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10 Vessel Terms and Definitions Even the Smartest Seafarers Misuse

Boat crew getting language right

The language of the Maritime Industry is almost as fluid as the water that it floats on, and as with most things, as time goes on, things change.  

We once relied upon the sun and stars for navigation, but that method was replaced by maps and compasses. We once used manpower to load and unload shipping cargo, but that was until electric and diesel motors came along though.

Captains of vessels once had to record their logbooks by hand, but now with the advent of digital technology, it is now much easier and more efficient to and record a ship’s logbooks electronically. This ensures much more accurate information and takes some of the drudgeries out of doing paperwork!

So, what’s with all of this? Well, along with the advancement of new technology, comes new terminology. Vessel terms and definitions are commonly used in our daily conversation, but some remain misunderstood or misused by even the smartest, most experienced of Seafarers.

Here is a list of the 10 most commonly misused vessel terms and definitions:

1. Gulf and Bay

Most nautical terms such as rivers, estuaries, lakes, seas, and oceans are self-explanatory but there is still quite some misunderstanding between terms such as Gulf and Bay and Straits, Channels, and Canals.

A Gulf is a large body of water that can be compared to an extended leg of a sea with a tight entrance along a strait and is nearly surrounded by land.

Examples of gulfs such as the “Hauraki Gulf” in Auckland.

Bays are almost the same as Gulfs, except that they are usually smaller and have a wider inlet. There are, however, exceptions to this rule. For example, the Bay of Bengal, between India and Thailand, is much larger than most bays in the world. A Bay is also not surrounded by as much land and resembles more closely the shape of the letter “U”. Think “Cockle Bay” in Auckland.

gulf vs bay2


2. Straits, Channels, and Canals

Straits are naturally formed, narrow bodies of water that connect two much larger bodies of water. The water flows in both directions and can depend on the tide.

Examples are the Cook Strait, between the North and South Island, and Foveaux Strait, which is between the South Island and Stewart Island.

Channels can easily be identified as wider Straits. The only difference being that a Channel is much larger and has better waters for navigation.

An example is the Tory Channel, in the Marlborough Sounds.

Canals are exactly like Straits and Channels, except that they are man-made. They are artificial waterways that were made to facilitate trade or to shorten distances between more natural routes.

Examples are the Suez, Panama and Kiel Canal's.

difference between strait and channels


3. Density and Specific Gravity

Density is defined as the amount of matter per unit of volume or Mass over Volume and is therefore measured in units of “kg/cm3”.

As an example, the Density of sea water is 1012kg/m3.

Specific Gravity is a ratio of the density of a certain material divided by another reference substance such as fresh water. The two units counteract each other; therefore, Specific Gravity is most often referred to as a ration.

As an example, the Specific Gravity of diesel is 0.86.

*Density and Specific Gravity will be the same if you are referencing fresh water.

density and specific gravity2


4. Fairleads, Chocks, Bitts, Bollards and Dolphins 

Our next set of terms and definitions are used in relation to a vessel’s mooring operations.

Fairleads are devices welded directly to a vessel’s structure and have rollers that are used to properly guide mooring lines towards a winch. Modern vessels use a Pedestal Fairlead to guide the mooring lines between the chock and the winch. These phased out Universal Fairleads that were used in earlier times.

Chocks are reinforced guides that lead the mooring lines to and from an anchor point. Chocks need to be solid and have a much higher SWL (Safe Working Load) than the mooring lines. This helps to withstand various forces and to avoid structural damage.

Bollards are basically short posts that are used to secure a vessel’s mooring lines. A single Bitt on a vessel, can also be referred to as a Bollard. Oil Tankers have special Bollards that are used for securing hoses during certain cargo operations.

Bitts are sets of two Bollards mounted vertical on a vessel. They are always found in pairs of two and are used to secure mooring lines to other vessels, tug lines or for special operations such as on Tankers.  

It is important for a tug operator to know the SWL (Safe Working Load) of a vessel’s Bitts. Without this information, he might exceed this force and cause damage to the vessel.

Dolphins are like bollards but aren’t quite the same. These are independent platforms on a jetty. These platforms have “hooks” or bollards and are used to secure a vessel’s mooring lines.

dolphins on a wharf for a boat to tie to2


5. Weathertight and Watertight Doors

These vessel terms are not the most popular with landlubbers but are very well known to those in the industry.

The key difference between these two terms is their location.

Weathertight Doors are located above the waterline and are designed to prevent water from flowing from outside to inside. Doors to the deck/accommodations on larger vessels, are usually weathertight doors. This type of door is designed to open outwards, ensuring a positive pressure, should there be a flood of water inside their room. 

Watertight Doors located below deck and are designed to prevent the ingress of water from both sides. This ensures the watertight integrity of the neighbouring compartment on the vessel. These doors open/close either upwards or sideward.

watertight and weathertight example


6. Gross Tonnage and Net Tonnage

These terms, while often used in everyday conversation, are often misunderstood on vessels.

Gross Tonnage is the volume of the sum of all enclosed spaces on ship, including the Engine Room and other non-cargo spaces. Gross tonnage is calculated by using a rather complicated formula.

Many Maritime Regulations such as SOLAS and MARPOL are based on a vessel’s Gross Tonnage.

Net Tonnage is the volume of the cargo carrying spaces only. Net tonnage can be used to determines the earning potential of an individual vessel – think Fishing boat for example. Port and anchor fees, if applicable, are usually based on a vessel’s Net Tonnage.

 net tonnage and gross tonnage


7. Derricks and Cranes

The next two definitions are quite similar. Derricks and Cranes are both lifting devices. The key difference is that Derricks are now only found on older vessels, as today, they have been superseded by the much more versatile Deck Cranes.

Derricks are built with one or more masts and are controlled by several lines attached to the top. These lines move the Derrick vertically and laterally, but the lifting and lowering of a load is accomplished by a separate line much in the same way a conventional crane operates.

Two separate Derricks are a popular choice, as this allows for much faster lifting operations. One downside to this is that it requires at least two operators. Another nail in the coffin is that it takes a long time to remove a Derrick, if you have to accommodate different types of cargo.

Cranes on the other hand, are much simpler and are usually designed to rotate a full 360 degrees. Unlike Derricks, one single operator can accomplish all functions. (Line up/-down, boom up/down, swing left/right) Cranes sometimes use safety limit switches to prevent operational damage.



8. Swinging Circle and Turning Circle

These terms are related to navigating a vessel and are quite often confused with one another.

Swinging Circle is used when a vessel is anchored and is the theoretical radius that the vessel is expected to swing around the anchor point. The formula used to calculate the Swinging Circle is (Number of Shackles x 27.5 meters + Length of the Vessel in meters). While at anchor, the Skipper or Officer of Watch must ensure that the vessel stays within this radius and that no other vessel enters it, and come into close quarters.

The Turning Circle is the diameter confined by a vessel when the rudder is put hard over when moving full speed ahead and is part of a vessel’s manoeuvring characteristics. During a vessel’s sea trials, the Turning Circle is determined in Loaded and Ballast conditions for shallow and deep waters. In larger ships, the Turning Circles are always posted on the bridge and the wheelhouse.

swinging circle while at anchor 


9.  Gangway and Accommodation Ladders

Although Gangways and Accommodation Ladders are both used to bridge the gap between a vessel and the shore, their rigging and usages are entirely different.

Gangways are rigged to the fore and aft line of a vessel and are mandatory for any vessel over 30 meters in length. Gangways must only be connected to railings that are designed for this specific use and must not exceed 30 degrees.

Accommodation Ladders are quite similar. They are rigged in the fore and aft direction and face astern. They are mandatory on vessel’s over 120 meters and must never exceed 55 degrees. They are usually fixed and are swung out using special winches.

gangway on a ship 


10.  Hatch and Hatchet.

A Hatch is a horizontal door in a floor, on a deck, or sometimes on a ceiling of a vessel, that is hinged and can be opened.

You tend to open a hatch to allow fresh air in to a space, or to store gear away, such as sails or other equipment.

A Hatchet is a small light axe, with a short handle. A hatchet may also be referred to as a tomahawk.

A hatchet is often required to be carried on board a vessel in case of a fire. A hatchet can cut through electrical wires, hoses, rope, lines, wood among other things.


There we have it folks! I hope this has helped you and your language around boats! If you would like to find out more, head across to Sea Blogs! We'll sea you there!