Fitter
The figure is a sort of technical assistant on the ship and is might vary from country to country. For example the director-general of shipping recommends two years training in the trade after completion of high school. The syllabi of the training includes diverse topics such as the theoretical understanding of the basic working principles of internal combustion engines and the combination of various workshops such as carpentry, forging etc. A proficient fitter in the engine room of the ship is certainly a great asset for the entire engine room staff.
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Motorman
The job of a motorman is quite similar to that of a fitter and depending on the company, they might require a motorman to a specialist training in a specific trade. A motormen has a vital role to play in keeping the engine room well maintained, and each motormen is normally assigned specific duties.
Oiler/Wipers
While the job of a fitter and motorman is quite technical in nature, there are other jobs such as cleaning the engine room, wiping the scattered oil and so forth and these are carried out by wipers. Their other job responsibilities could include tasks such as painting, chipping and assisting the engineers during routine maintenance operations.

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The above described jobs have been listed from the point of view of normal commercial vessels except cruise ships. The reason for this is that the cruise ship is much more requirement for staff and hence they could be specialised jobs such as air-conditioning technician, plumper and so forth. On non-passenger ships these jobs are carried out by the personnel described above for financial reasons.
It must also be noted that there may not be all the above personnel on all types of ships. This could vary with the size and type of the ship and in some cases their duties could overlap.
It is also worth noting that there are certain courses known as STCW courses which are mandatory for all ship staff irrespective of their rank or position. These courses include several short trainings such as firefighting, first aid, survival at sea etc.