Friday, 15 July 2016

SUPERSTRUCTURE AND FREEBOARD.

wikipedia:

The height and the weight of superstructure on board a ship or a boat also affects the amount of freeboard that such a vessel requires along its sides, down to her waterline. In broad terms, the more and heavier superstructure that a ship possesses (as a fraction of her length), the less the freeboard that is needed.

I have repeatedly stuck my neck out claiming that Waratah's registered freeboard of 8 ft. was too low for a ship of her size, which according to a simple period calculation, should have been length of vessel divided by 40 = freeboard. In the case of Waratah 465 / 40 = 11.6 ft.. An oversimplification this might very well be, but it gives one a glaring starting point - deficiency of 3.6 ft.. The above extract claims that more (in length) and heavier superstructure is, the less freeboard required. But this is a complex subject dependent on a number of factors not least of all the integrity and strength of the superstructure.

Note in the diagram below that the calculation above is based on the LBP (length between perpendiculars). The image illustrates that the difference between registered length and LBP depends on angle of the bow to the crossing point inside bow plate and upper deck. In the case of Waratah, illustrated in the photo below, this angle was much smaller than in modern vessels. But let's say for argument's sake that we need to deduct 5% from registered length i.e. 465 ft. ----> 441.75 ft. divided by 40 = 11 ft. NOT 8 ft.

Further to this 'anonymous' argues that Lloyd's would not have allowed a flawed steamer to be registered. We are talking about flawed in modern terms. Progress in 1909 was incremental and not perfect by any stretch of the imagination. The frame of reference for ship design and build was in evolution and innovation was encouraged = progress. One thing is certain, shipbuilders learned from the case of the Waratah. These incremental improvements accumulated resulting in the modern ships we have today. 

Extract from witness account:

He once called the engineer's attention to a movement of her promenade deck. The whole wooden structure moved bodily athwart the ship. Boltheads actually broke off owing to the strain, while woodwork around the saloon door was separated from the ironwork to the extent of a couple of inches.

The following extract is pivotal to the understanding freeboard in terms of superstructure:

The freeboard may be decreased provided that the ship has an enclosed superstructure covering at least 0.6L amidships. (where L = length of ship).

If one goes back to the Inquiry data, the bridge and therefore superstructure, was 175.5 ft. in length which is 38% the length of the steamer. This did not meet the requirement of being at least 60% the length of the hull and from period photos was not squarely amidships, but displaced somewhat aft - no doubt to enhance aft weight (steering etc) and compensate for heavy refrigeration machinery etc in front.

The following extract highlights hull shape design, which might have been one of the sticking issues alluded to at the Inquiry ref. letters from the owners to the builders:

26.7.6 Correction for sheer profile (Regulation 38) Sheer is defined as being the curvature of the freeboard deck in a fore and aft direction. Benefits of sheer include: * Greater reserve buoyancy at the ends of the ship, particularly forward, ensuring good lift in a head/following sea; * Reduces water shipped on deck; * Reduces risk of foredeck being submerged after collision thus improving survivability in the damaged condition and helps to maintain an acceptable angle of heel at which progressive down flooding takes place.

 Any deficiency in sheer will result in an increase in freeboard. ..due to reduced buoyancy.

The hull design of Waratah had very little sheer which was a further factor against a low freeboard. This low sheer design was born out by claims that Waratah ploughed into head seas and frequently shipped seas on deck, even in calm weather. 

The low freeboard meant smaller waves could board the ship, and the bridge and superstructure therefore had to be stronger to resist wave action.[8]

Back to:

He once called the engineer's attention to a movement of her promenade deck. The whole wooden structure moved bodily athwart the ship. Boltheads actually broke off owing to the strain, while woodwork around the saloon door was separated from the ironwork to the extent of a couple of inches.






http://160.75.46.2/staff/takinaci/Comp_Progs/Fribord/CALCULATION%20AND%20ASSIGNMENT%20OF%20FREEBOARD.pdf

2 comments:

Anonymous said...


Are we to believe that you are right and the shipbuilder with over 400 ships built by them, their naval architect, their scientific department, Lloyds, and the Board of Trade Load line committees all miscalculated her freeboard by 3.6 feet?. You are incorrect by using the length 495feet length over all, freeboard is always calculated on the L.B.P of the hull, Length between perpendiculars.

MARITIME STUDENT 3rd year.

andrew van rensburg said...

Thank you for your informed contribution. Noted.