(Rudder Effectiveness)

Was Titanic's rudder too small to be effective?

Where rudders are concerned, bigger is not always better. It's true that the larger the rudder, the more surface area there is to generate hydrodynamic torque, but larger rudders also generate more drag and require more pressure from the steering engines to turn them effectively. British naval architects during the first part of the century appear to have preferred the unbalanced rudder for merchant ships, where the blade of the rudder is entirely aft of the stock. This places the centroid of pressure (CP) relatively far out on the blade, increasing the pressure required by the steering engines to move the rudder. The balanced rudder was used for Lusitania and Mauretania (one that places the CP on the stock by having a portion of the blade ahead of the stock), but design of the stern for those two ships was dictated by Admiralty requirements. One of those requirements was that the entirely of the rudder blade had to be well below the waterline in order to protect it from being hit by hostile fire. This forced Leonard Peskett to resort to a balanced design with a very broad blade. Other factors affecting a rudder's design will be familiar to aviators...tip vortices (similar to cavitation), induced drag and stall. Another consideration was protection against grounding damage.

So what was the ideal design? To quote Lovett (W.J. Lovett's Class-Book of Naval Architecture, London, 1905), "Even the highest authorities are at variance in respect to the best form of rudder." The White's manual (W.H. White's A Manual of Naval Architecture, London, 1900) discussed four of the more common rudder shapes used by British shipbuilders (one balanced, three unbalanced) and weighed the pros and cons of each, comparative to the hull to which they were normally attached. For the rudder shaped like Titanic's, White mentioned that it was "a form now commonly used in the steamships of the Royal Navy," and that one major advantage of its shape was that "by tapering the rudder, the power required to put the helm over is made considerably less...these considerations would not have equal force in screw steamers where the rudder is placed abaft the screws; and then [the shape of the rudder used in Titanic's design] is to be preferred, as the broadest part of the rudder is much less likely to be emerged by pitching." The rudder shape accepted by the H&W architects therefore appears to have been a compromise (as most rudder designs were at that time) between weight and surface area, while taking advantage of the position of the centre screw and providing protection against potential grounding.

So, was Titanic's rudder big enough? White stated that "for steamships...the extreme breadth of the rudder [is often] from one-fortieth to one-sixtieth of the length...in merchant ships much smaller rudders are used, and values as low as one-hundredth have been met with." Without running a model in a tow tank, one can only judge by dimension and compare to White's stated guidelines. Titanic was 850' long along the waterline, her rudder was about 15' wide at the fullest part of the blade. That made it about one-fiftyseventh of the length and therefore followed White's rule of thumb.

Would a Titanic II have a differently-shaped rudder? According to the conceptual model published in Popular Mechanics, it would be much enlarged from the original. However, the shape of an enlarged rudder not only requires more powerful steering engines, but also introduces the risk of stalling the rudder at extreme rudder angles. Again, going back to White, he cautioned that (in a given example) a "broad rudder, with an area 37 per cent. greater than the narrow one, has therefore less turning effect by about 11 per cent."

There's no set standard for determining what the optimal shape for a rudder for Titanic ought to have been. Oftentimes, rudder shapes were determined by copying a shape that worked well for another ship of similar dimensions. But, given the methodology laid out in the contemporary Naval Architecture books, it appears that Titanic's rudder was of adequate design to effectively manoeuvre the ship.

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