Above the Waterline

Marine Outfitter

805 455 8444

Case Study -  Kapowai

Kapowai is an all-electric Catalina 30 powered by a Vetus 2.2 KW motor.  

All electric sailboats are limited in range by their battery bank size.   I recommend all-electric to daysailers, although there are circumstances where people have cruised extensively with all-electric boats.   The key being able to live within the energy budget available.

The all-electric boat recharges it's battery bank with shorepower, solar, wind,  and regeneration.   Shorepower can recharge any voltage battery with the right charger.     Two 55 volt solar panels in series produce 110 volts and can effectively charge a battery up to 84 volts.   The highest voltage wind generator is 48 volts.   Regeneration can recharge any voltage battery with the right controller.

Higher voltage translates into higher efficiency because less amperage is required to produce the same amount of power.   Remember that Power = Voltage x Amperage.

To calculate range you have to remember that a battery that is quickly discharged is far less efficient than a battery that slowly discharges.    If you double the size of your battery bank, you more than double the boats range.    If you motor slowly your range will be far greater than if you try to motor at hull speed.   Most batteries amp hour ratings are based on a 20 hour discharge.     For example of battery of 100 amp hours will have 100 amp hours available so long as it is only discharged at 5 amps.   If you try to discharge the same battery at 100 amps it would be lucky to have 50 amp hour capacity.   The best way to calculate range is to first calculate the size of your batteries in kilowatt hours, then divide by the power rating of your motor.   But remember to factor in efficiencies of your battery bank if you are drawing down faster than the 20 hour discharge rate.

Monohulls are better suited to have large battery banks than multihulls.   The extra weight, if placed low and central on the boat can enhance the sailing characteristics of the boat.

Multihulls regenerate far quicker than monohulls because of their speed but are very weight sensitive.

Regeneration is the ability of the boat to recapture energy from the propellor while sailing.

Bigger battery banks also result in higher power ratings for your motor.    When you apply a load to a battery the voltage on the circuit drops.    The more the voltage drops, the less actual power is being produced.   If you apply a 100 amp load to a 4D battery it might drop the voltage of the battery to 11 volts and you will get power of 100 amps x 11 volts or 1.1 KW.   If you apply that same load to a battery 10 4 D batteries and the voltage drops to 12.7 volts then you will achieve power of 12.7 volts x 100 amps or 1.27 KW.  

Kapowai is a Catalina 30.   She is powered by a 2.2 KW Vetus inboard electric motor running at 24 volts.   She has 6 - 8D batteries and 4 - 4D batteries for a total of 22 Kilowatt hours.   She achieves 4.2 knots top speed in calm seas and no wind.   Admittedly she is underpowered because she cannot achieve hull speed.   However because of the large battery bank and small motor she is capable of motoring for 10 hours and achieves a theoretical range of 30 knots in calm seas.   Because seas are rarely calm, and because we are not using the 20 hour discharge rate, but rather the 10 hour discharge rate, her practical range is  20 - 25 nautical miles.

We successfully cruise from Santa Barbara to the near side of Santa Cruz Islands.   It is my philosophy that a boat should be able to return home on motor power, in case of emergency.   We sail to the islands and keep our energy for the return trip if necessary.    Since we are looking at expanding our cruising grounds we have come to the conclusion that we will need to add a DC genset and Kapowai will become a Diesel Electric Hybrid.

The ultimate point of sailing for an all-electric sailboat is motor sailing.   In light airs applying a small amount of energy to the propellor produces apparent wind on the sails and all of a sudden you are scooting along at 2 or 3 knots and the boat next to you is not moving at all.   It is not like motor sailing with a diesel, because you can't hear the motor or feel the motor's vibration.     There are so many benefits to an electric sailboat - once you own one you can't go back.

Another major benefit to an electric sailboat is navigating in and out of the harbor.   If you are on a diesel you have to start it up, let it idle, then motor in and out of the harbor.   The motor is loud, it vibrates and it emits dangerous exhaust that inevitably you breathe.   Your relaxation does not begin until you turn the motor off and start sailing.   With an electric boat your relaxation starts from the time you step foot on her and continues throughout your entire voyage.

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Email James at       or telephone 805 455 8444

Above the Waterline, Ltd.,    6 Harbor Way # 226, Santa Barbara, CA, 93109, USA