Continuing our occasional series explaining lifeboat terms and operations Lifeboat Numbers...

There is a logic to the numbers seen on the side of lifeboats, and once you've cracked the code the combination can tell you quite a lot about the boat.

The numbers are the lifeboat's Operational Number, and the characters before the dash give the class and those after it denote both the chronological number within the class and can also provide a clue to the construction material.

Lifeboats up to and including the Arun class were built using imperial measurements, and the first two characters are the length of the boat in feet. Hence Aruns begin with either 52- or 54-, Tynes with 47-, Waveneys with 44- and Bredes with 33- .

From the Mersey onwards metric units were used, and the length is given in metres. Hence Mersey numbers begin with 12-, Trents with 14- and Severns with 17-.

Following the dash is the individual, two digit number within the class - issued in chronological order of building.

Hence 14-01 was the first Trent, 14-02 the second and so on.

But wait, what of those lifeboats with three digits after the dash? Well, here is your clue to the construction material.

The extra digit will always be a zero and always follow the dash - hence Tyne 47-001 or 47-012 and Mersey 12-001.

The leading zero indicates either steel or aluminium construction. All Tynes are steel and have threedigit suffixes but some Merseys aluminium (with the zero) and some are fibre reinforced composite (without the zero).

All-weather lifeboats also have another number, called the Official Number, which is simply a number issued in chronological order of building, irrespective of class or material. This number, prefixed by 'ON', is used mainly by the RNLI's operational and technical departments to track boat movements and maintenance.

Inshore lifeboats also carry an Operational Number - a single letter followed by a dash and a three-digit number.

The letter indicates the class and the number is once again a chronological number within the class.

Some early inshore lifeboats were conventional rigid boats and were allocated class A. The A class is now extinct, the last being withdrawn from Poole lifeboat station in 1995.

They were followed by rigid inflatables designated B class - first Atlantic 21s and then Atlantic 75s. There is a way to tell them apart - any B class with a 700-plus number is a 75! The smaller inflatables were allocated classes C and D, a C having twin engines and the slightly smaller D a single. Both are numbered chronologically and have progressed through various modifications over the years.

• As an aside a lifeboat uses her Operational Number as a radio call sign when on passage (perhaps to another station, on trials or when on her way for maintenance). Hence 'Salcombe Lifeboat', when on station becomes 'Lifeboat Fortyseven- zero-two-two' when on passage..