Ships are not always offered immediately at the shipyard, it may take several jumps to finally find it on sale.
Best way to test ships is not the shipyard, but the 'parade' of the start screen or the classical 'scripting a small encounter script'.
The first method is a quick way to see if your shipmodel (subentities and lights too) is working, you just need to place the name of your custom ship in the Demoships.plist .
The second method, is better for testing out the behaviour of the ship, lasers, exhausts, missiles etc. Some use it also to investigate their creations upclose from any angle, by temporarily replacing their AI by dumbAI.plist.
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exhausts are also hardcoded to be directed to the rear (or -Z) of the shipentity.
Sometimes you might want to direct them outward at angles to the main body, but this is not possible.
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In general, the best way to start out is to take the shipdata of a similar ship/entity to what you want to produce, then edit stuff.
When you want to add special behaviours or effects, look at how it is done in examples done by others.
-lights: look at police or orbs.
-turrets: look at behemoth or Armoured Transport.
-carrier: look at behemoth.
-other dockables: hermit, stations by OXPers.
-subentities: look at weeviloids, AT1.
-deathactions: Cloaked Asp.
etc, ofcourse there are many other examples of cool coding.
Don't forget to look in the elite-wiki at Alioth.net.
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Have fun!
ps Some less fun stuff:
If you want your ships to be accepted by the community in general,
be carefull not to create unrealistically powerfull ships.
Balance things out: Great speed usually needs a big engine so less cargo space, Offensive equipment is most effective when custom fitted, making a craft powerfull but less versatile in its configuration and difficult to upgrade.
Most ships are contemporaries and the classic designs in the Ooniverse have lasted for centuries. The Cobra3 is supposed to be a revolutionary design triumph and although it is a multi-role ship it is still superior to many older ships.
You could include code to limit your powerfull Nu-design to a later game time, but this is usually a bit beyond the casual OXPer.