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HMS Nelson runs aground on Hamilton bank, 1934

“Will the right hon. Gentleman in any event ask the captain to steer this extremely expensive craft a little more carefully? “

Reproduced from Gawn (1950)

On January 12th 1934, HMS Nelson was just leaving the Portsmouth harbour entrance at a speed of 15.5kts assisted by three tugs.  That is, the rpm of the propellers was set to an equivalent speed of 15.5kts but by the combined effect of shallow water and a following current of about 1-1.5 kts, her actual speed over ground was estimated by tracking fixed positions on land at 7-9kts.  As she was about to pass Blockhouse Fort speed was ordered to reduce to 12 knots, the tow of one tug was parted and she was ordered to wheel to port by 15 degrees. Before her rudder execute she was close the east embankment; shortly after Nelson started steering to starboard—against her rudder—and proceeded about 2.5 lengths before grounding herself on Hamilton bank  on the opposite side of the channel.

Fuel and shells were offloaded, the anchor lowered in a lighter and shoal removed by a dredger;  destroyers were ordered to pass her at 20 knots hoping the wash would raise her. It was even attempted to free her by having the crew jumping in unison on the quarterdeck for about an hour. Nelson would be refloated by the next tide. She suffered no physical damage and soon continued with her spring cruise, but the entire affair must have been an embarrassment for the flagship of the Royal navy, immovable for all to see and well captured by onlookers on nearby Southsea beach. The irony of HMS Nelson hitting a bank called Hamilton was impossible to miss by the British press.

It may appear a trivial navigation mistake that was initially attributed to wind conditions, but as with all things hydrodynamic—especially in shallow water— the matter is more complex than it appears. I found a paper in our archives by Gawn (1950) describing a follow-up studies explaining what had happened. A series of model tests were performed at the Admiralty Experiment Works at Haslar—opposite to Portsmouth—starting with a small unpropelled 4ft model in a model of the channel; the mass of the model and mass distribution were set to the relevant deep draft conditions as for the ship. These tests were performed with the model in the centre of the channel and various offsets to east and west, at speeds from 7 to 15 knots. The model followed the channel when sailing through the centre with a slight turn to port and responding to the rudder favourably. When the model was more towards the east side of the channel—as was the case on the 12th of January—the model turned to starboard for all observed speeds even with the rudder fully to port; a speed less than 7 knots and a rudder angle of about 20 degrees was required to avoid grounding. When the model was on the opposite side of the channel towards the West the model turned to port, though not as strongly as on the East side. Increasing the depth in the channel reduced to effect of steering against the rudder.

Reproduced from Gawn (1950)

A second series of test was performed with a medium-sized self-propeller model of 16ft at the large No2 ship tank. A false bottom at a 6.25 degree inclination was added to the tank. The model was tested at speeds of 6 to 14 knots and the rudder angle was set such that the model kept a course parallel to the bank.

Reproduced from Gawn (1950)

From these tests it followed that the rudder angle needed to keep a course parallel to the near bank increased with both higher  speed or lower water depth. At the depth and speed corresponding to the conditions in Portsmouth harbour a rudder angle of 35 degrees was not sufficient to avoid the model from veering away from the shallow region. Using a rudder about 20% larger in area would not have helped in avoiding Nelson grounding.

From these test it was concluded that Nelson sailed too much off centre in the channel at a speed at which grounding on the opposite side could not be avoided; an explanation rear-admiral Macnamara—captain when the incident happened—regretted not having sooner. He also commented that while the Nelsons were reported to have good steering properties that he found them to handle badly especially in shallow water or wind, although this opinion was not shared by other officers. Mr Stanley who worked on the design of the Nelsons as an assistant director of naval construction and who was also aboard when Nelson grounded was of the opinion that the grounding was not to be accounted for by any defect or deficiencies in the steering qualities of the ship; in fact, the Nelsons were known to handle better that the battleships of the Queen Elizabeth or R-class.

The cause of Nelson’s course instability is bank suction; in the restricted channel the flow cannot go under the bottom freely and the presence of the wall causes an asymmetry in the flow; the bow experiences a small apparent lift angle forcing it outward, while the stern experiences a low pressure region on the bank side pulling the ship in; a ship will typically yaw and then move away from the bank. The rudder must be moved towards the near bank to stay the course and then the ship can sail at a drift angle in a channel.

Nelson’s model test also showed the thrust of the port propeller was about twice that of the starboard propeller as her stern swung towards the near bank. From a more modern analysis we find that the flow in shallow water runs under the keel from the outboard towards inboard side and may even separate running over the skeg resulting in a region of ‘dead’ water. The inner propeller operates more as in bollard-pull conditions where propellers produce more thrust, forcing an even lower pressure on the stern making the situation worse (the contribution of the force imbalance to the yaw moment between the two propellers is most likely very small).

Reproduced from Vossnack (1979)

The rudder arrangement of the Nelson’s is a semi-balance rudder on an open skeg; these rudders offer good steering properties at high speed with relatively small turning circles and low resistance but are (now) known for their poor steering properties at low speed, particularly in shallow water and more so when their shaft lines diverge when running aft;  modern twin-screws container ships with a semi-balanced rudder were reported to hit the side walls of the Suez canal (Vossnack, 1979). A closed skeg may have been a better choice; however, a twin screw-twin rudder arrangement that allows for the propeller race over the rudder offers far better steering properties as the rudder’s effectiveness increases with the flow speed squared; it remains effective in the slipstream even at low ship speeds. It comes as no surprise that increasing Nelsons rudder by 20% had no appreciable effect. Placing the rudder in the slipstream does incur a resistance penalty, partially offset by increased propulsion efficiency as the rudder acts as a preswirl stator recovering rotational energy losses by the propeller.

A news clip can be found here

A wonderful serious of aerial shots of Nelson Aground are held by de Daily Herald archive and can be viewed via the image thievery website Getty

Consulted sources

Brix, Manoeuvring Technical Manual, Seehafen Verlag, ISBN 3-87743-902-0 Hamburg 1993

Burt, R.A., British Battleships 1019-1945, ISBN 9780 1 84832 130 4, Seaforth Publishing, 1993

Garthune, R.S., Rosenberg, D., Cafiero, D, & Olson, C.R., , The performance of model ships in restricted channels in relation to the design of a ship canal, David W. Taylor Model Basin report 601, 1948

Gawn, R.W.L., Steering and propulsion of HMS Nelson in a restricted channel, Transactions INA, volg 92, pp82-106, 1950

Hoydonck, W., van, Toxopeus, S., Eloot, K. Bhawsinka, K. Queutey, P.  & Visonnea, M., Bank effects for KVLCC2, J. of Marine Science and Technology 24(9), 2015

McNeil, Nelson & Rodney 1927-1949 The big battleships, Maritime Books, 2005

Vossnack, E., Good steering properties of container ships, the Motor Ship, November 1979

HMS King George V visits Melbourne

A set of five images of HMS King George V just landed in my mailbox,. As luck would have it, these images complement a set of three I bought a few years earlier but didn’t really tell a story on their own.

The camouflage pattern is an overall light grey and a slightly darker hull, matching the late 1945 pattern (meaning hull/superstructure painted in colours B20/G45), a pattern applied when she was in Australia [1]; the location was easily recognized as Station Pier in Port Melbourne. That puts the (probable) date between the 29th of October and 8th of November, 1945. This is the same period when the famous Allan C. Green photographs were taken, so now we have a collection from the port side well. Although these photographs were not all equally sharp, they do show a very large crowd visiting the ship. A brief search at the Australian War Memorial of HMS King George V’s visit indicates a few more shots taken from similar vantage points and even on board.

[1] Sovereign Hobbies, Royal Navy Camouflage Schemes, HMS King George V

Shelterdeck, part III

One of the issues that complicates any HMS Hood build is the area below the shelter (or boat) deck; the main deck parts should be fully painted before securing as the region cannot be accessed. The number of photographs of this region is modest and I made post earlier here. The ceiling of the shelterdeck was populated by structural elements, mainly a few larger longitudinal girders A & B). Some transverse girders appear perforated (C); I copied this style to multiple locations while playing with my milling machine. The outer girder appears to be placed where the shelterdeck used the end before it being widened in 1938. The bottom-right image is reproduced with permission from the HMS Hood association and shows the area below the boat deck quite clearly; other images from this 1940-41 album indicate a few minor changes to the model were in order!  A few details could be seen on other images: the hammock rails (D) and support stanchions (E)  placed below the boat crutches on the boat deck. These pillars have a white center, covered by a sewn-on canvas gaiter (painted stanchions are also observed on other ships). Now we can also find the ammo lockers for the UP launchers that according to Northcott were stored below decks (4 lockers per launcher, F) . A series of Denton rafts is found (G), a very large vent trunk (H).  At (I) a Dan buoy is found stored against the ceiling as identified on the Britmodeler forum; these buoys were used to mark a channel swept of mines. All warships were fitted with mine sweeping gear (paravanes) that was only effective only against contact mines and useless against magnetic, acoustic, or pressure mines, and there’s something counter-intuitive about using HMS Hood as a minesweeper and having her carry danbuoys. But once noticed I started spotting them in more places, see e.g. Jonhston’s & Buxton Battleship Duke of Yok, page 161, with a buoy next to the engine room vent. More on a small model of these buoys in a later post.

I bought a few original plans of HMS Hood earlier showing the shelterdeck and the structural details; these support beams are spaced 4 ft apart.  I previously added some random detail to the ceiling of the shelterdeck when my favourite modelling tool apparently was putty and everything was glued with Uhu plast (bottle with a small needle); I switched  to bottle of Plastic Magic and Tamiya (ultra) thin cement last year and this was really a step up in build quality. Some damage was collected during handling so the girders were readded in the form of strips. I had some fun with the milling machine and added a few larger perforated beams. I can only see one of these clearly on one image though but why not. I also changed the angled outline of the deck slightly, letting the angled parts end exactly on a beam end; the original plan set that I have does not seem to follow the outline very well and the model now matches the photographs much better. The position of the support pillars was drilled in earlier; the pillars themselves were made from three Albion-Alloy tubes, so that the centre can be neatly airbrushed white. Unfortunately, the pillars positions were added based on the general arrangements and do not end up exactly below the girders. I cheated a bit and lowered all pillars, the outboard row of pillars end up behind the main longitudinal girder and I added some strip at the ends of the inner row where all pillars should end.

The overall effect is quite nice… (pillar strip apparently not yet added here).

The hammock rails were added next, using a simple alignment tool to glue to etched parts in place. I put the rails right between the girders. If all goes well, none of the support stanchions and hammock rails intersect (did a lot of measuring and drawing to convince myself).
These are very fragile so the part is now much more difficult to handle..

The larger trunk is the the dynamo room vent and appears on the boat deck. It is open on the sides on some images and closed off on most others and copied that style. The trunk didn’t really end up nicely with the position on the main deck on my model. I already had a vent there, but that one was in the as-built configuration: I missed the extension to the boat deck. I gave it a slightly larger sweep to the side than on the original drawings. This was a really tricky part making an angle in the horizontal and to the side, and only after fitting the entire part I could see if it would align well; this took a few attempts. The main deck on the hull took some damage and Ill probably add a very denton rafts to cover it up.

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