Ships of my current Survey method

[Steve Walmsley]

I think Andrew's right - I remember we had a similar conversation when Steve put in the current breakdown rules.  My recollection is that you should pretend the 30kT ship is 5 * 6kT ships, and use that as the baseline for comparison,  The reason is that a breakdown on a 30kT ship costs the same as one on a 6kT ship.  In your example above, with your 5*6kT ships you can expect 5*70% = 3.5 failures per year.    With your 30kT ship, you can expect 1.5 failures per year, which is a lower rate of using up spares (this makes sense, since you devoted more mass to engineering).  If 3%*6kT holds enough supplies for 2 failures, then the 6kT ships will use up their spares in about 3 years and will be at 1 failure/ship (the point below which they risk system damage) after 1.5 years.  The 30kT ship, on the other hand, will have supplies for almost 30 failures, which will take almost 20 years, plus the time to be down to the 1 failure level will be almost the same.  Again, this makes sense, since you put almost 3 times as much engineering on board the 30kT ship.

So I guess the message is that one should look at (maint supplies)/(failure rate) as a rough metric for the survival time of the ship.

That's exactly right. Assuming similar systems, a 40,000 ton ship with 4% of its mass dedicated to engineering should use exactly the same number of spares over time as four 10,000 tons ships that each have 4% of their mass dedicated to engineering. The failure rate on the 40k ton ship will be 4x higher but it will also have approximately 4x as many spares and the individual failures will cost no more than those on the 10,000 ton ships.

Steve

[Steve Walmsley]

I have to admit I have never build anything above 15k tons, the larger ones in my example have only been thrown together in the ship design window to get the numbers, so I have no real experience with large ships.
That being said, I also have not realy thought the thing through from that angle, thanks for lighting me up  

I might have to reconsider my design philosophy (which currently is/was: smaller is better)
On the other hand, I have to look realy hard at what the advantages/disatvantages of large BBs vs. more numberous CA/CL are

The main advantage of larger ships is in economies of scale. For example, Aurora calculates the amount of armour required on the basis that ships are spherical. As the size of a sphere increases, the ratio of surface area to volume decreases. In other words, a 50,000 ton ship with strength-4 armour uses less armour than five 10,000 ton ships with strength-4 armour. Sensors (and to a lesser extent fire control) are another area where you save mass as the 50k ton ship will only need one set of sensors compared to five sets for the smaller ships. Having five smaller ships means more flexibility in terms of deployment, smaller maintenance facilities and cheaper development costs for systems such as jump drives. As with many other things in the game, it will depend on your situation and your general fleet design concept.

Steve