Re: Why superships cost more per mass

On Fri, 29 June 2001, David Griffin wrote:

> The question isn't is a Battleship more complex
> and harder to build than a Destroyer. The question
> is is a 200 mass battleship more trouble/complex/
> expensive to build than 4 50 mass destroyers all
> put together.

Yes. As Tom and I both pointed out, it's not linear. 4 mass 50
destroyers are 4 times as complex than 1 mass 50 destroyer. 1 mass 200
battleship is more than 4 times as complex as 1 mass 50 destroyer.

> Are there really more points of
> failure between say 1 big engine and 4 smaller
> engines?

Yes. For one thing, there's that whole question of inverse square and
inverse cube laws. Why can an ant carry many multiple times its own
weight in food when humans can't? A human being could never survive
being 50 feet tall because human bones just aren't strong enough (even
if they were proportionately bigger).

In a sphere, the surface area goes up as a cube of the diameter. This
means that a sphere twice the diameter has 8 times the surface area, and
requires 8 times the material to cover that area. 

> Is it easier to maintain 2 small honda
> motorcycle engines than 1 larger honda car engine?

I'd say yes. That's a good example. It's far easier to lift a motorcycle
engine off its mounts than it is to lift a car engine. The car engine is
just too heavy. You need special tools, and correspondingly more time to
lift a car engine than a motorcycle engine.
 
> Ok, so I'm not a naval architect or a construction
> engineer on ships. Is there anyone on the list who
> DOES have actual direct experience -- i.e. who is
> a real expert? If not, all we have is speculation.

Well, we're speculating about starships, so it will always be just
speculation. However a number of us work in technology industries where
complexity does not scale linearly.

I did some checking on the web. The only references I could find were to
CVN 71 "Abraham Lincoln" and DDG 82 "Lassen". Now, the Lincoln was built
in 1988, the Lassen in 1999. It took 40 million manhours to build the
Lincoln before it was christened, and 2 million manhours to build the
Lassen before she was christened. Their masses are about 104,000 tons
and 8500 tons, respectively (though I also saw a reference to over 9000
tons for the Lassen). 

This works out to 384.6 manhours per ton for the Lincoln, but only 234.3
manhours per ton for the Lassen. If complexity was linear with mass,
those two numbers should be about the same.

In another set of web sites, I found that the Lincoln cost approximately
US$4.5 billion to build. DDG 100 will cost $338.2 million. Note that
this does not take into account inflation and the true value of the
dollar compared to the building times. DDG 51, the first in the same
class, was built in the early 90s for $143 million.

In other words, the Lincoln is 12 times the mass of the Lassen. Yet, the
Lincoln took 20 times as long to build, and cost between 13.3 and 31.5
times the price (with the higher number being more accurate, as it's in
dollars of about the same inflationary value).

So, while not exactly scientific, I think it shows that in today's real
world applications, a bigger ship costs more to build -- in time and
money -- per ton than smaller ships.

Allan Goodall - agoodall@canada.com
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