TECHNICAL CALCULATION

Simplified Calculation for Choosing a Propeller

1. Gather the necessary information:

  • Total weight of the loaded boat (in kg): Include passengers, fuel, and equipment.
  • Length of the boat (in meters).
  • Engine power (in HP).
  • Optimal engine RPM (in rpm): Indicated in the manual or on the engine plate.

2. Formula to estimate the propeller pitch: To calculate the estimated pitch (in inches), use the following formula: Estimated pitch (in inches) = (Target speed (in knots) × 1056) ÷ (Optimal RPM (in rpm) × (1 - slip))

Explanation of variables:

  • Target speed (in knots): The desired speed. Generally, 20-25 knots is a good estimate for a standard boat.
  • Slip: The speed loss due to water resistance. To simplify, use 0.15 (15%).

3. Practical example: A 6-meter boat weighing 1200 kg fully loaded, equipped with a 90 HP engine with an optimal RPM of 5500:

  • Assume a target speed of 25 knots.
  • The calculation for the estimated pitch would be: Estimated pitch ≈ (25 × 1056) ÷ (5500 × (1 - 0.15)) ≈ 11.2 inches

In this example, a propeller with a pitch of 11 or 12 inches would be a good starting point.

4. Practical estimation table (in inches):

Boat length (m) Total weight (kg) Engine power (HP) Recommended pitch (in inches)
4 - 5 m 500 - 800 20 - 40 7" - 9"
5 - 6 m 800 - 1200 40 - 90 10" - 12"
6 - 7 m 1200 - 2000 90 - 150 13" - 15"
7 - 8 m 2000 - 3000 150 - 300 16" - 20"

5. Propeller diameter: The diameter is generally determined by the manufacturer based on the pitch and engine power. The higher the power, the larger the diameter. Here are some guidelines:

  • 20-40 HP engines: 8"-10" diameter.
  • 40-100 HP engines: 10"-14" diameter.
  • 150 HP and above: 15"-18" diameter.

6. Tips to refine performance:

  • If your boat takes time to reach planing, choose a smaller pitch.
  • If the engine exceeds its maximum RPM, choose a larger pitch.
  • Always test on the water to make adjustments based on actual performance.

Disclaimer: The information and advice provided in this section are intended to help you choose the most suitable propeller for your needs. These elements are based on general principles and standard recommendations and do not constitute personalized professional advice. We encourage you to consult an industry expert for an accurate assessment and to ensure your choice is suitable for your boat. We disclaim any responsibility for errors, misinterpretation, or issues resulting from the use of this information.

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