PLUG-IN BOATS A Primer for Converting to Electricity

Excerpt from Tom Hruby's new book

Preface

Boats were propelled by electricity decades before gasoline and diesel engines appeared.  The first electric boat was the Elektrokhod that sailed on the Neva River in St. Petersburg in 1838.  It was not very successful because the batteries at that time gave off too many fumes (Desmond, K. 2017. Electric Boats and Ships – A History).  By 1881 however, battery technology had improved and Gustave Trouvé of France patented an electric boat and the first outboard motor.  Remarkably, the first outboard was electric rather than gas, diesel, or steam.  The back cover has pictures of two of Trouvé’s electric boats.

Gustave Trouvé’s electric outboard (from George Barral, L’Histoire d’un Inventeur, 1891)

Unfortunately, with the advent of inexpensive gasoline, the interest in electric boats waned, and only recently has the interest been increasing.  Today, there are several companies developing and building electric boats.  There is also a renewed interest in doing your own conversion, but the information on how to do this is scattered and hard to find. 

The purpose of this book is to provide some of the basic information to get you started on your own conversion.  It is based on my 25 years of building and running three electric St. Pierre Dories.  By sharing what I have learned over these years I hope to encourage you to try it, and avoid some of the mistakes I have made.

Chapter 3: How much power do you need?

Given the large difference between how far one can travel on a kilowatt-hr of electricity vs. a kilowatt-hr of  gasoline, we need to look at new ways to calculate how much power we need.  The discussion below assumes speeds that are less than or equal to hull speed in a displacement hull. Converting a semi-displacement or planning hull to electric requires a different set of assumptions and calculations.  These will not be addressed here.

If you search the web for information on how much power you might need, you will find a wide range of answers and all of them are for gasoline or diesel engines.  I found numbers that range from 1 hp to 15 hp per 1000 lbs of displacement.  In addition, the confusion is increased because there are several different ways horsepower is presented in combustion engines for boats (rated hp, brake hp, manufacturer’s hp, shaft hp).  To find equations that will estimate power requirements for electric motors one has to look for equations that calculate shaft horsepower.  Shaft horsepower (SHP) is the power delivered to a propeller shaft, or conversely, how much power is needed at the shaft to move a boat a given speed, regardless of the type of motor used.  This eliminates having to account for losses due to transmission, combustion, heat loss, engine efficiency, etc.  Shaft hp can be easily converted to watts since 1 SHP = 750 watts.

All simple equations that estimate power needs for a boat are approximations because there are too many factors involved in calculating a precise value.  A motor must continually overcome the resistance of water that changes with wave height and temperature, and air that changes with wind speed.  Calculating a precise value for resistance is a costly endeavor that requires computer modeling and tank testing of models.  Over the years, however, a number empirical formulas for predicting power needs and speed have been refined.  These formulas take into account basic factors such as hull type, shape, and total weight.  When used with common sense they can yield fairly accurate estimates.  One such empirical equation that has been successfully used for over 30 years was developed by Dave Gerr in his 1989 Propeller Handbook (International Marine, 1989, 2001).  The equation gives boat speed as a function of power, but it can also be used to estimate power required as a function of speed.

The basic Gerr equation is:

SL ratio = 10.665/∛(LB/SHP)

Where SL ratio (speed to length ratio) = [knots/ (√LWL)]  

LWL = length of waterline in ft

LB = weight of boat in lbs

SHP = Shaft horsepower (1hp = 750 watts)

The graph in Figure 8 compares actual power-speed data for my boat (4000 lbs, 19 ft WL and two Minn Kota® trolling motors) compared to the estimates provided by the Gerr equation.  The graph shows mph rather than knots because those were the units of my GPS. 

Figure 8: Speed as a function of power for my 26ft. dory. (If you are good with a spreadsheet you can develop a graph for your own boat.)

If one uses the standard equation for hull speed in knots [kn] as HS = 1.34 x √(WL) then the equation yields the following power requirement: 1490W are needed to propel 1000 lbs of displacement to hull speed.  This value is independent of the length of the waterline, as a longer boat will take less power to move at a given speed than a shorter boat of the same weight.  Experience, and the equation, also tells us that much less power is needed at speeds that are lower than hull speed.  To achieve 50% of hull speed only requires 185 watts. The table and figure below summarize the data from the Gerr equation.

Figure 9: Power required per 1000lbs as a percent of hull speed.

For more information you can find Tom’s book for sale at Duckworks.com.