Battery Size and Range Calculations for 33′ Electric Sailboat

Battery Size and Range Calculations for 33′ Electric Sailboat

One common question I hear when talking about repowering sailboats to electric, is “what kind of range can I expect using just the motor, for moving around without using the sails?”

The answer to this question is probably most easily understood by just walking through an example from our very typical 33’ sailboat we use for system testing.  From there, variations can be made, considering different types of boats, and larger or smaller battery packs.

On our Hunter 33 sailboat (for which we have tons of data available), we can talk about a few use cases of sailing around Southern California, in Newport Beach and Long Beach harbors, and at Catalina Island.  With the sails down, and just using the motor, the power needed to move the vessel at various speeds is shown in Figure 1 below.

Graph of Power vs. Speed for Hunter 33 Sailboat

Our test vessel has a 10 kWh LFP battery dedicated to the propulsion system. Since we never discharge the LFP below 10% of its total capacity, our 10 kWh battery actually has 9.0 kWh of usable energy in it when fully charged.

Using some simple arithmetic, we calculate the run-time and ranges for given speeds.  Below is a table of calculations showing speed, power, and motoring times for the Hunter 33 sailboat, with three different LFP options, 10, 20 and 30 kWh.

Power and Range Estimates H33

Table of expected ranges and motor run-times at various speeds for different battery sizes

Again, the calculations in this table are for hours of motor run-time with no sail-power being used at all, we’re only using the battery powered motor here.

When motoring for a long distance, particularly without wind, I’ll typically use a cruising speed to about 60% of hull-speed or about 4.5 knots or less, to conserve battery power while still moving relatively fast.   When the wind is blowing strong, I’ll set the throttle so that the motor becomes a generator, and capture a few hundred watts of power through the process of prop-dragging.

An important thing to recognize is that since your range, or motor run-time, increases so dramatically as the speed of the boat is reduced,  we can always simply go slower to extend our range if it is really needed.

In a couple blog posts here, I’ve talked about some recent sailing trips I’ve taken to Catalina Island from Long Beach in our Hunter 33, which is currently outfitted with a single 10 kwh battery pack.  During those and similar trips, I’ve never had a serious concern about running low on battery power, because I always consider how much battery power I’ll need as I continue my journey.  Also, the EVMS cockpit computer does of great job of providing estimates for run-time and other information about the power being used, the vessel speed, and it displays the current and historic information about the system in general (including current flows, amp-hours remaining, etc.).

Sailing electric in 2022 is completely practical, and totally fun. The charging infrastructure is already built into just about every slip in Southern California, which will keep the LFP battery charged and ready to go, and it’s free (or included in your slip fees). Sailing electric is not only environmentally friendly, it costs less.

Walt White

Founder and lead systems engineer, working to bring new products to market that make a positive impact to the environment. Our products and services help boats make the switch to battery electric propulsion, harness renewable energy, and solve other challenges involving marine electric propulsion systems.

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