Anatomy of a Sail Boat - Part VI : The Bow

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BOW - Well, we aren't talking about the weapon but instead of that we are talking about the foremost part of the boat - the point that is most forward when the vessel is underway.


ORIGIN

The term bow comes from the old days of timber boat building. It is derived from the use of the trunk and a bow (bough) or large limb of a tree where the natural strength from the grown curved fibres of the wood provides the strength for this most vulnerable part of the ship or boat. The shipwright would cut the bow and trunk vertically through the felled tree to find the most natural curved form.

BASIC DESIGN

The bow is designed to reduce the resistance of the hull cutting through water and should be tall enough to prevent water from easily washing over the top of it. On slower ships like tankers, a fuller bow shape is used to maximise the volume of the ship for a given length.

The forward part of the bow, usually on the ship's centreline, is called the stem. Traditionally, the stem was an upright timber or metal bar into which side planks or plates were joined.

THE BULBOUS BOW

A bulbous bow, a feature of many modern ship hulls, is a protruding bulb at the bow below the waterline. Usually visible only when a ship is in drydock, the bulb modifies how water flows around the hull, reducing drag and increasing in speed, range, and fuel efficiency. Ships with bulbous bows generally have 12 to 15 percent better fuel efficiency than similar vessels without them.

HOW BULBOUS BOWS WORK ?


The fluid dynamics of bulbous bows can be calculated.

Long waves are faster, so a ship that wants to go fast has to excite long waves and not short ones. In a conventionally shaped bow, a bow wave forms immediately before the bow. When a bulb is placed below the water ahead of this wave, water is forced to flow up over the bulb. If the trough formed by water flowing off of the bulb coincides with the bow wave, the two partially cancel out and reduce the vessel's wake. While inducing another wave stream saps energy from the ship, canceling out the second wave stream at the bow changes the pressure distribution along the hull, thereby reducing wave resistance. The effect that pressure distribution has on a surface is known as the form effect.

Some explanations note that water flowing over the bulb depresses the ship's bow and keeps it trimmed better. Since many of the bulbous bows are symmetrical or even angled upwards which would tend to raise the bow further, the improved trim is likely a by product of the reduced wave action as the vessel approaches hull speed, rather than direct action of waterflow over the bulb.

The first bulbous bows appeared in the USA being fitted to the USS Delaware which entered service in 1910.

SONAR DOMES


Some warships specialized for anti-submarine warfare use a specifically shaped bulb as a hydrodynamic housing for a sonar transducer, which resembles a bulbous bow but has only incidental hydrodynamic purpose. The transducer is a large cylinder or sphere composed of a phased array of ultrasonic acoustic transducers. The entire compartment is flooded with water and the acoustic window of the bulb is made of fiber-reinforced plastic or another material (such as rubber) transparent to the transmitted and received underwater sounds.

Also, some ships have specialised bows in order for tasks like breaking through snow for polar expeditions.

An important thing for sailors to note is...that in sailing parlance - both the terms fore and forward mean towards the bow.