High Guard says this, but never defines what an "Average Commercial Shipyard" is.
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Is an average shipyard a shipyard at a Class-C Starport, since Class-C starports are considered "average"? Except that the Traveller Companion has this to say about average starports.
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So, this says that Class-C starports are unlikely to have a shipyard of anykind, and yet...
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This clearly states that to be considered a Class-C starport, it must have a minimum of 200 tons of shipyard.
Which leaves Me severely confused as to what any of this is supposed to mean. Which takes me back to my original question...
What is an "average commercial shipyard"?
I think this is another one of those places that the rules can only be used as general guidance and you have to toss out specifics. An "average" shipyard should be an oxymoron because they shouldn't exist. If we use actual history as a bellweather you'll find that shipyards are going to be all over the map - depending on time (as in what century), available technology, local resources, and most importantly - need.
Time and available technology - As TL increases we see fewer and fewer actual shipyards that exist for a variety of reasons - chief among them is cost. A large shipyard able to churn out the largest vessels has, since around the industrial era, been getting more and more scarce. It takes a lot of resources just to have one, so they aren't built unless necessary due to costs and maintenance. The basic infrastructure feeds into the need as well. If the market only supports limited numbers of ships of XXX size then you'll build them at a rate that makes economic sense. For Traveller, since ships are supposed to be able to last for quite a while, an entire sector might only have 1-2 shipyards capable of building starships in excess of 10k Dtons (tonnage is just used as an example) because demand for such ships, per year, simply isn't enough to justify a shipyard of that size at every class A starport. Today we see a multitude of smaller shipyards that churn out the more numerous smaller ships and yachts, but just a handful that can build 100,000 ton cruise liners, or 20,000 TEU container ships.
Local Resources - This is where it gets interesting as your TL goes up. As your capability increases to transport whole engines that can power the largest of ships, it means you are no longer tied to having your boiler shop located next door to your fabrication facility. Even entire sections of ships have been built and towed elsewhere to be mated to the final build. So a shipyard MAY have local resources that build your jump drives, your maneuver drives, your hulls, your electronics, etc, nearby or even in the same system. Then again, it's quite logistically possible to have manufacturers located in different systems and the parts are manufactured elsewhere and transported to the final assembly location. Thus your "average" shipyard is just the place where you assemble all the pieces (or at least could be). This also means that you may not need that many shipyards capable of building the largest of ships. You just need a logical assembly location for these types of ships (the logic works as well for smaller vessels).
Need - This one is more a result of a basic understanding of what you are trying to portray. Obviously the pre-defined sectors fall within the rules, but the rules have large holes in them that don't hold up when you try to break them down using common sense. Plus the contradictions already present make for too many head-scratchers. With the official published rules it's best to just accept them and not question them. But if you plan on using shipyards for your campaign, then you'd want to fix them by asking yourself - just how many shipyards would this sector actually NEED in order to satisfy shipping capacity (which in and of itself is a referee question as well). Keep in mind it may actually be more economical to build the largest of ships in a subsector over and just fly it to it's intended operational area. They are, after all, starships, and are quite capable of being moved a reasonable distance to start their lives as transports. Even if your planetary economy is tech-capable it may not do this. A small TL-12 world of a few million people doesn't have the screaming need to justify building all the necessary industries to support a shipyard. It may repair or overhaul some, but size and capacity should be driven by the local need.
The above is for large ships that PC's may interact with, but not actually use for an adventure. So anthropomorphizng the technology for large starships is not necessarily a bad thing. It makes the hand wavium easy.
PC type ships, or say those 10kdtons or smaller, is probably where the most need is going to be. I don't see an actual need for every Class A starport to be a hub for building starships. The exception to that need, of course, is that world A is probably going to be loathe to let world B be the place where they have to go to get their ships built. At some point the human condition kicks in and the locals are gonna be nationalistic and look to justify being independent. Which works for the worlds that can afford it - the rest may cobble somethings together, but not really be able to do much beyond assembling a hull and installing (at least some) imported components. Some things just may be beyond their capabilities. And, lets face it, you can build a 'shipyard' anywhere you can assemble components or float them into place. It may not be the most efficient or fastest, but tech wise you can do it. The question of IF you would do it is different.
Bottom line is that hard and fast rules are just going to be woefully inadequate to give you the answers you want. If you want hard rules, just hand wave them. Otherwise concentrate on smaller areas and detail them out as opposed to just blind die rolling, and establish your shipyards and industrial concentrations that way.