Unit
- Total ship resistance: The sum of frictional, wave-making, form, and air resistance determines the thrust required from the propulsion system. Frictional resistance dominates at low speeds, while wave-making resistance becomes significant at higher Froude numbers.
- Froude number: A dimensionless parameter equal to the ship speed divided by the square root of the product of gravitational acceleration and waterline length. It governs wave-making resistance and helps scale model tests to full-size vessels.
- Effective horsepower: The power needed to overcome total resistance at a given speed. Calculated as the product of resistance and speed. It represents the ideal power requirement before considering propulsion efficiency.
- Propulsive efficiency: The ratio of effective horsepower to the power delivered to the propeller (shaft horsepower). Typical efficiencies range from 0.60 to 0.75, influenced by hull form, propeller design, and operating conditions.
- Wake fraction and thrust deduction: The wake fraction describes the reduction in propeller inflow velocity due to hull boundary layer. Thrust deduction accounts for the pressure field effect of the propeller on hull resistance. Both are critical for propeller design.
- Model testing methods: Scale models are towed in test tanks to measure resistance and predict full-scale performance. Towing tank data informs the design of propellers and hull forms using empirical correction factors like the ITTC-1978 method.
- Computational fluid dynamics: CFD simulations model flow around the hull and propeller using RANS or panel methods. These tools reduce reliance on physical testing and allow optimization of energy-saving devices like ducts and fins.
- Energy-saving devices: Pre-swirl stators, rudder bulbs, and propeller boss cap fins improve propulsive efficiency by reducing rotational losses and improving inflow uniformity. Their adoption can yield fuel savings of 3 to 6 percent.
- Cavitation: The formation of vapor bubbles on propeller blades due to low pressure reduces thrust and causes erosion. Cavitation inception depends on blade loading, ship speed, and depth. Mitigation strategies include adjusting blade area ratio, using skewed blades, and adopting surface-piercing propellers for high-speed craft.
- Propeller blade design: Blade geometry parameters like pitch ratio, expanded area ratio, and skew angle are optimized for efficiency and cavitation avoidance. Modern propellers often use controllable pitch or ducted configurations to match varying operational profiles, improving overall ship fuel economy by up to 5 percent.
- Shafting and bearings: The propulsion shaft transmits torque from the engine to the propeller and must be designed for torsional vibration, alignment, and thrust loads. Ocean-going vessels typically use oil-lubricated stern tube bearings with sealed systems to prevent seawater ingress and maintain reliability over long voyages.
- Controllable pitch propellers: These allow blade angle adjustment to optimize thrust and speed without changing engine RPM. They improve maneuverability and efficiency in multi-mode operations such as towing, trawling, or dynamic positioning. Maintenance is more complex due to hydraulic mechanisms inside the hub.
Code Example
Python
def total_resistance(displacement, speed, wetted_area):
# Empirical approximation using Holtrop method
rho = 1025 # sea water density kg/m3
frictional = 0.5 * rho * speed**2 * wetted_area * 0.075 / (log10(Re) - 2)**2
wave_making = 0.5 * rho * speed**2 * displacement**(2/3) * 0.01
return frictional + wave_makingThis Python function implements a simplified resistance calculation for preliminary design. The frictional component uses an ITTC-1957 correlation line, while wave-making is estimated using an empirical coefficient. Real applications require integration with model test data or CFD results.
Quick Revision
- Total ship resistance includes frictional, wave-making, form, and air components.
- Froude number and Reynolds number govern scaling from model to full size.
- Effective horsepower is the power needed to overcome total resistance.
- Propulsive efficiency depends on hull-propeller interaction parameters like wake fraction and thrust deduction.