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Decomposition of forces (in horizontal cross-section) acting on a sail, generating lift. F T is the Total Force acting on the sail for the Apparent Wind (V A), shown. This resolves into forces felt by the sail, Lift (L) and Drag (D), with vectors shown in red and angle of attack noted as α.
In other words, for an object floating on a liquid surface (like a boat) or floating submerged in a fluid (like a submarine in water or dirigible in air) the weight of the displaced liquid equals the weight of the object. Thus, only in the special case of floating does the buoyant force acting on an object equal the objects weight.
This motion is generated directly either by the water and wind motion, particularly lateral wave motion, exerting forces against the hull or by the ship's own propulsion; or indirectly by the inertia of the ship while turning. This movement can be compared to the vessel's lateral drift from its course.
If the body is in motion, then there are also hydrodynamic pressures that act on the body. For a displacement vessel , that is the usual type of ship, three main types of resistance are considered: that due to wave-making, that due to the pressure of the moving water on the form, often not calculated or measured separately, and that due to ...
A propeller imparts momentum to a fluid which causes a force to act on the ship. [1] The ideal efficiency of any propulsor is that of an actuator disc in an ideal fluid. This is called the Froude efficiency and is a natural limit which cannot be exceeded by any device, no matter how good it is.
The forces transmitted via the sails are resisted by forces from the hull, keel, and rudder of a sailing craft, by forces from skate runners of an iceboat, or by forces from wheels of a land sailing craft which are steering the course. This combination of forces means that it is possible to sail an upwind course as well as downwind.
The total force vector acting at the center of pressure is the value of the integrated vectorial pressure field. The resultant force and center of pressure location produce equivalent force and moment on the body as the original pressure field. Pressure fields occur in both static and dynamic fluid mechanics.
German engineer Anton Flettner was the first to build a ship that attempted to tap this force for propulsion. "The idea worked, but the propulsion force generated was less than the motor would have generated if it had been connected to a standard marine propeller." [1] Ships using his type of rotor are sometimes known as Flettner ships. [2]