Multi-hull warships

[Sea Skimmer]

Tank testing is never useless. It is only a matter of how far it will get you past guess work, and how large of a model tank you are prepared to pay for.

A couple of them are completely huge and allow for much freer movement of the models, some with there own scale propulsion which is important for this. Traditional WW2 style tank test was typically just a straight line tow, and that's a problem if were talking about a twin hull vessel propelled by massive water jets on the stern and sucking from the bottom. High speed ships generally dig in the stern under power, but that doesn't happen the same way if your towing a model. Model towing however is much cheaper, and produces reasonable results quickly.

Japan brute forced the hell out of the Yamato hull form and bulbus bow via a huge number of tow tests, and the results worked out better then even expected, but for most world war era stuff you weren't doing many runs.

The limitations of tank testing are why completely novel hullforms are usually tested at some kind of reduced scale on lakes or the actual ocean though. For DDG-1000 for example several progressively larger models were lake tested up to one quarter scale with an actual crew on the model controlling it it was so big.

Lakes might sound stupid for testing a seagoing ship, but they have the advantage that you can actually try to control the wave conditions on a repeatable basis. Usually they use the wakes of other larger vessels to make the required wave. The wave making vessel being the exact same size and speed each time means same wake, within the error produced of the actual lake waves which is small.


DDG-1000 R&D model demonstrating that it at least wont sink in a calm turn!

[The Duchess of Zeon]

Argh-bargh, I'm having like five job interviews a week right now.

Okay, yes, fluid dynamics is not a solved problem. I would have said exactly the same thing as Terraltha when I first started my master's education. But my supervising professor corrected me. What we do, of course, is finite element modeling of fluid flows. We use these kinds of models to correct for deficiencies in tow tank results. Tow tanks remain absolutely necessary; I simply meant to imply it wasn't possible to achieve the desired levels of performance using tow tanks alone. The other thing of course is the NIH syndrome in the 19th/20th century to be sure.

Design of ship hulls was art rather than science until we could do finite element modeling in fluid dynamics. Now we have a kind of fuzzy granularity to our results: We know where something must be, but of course we still can't solve the N-S equations, so we cannot pin down exact answers. This causes problems in regimes like around the stern of a ship--the interaction of hull, screw, and rudders--but otherwise we can effectively characterize the design by combining methods.

You may notice that the catamaran has fallen out of favour to the trimaran in a lot of military design work, and that's no accident or mere preference. Trimarans work better in terms of flow and propulsion--in the end a main hull with outrigger hulls is just better than two equal sized hulls. There are still plenty of applications where a catamaran is seen as desirable. I used to not like multihulls at all, but after my education, have an affinity for trimarans. The trimaran is a logical powered evolution of the Polynesian outrigger, so it really is an NIH sort of thing too; I'm not sure if Europeans would have thought to even put one in a tow tank in the 19th century.

So, the essence of it is that we've by no means solved fluid dynamics, but we can do a lot of neat things to properly characterize hull and ship performance through a combination of testing and computer modeling, and those things let us reliably design multihulls even if the design of course cannot be based on an analytical solution to the N-S equations since that's impossible. This was what let to my integration of simple mathematical simulation with testing through an autonomous rig in my thesis work, which is being used to characterize interactions at mid-level Reynolds numbers i.e. technically turbulent regimes but below say 50,000 where there are still recognizable pattern behaviours. This is important because in nautical land, ships and platforms and rigs are REALLY BIG, so the Reynolds number can be in that range even for an object simply drifting in current.

Now, here's the final component I hadn't addressed at all before: Sailing. Catamarans and trimarans require unique behaviours during sailing because sailing ships heel, they tack, they engage in operations with a propulsive system that requires the hull to change its attitude in respect to the water surface. This can result in really stupid high loadings through multihull connections. It's why the Polynesians used light outriggers for stability purposes, and the big catamarans were mostly oared or used a very simple rig. So in the end combining multihulls with an advanced 19th century late European sailing rig is something I believe would just break down.

Probably by far the most interesting use of the multihull in the 19th century was the steam USS Demologos, with her false bulwark upperworks around two immersed catamaran hulls, being used to engulf and protect a single centreline paddle wheel. This kind of design might have seen a lot more traction if a major war between European powers had occurred in the 1820s or 1830s.

[Sky Captain]

From my experience with multihull and monohull sailboats I can tell catamarans and trimarans are very fast only if kept light. A lightweight 5 - 6 m long catamaran with 2 people on board can sail at 15 - 20 knots - more than twice as fast as similar size monohull sailboat. Load it up with 5 people and gear for weekend camping cruise and that speed advantage is lost. There is no reason to use multihull and many reasons to use monohull if primary mission is to carry heavy cargo.

Looking at existing multihull designs they mostly are used for fast ferries and sailboats. None of those are meant to carry heavy cargo. There are no multihulls among oil tankers, bulk carriers and container ships which are built to carry heavy stuff.

Also structural loads on crossbeams get very large when size of multihull vessel is increased. All high end ocean racing catamarans and trimarans are made from carbon fiber composites (very expensive) because only that can take the loads and still remain light.

For large warships I think aircraft carrier may have some advantages if it was catamaran. It could have much larger flight and hangar decks than similar sized monohull carrier and better stability in rough seas which is important for flight operations.