Ship resistance is a field of study in naval hydrodynamics that examines the interaction between a vessel's hull and the surrounding water. It involves determining the propulsive thrust required to overcome resistance and move the vessel through the water.
Historically, this was one of the first topics studied in naval hydrodynamics, starting in the 18th century with the work of D’Alembert, Reech, and Froude—scientists who used scale model experiments very early on.
The resistance force (expressed in newtons) opposes the vessel's motion through the water.
It is useful to break down the total hydrodynamic resistance (Rh) by distinguishing three main causes of resistance:
Viscous Resistance (Rv), composed of:
Wave-making resistance (Rw), which depends on the vessel's speed, hull length and shape.
Aerodynamic drag (Ra), caused by airflow around the vessel's exposed surfaces and superstructures.
Froude is credited with the idea of studying viscous resistance (Rv), wave resistance (Rw), and aerodynamic resistance (Ra) separately.
As a vessel moves through the water, it generates a characteristic wave pattern.
This wave pattern contributes to the vessel's total resistance. It affects the bow and the stern of the vessel.
At the stern, waves form at a 20° angle—these are the accompanying waves. T
here are two types of accompanying waves: transverse and longitudinal waves.
Froude’s legacy in naval science is his eponymous number (Fn, as shown in this article’s illustration), a key parameter in hull hydrodynamics. It’s used to categorize navigation regimes and classify vessels into the three hull types.
It is defined as the ratio between the vessel's speed and the square root of the product of gravity and the vessel's characteristic length. But don't worry, it's just the value that matters here.
At low speeds, the vessel generates waves with wavelengths significantly shorter than its waterline length.
This means wave resistance is very small, but frictional resistance is considerable, about 90% of the total resistance. These hulls are commonly found on heavy displacement vessels.
In short: In recreational boating, low-powered motorboats, trawlers and motorsailers commonly have displacement hulls.
When the vessel's speed increases, the wavelength of the waves also grows until it matches the length of the vessel (L approximately equals λ).
At this point, the vessel effectively travels within its own wave system, with crests near the bow and stern and a trough amidships. The Froude number is then around 0.4, and this is called the "theoretical hull speed."
When the vessel's speed continues to increase, the crest moves away from the boat, and The vessel adopts a pronounced bow-up trim—This results in increased resistance and reduced propulsive efficiency. This corresponds to a Froude number between 0.4 and 0.7, with a wavelength between L and 3L.
At this point, wave resistance predominates (about 70 to 80% of total resistance). The vessel generates significant wave-making resistance, resulting in high fuel consumption. This also corresponds to the transition towards planing in a high-speed motorboat.
In brief: Light trawlers (Rhéa), offshore rescue vessels from SNSM, as well as some cabin cruisers, are equipped with semi-displacement hulls.
A planing hull where the ship’s length L is shorter than the wave’s length λ. Finally, as the vessel's speed increases further, the entire ship length is contained within the wave, allowing it to plane once the hull lifts from the water. This corresponds to a Froude number higher than 0.7.
These hulls are designed to ride on the water surface at high speeds, reducing the wetted surface and water resistance in favor of speed and fuel efficiency.
Planing hulls are effective at Froude numbers above 0.7, where lift exceeds gravity, allowing the vessel to "plane" over the water.
In brief: Rigid inflatable boats (RIBs), dayboats such as the Bénéteau Flyer 10, and most pleasure boats equipped with powerful outboard engines feature planing hulls.
