(Related to Textbook Section 9.3.3 - Drag Coefficient Data and Examples)
The drag on a boat comes from friction drag along the boat's surface and form drag that manifests itself in the form of waves. The energy needed to produce the waves is provided by the boat's motor.
At very low speed, a jet ski plows through the water and produces large waves in its wake. The form (wave) drag is very large. Under such conditions the boat is not an efficient mode of transportation. However, when properly trimmed at higher speed, the jet ski planes across the surface with a minimal wake. Most of the relatively small drag is friction drag; very little energy is lost to wave making.
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