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Congratulations with this website Hendrik.
By reading in the Timeline I came across the following statement:
"Marius is just 19 when he passes for his first exam as ship's engineer in The Hague on 22 September 1923."
In those days you were either trained as an Engineer or a Deck Officer.
He finally was the Captain of the "Boelongan" so he was trained as Deck Officer, I presume.
Note. Nowadays students can be trained as "Marof's", Maritime Officers. So they can perform their duties as well in the Engine room, as on the Navigation Bridge.