An Illustrated Dictionary of Key Nautical Terms

This brief dictionary has started as a simple one Web page. As it gradually grows, at some point it will be reorganized to efficiently provide the answers you are looking for. The entries are alphabetically sorted.

anchor - a contemporary anchor is a specially shaped metal device for securing a vessel to the ground beneath her. It is lowered into the water using a rope or chain, or a combination of the two with the anchor attached to a few meters of chain continued with a rope the end of which is secured to the bow of the boat. The anchor is designed that way that, when pulled by the shank, it digs itself into the ground holding the boat. Using the chain adds additional weight parallel to the ground, improving the anchor's holding. The effective holding power of an anchor and its chain and rope arrangement depends on its weight and its design, and the length of the line (chain and rope) laid out from the bow. As a general rule of thumb, the line should be at least three times longer than the depth below the boat. When at anchor, the boat can swing around a full circle around it as driven by the wind and current. The anchor is retrieved (sometimes with difficulty) by taking the boat above it and pulling on the anchor line straight up to get the anchor off the ground. The most common types of modern anchors are CQR, Danforth and Bruce. The often used anchor symbol represents the more traditional and today rarely used fisherman's anchor.

athwart - a direction across that taken by the vessel, not necessarily at the right angle.

athwartships - from one side of the boat to the other, at the right angle.

beam - the measurement of a vessel from side to side at its widest point. Typically the vessel's beam is 1.1 to 1.2 times the Bwl (beam on the waterline).

BHP - Brake HorsePower, the measure of an engine's horsepower without the loss in power caused by the gearbox, generator, differential, water pump, and other auxiliary components such as alternator, power steering pump, muffled exhaust system, etc. "Brake" refers to a device which was used to load an engine and hold it at a desired RPM. During testing, the output torque and rotational speed were measured to determine the "brake horsepower". Compare the BHP to the SHP (Shaft HorsePower).

bilge - the lowest part of the interior of a vessel on either side of the keel. It is the area where all the internal water, and occasionally dirt, collects.

bilge pump - a pump installed to collect the water accumulating in the bilges and pump it overboard. In the old days it used to be a mechanical pump with a critical safety function to pump the water out in case there was a leak in the hull. Nowadays, an efficient electrical pump is typically used.

bow - the very front of a vessel, the opposite end of the stern (the back end).

Bruce anchor - a specially shaped anchor with no moving parts and no sharp projections:

Excellent holding power on all types of ground
Because of its shape, difficult to stow aboard unless it hangs over the bow
Not so frequent, yet very popular with boat owners who have tried it

Bwl - beam on the waterline, the widest measurement from side to side of a vessel at the waterline level.

canoe stern - the shape of the stern where the sides meet along the centerline ending up in a pointed stern, similar to the bow. This design, sometimes called double-ender, results in a low-speed category boat, yet it is often used in small cruising boats as it gives a drier vessel in rough, following seas.

capping, propeller - the twist in the trailing edge of a propeller blade. It increases the propeller grip on the water, reduces cavitation and increases the speed of the boat. It also decreases the engine top RPM (Revolutions Per Minute).

cavitation - an effect caused by bubbles of partial vacuum caused by high propeller speed and/or loading. High RPM, excessive slip, excessive pitch and high blade tip speeds, nicked, square or sharp leading edges of the propeller blade all encourage the appearance and increase of cavitation. In extreme cases, cavitation bubles of gas erode the blades through "exploding" and "burning" on their surface. A cavitating propeller can still produce lots of trust, but as the bubbles form irregularly, this can be felt as vibrations. The force of exploding bubbles is so great, that it literally sucks metal off the surface of the blades causing irregular pitting. This in turn produces uneven wear of the propeller blades, imbalance and even more vibration. Cavitation is rarely a problem on slow speed boats with low RPMs. Keeping RPMs down, shaping the blades so that they produce negative pressure at the back, and keeping the pitch as low as practical all help eliminate or reduce cavitation. Cavitation is a different effect from the propeller ventilation.

chine - the line at which the side of the boat meets with the bottom. A smooth, gradual transition represents a soft-chined form, also called round-bilge. In contrast, a sharp angle along the line where the side and the bottom meet is the hard-chine form, often seen with the high-speed flat bottomed craft. To produce more gradual transition, there could be two, three or more chines, where we speak of a multi-chined hull. (see picture at deadrise)

chine rail - a spray rail running along the chine. Also: chine spray rail. (see picture at deadrise)

CQR anchor - (from Chatham Quick Release; pronounced phonetically "secure"), also known as plough anchor:

Good overall holding power
Ideal for mud or weedy grounds
Not at best on a rocky bottom
Difficult to stow overboard, unless it hangs over the bow
Very popular

Danforth anchor - one of the most often used contemporary anchors, with good holding power on all types of ground. It is also generally low cost. Its main disadvantage might be its awkward shape, making its handling while on the boat a bit cumbersome. Because of its lower price, it is often used even for trailer boats, where if it is stored on the bow so that it sticks partially out - there it represents a constant danger for everyone passing close by (which its honest owner can often prove with the numerous bumps on his head).

deadrise - the angle the bottom makes with a horizontal line drawn from the centerline atwarthships. The deadrise varies along the bottom. For both power boats and sail boats it may be an average of about 15⁰ midships. With power boats, where the deadrise difference is typically more pronounced, it might be some 25⁰ or more at the bow (called the deep vee), to drop to some 12⁰ or less at the stern.

deck - the nautical equivalent of the floor. There may be one or more decks, depending on the design. Not all decks may extend the whole length of the boat, yet they all reach from one side to the other. The top deck is a part of the boat's outer shell.

diameter, propeller - twice the distance measured from the tip of any blade to center of the hub of the propeller. Shown usually in inches, the diameter and the pitch determine the propeller size, e.g. 13" x 19" (D x P)

displacement - the amount of water (weight) displaced by the vessel afloat. Since the weight of the displaced water equals the weight of the vessel, the displacement is often used as a measure of the vessel's size.

displacement boat always displaces water equivalent to its weight

displacement boats - boats designed to move through the water, always displacing a certain amount of it equal to its total weight (see displacement). All sail boats and most (especially larger) power boats are displacement boats. See planing boats as another design option.

displacement speed - another name for the hull speed. (see hull speed)

DWL - Design-Water-Line, another name for the LWL.

Eskimo Roll - a self-rescue technique of rolling up with your kayak after a capsize, where you do not leave the kayak but use your paddle (more skilled individuals can do the roll with their hands only) to roll up again.

fisherman's anchor - a very traditional anchor:

Good holding power on rocky seabeds
Not good on mud or sand
Comparatively inefficient for its weight
Very awkward to stow onboard, unless the stock can be taken out
Easily fouled up with its own chain and rope, as one arm always sticks out of the ground
Nowadays rarely used

frame - extending out and up from the keel like ribs from a backbone, made in symmetric pairs, frames support the sideboards or plates, give rigidity to the hull and determine the shape of the boat.

helm - the steering wheel or the tiller through which the rudder is controlled.

hull - the body of the boat. The hull includes the deck, the sides and the bottom as the elements of the boat's outer shell. while it does not include the above-deck superstructures, masts and rigging.

hull speed - the maximum economical speed of a displacement boat. At that speed, two distinct waves are formed by the movement of the hull through the water, one in front at the bow, and the second at the stern, with a distinct trough running along the hull. The hull speed occurs at the speed/length ratio of 1.34.

keel - the lowest member of the boat's framework running fore to aft. Usually stronger, in wooden boats it supports the frames resembling the backbone and the ribs. In metal boats the keel is formed by the lowest continuous joining line of the bottom plates. Sailing boats commonly have deep narrow keels designed not only to form the boats backbone, but also to provide stability and resistance to the sideways push of the wind and waves.

knot - an international nautical unit of speed. 1 knot is a speed of 1 nautical mile (1,852 m) per hour. Usually abbreviated as kn.

LOA - Length-Over-All, the longest measurement of a vessel from the very front of the bow (the front end), to the stern (the back end).

LWL - Length-Water-Line, the design measurement of a vessel at rest on a calm water, from the front to the back along the waterline. The LWL of a displacement boat, or even a planning boat before it starts to lift up to a plane (while it behaves as a displacement boat), determines its maximum economical speed.
lwl (in lower case) is sometimes used as load-water-line, the line at which the vessel floats when fully loaded.

moor, to - to secure a boat with ropes or chains to the ground beneath her (using her anchor or special mooring lines) or to the shore.

mooring - a permanent place in a harbour or a protected bay to moor a boat. Moorings are often marked with lightweight mooring buoys. To moor your boat, you need to pick up the buoy out of the water. Then, by pulling the lighter rope attached to it, you will bring up a heavier rope or chain attached to a heavy anchor permanently secured to the ground.

motor-sailers - boats equipped with rather capable sails and one or more engines, so they can use either means of propulsion, depending on the weather conditions, capability of the crew and the skills and the mood of the skipper. As a hybrid design, a motor-sailer is typically not so good under sails as a comparable sailing boat, and it is not as good under engine as a comparable power boat. Still, motor-sailers have their good sides appreciated by many boat owners. Compare them with power boats and sail boats as the two contrasting options.

pitch, propeller - theoretical movement of the propeller through the water in one revolution. For example, a propeller with a 19" pitch should theoretically move 19" through the water in one revolution. In reality, there is always a loss depending on the match of the boat, the engine, and the propeller. This loss, called slip, is typically 10-20%, not an insignificant value.

plane, to - to move over the water with most of the hull raised out of the water, only the very end of the (flat) bottom sliding over the surface (see planing boats below).

planing boats - boats specially designed with flat or moderate bottom and equipped with engines powerful enough to lift them above the water at higher speeds, so that they ride on the water on that flat bottom area at the stern end. Typically these boats have a moderate V-bottom in front, to give them better directional stability at lower speeds when the boats still moves through the water as any other displacement boat.

plough anchor - another name for the CQR anchor.

power boats - boats whose main means of propulsion are one or more engines (as contrasted by sail boats, and motor-sailers as the hybrid between the two options.

propeller size - usually shown as propeller diameter D and pitch P in D x P, both shown in inches. Example: 13" x 19"

rudder - a flat board mounted in the center at the back of the vessel. By turning the rudder, the boat is steered. Different mechanisms, all called helm, have been invented to control the rudder. Most often met are the tiller (especially on the smaller boats) and the steering wheel.

rudder stock - a strong upright member to which a rudder is attached, so that turning the rudderstock turns the rudder.

sail boats - boats whose main means of propulsion are sails (as contrasted by power boats, and motor-sailers as the hybrid between the two options.

slip, propeller - the loss, usually 10-20%, between the theoretical expectation how much should the propeller (and the boat) move for each revolution, as expressed by the propeller pitch, compared to the actual value.

SHP - Shaft HorsePower, the "net output" power of the engine at the driving shaft (at the propeller), after the gearbox and all auxiliary units. Compare it to the BHP (Brake HorsePower). As a rule of thumb, the SHP is about 10% less than the BHP.

speed/length ratio - a boat design parameter: The boat speed given in knots divided by the square root of the LWL in feet. For displacement boats the hull speed happens at speed/length ratio of 1.34. To be able to lift up and plane, a boat must have a speed/length ratio of 3 or higher.

stern - the very back end of a vessel, opposite from the bow (the front end).

spray rails - triangularly shaped rails with their bottom faces horizontal, that run along the chine up to the bow. Other, smaller sets of spray rails may be fitted closer towards the centerline. They deflect the water spray at high speeds, providing additional lift and narrowing the effective beam, a desirable effect to achieve even higher speeds. They are effective only on boats capable of planing (speed/length ratio 3 and above). In slower boats, they would have no other effect but to increase the wetted area and thus slow the boat down. (see picture at deadrise)

tiller - a long arm attached either directly or through a simple mechanism to the rudder to steer the boat. In smaller boats the tiller is most often directly attached to the rudder stock, which gives a very positive feeling of the position of the rudder and a fine control of the boat. With larger boats the forces at the rudder become too big to handle, and the tiller is replaced with a steering wheel operating the rudder through a system of pulleys or through hydraulic connections.

transom - the flat area at the very back end of a boat forming the stern (the back end). Not every boat has a flat stern area, so not every boat has a transom.

trough - the area between the crests (tops) of two adjacent waves.

ventilation, propeller - an effect of the air from the surface (if the engine is trimmed too shallow or the propeller diameter is too big) being sucked into the propeller, disrupting the flow of water across the propeller blades. Its effects are usually not as severe as the effects of cavitation, but they can lead to vibrations and a sudden loss of trust at increased engine RPMs (Rotations-Per-Minute). The best way to prevent ventilation is by trimming the propeller deeper, and choosing a propeller with a smaller diameter. Using a propeller with blades raked slightly aft also reduces this effect. Ventilation is a different effect from the propeller cavitation.

waterline - the line along the boat's side at which she floats. The larger vessels often have their design waterline distinctly painted to help monitor the correct distribution of its load. The length of the waterline (see LWL) is an important element that determines the boat's maximum economical speed.

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