ABYC Article Part 1

This article is published in the ABYC journal “The Reference Point,” Spring 2012 issue.  Part 2 will be published in a future issue.

HYBRID PROPULSION

Part One

by James Lambden, ABYC Member

Long time readers will recall that a James Lambden article appeared the inaugural issue of the Reference Point as a counterpoint to the then recent 100th anniversary of the marine engine.  James has continued to focus on the benefits of alternative energy for marine craft and provides an updated report on 2012’s viable hybrid option.  When one considers the advancement of hybrid automobiles during the past 5 years, the marine industry has some catching up to do.  Perhaps another title for this article could have been “Marine Goes Green!”

Hybrid propulsion enhances life aboard a sailboat. The word is spreading quickly and hybrid propulsion is becoming the fastest growing form of propulsion for sailboats today. Last year there were more vendors at the San Francisco – Strictly Sail Boat Show selling electric propulsion motors than there were selling diesel motors.

Electric propulsion for boats has a rich history going back over a century, preceding propulsion by diesel and gas engines. Over the last 115 years, hybrid propulsion has been used to power submarines, and pleasure boats owned by electrical hobbyists and some very wealthy individuals. Today the affordability and technical advancement of motors, controllers and batteries is creating a revival of this old and storied industry.

The reasons for replacing an internal combustion engine with an electric motor are compelling. For day sailors there is no better option. The cost is less than a diesel engine of comparable performance and, as an extra bonus, control is improved. For coastal cruisers, large battery packs with solar or wind generation can provide the necessary and improved range. For cruising sailboats, a hybrid diesel-electric system allows for long motor-sailing times and short engine run time. Reducing the run time of the iron genny is on every sailors mind, and there is no better way of accomplishing this than with hybrid propulsion.

 

The Hybrid Advantage

Adding less than one half of a horsepower to the propeller of a 30 foot sailboat sailing upwind in low wind conditions will double the boat speed, taking the vessel from 2.5 knots to 5 knots quickly. The friction of the driveline is overcome with as little as 50 watts and the propeller spins the speed of the water with no resistance. The next 100 watts moves the low pressure zone behind the boat, to forward of the propeller, and smoothes out the wake. The sails fill out in the newly created apparent wind and the boat leaps ahead like a finely tuned race boat.

Flipping a switch to add 7 hp to the propeller of a 30 foot sailboat in any condition, whether tacking, gibing, or going into a large waves, gives complete control of the boat to the boat owner. Power on demand can only be accomplished with an electric motor.

Powering quietly through a dense fog while listening for other boats, or waves upon the shore, can only be done with an electric motor.

The large battery packs and fast charging capabilities of a diesel generator look after house loads easily. By consolidating power generation of house and propulsion loads, increased efficiencies once again lead to less engine run time.

Every electric boater I know has their own particular reason why they love the experience of owning an electric boat. Some cite the lack of maintenance, others the ease of operation, or the quietness and similarity to sailing. For me it is a technical challenge to be able to power all of my boats systems from energy that naturally exists.

For many years I thought that electric motors were only suited for sailboats. That, too, is now changing. Advancements in the safety of DC systems allow higher voltage hybrid systems to be designed for displacement trawlers. A new electric speedboat you can water ski behind, is the must-have tender for large expedition yachts.

There is no other boat builder’s option, that offers more to enhance life aboard, than hybrid propulsion.

 

Electric Motor Fundamentals

It is essential for an installer to understand how the power equation (P = V x I) effects an electric drive system. To a motor, voltage is rpm, and current is torque. The motors have an rpm to voltage constant and a current to torque constant.

Current creates heat in a motor. A motor will overheat if overloaded. It is counter productive to run a motor hot – hot windings are less efficient due to increased resistance. The controller should monitor the internal windings of the motor and scale back current if the motor gets too hot.

Electric propulsion motors are used continuously therefore they should be rated continuously. A properly rated marine propulsion motor will give an operating temperature at load after one hour of operation. Although insulation on windings can be rated as high as 150 C, we do not recommend operating over 120 C and prefer that the motors do not exceed 100 C.

A motor only draws the amount of current that the system asks from it. If there is no load on the motor, the motor will draw very little current. How much current the motor draws is a function of the size and pitch of the propeller and the gear ratio. A properly balanced system will draw approximately the same amount of DC current feeding the controller, as AC current feeding the motor. This relationship between AC and DC current is determined by the gear ratio. The AC current feeding the motor goes up when the gear ratio goes down, because the load increases on the motor.

A motor draws less current at higher boat speeds at the same propeller rpm. This is counter-intuitive so we need to understand this clearly. As a boat speeds up, the propeller has less bight on the water because the speed of the propeller with respect to the water is less.

 

Electric Motor Sizing

For fair weather sailors, day sailors and lake sailors we recommend 1 KW per ton of boat displacement. For offshore cruisers that can encounter adverse conditions and lee shore situations, we recommend 2 KW per ton of boat weight. This is usually substantially less power than the recommended diesel motor. There are several reasons for this:

1. An electric motor has a very flat torque curve which can turn a larger, higher pitch propeller slower with far higher efficiency. An electric motor can produce the exact amount of power that the propeller requires in all conditions by varying the ratio between voltage and current.
2. A diesel motor needs rpm to create torque and has a hump for a torque curve, that doesn’t look anything like the torque curve of the propeller, which increases exponentially. The diesel is sized large enough to be able to turn the propeller at idle, and for the curves to meet at maximum rpm. The area between the curves is only utilized when the boat starts powering into heavy weather.
3. Many diesels don’t utilize their power because the propeller is not large enough and/or the reduction gear is not low enough. Customers have demanded larger motors, and in many cases, the extra power is never used. This unloading of the diesel engine is not efficient, and not good for the engine.
4. Diesels must also run the alternators and water pumps.

My own boat, a Catalina 30, has a 5.5 KW electric motor – about 7 horsepower. This motor replaces the standard 21 h.p. diesel motor. Top speed is 6 knots. When beating directly into 25 knots of wind and waves, the speed drops to 4 knots. I have never felt unable to control my vessel with this amount of power and have extensively cruised the Channel Islands off the coast of Santa Barbara. For some this may not be enough. What is important is the customer knows what he is buying, and what he might expect in the way of performance in both calm seas and inclement weather.

 

Types of Hybrid Propulsion

The advantage of a hybrid propulsion system is the capability of providing the very small amounts of emission free power that is typically needed most of the time, but also being able to deliver more than the power of a diesel engine on demand. All hybrids have water generation from the electric propeller turning while under sail and incredible electric motor-sailing capabilities.

There are three different types of hybrid propulsion:  Serial, Parallel and Diesel Plus Electric.  Serial hybrid propulsion has an electric motor dedicated to the propeller shaft and a separate diesel DC generator. The advantage of this system is being able to mount the generator out of the living spaces of the boat, and to have a powerful and efficient generator for house power.

The parallel hybrid has both the electric motor and the diesel motor on the same shaft. The advantage of this system is compactness. The electric motor acts as a generator when the diesel is running.

On the diesel plus electric, the boat has a large diesel motor on one shaft, and a small electric motor on the other shaft. A large alternator on the diesel motor recharges the propulsion batteries. The advantages of this type of hybrid are a redundant get home electric option and superior docking control over the boat.

 

Duty Cycles

The conditions that the boat ventures into determines the duty cycle of the diesel generator. When pushing into weather or escaping a lee shore in high winds, the duty cycle is 100 %. Harbor motoring can be done on batteries charged by shore power and the duty cycle becomes 0%. When motor sailing, a 30 foot sailboat requires several hundreds of watts to substantially increase the boat speed, and the duty cycle can be as low as a four hour generator run time for every two days of increased boat speed, about an 8 percent duty cycle.

 

Go Hybrid to Grow your Business

Hybrid systems can grow over time. As your customer wants to explore further distant shores, you can add solar, wind and a DC generator to the initial battery only package. As a business this is the best of news: a customer with a growth potential for the future that adds to the enjoyment of their boating experience.

James Lambden is a propulsion consultant and builds hybrid-electric drives for boats of all sizes. For more detailed information on electric propulsion technology and solutions, please visit www.electricboatdesign.com and www.propulsionmarine.com.

James Lambden
805 957 1797

 

Hybrid Propulsion – Part 1 (pdf) as published in ABYC The Reference Point