ORIGINAL: Elladan
The production time in game (in days) is the lower of (10*durability) or (time to arrival). That's how long you pay for a given ship assuming you don't accelerate.
So a sub with durability value 49 takes 490 paid days to complete, costing Japanese 24010 shipyard points in total.
Did I ever say how much I hate hard code? What is so hard about letting US enter the correct data in a field? A German submarine takes about 9 months to build. A German sub also has a diving depth on the order of 650 feet = durability = 65 = 650 days to build. This is the source of the problem. We can decrease durability - but then pay the price in a sub too easy to sink. Since ALL subs are too easy to sink the way code works - I hate do do that.
Another problem may be the way devices are coded. Stock used 9999 for big guns - and I use the actual weight.
So even a gun that weighs huge amounts - hundreds of tons - is much less. It makes battleships relatively cheaper in RHS. On the other hand, 9999 is code for "immobile" - and I have MOBILE heavy guns. Also 9999 is used for people (in land units) - and it makes the reports seem out of whack. Immobile is meaningless for naval guns anyway. What a system.

