danebrog wrote:I would honestly be interested in an approximate calculation just to ensure the necessary fuel requirements from an operational point of view
It may sound nerdy, but I find such aspects interesting.
It's totally nerdy, which is totally the intended point of this thread. I was hoping to talk more about specific fuel consumption and Froude Numbers than anything else.
So for the megaship to operate in a sustained manner as a cargo transporter (after destroying Japan's navy and merchant marine, for example, after which the Pacific part of the Pacific War is mostly over and the theater goes into quiet siege):
- At max speed, megaship uses 3mil HP to do 40kn.
- To figure HP at reduced speed, good estimate is ((speed-RPM delta)^3) * (Wave factor) ---- Here wave factor is the proportional decline in wavemaking resistance's share of total resistance.
- For 33 knots, I estimate a wave factor of 5%, thus HP = ~1.6mil.
- Megaship is going to have diesels for cruise and auxiliary generation, turbines for highest speed. About 2mil HP of each. For our transporter ops, we'll run on diesels.
- Diesel SFC at this time was around .35 lb/HP-hr. Therefore 250 long tons per hour, 6,000 tons/day, ~7.6 tons/nautical mile.
Note that, as the megaship ballasts close to a million tons to reach 110ft draft, we could also specify oil instead of ballast. By the above calculation, a million tons of oil would give the megaship ~130,000nm range at 33 knots. And/Or the megaship could act as fleet oiler.
daneborg wrote:And my shipbuilding expertise is not worth mentioning, but logistics issues are a bit different.
Logistics is fascinating. A "What If" that's incubating: What If US had invented containerized and RoRo shipping before WW2? IMO it removes many of the shipping constraints on American power projection, easily enables landing in France in '43 or earlier. Maybe gets the US Army into unnecessary trouble on the Japanese home islands.
daneborg wrote:Ensuring operational capability requires certain maintenance intervals (at least for the propulsion systems).
True. As megaship has is using ~80% of diesel HP in our 33kn example, it can keep 20% of its generators off line at all times. As there's enormous room to accommodate maintenance personnel and spare parts, I'd guess much maintenance can be done under way.
As the remaining 20% may not be sufficient for onboard hotel power draw, we may have to reduce speed to 30kn at times or - in a pinch when a tight cargo schedule demands making up lost time - activate a boiler/turbine set or two.
It all goes back to the magic of square/cube dynamics, Reynolds numbers, and Froude numbers, all of which make propelling a superlarge ship super efficient per ton, thereby making spare propulsive capacity relatively cheap.
daneborg wrote:What I was hinting at with the example of submarines (and/or bombers, for that matter): It was not about sinking, but about constant attrition.
Yeah I get that. But the ship's ability to be suffer damage without material impact on its combat capability is one of the virtues of designing an over-large ship. It's one of the reasons every iteration of the megaship has seen the benefit of combat draught only after taking on ballast: That ballast is a "damage reserve" whereby you're pinpricking a ship that intends to half-sink itself before battle anyway. Among other things, it means that torpedo and diving-shell hits during battle cause no immediate list, unlike other ships for which the primary non-survival goal of damage control is to counterflood against list, thereby preserving aiming accuracy and combat effectiveness.
This means that a megaship that has even dozens of holes in its hull can continue operating, continue seeking the next battle. It need go into the yards only when its combat capability has been eroded by damage exceeding its ballast allowance (as estimated upthread, up to 500 torpedoes - though fewer if it loses shafts).
daneborg wrote:(It´s just a WI after all)
Normally this would get my hackles up haha. Within WI's there's counterfactual reasoning, which seeks to test a historical narrative by examining the absence/reversal of the supposed causal factor. IMO that's a intellectually valid form of analysis with primacy of place in empirically-oriented social sciences and improperly undervalued by historians.
This What If is a bit different in that I'm not necessarily suggesting that the historical conditions for megaship's construction were feasible.
At its heart, the inquiry is this:
The US possessed the economic means to "bully" other seapowers prior to WW2 but for many reasons did not exercise it (we certainly did only ~5 years later though, and continue to do so today). When we chose to swamp potential naval challengers with our industrial might, we did it mainly via quantity. Our ships were as good any others but not qualitatively anomalous to the degree that our quantity was anomalous (of all major fleet and support types by the end of WW2 and ever since).
Had we chosen the path of qualitative anomaly along with quantitative, IMO it's inevitable that we would have built battleships larger than other powers' battleships - not merely more of them. This was indeed the path we followed in the supercarrier era. IMO this would have been the right path for a prewar hegemonic USN, given the inexhaustible economies of scale for ships (square/cube, Froude number, Reynolds number, water pressure at depth).
Whether pursuing qualitative superiority via scale would have looked exactly like this megaship is, of course, open to debate.