Wednesday, October 21, 2009

quick Trav item

A glance at this wikipedia page suggests that the largest real world commercial ships top out at about 5 times the displacement of the largest military vessels, with many commercial vessels three times the size of the largest warships. So if the largest military vessel seen in your Traveller sector is a 60,000 ton cruiser, then the biggest merchant vessel one would expect to find would be 300,000 tons, with 180,000 ton vessels more common.

I've long wanted a rule of thumb like that.


  1. Would that rule transfer from ocean to space? Displacement is an issue due to weight. Ships need to float.

    In Trav, ship size matters due to momentum... but I don't know that there is any equivalency there. What would stop some merchant vessels from being even bigger?

  2. Even bigger, potentially, since Traveller should have a class of ship that doesn't exist on Earth - the colony ship. Since upon its arrival it will be salvaged down to the last rivit and wire for the new colonists, it needs to be accelerated only once. Plus defensiveness could be sacrificed. This would eliminate many of the size restrictions on the vessel. Colony ships could be ten or twenty times the size of military flagships.

  3. Wouldn't that ratio vary depending on various factors.

    In modern times, military ships don't have the same value they did in earlier times, and there are trade-offs between size and cost and crew. Merchant ships can be quite large because the crew size doesn't increase all that much.

    In WWII, I think battleships and carriers were the largest ships and merchant ships were comparatively small.

    How about in the age of sail?


  4. If one goes back far enough into the age of sail, the distinction between merchant vessel and war ship becomes largely academic.

    The mere fact of military space ships, designed for military requirements means that a Navy vessel will attempt trim out every last unneeded bit of mass for; the best acceleration and lowest target profile, and try to cram the largest crew, most redundant and powerful systems, and best weapons for the role, at the cheapest price.

    Those requirements are in fact mutually exclusive so every design will be a compromise with budgetary realities and what can be sold to the naval requirements board.

    A civilian merchant vessel on the other hand, needs to minimize crew complement (fixed costs) and maximize cargo carried.

    On the gripping hand, once piracy becomes a major issue (when the insurance rates become more expensive than increasing fixed costs) merchantmen will want larger crews and will likely arm their vessels.

  5. Would that rule transfer from ocean to space?

    Only if I say so.

  6. I looked at that chart and saw two different rules of thumb:

    I think the supertankers are a bad comparison for traveller: ignoring the physics comparison between water and space, a ship that huge, valuable, slow and *defenseless* would be such a magnet for piracy that it wouldn't exist imho.

    So if you exclude supertankers, the largest aircraft carriers are only about 50-60% smaller than the largest container ships and largest passenger ships.

    Military ships for 'cruising' tend to be a lot smaller - even a huge one like the Ticonderoga class is about 10 times smaller than the average container/passenger ship.

  7. Anonymous12:04 PM

    I think it makes sense that large transport ships would be designed for areas that are well-patrolled and very safe. They should have minor defensive weapons but most of their power output goes to propulsion. This is why they can be so big compared to warships that need to be able to power weapons and such.

    More mass means less maneuverability as well. WHile a merchant ship doesn't expect to need to be able to turn easily, a warship does. This further limits the warship's mass.

    A warship might have more complex systems than a merchant ship, and may need to have marines for repelling boarders or boarding actions of their own. More people means more support systems that require more power. Less power for propulsion of mass.

    These large, lightly armed merchant ships would occasionally need to travel beyond the well-patrolled zones. Or if piracy became a problem. In these cases perhaps a warship would escort the merchant ship, or a special booster ship attached to the hull to provide extra defensive weaponry or simply a speed boost to evade pirates. This booster ship may simply be a drone carrier. Of course, to cut down on costs occasional merchant consortiums will attach a dummy drone booster to deter pirates that lack sophisticated scanning equipment ...

    Note that pirates don't want to just destroy the merchant ship. They want to capture it. A military escort can deter pirates even though the merchant ship is still vulnerable in a fight.