I. Core Dimensions of Propeller Size: Diameter and Pitch

As the core power output component of an electric outboard motor, the propeller's size (diameter and pitch) directly determines the efficiency of converting power into boat speed. This article combines professional test data and engineering principles to explain how propeller size affects speed and provides guidance for propeller selection.

The "size" of a propeller is not a single parameter, but is jointly defined by diameter (the diameter of the circle formed by the propeller blade tips during rotation) and pitch (the theoretical distance advanced per full rotation). It is typically labeled in the format "Diameter × Pitch" (e.g., 14×19 indicates 14 inches in diameter and 19 inches in pitch). These two parameters, similar to the gears in a car's transmission, regulate "traction" and "transmission efficiency" respectively, and together influence the final boat speed.

II. Two-Way Impact of Diameter on Speed: Balancing Thrust and Resistance

Diameter determines the propeller's contact area with water, directly relating to thrust output and high-speed resistance. It exhibits a typical characteristic of "low-speed gain, high-speed bottleneck":

1. Large-Diameter Propellers: Significant Low-Speed Advantages

Thrust Enhancement Mechanism: A larger diameter means more blade area in contact with water. Just like increasing the contact area of a tire with the ground, it can more efficiently convert the torque of the electric motor into thrust. For example, Torqeedo equips thrust propellers with a diameter of up to 12 inches for 6-ton large boats, enabling the heavy hull to quickly plane at low speeds.

Speed Limitations: When the boat speed exceeds 30 km/h, the hydrodynamic resistance generated by large-diameter blades increases sharply. Tests by Mercury Marine show that the resistance of a 16-inch diameter propeller on a high-speed boat is 23% higher than that of a 14-inch model, resulting in a 5-8% decrease in maximum speed.

2. Small-Diameter Propellers: Higher Efficiency at High Speeds

Low-Resistance Characteristic: Small-diameter blades generate less resistance when cutting through the water, making them particularly suitable for high-speed electric motors. The 1.2kW outboard motor from Baisheng uses a small-diameter propeller of 9.25 inches, which, combined with a high rotational speed of 1,750 rpm, achieves a maximum speed of 8.7 km/h.

Application Limitations: Insufficient blade area leads to weak low-speed thrust. If used on heavy hulls (e.g., yachts over 5 tons), it may cause "high rotation speed but slow boat speed" (slippage), increasing the acceleration time by more than 40%.

III. Direct Regulation of Speed by Pitch: Conversion Between Rotational Speed and Propulsive Efficiency

Pitch is the key factor determining the "distance advanced per rotation," and its impact is similar to bicycle gearing:

1. High-Pitch Propellers: The Core of Maximum Speed Improvement

Theoretical Efficiency Advantage: A higher pitch means a longer distance advanced per full rotation. Actual tests by BoatTEST show that at the same rotational speed of 4,000 rpm, a 21.5-inch pitch propeller increases boat speed by 15% compared to an 18.5-inch pitch propeller (from 26 knots to 30 knots).

Compatibility with Electric Motors: The constant-power characteristic of pure electric outboard motors is suitable for high-pitch designs. The 25kW model from Pure Watercraft is equipped with a propeller of 16 inches in diameter and high pitch, achieving a maximum speed of 35 km/h at only 1,500 rpm.

2. Low-Pitch Propellers: Guarantee for Acceleration and Heavy Loads

Torque Compatibility: Low-pitch blades have lower load, allowing the electric motor to quickly reach its rated rotational speed. Torqeedo configures a 10.5-inch low-pitch propeller for its 3.0kW outboard motor, reducing the planing time of small boats to 8 seconds.

Speed Trade-Off: A 1-inch reduction in pitch usually leads to a 3-5% decrease in maximum speed. However, in rough sea conditions, low-pitch propellers have stronger "water-gripping ability," allowing the hull to maintain a planing state at lower rotational speeds, which is safer and more efficient than high-pitch models.

IV. Special Adaptation Principles for Electric Outboard Motors

Compared with fuel-powered models, propeller selection for electric outboard motors requires additional attention to power characteristics and energy efficiency balance:

Power Matching Formula: According to engineering calculations from Doc88, the propeller diameter should be positively correlated with the motor power — a 600W motor is suitable for a 9-10 inch diameter, while a 10kW motor requires a 12-13 inch diameter. A deviation of more than 1 inch will reduce efficiency by over 20%.

Rotational Speed Coordination: High-KV (high rotational speed) motors should be paired with small-diameter, high-pitch propellers (e.g., an 1,100KV motor with a 10×17 specification), while low-KV (high torque) motors require a combination of large-diameter and low-pitch (e.g., a 500KV motor with a 13×12 specification).

Energy Efficiency Priority Strategy: The range of electric outboard motors depends on power conversion efficiency. Excessive pursuit of high pitch may cause motor overload. The optimal configuration for Baisheng's 1.2kW model is 9.25×5.1 inches; if replaced with a 10×7 high-pitch propeller, although the maximum speed increases by 0.3 km/h, the range decreases by 30%.