How to Match an Aluminum Boat to a Motor

When you want to hang a motor on the back of your aluminum boat, you need to find out "how much motor" your boat can handle -- both in horsepower and weight. If either is larger than the boat was designed for, you might end up with more motor than the boat can handle. With a bit of research and some simple arithmetic, you can match boat and motor with little effort.

Instructions

  1. How Much is "Too Much"

    • 1

      Locate the Coast Guard's "Capacities" label on the inside of the stern -- that's the back of your boat -- and look for the "Maximum Horsepower." Usually the label is found on the right, or starboard, side of the boat's transom. Record the horsepower information. Inspect the label further and locate the "Maximum Weight" allowed for the boat. This is the maximum weight the boat can carry without being overloaded. It includes the weight of the motor.

    • 2

      Multiply the maximum horsepower stated on the “Capacities” plate by 1.1. The result is 110 percent of the maximum horsepower. If your boating usually finds your boat heavily burdened, and if the motor with 110 percent of the maximum horsepower is light enough that your boat is not overloaded with weight, an additional 10 percent of power will not cause the boat to exceed the hull design speed; rather, it will provide the best performance available while under load.

    • 3

      Multiply the maximum horsepower by 0.9. The result is 90 percent of the maximum horsepower. If you usually run “light,” that is, without passengers and their gear, or other encumbrances, you can operate more economically by using a motor with lower fuel consumption, while still achieving respectable performance.

    • 4

      Subtract the weight of the motor that provides the maximum horsepower specified on the “Capacities” plate, from the “Maximum Weight” allowed for the boat. If the combined weight of you, the motor, your buddies and their gear exceeds the maximum weight, you face a choice: lighter buddies and gear, or a smaller motor.

    • 5

      Locate a motor, using the difference between the load and the maximum weight. The difference between the combined weight and the maximum weight specified by the Coast Guard -- the point where your boat becomes overloaded -- isn’t flexible. Because most aluminum boats have low sides, a condition known as “low freeboard,” the heaviest motor your aluminum boat can carry without the threat of sinking may be smaller than you want.