What’s a helmet?

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The hull is the basic structure of a ship, with three main designs: displacement, planing, and semi-displacement. Each has advantages and disadvantages, with displacement being safer and more fuel efficient, planing being faster but less efficient, and semi-displacement being a compromise between the two.

The hull is the basic structure of a ship, not including masts (if any), rigging, above-deck construction, or accessories of any kind. Typically, it is curved along the bottom, angular towards the bow (front) and somewhat square at the stern (rear). There are flat-bottomed boats and different bow and stern shapes, but most hulls are specially designed with hydrodynamic considerations as described above.

There are three basic hull designs, and no matter which one is used, all boats create a bow wave as they push through the water. At slow speeds this is not a relative factor, but at higher speeds the wave gets bigger and higher and boats will try to climb it. Some boats are not built to ride a bow wave and will stay on the bottom of their own waves. Others are purposely designed to ride up the bow wave and ride on it, while other hulls will only partially lift.

The first type is called a displacement hull. Any boat of this design pushes through the water, displacing it by a volume equal to the weight of the boat. Once the highest speed is reached and the bow wave is of sufficient size, the displacement hull cannot rise to ride the wave and is therefore limited in speed. Longer hulls of this type can achieve higher speeds due to their greater weight which, in turn, displaces a greater volume of water. To calculate a theoretical top end speed in such a way, it is first necessary to find the square root of the length of the boat. This is multiplied by 1.34. The resulting number is the estimated maximum speed the vessel can achieve, measured in nautical miles per hour (knots).

The second type of design is called a planing hull. This type is designed to ride up and ride the larger bow wave created by faster speeds. Once the boat is lifted onto the plane, the energy previously required to push the water aside is reused in a forward motion over the water. When operating at such a speed, the planing hull design displaces less water than its own weight. For the most part, the basic design feature that allows planing is found in the shape of the bottom surface of the hull. It is usually less rounded than displacement hull designs.

A third type of design is a hybrid of the previous two types and is commonly known as a semi-displacement hull. This guy can raise the boat to a medium plane; however, it cannot reach a true planning position.

Knowing which type of helmet is best requires understanding the advantages and disadvantages of each. Displacement designs are often safer, more fuel efficient, and easier to handle in harsh weather and rough seas. Freighters are a good example of a displacement hull in practice. Planing hull designs are fast, less fuel efficient, and perform better in calm water. Speedboats and racers are good examples of this type.

Semi-displacement hulls aren’t as fast as a planing hull and aren’t as fuel efficient as a displacement hull, but they can go faster than the latter and handle rougher seas than the former. Recreational cruises are good examples of this type.

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