Thursday, 19 January 2012

Cruise Ship Design

When I was young, I thought of 'tender' as the description of a boat that serviced a ship.  The last tender I owned for a while was used on the annual raft race on the river Dee each April.  Those with experience of Aberdonian weather will realise that being in survival gear on the safety boat was preferable to riding a bed-frame and barrels in fancy dress!  That tender was named the "cow-pat" for the simple reason that it serviced a 32 foot Albin Express racing yacht by the name of "Aberdeen Angus".  In terms of the use of the names "ship" and "boat" this was the opposite end of the scale to HMS Nottingham's return home from Sydney in 2002 on board, and welded to the deck of, the heavy lifting vessel MV Swan.

It now transpires that a "tender ship" is actually a term for a design where the centre of mass is rather too close to the centre of bouyancy, resulting in a ship that is perfectly stable in good conditions, but is rather likely to respond to steering movements or heavy water with more roll that would be appreciated by passengers, especially those eating their soup!  Nautilus International spokesman Tony Minns told New Scientist about this in response to the Costa Concordia disaster recently:

The height of cruise ships is a problem, too, says Minns. “It is known in sea trials that these vessels are what we call ‘tender’ in stability terms – they are very stable but have a quick rate of roll when the rudder is moved a few degrees.” In other words, they are quite sensitive to being upset.

Is there a fundamental issue here where the profit motive is leading to the commissioning of vessels that are, in the worst possible way "fair weather sailors"?

I would welcome the sage words of the Monk here as his knowledge of such things goes beyond my mere dabbling and desultory interest.


  1. I was surprised to learn while working at Lloyds Register that these ships are designed with "marginal" stability and rely on their stabilisers to hold them steady in a seaway. As you rightly say, they are best described as being "tender."

    Something that perhaps not everyone appreciates is that the SOLAS Rules require a lifeboat/liferaft provision for 100% of crew and passengers, which is fine as long as all the boats and rafts can be launched. Cargo vessels on the other hand have a 200% capacity in lifeboat/liferaft provision. As we have now seen in several passenger liner accidents, it is not always possible to launch all the boats and rafts. A quick look at the Costa Concordia pictures shows several on the "upper" side (Port side of the hull as she is lying on her Starboard side) that a number of the boats and rafts (I count two of the large 'tenders' still in their davits and two liferafts still on the hull) could not be launched due to the list the ship had taken on. An early picture also shows some boats on the now submerged Starboard side still in their davits. As the rafts carry around 100 people each and the boats are rated at around 200, that suggests there weren't enough functional boats for everyone on board in this case.

    The stability is a very tricky question. As Josephus has mentioned, when the Centre of Gravity and the Centre of Bouyancy should, ideally be vertically opposed and fairly close to each other. If the C of G is too high, the ship will be unstable, to low and she is likely to be very "stiff" ( A problem with bulk ore carriers). As a ship rolls, the C of G and the C of B move in opposite directions, one acting in an upward direction (Bouyancy) and the other downward. If the movement is extreme - as in the case of a high C of G - the roll will be dramatic and can be dangerous.

    When a ship has flooding within the hull this changes both the C of G and the C of B and as she lists, the difference between the location of each "centre" shifts further and further out of line vertically, increasing the instability of the ship and the likelihood of it capsizing.

    One reason the Monk and Mausi are not keen on travelling on Cruise Liners ...

  2. You're a brave man Josephus: you asked the Monk a ship-related question!

    Now, what does everyone think of the captain's claim that he was trying to launch life boats and not abandoning ship as accused by the coastguard?

  3. The Postulant is probably unaware of an e-mail sent quite tongue in cheek some years ago after the first screening of "who sank the Bismark?": it read, in the best tradition of Royal Naval signals "what deck armour, Hood?" The reply ran to thirteen pages and included the inside leg measurement of the left handed riveter who set the keel-plate!

    To answer your question, the Master of the vessel in question quite obviously missed his vocation; he should have been in politics.