A 40 horsepower (HP) outboard motor is a common mid-range power choice for recreational boating, often found on smaller fishing boats, aluminum skiffs, and compact pontoons. While 40 HP offers a good balance of efficiency and performance, the maximum speed is never a fixed number. Performance is heavily influenced by the specific vessel configuration and the conditions on any given day.
Expected Speed Range
For most common applications, a 40 HP motor will push a vessel at speeds ranging from 15 miles per hour (MPH) up to 30 MPH. A light, unladen 14-foot aluminum fishing boat might easily reach the higher end, achieving speeds around 28 to 30 MPH. Conversely, a heavier, small pontoon boat carrying a full family will likely travel closer to the 15 to 20 MPH mark.
Achieving speeds above 20 MPH usually requires the vessel to transition from displacement mode to planing. In displacement mode, the hull pushes water out of the way, creating significant drag that limits speed. Planing occurs when the hull lifts partially out of the water, reducing the wetted surface area and allowing the boat to skim across the top. A 40 HP engine is sufficient to get lighter, appropriately designed hulls onto a plane, increasing the top speed.
Vessel Design and Load Factors
The design of the boat’s hull is the primary determinant of speed for a given horsepower. Planing hulls, characterized by a flatter surface toward the stern, are designed to lift and ride on the water surface, minimizing frictional resistance. Displacement hulls, such as those found on heavy sailboats, are rounded and designed to cut through the water, meaning they will never achieve high speeds with 40 HP due to the drag created.
Within planing hulls, the geometry significantly affects performance. Deeper V-shaped hulls offer a smoother ride in choppy water but create more drag and require more power to push onto a plane compared to flatter-bottomed skiffs. Catamaran hulls, which feature two slender hulls, benefit from reduced wetted surface area at speed, often achieving good efficiency, though their design presents unique challenges for mounting a single outboard.
The total mass of the vessel, including the boat structure, engine, fuel, gear, and passengers, directly opposes the motor’s thrust. For every additional unit of weight, more horsepower is required to maintain the same speed. A boat rated for a maximum of 40 HP will show a noticeable decrease in top speed when carrying a full load compared to when it is lightly loaded.
External factors like hull fouling and water conditions also introduce significant drag. Marine growth, such as algae or barnacles on the hull bottom, disrupts the smooth flow of water and prevents the vessel from planing efficiently. Similarly, operating in choppy water or waves requires the engine to constantly overcome the added resistance and impact forces, reducing the overall sustained speed.
Propeller Selection and Motor Setup
The propeller translates the engine’s rotational energy into forward thrust, making its characteristics paramount to speed. Propeller pitch is the theoretical distance the propeller moves forward in one rotation. A higher pitch propeller delivers greater top-end speed but requires more effort from the engine, similar to a high gear in a car.
Selecting the correct pitch involves a fundamental tradeoff between acceleration and top speed. If the pitch is too high for the boat and load, the 40 HP engine will be unable to reach its optimal wide-open throttle (WOT) revolutions per minute (RPM) range, causing it to lug. Conversely, a low-pitch prop allows the engine to easily hit its maximum RPM, offering strong acceleration, but limits the boat’s ultimate velocity.
Beyond pitch, the propeller’s diameter and the number of blades also affect performance. A larger diameter or an increased number of blades, typically three or four, moves more water, which can provide better thrust for heavy loads and improved handling. However, a greater surface area can also introduce more drag, requiring careful balancing to maximize the potential of the 40 HP motor.
The height at which the 40 HP motor is mounted on the transom impacts both drag and efficiency. If the motor is mounted too low, the lower unit creates unnecessary drag, slowing the boat down. If it is mounted too high, the propeller can ventilate or cavitate—drawing in air or experiencing water vaporization—which causes the engine to suddenly over-rev and lose thrust.
Adjusting the motor’s trim angle, which controls the vertical angle of the propeller shaft relative to the bottom of the boat, is a fine-tuning mechanism for speed. Trimming the bow up slightly reduces the wetted surface area of the hull, decreasing drag and often increasing speed, especially on planing hulls. Trimming too far up, however, causes the boat to porpoise or lose steering authority.