How To Choose an "Electric" Boat and Estimate Its Performance


By James M. Graham III


Choosing the best boat is a very personal process that involves a number of different, and sometimes opposing, factors. Each person has different ideas and preferences, but needs to understand that any choice is of necessity a compromise between function, performance, comfort, appearance and cost. This article deals with the issue of performance and is aimed particularly toward fishermen and others who wish to purchase a boat for use with an electric motor. I do not presume to tell you which boat to purchase; I only want you to be aware of what is involved so that you may make an informed choice.

The speed any boat will achieve is determined by a number of interrelated factors. The hull design of a boat, the weight to horsepower ratio, and the waterline length all have a bearing on the maximum speed the boat will reach.

Many boats built today are designed primarily as planing hulls and are quite efficient when they are powered by the properly matched gasoline outboard motor and are "up on plane." It may be quite a different story, however, when they are propelled by a low horsepower electric motor.

A planing hull is designed for speed and, when powered by a motor of sufficient horsepower, slides up and over the water with most of the hull out of the water, reducing drag and allowing high speeds. Weight is positioned toward the stern to allow the bow to be clear of the water.

When a planing hull is traveling at low speed, with the hull fully in the water, the width of the boat and the heavy stern produces a large frontal area pushing against the water, requiring a large volume of water to be moved out of the way for the boat to pass, and producing large waves. The deeply submerged transom also sucks water along behind it as it moves along, increasing the effective weight of the boat (the weight of the water being dragged), as well as creating more turbulence. This condition is seen when a planing hull is initially accelerated from a standing start; the stern "digs in", and a lot of power is required to get up on plane. At less than planing speeds, this hull design and weight distribution has much more drag than a displacement hull. The only electric boats that are capable of planing are very lightweight specialized racing boats that are impractical for other purposes.

The most efficient hull shape for displacement speeds is long and narrow, like that that of a canoe. This shape moves the water aside and lets it return behind the boat with the minimum of turbulence. Turbulence causes drag and this consumes power.

Many fishermen want the comfort of a wider, more stable fishing platform, rather than a narrow canoe, but want an efficient electric powered boat to fish restricted reservoirs. These fishermen should choose a boat that is relatively narrow (especially at the transom), has a flat bottom that is not deeply submerged at the transom, and has a bow that does not plow through the water at low speed. In general, a jon boat style hull with a gently upswept square bow is more efficient than a semi-vee. Weight should be distributed so that the bottom of the boat is parallel to the water surface when the boat is at rest.

The weight to horsepower ratio is the next consideration. The lighter a boat is for a given horsepower, the faster it will move. It is important to eliminate all unnecessary weight from the boat. When considering which boat to purchase, the hull weight is one of the important considerations. A boat without a lot of heavy decking is an advantage, both because it is lighter and also because it is easier to position the battery weight to achieve level flotation.

The last consideration is the waterline length of the boat. For a given weight of boat, and a given horsepower, the longer a boat is, the faster it will move. As a boat moves through the water waves are generated. The distance between the waves increases with the boat speed. As this distance increases, the stern of the boat drops into the trough between the waves so that the boat is climbing the bow wave rather than bridging across the wave train. A large amount of power is needed to increase the speed beyond this point. (This is the point that a boat must "get out of the hole" to get up on plane.) A long narrow boat has less frontal area so a smaller volume of water must be moved out of the way to allow the boat to pass. This reduces the size of the waves and wake, using less energy.

Be sure that whatever boat you choose has enough carrying capacity to accommodate the extra weight of the batteries you will be using. 12 volt deep cycle batteries weigh about 60 lbs. each, and 6 volt golf cart batteries weigh about 70 lbs. Remember that many boaters would like to have a boat that is larger, but very few are sorry they didn't get a smaller boat!

Speed Calculations

We now come to the calculations which may be performed to estimate the performance you may expect from your boat. These calculations are based on the assumption that the boat has an efficient hull shape, is properly balanced, and has a typical open propeller. A boat with a less efficient hull design will reach a lower speed. Boats with twin hulls (catamarans, pontoon boats) or with extremely long and narrow hulls will be faster.

Accurate assessment of the total loaded weight of the boat, including passengers and gear, and knowing the true developed horsepower of the motor are essential to the accuracy of these calculations.

The developed horsepower of an electric motor is found by:

  1. Measure the battery current (Amperes) while under load while the boat is underway.
  2. Multiply the current by the battery voltage to find the Watts.
  3. Multiply the Watts by the motor efficiency (about 65 to 70% for a series wound electric motor and 80 to 83% for permanent magnet motors) to find the effective Watts.
  4. Divide the effective Watts by 746 to find the horsepower.

The ducted propellers used on Reservoir Runner electric outboard motors are 50 to 70%
more efficient than open propellers, so you should multiply their horsepower by 1.6 to find the effective horsepower before performing the calculations.

Speed calculations are based on the speed to length ratio (SL RATIO) which is multiplied by the Square Root of the waterline length of the boat to give the speed of the boat in miles per hour. The following tables give the SL RATIOs for various weight to horsepower ratios (Pounds/Hp), and the square roots for various waterline lengths. For numbers that fall between the numbers on the tables, estimate the number.

To make your calculations, add up all of the weights of your boat and its contents (don't forget the weights of passengers, gear and the weight of water in the livewell). Divide this total by the horsepower of the motor. Look at Table 1 for the speed to length ratio (SL RATIO) corresponding to this weight to horsepower ratio, and multiply it by the square root of the waterline length of the boat as found in Table 2. The resulting number is the speed in miles per hour.

Speed estimates are generally accurate within about 1/4 to 1/2 mph, but may vary considerably more if the weight in the boat is not properly balanced or if the hull design of the boat has high drag at displacement speeds. Boats with multiple hulls such as pontoon boats and catamarans may be somewhat faster than the calculations.

Tables Used for Speed Calculations
Table 1 Table 2
Lbs./Hp. = SL Ratio
Lbs./Hp. = SL Ratio

WL Length = Square Root
840 1.3
230 2.0

11 Ft. 3.32
672 1.4
200 2.1

12 Ft. 3.46
547 1.5
175 2.2

13 Ft. 3.61
450 1.6
150 2.3
14 Ft. 3.74
375 1.7
130 2.4
15 Ft. 3.87
315 1.8
115 2.5
16 Ft. 4.00
270 1.9
105 2.6
17 Ft. 4.12

A sample calculation for a typical 16 foot jon boat, powered by a Reservoir Runner 500 with a ducted propeller, operating at 60 volts, and having 10 golf cart batteries, a foot controlled 12 volt bowmount motor and battery, and 2 persons with their gear on board is as follows:

Total boat weight = 1600 Lbs., Waterline length = 14.5 feet
A Reservoir Runner model 500 with a ducted propeller develops 6.5 horsepower, multiplied by 1.6 for effective horsepower = 10.4 Hp
1600 Lbs. divided by 10.4 = 154 Lbs./Hp = SL RATIO of 2.3
The Square Root of 14.5 feet = 3.8
SL Ratio of 2.3 multiplied by 3.8 = 8.7 MPH

In this example, if the motor had an open propeller, the Lbs./Hp would have been 246 and the SL RATIO would have been 1.95, Yielding a speed of 7.4 MPH. The added efficiency of the ducted propeller increased the thrust of the motor by 60% and the speed by 1.3 MPH without increasing the electrical power consumption. An open propeller would have required 60% more horsepower from the electric motor to reach this speed. This would have reduced the run time, and the distance that could be traveled on a charge, by over 40%.

© 1996, James M. Graham III

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