Last time, we discussed multihulls, the catamarans and trimarans that are make up a small but significant portion of the world's fleet. Most multihulls are designed primarily for high speed, but for high speed to be useful, it needs to be practical if the sea is less than calm. Fortunately, naval architects have worked out ways to solve this problem, some of which can also be applied to conventional ship, and have even come up with a catamaran derivative, known as SWATH, that provides unmatched seakeeping for its size. But we'll start with an illustration of the basic principle at its simplest with the wave-piercing bow.

Axe-type wave-piercing bow and conventional bow, showing the different shapes
The basic concept behind a wave-piercing bow is simple. Imagine a normal ship is sailing along over a flat ocean, then runs into a wave. The bow is going to reach the wave first, and as the ship is horizontal, it will be pushed into the wave. This means that the waterline there will be higher than normal, and on a conventional bow there is quite a bit of flare,1 so the bow displaces more water, which means more buoyancy, which in turn pushes the bow up. The end result is that the ship will pitch up and down as it passes through the waves, which is the major cause of seasickness and also impairs decision-making. A wave-piercing bow has less buoyancy than a conventional bow, particularly above the normal waterline, and as such, pitches the ship less when it runs into a wave. The downside to this is that the bow rising keeps the water further away from the deck, so ships with wave-piercing bows tend to be pretty wet forward. This is a serious problem if the designer hasn't taken it into account,2 but can work if nobody is expected to be standing in the areas that keep getting wet. Read more...
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