Zumwalt DDG

[locarno24]

The tumblehome sides are supposed to improve sea-keeping whilst being stealthy as well.
Compared to what you might call 'proper' geometry, it puts a lot of mass lower in the hull, which lowers the centre of gravity and hence makes the ship less prone to minor rolls.

On the other hand, it also makes the ship fatter below the water, which makes for more drag - so it eats more fuel per mile, is more of a maintenance hog on its engines and is generally less efficient at manouvring.





In Traveller scale, your best weapon for a multi-role ship is the torpedo barbette - it allows to switch from ship-to-ship engagements to ship-to-surface engagements without needing to re-arm.

Ortillery 'earth-shattering kaboom' railguns are a distinctly different weapon to a ship-to-ship autofire weapon, and both are bay mounts. Having only one bay mount and just feeding different ammunition types through it is generally more efficient.

An independently operating ship in Traveller is going to want small craft. 40 dTon pinnaces provide a decent chunk of capability - a proper cargo door and a bay that can pack in meaningful cargo and/or light ground vehicles, twice the endurance capacity (2 weeks) of the other generic smallcraft, and the capacity to mount a pair of lasers to provide a fairly credible armament for a runaround.

A hundred and ten dTons buys you a bay capable of operating a pair of pinnaces (so there's one available when the other is down for maintenance).

 

[phavoc]

When a normal ship takes on more water, it becomes more buoyant.

Umm, nope. ANY ship taking on water, becomes less buoyant as the water is filling space formerly occupied by air. Whoever wrote that on the blog was taking copious amounts of hard drugs.

According to this blog (I found it) http://navy-matters.blogspot.com/2013/04/whos-watching-zumwalt.html - the specific design of the Zumwalt hull is less buoyant. Here's the passage:

Survivability – Because of the tumblehome hull form, the waterline cross-sectional area decreases as the ship sinks. This means that as the ship takes on water from flooding damage, the ship will have decreasing buoyancy. This is the reverse of a conventional ship where the cross-sectional area increases.

I don't know enough about naval architecture to dispute the idea. I did run across another post elsewhere that was critical of the bow of the Zumwalt. The guy may have not been right, but damn he did a helluva job with math and lots of examples of sea handling and the idea that a wave-piercing bow may come to bite the Navy in the ass if the Zumwalt's ever have to go through really heavy seas.

 

[Rick]

Which is presumably why they had to cancel the new cruiser concept and cap the Zumwalts at just 3 - its going to be the new technology testbed for the next generation of USN vessels.

 

[sideranautae]

When a normal ship takes on more water, it becomes more buoyant.

Umm, nope. ANY ship taking on water, becomes less buoyant as the water is filling space formerly occupied by air. Whoever wrote that on the blog was taking copious amounts of hard drugs.

According to this blog (I found it) http://navy-matters.blogspot.com/2013/04/whos-watching-zumwalt.html - the specific design of the Zumwalt hull is less buoyant. Here's the passage:

It is irrelevant what blog entry says. Physics makes it otherwise. You fill any containers air filled spaces with water (for a container floating on water) and it becomes LESS buoyant.

 

[Rick]

It just means that if it takes on water, it'll settle a tiny bit lower in the water than would a conventional ship.

 

[locarno24]

When a normal ship takes on more water, it becomes more buoyant.

Umm, nope. ANY ship taking on water, becomes less buoyant as the water is filling space formerly occupied by air. Whoever wrote that on the blog was taking copious amounts of hard drugs.

According to this blog (I found it) http://navy-matters.blogspot.com/2013/04/whos-watching-zumwalt.html - the specific design of the Zumwalt hull is less buoyant. Here's the passage:

Survivability – Because of the tumblehome hull form, the waterline cross-sectional area decreases as the ship sinks. This means that as the ship takes on water from flooding damage, the ship will have decreasing buoyancy. This is the reverse of a conventional ship where the cross-sectional area increases.

I don't know enough about naval architecture to dispute the idea. I did run across another post elsewhere that was critical of the bow of the Zumwalt. The guy may have not been right, but damn he did a helluva job with math and lots of examples of sea handling and the idea that a wave-piercing bow may come to bite the Navy in the ass if the Zumwalt's ever have to go through really heavy seas.

Okay - bad wording on the blog. It's not "water makes me lighter".

As a ship takes on water, it settles in the water - i.e. more of it goes "below the waterline" until the increased displacement from the volume of the hull now under the water balances the lost bouyancy from the flooded compartment. It's like a boat settling deeper in the water as you load things into it. Makes sense?

Now on a 'normal' ship, which looks like a -V-, each deck is wider and hence has a greater volume than the one below it, so your rate of sinking is a decreasing curve - a depth of water X on deck 1 only pulls a lesser depth Y of deck 2 above it below the waterline to compensate.

On a tumblehome ship, the reverse is true - because each deck is wider than the one above, the same depth X of water on deck 1 needs to be offset by a greater depth being pulled below the waterline, so the ship sinks in an accelerating fashion.

 

[locarno24]

I don't know enough about naval architecture to dispute the idea. I did run across another post elsewhere that was critical of the bow of the Zumwalt. The guy may have not been right, but damn he did a helluva job with math and lots of examples of sea handling and the idea that a wave-piercing bow may come to bite the Navy in the ass if the Zumwalt's ever have to go through really heavy seas.

Note that I said "minor rolls" above. It's more stable in normal and mild seas.

In really BAD weather, the shape encourages big waves to roll over you (because they meet a slope leading up over the ship) rather than lift you along with them (like a 'normal' ship which has an outward-sloping hull at the waterline), which means you can start experiencing what I believe naval crew call Very Bad Things.




If you want a quick primer on naval hull design, look at megayachts and heavy freighters. They get designed and built continuously (where warships don't) and are sufficiently pricy to attract all sorts of hightech software tools and very clever people into their design process.

Tumblehome hulls are common in the former and rare in the latter, because yachts (a) want stable hulls in good weather (b) don't go out in bad weather, (c) don't go far, and (d) don't tend to be owned by people who balk at running costs unless they're obscene.

A freighter or liner, by comparison, needs to be a paying proposition, which means running costs and fuel efficiency are important, and can't afford to stay in port or divert around bad weather systems which might occupy half an ocean, and I've yet to see a ship in that class built on a tumblehome.

Translating into warships, tumblehome hulls make good sense for things like Littoral Combat Ship (and I believe one of the designs is built on one) but are much riskier for a deep water ship.

Of course, ultimately even a DDG isn't really a deep water superiority ship. The deep water superiority vessel of the last 60+ years has been the submarine. Other ships are good for local area or point defence of a convoy or carrier.

 

[Rick]

Tumblehome hulls are not a new technology - its what was used for warships from the ancient Egyptians right up to just pre-WW1. Why they were considered sea-worthy in all conditions then, but not now, is a bit of a mystery to me. Historically, several tumblehome ships at the battle of Tsushima became more unstable after they had taken hull damage (Borodino class), and "watertight integrity was breached", other than that they were considered more seaworthy than the hull types that are usually used today.