ShipShape

Ship building has advanced so that ships of a wide variety of shapes can be built, like Swath and Seaknife. I start this tread to gather discussions on the merits and demerits of ship configurations for specific purposes.
My first subject is inspired by a description of a video showing Kuznetsov sailing stolidly through a storm while an attending Udaloy looked most unhappy.
If a tricat destroyer has a long, thin main hull with a large draft and pretty long and thin side hulls with equally large draft she can have hydrofoils spanning from the side hulls to the main hull fore and aft. These hydrofoils will be in water that is less subject to wave action, so the movement of the ship is damped. They might even contribute, a little, to propulsion, compensating, also a little, for the larger wetted surface and increased friction drag. At high speed the wave drag should be smaller than for a conventional hull. The electric motors and propellers are mounted one each under each hull, giving a higher propulsive efficiency than with two or more propellers under a monohull. Together this reduces the power required and thus the cost of the power plant for a given design speed.
The new shape would provide a wide and stable platform for operating helicopters. It might also be possible to mount runways, with ski jump, over one or both of the side hulls to launch and recover small unmanned aircraft or even to operate manned communication aircraft with a shape similar to Facetmobil, i.e. with a very low aspect ratio.
I'd love to see your comments.

Hydrofoils under the hull start to become a real shortcoming when you begin to talk about ships the size of an aircraft carrier. Many naval bases require extensive and on going dredging just to accommodate the increasingly deep drafts of modern monohull aircraft carriers.
Catamarans and tri-hulls have more drag than monohulls, and their stability advantages decrease with tonnage. Also, while a cat or tri-hull is stable in roll, they tend to be shorter for a given displacement than a monohull and pitch more in a heavy swell. This can be somewhat overcome with a long, thin wave piercing hullform, but you get a very wet deck. This won't work for an aircraft carrier. If the structure between the hulls is far forward on the hull you encounter wave slamming. All of this puts heavy strain on the structure.
Tri-hulls and cats have very thin hulls, and this limits such a ship's internal volume. For some designs it would be acceptable to move functions out of the hull and into the superstructure, such as gas turbine/alternator sets to power the propulsion motors, weapons and sensors, but for a capital ship like a CVN, you prefer a deep draft monohull with multiple bottoms, and lots of void spaces for protection from torpedos, and to be able to bury the magazines and reactors well down in the hull where they are protected from torpedos and from missiles and bombs. The necessary protection requires a very deep and wide hull to build in those necessary void spaces and fuel bunkers.

Some comments from what I understand to be the characteristics of catamarans and trimarans:

1. These hull-forms are great for straight, high speed runs (this makes them more fuel efficient for long, straight transits). Better than the traditional mono-hull. But, they fare badly in high speed turns as compared to mono-hulls. Low speed turns are compensated for by varying the propulsion on the different hulls.

2. The structure between the hulls need to be very strong to take the stress, particularly if the individual hulls have substantial displacement. Wouldn't want the hulls breaking apart in a severe storm.

I was looking specifically at destroyers, so less than 10 000 tons, and not shorter than an equivalent mono hull. The draft will be somewhat larger than the mono hull. I doubt if what I want here can be achieved in a frigate. The matter is different again in a larger ship.
The side hulls have about half the length of the main hull and their middle is slightly abaft the middle of the main hull. The superstructure is extended over the middle of the side hulls and the side hulls are braced to the main hull by the hydrofoils, thus connecting the three hulls adequately. You don't want slamming on the underside of the bridging structure.
The hydrofoils are expressly meant to reduce pitching and you then accept a wet fore deck. No-one should have to work there in bad weather. I remember two Dutch sailors being killed on the fore deck of a naval ship in the Mediterranean by an unexpected wave.
The propulsion motors might be pod mounted and used for steering, getting rid of the rudders.
The amount of steel used will be larger than in the mono hull. You would win some weight back from a lighter power plant but the main purpose is reducing the tiredness of the crew in a storm.

delft said:

I was looking specifically at destroyers, so less than 10 000 tons, and not shorter than an equivalent mono hull. The draft will be somewhat larger than the mono hull. I doubt if what I want here can be achieved in a frigate. The matter is different again in a larger ship.
The side hulls have about half the length of the main hull and their middle is slightly abaft the middle of the main hull. The superstructure is extended over the middle of the side hulls and the side hulls are braced to the main hull by the hydrofoils, thus connecting the three hulls adequately. You don't want slamming on the underside of the bridging structure.
The hydrofoils are expressly meant to reduce pitching and you then accept a wet fore deck. No-one should have to work there in bad weather. I remember two Dutch sailors being killed on the fore deck of a naval ship in the Mediterranean by an unexpected wave.
The propulsion motors might be pod mounted and used for steering, getting rid of the rudders.
The amount of steel used will be larger than in the mono hull. You would win some weight back from a lighter power plant but the main purpose is reducing the tiredness of the crew in a storm.

Click to expand...

I can see the utility of a tri-hull for a destroyer. Interestingly, the USN is not using podded propulsors because the electric motors necessary for the sorts of speeds required for something like a DDG-1000 are simply too large for a podded propulsor. Combat ships need quite a bit more power than commercial ships so, at least for the fastest ships, podded propulsors are out. What would work are shrouded pump jets like submarines have. The USN was experimenting with these on frigates when gas turbine propulsion and reverse pitch props made pump jets impractical. With electric motors that are easily and quickly reversible (why icebreakers use them) a pump jet would be beneficial.
Now, the $64 question, is why the USN hasn't fallen head over heels for the General Dynamics/Austal tri-hull design and down selected to that design over the mono-hull Lockheed-Martin design. Apparently each design offers something the other does not so the US Navy is planning to build more of each. Maybe the choice is not so easy to make? Too bad it will be years before we are told the Navy's reasons.

I suppose a super-conducting motor might be small enough, but I wouldn't dare to use it in a podded installation. That means using steering by main motors for fast changes in direction. Wouldn't a pump jet be much less efficient than a conventional prop?
A Littoral Combat Ship is a kind of shallow water frigate, so the purpose and the shape are very different from what I am considering, but I think the concept very interesting. General Dynamics hasn't provide a runway for recovering UAV's.

No, the superconducting motors in the DDG 1000 are too large for pods. Pods were considered and rejected for this reason.
A shrouded pump jet is very efficient because cavitation is greatly reduced. It is also very quiet. USS Glover, a modified Garcia class frigate used for ASW experiments carried a shrouded pump jet. The drawback was the size and expense of the gearbox necessary for the counter rotating props.
With the advent of gas turbine propulsion, shafts could not be reversed and shaft speed has to be kept constant with power varied by changing the pitch of the props. Adding variable pitch to a counter rotating prop was too much complexity for something that has to be cheap and easy to produce like a frigate. Pump jets were abadoned.
With electric drive, shafts are easily reversed and pump jets could be very practical.

This sounds very reasonable. Remarkable how USN goes from steam-electric to steam-electric in a hundred years in its largest ships, from early dreadnoughts to Ford class.

delft said:

I was looking specifically at destroyers, so less than 10 000 tons, and not shorter than an equivalent mono hull. The draft will be somewhat larger than the mono hull. I doubt if what I want here can be achieved in a frigate. The matter is different again in a larger ship.

Click to expand...

Why would the draft be larger than a mono-hull?

1 of the key advantages of catamarans/tri-marans over mono-hulls is that they have a shallowed draft as their internal volume (and hence displacement) is distributed between more hulls. This allows a cat/tri of comparable displacement to a mono-hull to have a shallower draft, which contributes to better fuel efficiency (deep draft increased hydrodynamic drag).

The purpose of the deep draft is to have the hydrofoils deep enough to feel little of the waves, thus damping the pitching and rolling, giving more comfort to the crew and let that crew work better also under a bad storm.
The hulls are narrow and the result is a higher wetted surface and more friction drag, but a lower wave drag which is the major drag component at high speed.