Resistance to movement is a discipline in itself that deals with the contact between the vessel and the water. It involves determining the propulsive force necessary to move the boat by overcoming the resistance forces.
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 ship’s course.
It is useful to break down the total hydrodynamic resistance (Rh) by distinguishing three main causes of resistance:
Viscous Resistance (Rv), composed of:
Wave Resistance (Rw), which depends on the vessel's speed and length.
Aerodynamic Resistance (Ra), which comes from the superstructures exposed to the wind.
Froude is credited with the idea of studying viscous resistance (Rv), wave resistance (Rw), and aerodynamic resistance (Ra) separately.
In practice, as the vessel moves, a regular wave field forms as it moves through the water.
This wave field is responsible for resistance to movement. 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.
The ship is slow, and its accompanying waves have a wavelength significantly shorter than the ship's length.
This means wave resistance is very small, but frictional resistance is considerable, about 90% of the total resistance. These are the hulls of heavy vessels.
In short: In pleasure boating, lightly motorized boats, dinghies, trawlers, and motorized sailboats 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 boat positions itself on a wave with two crests at the bow and stern and a trough in the middle. 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 finds itself in a "bow-up" position—very unfavorable in terms of resistance to movement. 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 boat pushes a lot of water, and its fuel consumption is high. This is also the transitional phase of a fast motorboat planing.
In brief: Light trawlers (Rhéa), offshore rescue vessels from SNSM, as well as some habitable motorboats, 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: Semi-rigid boats, dayboats like the Bénéteau Flyer 10, and most pleasure boats equipped with powerful outboard engines feature planing hulls.
