Class hull and machinery is not issued under the authority of the flag state which means it is not statutory certificate, however some flag states require approval for the issuance of the class hull and machinery certificate

The importance of the class hull and machinery certificate arise from the traditions in maritime industry as it historically use to address the condition of the ship, accordingly all parts involved with the ship , such as insurance companies, port states, charterers etc, give this certificate higher importance

The concept of classification of ships:

Classifying a ship means simply to specify the purpose the ship can be used for

Classification process starts at the design stage by reviewing and approval according to the class society rules

second stage is the construction stage where construction procedures are verified to be in compliance with the approved design

Latest stage is the in service stage where the vessel is surveyed to make sure that it is maintained in the condition complying with the class society

Nowadays the concept of classification of ships extends to cover the hull, the machinery, the boilers, the tailshaft, the electrical installations, the cargo gears and even some specific safety equipment

Survey loop for class hull and machinery certificate:

Initial survey:

class hull and machinery certification starts with the initial survey which is to be conducted at the time when the ship is firstly certified by a class society

Annual surveys:

Annual surveys are carried out at the anniversary date of the initial survey with a window of three months before or after.

annuals surveys are carried out at the first, second and fourth anniversary dates

Intermediate survey:

normally it is due at the middle of the five years loop with a window of 6 months before or after, but practically is carried out at the third anniversary date 

special survey:

Special survey is carried out every five years interval, so it is normally done at the initial survey and at the renewal date after five years

Dry dock survey:

Dry dock survey is carried out in the dry dock at the interval of 30 months and maximum at 36 months in condition that dry dock survey to be carried out twice every five years

Dry dock surveys are considered part of class hull and machinery and cargo ship safety construction certification

In many cases it is done as part of initial survey

Bottom surrey:

Bottom survey are always carried in dry dock, and sometimes additional bottom survey can be carried afloat and called under water bottom survey

Tail shaft surrey:

Tail shaft survey is carried out every five years intervals where tailsahft can be withdrawn for further inspection and test

In many cases it is done as part of initial survey