The Auxiliary Motor

My desire was to sail and not motor. However, there is the practical problem in that you cannot easily sail into one of the narrow and crowded marinas. This close quarters maneuvering task is best left to a sailboat under motor power. The Com-Pac 19 did not have a motor when I purchased it. The broker indicated that he had been using a two stroke Tohatsu 5 horsepower long shaft motor without problems. A few queries to the Com-Pac owners association E-mail list seemed to indicate that anything from 4 to 6 horsepower would be fine with two stroke engines in the majority. This agreed with the manufacturer’s recommendation as found in the manual. Most owners reported only using the motor for 15 to 20 minutes per day. Just enough to get them away from the dock at the beginning of their trip and back to their slip, mooring, or trailer at the end of the voyage. Other’s would use their motor more frequently; especially to get back home under adverse conditions, no wind conditions, or when the schedule dictated.

Several owners reported on the benefits to be derived from using a four stroke engine but at this time, the only small (five horsepower and under) four stroke engines were built by Honda. A five horsepower engine from Honda was quoted to me at a cost of $1,600 for a long shaft while several two stroke long shafts were quoted in the $700 range. Even though a four stroke Honda engine would be much quieter and less polluting than a comparable two stroke engine, I did not want to spend that much money until I had some more experience with the boat. At this time, I was also reading extensively about the pollution problems being caused by the current crop of two stroke engines. I felt that four stroke engines would become more popular with time and the price was sure to decrease with competition from other manufacturers.

My experience with other small gasoline engines led me to be skeptical of engines that receive very limited and intermittent use. Pull starting a cranky two stroke for 15 minutes of operation did not really appeal to me. Additionally, the concept of spending $700 to $1,600 sent me back to the books for a cheaper alternative. The obvious choice to me would be an electric trolling motor. After all, they were readily available in any discount sporting goods store for about $150 to $200. The real question is would this radical scheme work? Since I had never seen it in use, I had to assume that there was something wrong with the concept.

Auxilliary Power Requirements with Gasoline

According to the references, additional queries on the Internet, and some simple calculations, I found that the typical small gasoline motor of 5 horsepower was really designed to operate a lightweight boat with a planing hull shape at a top speed of 8 to 13 miles per hour. This was due to it’s small diameter propeller operating at a relatively high speed. Most of these engines used about a 2 to 1 gear reduction system with a engine operating at about 5,000 rpm to produce the rated horsepower. Therefore, in pushing a sailboat at 5 miles an hour, with one of these engines, most of the horsepower would be wasted.

Occasionally, manufacturer's have made high thrust versions of their small outboard motors available.  The market for these appears small so they are not too popular.  They are characterized by a larger diameter propeller, a lower pitch, and an enhanced heavy duty gearbox.  These motors in their long shaft versions are much more suitable for a small sailboat or other heavy displacement hull shape.

Auxilliary Power Requirements with Electricity

More queries to the Internet and reading Electric Boats – The Handbook of Clean, Quiet Boating brought some answers and more questions. Essentially, I felt that for a boat with a displacement of less than 2,500 pounds (boat, crew, supplies, and equipment), an electric trolling motor as auxiliary propulsion was a viable alternative. The major issue to be resolved would be battery storage capacity and recharge capability. I therefore set a goal of having enough capacity to motor for at least 5 miles under conditions of no wind and no current. If there was wind, I would be forced to sail until I got close to my destination. I also wanted to have enough thrust to achieve two to three knots of boat speed with no wind for a limited time period. Therefore, I hoped to have enough capacity to motor against some significant wind or current resistance for a brief trip into a marina or up a short narrow channel to shelter.

A typical trolling motor is designed to push a displacement hull at a maximum of about 2 to 3 miles per hour. Therefore, I determined to use the highest thrust 12 volt electric trolling motor that I could purchase and see where that would lead. In this case, a Motorguide model T47 motor with a 36 inch shaft length was commonly available at the local Kmart stores. This motor draws about 50 amps at maximum speed and is rated to produce 47 pounds of thrust. Replacing the propeller with a two blade (power) design from the three blade (weed free design) included with the motor should result in the required combination of low shaft speed and high thrust. This motor, combined with a bank of two new fully charged group 27 deep cycle batteries, could operate at maximum thrust for about 2.5 hours. In theory, the calculations said that this combination would propel my Com-Pac at just under three knots. This would meet my stated goal of a 5 mile range under power with some safety margin.

The Electric and Propulsion Systems Shopping List

• Motor Guide T47 Electric Trolling Motor with 36 inch shaft - about 160 dollars at Kmart
• Two Group 27 Battery Boxes - about 8 dollars each at Wal-Mart
• Two Group 27 Deep Cycle Batteries - about 55 dollars each at Auto Zone
• One Kyocera 40 Watt Solar Panel (Model KC40) rather than the minimally required 30 watt panel - about 220 dollars at an internet site - Since this panel was being discounted at the time, it was actually about 25 dollars less than the 30 watt panel I had originally intended to use.  It was also about 50 dollars cheaper than the exact same panel offered by a major marine supplier.  This panel is 20 3/4 by 25 3/4 and comes with a sealed junction box already mounted and containing blocking diodes.  For the curious, the mounting details for the solar panel are on the photographs page.
• One Morning Star Pro Charge Controller (Model PS20) - about 110 dollars at an internet site.  While significantly overrated for the 40 watt panel, I went with the higher capacity since it includes a load controller.  This will disconnect the electric supply to the boat if the battery voltage drops to a dangerously low level (11.5 volts) and reconnect once the battery is recharged (12.5 volts).  It will control a total load up to about 16 amps.

This page was last updated:  09/14/00 05:11:10 PM