S/V Barbara Ann

Hybrid Propulsion Design

When we started our systems refit, I thought that the time was right for hybrid technology to meet sailboats.  I decided to convert SV Barbara Ann to electric hybrid propulsion. I had a couple particular goals in mind.  My goals turned out to be different from the goals of many of the hybrid boat projects I’ve found. It’s really important to understand your goals BEFORE you start a project like this because you might find, as I have, that your goals may be different from the norm and actually simplify the job for you.

In short, you’d better know what the hell you’d like to end up with.  In my case I wanted to move the weight in the boat forward to improve the trim; I wanted to be less dependent on diesel; and I wanted make the boat more environmentally friendly because it’s a good thing to do.   And I love new gadgets.

Our trim problem was a result of having to change our mast a few years ago.  The replacement carbon fiber mast and boom is much, much lighter and we discovered after it was installed that the boat is now a bit heavy in the rear.  The reason the old mast broke was mostly because of defective laminate, but I think we aggravated the problem by sailing through a hurricane.

One of the big benefits of going electric is that your power plant may be placed anywhere that’s convenient.  In our case, we are replacing our 5kW AC genset and our 100hp propulsion diesel with a single 34kw diesel powered generator and placing this sufficiently forward to correct the trim problem.

Following the Leaders


Several electric boat companies have advocated very small electric drive motors as being enough to match the performance of much larger internal combustion drives.  This is very misleading.  The two kinds of motor are just different.  If you only need your top power for a very short time with electric, you can probably get by with an electric motor that’s about a third to a half your required shaft horsepower at hull speed. The Barbara Ann needs 70 sHp to drive her at hull speed.  So I should be able to do this for a short time with a 16kw (20 Hp) electric motor, which is the size many electric boat companies would recommend. HOWEVER, if I want to run continuously at hull speed then I need 50kW. 

With a diesel motor, you just plain need the peak horsepower to get to top speed.  So I’d need the 70 Hp motor if I ever want to get to hull speed.

Electric powered sailboats have been around in a limited way for a while.  Electric powered ships and submarines are the norm.  Regenerative power, at least for braking, is in every Prius. The car companies are the ones investing to create the technology for hybrid power.  I wanted to follow them, and hopefully not into bankruptcy,  using the parts they’ve spent so much to develop.  Suitable propulsion motors are readily available but it’s still early in their development and you have several very different approaches to pick from.  There are zealots promoting all these different approaches as the one true way.  Each of these folks promotes himself as the only person on earth who knows how to do this the one true way.  Don’t believe.  There are many ways to skin this cat. 

The Tradeoffs

Sailboats have a different set of tradeoffs than an automobile or a powerboat. Medium or heavy displacement boats have different tradeoffs than a featherweight racer.  The first really important tradeoff is that going green in a boat means not powering at top speed.  The Barbara Ann requires about 70 horsepower at the propeller shaft, or 50kW, to move her at her hull speed of 9.3 knots.  Displacement boats, those that don’t plane over the top of the water, are limited by how much energy it takes to push the water out of the way.  The amount of power that it takes to get close to the calculated hull speed goes up almost exponentially.  It’s sort of like breaking the sound barrier.  If I compromise on speed and I’m willing to live with one knot less under power, or 8.2 kts, then the 10,000 pounds of thrust force required drops from 70 to 48hp.  If I’m willing to have a maximum continuous motoring speed of 7.2 kts, then I only need 30hp. More importantly from the “going green” standpoint is that the extra two knots of boat speed doubles my fuel consumption.

When you are repowering with an electric motor turning your prop, you can get a bit of this compromise back, especially if you are doing hybrid electric instead of diesel electric.  Some definitions are in order here. 

A lot of ships are electric powered.  All submarines are.  Both use a diesel driven generator to make the power and then an electric motor to push the boat.  This alone makes good sense for a lot of reasons relating to efficiency, engine placement and redundancy.  This is diesel electric. 

World War II submarines were hybrid electric.  They used diesels to charge a battery bank and then powered off the batteries while submerged.

Most hybrid electric cars are also regenerative.  They use a generator (or plug in periodically) to make the electricity and use a large bank of batteries to store the electricity.   When these cars are braking, the drive motors run in reverse to re-generate more electricity to be stored in the battery banks.

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Bill & Barbara Southworth
39 Pickering Street • Portsmouth NH 03801 USA
Cell: (617) 905-6800 or (617) 905-6803 Fax:(888) 300-8888