Electrical Design Overview

The range of an electrical or hybrid powered boat is determined by the same rules and principles that apply to a boat, which governs the resitance or drag it creates in the water. Then the size of the motor, or more specifcally the power, which dictates how fast the boat can move. And the fuel tank(s) or battery packs.

All of these affect each other and usually finding the right balance is the recipe for a boat with good motoring capabilities. It’s a question of having the right size engine and fuel tank for a certain sized boat. If the enigne is too big, it adds unnecessary weight and uses an un-economical amount of fuel, which leads to a shorter range or having to add more fuel that negates the effects of the larger engine.

Boating people have grown to be accustomed to expect and accept certain performance envelopes and limitations from boats. People await to be able to motor 60-100nm on a high power planing motorboat, four hours at high throttle. But would accept that on a daysailer iwth an outboard and a 15l portable fuel tank you would maybe be able to motor 30nm – six hours at five knots. The type of boat, its size, engine power and fuel tank capacity dictate all of this.

For a boat electric propulsion the basic principles are exactly the same. The power of the motor is typically stated in kilowatts (kW) and the size of the fuel tank is expressed either in kilowatt-hours (kWh) or amps and ampere-hours instead of horsepower and liters.

To be objective in this white paper, we first have to accept some facts. One fact is that a liquefied fossil fuel (gasoline, diesel or liquefied gas like propane or LNG) is an extremely efficient way to store energy in a compact form. The amount of energy stored in a kilogram of diesel fuel is 35800Kj compared to a standard marine lithium battery which would be able to house anywhere between 900 to 2630Kk/kg. The drawback of this is that burning one kilogram of diesel fuel has a significant environmental effect, while you can charge the lithium battery thousands of times.

The key benefit of the battery pack is that it can be charged with a renewable enrgy source onboard like panels, a wind generator or if a modern electric motor is used then by using the motor as a generator while sailing.  This enables an energy autonomous operation of the boat in suitable conditions.  And when the boat is in a harbour the batteries can be charged from the shore power rapidly.

Again, to be objective we must accept that a pure electric boat has range limitations.  This is determined by the size of the battery pack, which is economical to carry onboard in two senses: weight and cost.  Typical battery packs installed in today’s monohulls are in the range of 6-16kWh.  This enables the boat to motor 4-10h at an average speed of 4-7kts.  Meaning that a boat will have a practical range of 30-40 nautical miles with battery power alone.  The graphs further down explain this in more details.

You have probably at some time in your life rode a bicycle. Notice how after a certain speed pushing even a little bit more requires significantly more energy than the relative speed gain? That’s because the resistance of the bike to airflow builds at a square of the speed. The exact same principle applies to a boat in the water. The graphs represent a typical sailboat that is 10m long, 3.3m wide and weighs 5000kg. The equations are standard marine engineering calculations and have nothing to do with electric or hybrid power; they will also apply for a diesel engine or an outboard.

As the ‘Power requirements’ chart shows, when the hull speed is around 7 knots, it requires 8kW of power to move the boat and a very economic speed is somewhere around 5 knots where you need 2kW to move the boat in flat water. The increase of power from 5kW to 10kW only yields a knot of more boat speed and the next 5kW increment only 0.5 knots.

This brings us to the discussion of range. ON the ‘Range’ chart you will see range calculations with three different battery pack sizes for the same boat. You witness the same effect of resistance building up when speed grows, this of course means that with less speed you have less resistance and you have longer range.

There are two ways to gain more range in a boat. You must stop and charge the batteries. Or you must go for a hybrid solution. Meaning that you must have an alternative source of generating power onboard.

Today, the only real alternative source of generating enough power (for example 2kW) to enable you to motor 400 nautical miles if you are stick in the middle of a massive high-pressure system is to have a generator onboard.  You might rightfully ask, doesn’t this defeat the whole purpose of going electric in the first place?  The answer however is not that simple.  First we must take into account the usual usage profile of a boat.  Most boat owners motor in and out of the marina using the engines maybe 0.5-1 hour before setting sail.  If the wind calms they might motor 4h back to a marina.  This is not an issue for an electric motor and any size battery pack.  They use the engines for extended periods only in calm seas when sailing is impossible.  Or in some special circumstance where sailing for some reason is not a practical solution.  The second thing to understand is that a modern marine generator is not some horrible noisy, large bulky thing.  They are engineered to be lightweight, compact and very silent.  A typical marine DC generator will produce about 60dB (equal to the noise level of a normal conversation) at a 1 meter distance without any sound insulation.

A key driver for hybrid yachts is the fact that the generator is much more fuel efficient than a diesel engine.  Fuel efficiency comes from the fact that the generator is built to operate at an optimum 2800-3200 RPM range.  A diesel engine in a boat usually runs at 1500-1800 RPM, which is a very inefficient operating speed for a diesel motor.  In a typical real-life scenario, owners notice that they are able to motor roughly three times further with the same amount of fuel, when using the generator and electric engines, compared to only diesels.  This is exactly the same principle that is applied in today’s shipping industry – all modern ships have diesel generators powering electric engines due to this efficiency gain.

Information provided by Oceanvolt.

Electrical Graphs

Hydrogeneration Estimate for Twin Saildrive 15 & Twin ServoProp 15

Resistance Prediction and Electrical Power Estimator for Catamarans 30-60 feet.
V2.1 (2/4)

Resistance Prediction and Electrical Power Estimator for Catamarans 30-60 feet.
V2.1 (3/4)

Resistance Prediction and Electrical Power Estimator for Catamarans 30-60 feet.
V2.1 (4/4)

Electrical Specifications

SD15 Dual Propulsion System

Dual 15kW SD15 motors with a combined power of 30kW. Motor system includes: Motors, Sail drives, Motor controllers, CAN-cables and Displays. Excluding cooling kit.

Gori 3bl Propeller 16.5×11 LHS

Goris 3-blade 16.5×11 left hand propeller for sail drives.

Oceanvolt Dual Control Lever

Dual control lever. Top mount application.

Oceanvolt Cooling Fin


Oceanvolt Cooling Kit

Including pump, expansion tank, clamps, valve and hose connectors 19mm.

Valence 17.5kWh 48VDC Battery Bank

10x Valence U27-24XP 24V modules. 2x Lynx distributors and 8x mega fuses included. BMS not included.

Valence 600A 48VDC BMS

Max. consumption 28.8kWh. One BMS mandatory for each battery bank.

Fischer Panda AGT-DC 15000-48V

With Oceanvolt FP control.

Fischer Panda Premium Install Kit with Exhaust and Water Separator


Victron Energy Skylla TG 48/50

2.8kW charger with custom Oceanvolt configuration for the 48 VDC battery bank that will charge the battery bank from shore power.

Victron Energy Skylla Control

Remote control panel for the Skylla chargers.

Victron Energy Colour Control GX

The Colour Control GX is a system monitor that allows remote access.  With custom Oceanvolt configuration.  Wi-Fi and SD-card.

Oceanvolt DC/DC Converter 48/12V 30A

360W Oceanvolt DC/DC 48V/12V 3-0A Charger converter with lithium charging logic.

Victron Energy Lynx Distributor

Lynx distributor. Including 4x Mega fuses. 

Optional accessories

Victron Energy Multiplus 48/3000/35-16

2kW combined charger/inverter with custom Oceanvolt configuration that charges the 48 VDC battery bank from the shore power and provides 230 VAC power from the 48 VDC battery bank.

Victron Energy Multiplus 48/5000/70-100

4kW combined charger/inverter with custom Oceanvolt configuration that charges the 48 VDC battery bank from the shore power and provides 230 VAC power from the 48 VDC battery bank. With custom Oceanvolt configuration.

Victron Energy Digital Multi Control 200/200A GX

Remote control panel for the combined charger inverters.