What two primary factors increase a ship's frictional resistance as it moves through water?

The two primary factors that increase a ship's frictional resistance as it moves through water are its wetted surface area and its speed. Frictional resistance is the drag force created by the friction between the ship's hull and the water, primarily due to the water's viscosity, which causes shear stresses as water layers slide past each other.
Firstly, the wetted surface area is the total external surface area of the ship's hull that is submerged in the water. A larger wetted surface area means that more of the ship's hull is in direct contact with the water. This increased contact area allows for a greater cumulative effect of the shear stresses between the hull surface and the water layers, directly increasing the overall frictional resistance. For instance, a ship with a longer or wider hull will generally have a larger wetted surface area, thus experiencing more frictional resistance, assuming all other factors are constant.
Secondly, the ship's speed is a critical factor. As a ship moves faster through the water, the relative velocity between the hull surface and the adjacent water layers increases. This higher relative velocity intensifies the shear stresses at the hull-water interface and within the boundary layer, which is the thin region of water directly affected by the hull's movement. This intensification leads to a substantial increase in frictional resistance, which typically rises approximately with the square of the ship's speed. Therefore, even a small increase in speed can lead to a significantly larger increase in frictional resistance.