This section may be getting a bit technical and I will dedicate one blog post on the selection of batteries and electrical sailboat systems in general. Batteries come is all seizes and materials. BarcelonaSail requires batteries that work, are tested with a proven track record and have a long lifetime, with smallest possible affect on the environment.

In general there are two battery options: Lithium-Ion batteries and Lead-Acid batteries. The batteries that you have in your car are Lead-Acid batteries. Lead Acid batteries are the least environmentally friendly, heavy, have a limited lifetime and are very heavy. Calculate with at least 600kg of Lead Acid batteries to get some autonomy.

BarcelonaSail selected the lithium-ion batteries because of the environment and weight. We used the following technical criteria to determine the right lithium-ion batteries :

• Charge and discharge rate, or C-rate,  (C2 = the batteries rated capacity is provided in 30min.)
• Price
• Weight
• Stable, not explosive
• Long life time
• Memory effect (how many cycles before the battery looses power capacity)
• Self discharge  (what is the power after 3 weeks no sailing)

Lithium Iron Phosphate batteries, LiFePO4 came out best for our usage. Lithium Iron Phosphate batteries, the so called LiFePO4 (3.2V/100Ah) have a long life time, high discharge rate and are stable. Besides the memory effect and self discharge rate of the selected batteries really good.

Step 4.) Select solar panels, battery charger and battery controller.

Samba is a very light regatta sailboat and therefor normal solar panels, that you see on homes, do not fit the sailboat. BarcelonaSail uses two flexible solar panels (200W) of each 2 kg that we store inside the sailboat when sailing.

Solar panels provide relatively little power compared to the designed battery pack for the electrical BLDC motor.  The main power comes from the propeller that functions as a generator when we sail. We estimate that 10 min motoring requires one hour sailing for recharging the batteries.

Battery charge controllers and battery controllers are different parts of electrical system, see image below. Battery charge controllers control the charging of the batteries, in other words, making sure the batteries do not get overcharged. The battery controllers match the electrical demand from the electric motor with the battery capacity.

Step 5.) Design, and drive (propulsion) system

BarcelonaSail attained Teixido Harrold for the design and production of our electrical motor system. We know now that we want an electric propulsion solution, but the drive system is not that clear. Four options we investigated early on:

1. Shaft drive, using the existing shaft and propeller
2. Sail-drive
3. Pod-drive
4. Outboard drive

Next blog post we look into the pro’s and cons of each drive solution. With the electrical BLDC sailboat motor program we defined the technical requirements and now we can start making a selection of products.  Before we will have to agree on the drive system.