This task involves annotating the answer type to a given question that involve some kind of complex reasoning (including numerical reasoning). Note that the questions require looking at more than one part of the passage to answer. There are 3 possible answer types (i) spans, (ii) numbers and (iii) dates. If the answer can be found in the passage, label it as "span". If the answer is a number, label as "number". Similarly, label "date" if you think the answer to the given question is a date.

Passage: Since 1912 the Royal Danish Navy has operated aircraft either as a functional part of the navy or as flights conducted by the Royal Danish Air Force. Aviation pioneer Robert Svendsen purchased the first aircraft, a Henry Farman, and gave it to the navy who subsequently named it Glenten . Later the same year, a private funding effort made it possible to purchase two Donnet-Leveque flying boats named Maagen and Ternen . At the outbreak of World War One the navy had two operational flying boats and five educated pilots making it possible to make daily reconnaissance flights over the Sound, monitoring German mine-laying activities. From 1914 to 1917, the Orlogsvrftet produced eight OV-flying boats and in 1915 and 1916 two naval flying stations were established in Copenhagen and Nyborg, mainly focusing on the two international seaways resund and Storeblt. After the war the Danish production of aircraft was stopped due to several crashes and an international surplus of warplanes. Following British advice, five Avro 504 were purchased for training purposes and based in Ringsted between the two important straits.
Question: What year did production begin on the Orlogsvrftet producing eight OV-flying boats?
date