How is the speed calculated when designing a ship?

The calculation of the speed when designing a ship is a complex process, which requires the comprehensive use of theoretical calculations, model tests, and empirical formulas to ensure the accuracy and reliability of the design speed.

A. Theoretical calculation

1. Resistance calculation

When a ship sails in water, it will be subject to various resistances, including friction resistance, wave resistance, and viscous pressure resistance. Through empirical formulas and theoretical models, such as Froude's law and ITTC formula, the resistance values at different speeds are calculated according to the main parameters of the ship (such as length, width, draft, line shape, etc.).

2. Propulsion power estimation

According to the resistance calculation results, combined with factors such as propeller efficiency, the power required to propel the ship is estimated. Generally, empirical formulas or regression analysis of model test data are used to determine the relationship between power and speed.

3. Speed determination

After obtaining the relationship between power and speed, the speed of the ship under the design conditions is determined according to the main engine power given in the design requirements or the expected propulsion system performance. For example, if the design requires the main engine power to be a certain value, by looking up the power and speed curve, the speed value that meets the power requirement is found, which is the design speed.

B. Model test

1. Ship model production

The ship model is made according to a certain ratio, usually between tens and hundreds. The model should reflect the line type, scale and structural characteristics of the actual ship as accurately as possible.

2. Tank test

Put the ship model in a towing tank or a self-propelled tank for testing. In the towing tank, the ship model is towed by a trailer at different speeds to measure the resistance of the ship model; in the self-propelled tank, the ship model is equipped with a power device to simulate the self-propelled state of the actual ship, and the speed, propeller thrust and other parameters are measured.

C. Experience correction

1. Consider practical factors

During the design process, it is also necessary to consider the impact of some practical factors on the speed, such as the ship's loading conditions, sea conditions, wind resistance, etc. These factors may cause the actual speed to deviate from the theoretical calculation and model test results.

2. Adjustment of empirical coefficients

Designers will introduce empirical coefficients to correct the calculation results based on previous design experience and actual ship operation data. For example, for a specific type of ship, a speed correction coefficient may be determined based on experience, and appropriate adjustments may be made based on theoretical calculations or model test results to obtain a design speed that is closer to the actual situation.