Schneier on Security

Clive Robinson • May 7, 2012 1:14 PM

@ askme233,

Surprised it looks so like a traditional boat. I think if one were designing something that need to be able to travel long distances on stored fuel and manage weather itself, you would end up with a mutihull or some other more efficient model.

Two of the primary requirments after seaworthyness are platform stability and self right ability.

These two are conflicting requirments in that a bihull such as a cat can be made very stable but won’t self right. A monohull can however be made reasonably stable and also self righting as the design of RNLI boats has shown.

There are other problems with multi-hulls including a larger draft and less manoeuvrable having a far larger turning circle. They also tend to “cut through” rather than “ride over” waves which is problematic for an observation platform.

Then there is the question of “stealth” a multihull boat is going to be a lot harder to make stealthy than a low riding monohull. Not just to EM radiation (radar etc) but also acoustic radiation (Sonar etc).

The price you pay with monohulls is speed, because they ride over not cut through waves and they create their own bow wave there is a limit to how fast the hull can go whilst fully wetted directly proportional to the hull length. To get the hull above this speed then the hull has to “lift” out of the water and the boat goes onto a “plane” this requires a considerable energy input to achieve or clever ballast changing techniques (in sail boats you get all the ballast up front and when the bows start to sing you get all the ballast aft as qquickly as possible and if you are lucky the singing terns to a scream).

Clive Robinson • May 8, 2012 8:59 AM

@ Jon

And in sailing ships, cats have much shallower draft, because they don’t have to have a deep keel

Err first off I was not talking about sailpowered hulls, secondly “deep keels” are historicaly a very modern idea and did not exist on the majority of working sailing ships (and still doesn’t today).

The “deep keel” has a couple of benifits for racing/pleasure yachts but several disadvantages.

In general the “deep keel” is a passive system designed to support the modern sail rig and provide improved windward performance. It does this by providing a leaver on which a high density weight is added to provide contra rotation arround the assumed pivot point thus allowing a much larger sail area on much taller masts. The same lever can be extended into a blade which helps provide a counter vector to that caused by the wind thus improving the windward performanc from a braod reach to close hauled.

The bigest problems are a very much increased wetted area which significantly reduces efficiency, and the significant change in main hull structure to handle the changed and increased stresses.

It is also not a lot of use to most cargo carrying vessels because they don’t want either the deep draft or structural supports that reduce cargo carrying capacity.

From the pleasure boat perspective a deep keel iss also a nuisance, due to mooring up. Unless it’s a deep water harbor the boat needs to either more up sufficiently far from shore to prevent the keel touching bottom or take precautions to ensure the boat will either not topple or topple in the desired safe direction (usully towards shore).

Frequently the solution in cruising craft is a duel keel with sufficient strength to take the full weight of the boat etc.

Even on racing yachts the traditional passive deep keel is going out of popularity, the keel weight to stop wind rotation has been replaced with winglets that use hydrodynamics to provide contra rotational forces and in some cases where race rules alow “active ballast”. One such rule is in the Whitbread race where there is a rule about not having excessive quantities of bear on board. The reason was that in earlier races the barrels and crates were used as “active ballast” over and above having the crew ssit on the windward side.Basicaly the “Winch Gorillas” would on making a change of sailing point go below and move the barrels and crates to the windward side to provide a little extra “trim adjustment” allowing for a little extra sail hight and thus a considerable advantage.

With regards stealth one of the problems as you probably know is “tri-corner” and “right angle” reflectors vastly increasing the returned signal gain. The traditional design of multihulls ends up with a “trough” under the boat between the two hulls. This trough consisting of the inside walls of the hulls and the sea is a fairly high gain reflector at some frequencies and even at others still provides quite a high gain to airbourn radar. The solution is to “over deck” the hulls but this has to be done with considerable care and introduces another problem.

A secondary issue with multi hull vessels is they cut through rather than ride the waves, this has a consiquence in that the deck hight consiquently has to be very much higher esspecialy if over decked, thus vastly increasing the vessels detectability horizon.

Further certain kinds of stealth are kind of going out of fashion because they have a high cost to efficiency and in the case of vehicle surfaces with non 90degree and non curved surface reflectors these can be picked up with “offset radar” where there is a considerable distance between the pulse emmiter and echo receivers.

The reason for the oddity of offset radar is not stealth it’s self but “anti-radar missile technology”. In essence the missile flys down the pulse emmiter beam and destroyes it (or works out it’s location and flys down to the GPS coordinates).

However the technology behind the pulse emmiter is generaly very low cost compared to that of the nearly passive receiver technology. Thus having a number of emitters that can be turned on when others have been taken out makes the anti radar misssiles a very expensive option for an attacker (the new GPS guided artillery shells are redressing this though but still require the emitter location to be known which with a shell flight time of over tthirty seconds alows for a fairly easy defence).

For instance there is only a limited number of these missiles that can be carried and the firing of them marks the aircraft out for optical and IR passive anti aircraft defences. The fact that the missile has been fired is a very large indicator that another attack is imminent thus bringing up a second emmiter a short while later will enable these aircraft to be detected and located for other defence systems.

Further the anti-radar missile needs either a static emmiter or a continuous emmiter for it to work. Thus if the emmiters are of the “single pulse” type on highly manoeuvrable or fast moving platforms the anti_radar missile is effectivly mitigated as is the GPS guided artillery “smart shells”.