Sailing Advantage!
Electric Sailboat Outboard Motors

Is it right for you?
Understand your goals

First, clearly define your goal: the purpose of the motor and your sailing habits. This is very important because, unlike gas or diesel motors, “refueling” (recharging batteries) may be an over-night process and require advanced planning. These electric outboards are mostly designed to be trolling motors and not speed-boat motors. A trolling motor is good for low speed, instant access boat propulsion.

Do you intend to use the motor just to get in and out of the marina; to pull away from the dock, or mooring? Should it simply have enough power and running time to get you and your boat out to open water, so you can enjoy a day under sail and then, at day’s end, motor your way back to the dock? Will you only need the motor for an hour or two or three before recharging the batteries? Good! This is an excellent use case for an electric sailboat motor.

Or, maybe you want the security and confidence of knowing you have instant access to auxiliary power when needed. Quickly get out of a dangerous situation; avoid flotsam, rocks, or another boat in tight quarters and limited maneuvering space. The instant "on" and quick change from forward to reverse of an electric kicker can be a lifesaver! Again, this is a perfect use case for an electric sailboat motor.

Are you often impatient and want to get somewhere in a hurry because sailing just isn't fast enough? Well, if you want to use the motor for an extended time, or to increase boat speed, then electric may not be the best solution for you.

Or do you want to spend substantial time motoring out to a distant sailing destination? Maybe you want to get out of the marina before dawn and motor your way to a local island or bay. Then after a pleasant day sailing, motor back to the marina at night. You may need six or more hours of battery running time. So, unless you plan to spend the night and recharge your batteries with shore power at a guest dock before returning home, an electric outboard may not be ideal for you.

Generally, an electric outboard motor is not ideal if you plan to use the motor for prolonged periods of time, or in rough seas, into strong wind, or against strong current. Electric outboards do best in calm, protected areas such as in and around a marina or protected cove. Be sure to quantify your required number of hours of running time before battery recharge and consider the wind, waves and current environment where you plan to use the motor.

With an Electric Motor
How fast will my boat go?

 How fast will my boat go with an electric outboard auxiliary engine? Let's consider the two basic types of boat hulls: planing and displacement.

Planing Hulls: Boats with planing hulls are designed for speed. As they go faster, the bow starts to rise and then nearly the entire boat hull starts to glide on top of the water. With enough power and speed, these boats have very little water resistance. They act like displacement hulls when at rest or at slow speeds but climb towards the surface and “plane” as they move faster.

Displacement Hulls: Boats with displacement hulls move through the water by pushing the water aside. As the boat pushes water aside, it creates a "bow wave" cresting just in front of the bow. As the boat goes faster, the bow wave gets longer, moving the wave trough amidships and making another crest at the stern. At "hull speed" the boat would have to climb its own bow wave to go any faster.
Most single-hull sailboats have displacement hulls, allowing them to move smoothly through the water, but at slower speeds than those with planing hulls. Other than when the boat is planing, or under some circumstances “surfing”, a displacement hull sailboat is limited to its “hull speed”. Calculate your boat's hull speed with this formula:
hull speed (knots) = 1.34 x the square root of the waterline length (feet)

Examples of boat Waterline
Lengths and Hull Speeds:
  10 feet     4.2 knots   (4.8mph)
  15 feet     5.2 knots   (6.0mph)
  20 feet     6.0 knots   (6.9mph)
  25 feet     6.7 knots   (7.7mph)
  30 feet     7.3 knots   (8.4mph)
  35 feet     7.9 knots   (9.1mph)

With an electric outboard motor, or any kind of auxiliary motor, boat speed depends on the hull type, waterline length, and total displacement weight (including passengers, food, and baggage), as well as the motor thrust. Speed factors also include the waves, current, and wind, relative to your heading. Sailboats with high freeboard are more affected by wind. With a typical displacement hull sailboat, even without adverse weather conditions, exceeding 80% of hull speed should not be expected.

Examples

There are so many different kinds of sailboats. They vary in hull design, length, weight, cockpit and cabin styles. These pages are designed to help you understand the options, design principles, and reasons to choose from among the many electric motors, batteries, chargers, and mountings. This site is focused on electric outboard motors for sailboats up to 27ft, and up to 5,000 lb. displacement.

Before we get started on the various choices, here are some examples to think about as you move through the process:

Example One: For your 18ft sloop, with 1800 pounds displacement, you chose an electric auxiliary motor with 55 lb/thrust, drawing 55 amps with 12 volts (one battery).

If you get a 12-volt battery rated at 75Ah, then you might expect about an hour and a half running time at full power. And, yes, for these calculations you should always assume running the motor at constant full power, just to be conservative with running time expectations.

You should get a 2-bank charger so that you can charge the motor battery as well as your "boat systems" battery with the same charger at the same time.

Example Two: Same 18ft sloop, but you decide to get a more powerful motor. You chose an electric auxiliary motor with 80 lb/thrust, drawing 80 amps with 24 volts (two 12-volt batteries).

If you get two 12-volt batteries rated at 90Ah, you still might expect about an hour and a quarter running time, or more because you won't need to run the motor at full power all the time.

You should get a 3-bank charger, so you can charge the motor batteries as well as your "boat systems" battery with the same charger at the same time.

In the second example, although you can expect pretty much the same speeds under ideal conditions due to hull-speed limitations, the 80 lb/thrust motor has some "power-to-spare" and will be more responsive in urgent "must move" circumstances. Also, you can take that motor with you when you move up to a slightly larger boat. Remember, you can never have too much power, so get the largest motor you can reasonably fit into your boat and budget. Consider all aspects: power, weight, cost, and your sailing objectives.

Review

REVIEW:  We are sailors because we enjoy sailing! Having a motor is a convenience to be used when necessary to avoid dangers or to easily leave and return to the dock. The noise and pollution associated with gas or diesel motors seems counter intuitive to the joy of sailing. The advent and recent improvements in electric outboard motor design provides us with a welcome alternative to combustion engines.

So, after reading and considering numerous postings by sailors sharing their experiences, as well as my own experience, I have come to these conclusions:

1.) There is no single rule or equation that clearly and accurately compares electric outboard motor power equivalents to gas or diesel motor horsepower ratings.

2. Personal preference for power varies among sailors.

3.) More power is almost always better.

4.) More power is a trade-off due to increased weight of the motor and batteries.

5.) Buy a motor rated for saltwater use, even if you don't plan on using it in the sea. It will be more resistant to corrosion and, besides, you never know when you might take it to the ocean! Why limit yourself.