Depends on what kind of tender you want since the term has been applied to very different ships capable of performing quite different tasks over the years.
Anyway to get the discussion rolling here's what I think a sub tender needs.
- Long endurance
- Large holds
- Tanks for fuel and water and other liquids needed by its charges
- Cranes for transferring said stores in a fairly quick fashion
- Extra crew facilities
- Extensive workshops
On the upside that's the description of an ofshore supply ship, but on the downside people tends to keep a pretty close eye on that kind of ships.
I'd shy away from the catamaran salvage ship though since they're impossible to hide from even the most superficial inspection and because if you need to salvage one of your boats you'll have bigger problems than hiding it.
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