Method for determining manpower needed to crew a starship based on size

Question

In the setting I'm building up, there are starships ranging from several meters in length to 1.6 kilometers. I'm trying to devise a means to determine what sort of manpower each ship would likely need.

My current method is looking at naval vessels of similar roles to their fictional counterparts (anywhere from World War II to cutting edge modern ships) and scaling their crew by the difference in size of the two ships.

I was curious to hear of other methods for determining crew requirements and to see if the numbers from my method sounded plausible.

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Example of my method:

• Fictional ship: Heavily armed battleship, with a focus on firepower, length of 600 meters.
• Comparable ship: Yamato-class battleship, length of 256 meters (crew 2400-2700)
• 256/600 = 2.34
• Fictional ship crew: 2.34 * 2500 = 5859 maximum crew

Answer 1

Expanding on Sasha's answer, warship crews are not so much about the length of the ship, but about their displacement and whether they're specialized or not. But really it's all about technology level.

For example, HMS Victory, a 1st Rate Ship Of The Line launched in 1765 with a length of 70 m, it had a crew of about 850 men to sail the ship and work the over 104 guns. Most of the crew were necessary to work the sails and guns, but nearly everyone pulled double or triple duty.

Admiralty regulations stipulated the crew required by each type of gun, based on a rule-of-thumb of one man per 5 cwt [about 100 lbs] of gun, which worked out as 14 men for a 32-pounder, 11 men for an 18-pounder, and so on. These were the absolute minimum required and if, as generally happened, only one side of the ship was engaged with the enemy, then the crews on the unengaged side crossed the deck to assist... The crew of each gun was lead by a gun captain but most of his crew had another designated task and when called for by the pipe had to leave the gun. Such tasks included trimming the sails, manning a boarding party, fetching powder, acting as a boat's crew, and so on.

Patrick O'Brian's Navy by Richard O'Neill, Chris Chant, David Miller, and Dr Clive Wilkinson, p 76

Fast forward to a South Dakota class battleship [I'm using them as an example of a WWII battleship because they were never upgraded]. At 210 m long if we just scaled up the crew from HMS Victory based on length we'd need 2550 men... which is surprisingly close to the actual 2364 number, but this is a ruse.

Length is not a good choice for scaling a ship, displacement is. It's a measure of how much water the ship displaces when floating, and it gives you a rough idea of the size of the ship. HMS Victory is 3500 tons, but USS South Dakota was 45,000 tons! Over 10 times the size of Victory, but only 3 times the crew. Naval warfare changed a lot between Napoleon and WWII.

What about a modern ship? We don't use battleships anymore, our modern "do it all" surface vessel is an Arleigh Burke class destroyer. 155 m long and displacing 9,000 tons, highly automated, it has a crew of just 300 men.

But an Arleigh Burke is a design from the 90s. The new Zumwalt class destroyers just being commissioned are state of the art. They're 180 m long displacing 14,000 tons with a compliment of just 140.

A closer comparison to a starship might be a submarine. A nuclear submarine is required to be self-contained for long periods of time, must be sealed off against the environment, is very tight on space, and has a highly technical engine and weapons. A Virginia class, state of the art, has a length of 115 m, displaces 8000 tons, and a crew of just 135. The trend line of submarines is not so clearly downward as their size and capability have increased dramatically over the decades, but as you can see it's a pretty small crew.

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You could make a plot of year, displacement, and crew and extrapolate, but as you can see the trend line for warship crew is inexorably downward.

This is of great benefit for warship designers. Less crew means less weight and space for that crew and all their amenities: less food, water, bunks, storage, kitchens, mess halls, entertainment, air conditioning, laundry, chaplains, doctors, medical facilities, medicines, etc...

On the other hand, less crew means more electrical power to run all the automated devices on the ship; overloaded electrical systems are a big hassle for upgrading navy ships. Less crew also means less people available for damage control and maintenance, until you have reliable repair robots.

So it depends on your tech level. If you're in a starship you can reliably assume a tech level at least equal to our own, but maybe you're in a retro-future Apollo-era space-race-run-wild setting with 60s level automation.

If you want to be realistic, you'd use as small a crew as possible. Starships are all about doing as much as you can with as little mass as possible, because $Force = mass \times acceleration$. Less mass means less force for the same delta-V means less fuel means less mass means less force.... More crew means more mass; not just for the crew itself, but for all the support structure that goes along with them (see above).

Answer 2

I guess the biggest question you need to answer is the level of automation available and how advanced is the drone and robot technology of the setting?

With enough tech you could easily have a crew of say 12 people on the command center, with every system automated and using drones when they need to go anywhere.

Answer 3

Rather than focus on the size of the ship first I would suggest you focus on what you want your crew to do. Break down your crew into it's component departments and work from there.

Presumably you're going to have a set of officers who run things from the "Bridge", some sort of engineering crew to run the engine rooms, other staff for things like medical and food prep and stuff like that. Do you need to carry troops? Add a "marine" squad. Fighters? You need pilots and people to maintain them. If your weapons aren't controlled from the bridge then you're going to need on mount crews for each gun.

Work out how many people you want for each. Presumably the story is going to be about these people anyway, right? Once you have that sort of detail nutted out you start working out how many people you need for each. Bigger ships have multiple engine rooms? More engineers. More guns? More on mount crews and so on.