-H.M.S. Hood Reference Materials-
ADM 226/39: H.M.S. Hood - Suction Effect on Sea Bed
Updated 30-Nov-2008

This document is a modern transcription of a portion of Admiralty record ADM 226/23. It concerns the findings of tests conducted to determine the degree of sea bed suction generated by H.M.S. Hood in shallow waters. The original file is held at the The National Archives at Kew, London. This Crown Copyrighted material is reproduced here by kind permission of The National Archives.

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Admiralty Experiment Works,
25th March, 1937



H.M.S. "Hood" - Suction Effect on Sea Bed

D.N.C.'s reference dated 20th March requested information as to running sinkage and trim of "Hood" in shallow water, and this is shown on the accompanying Tracing. The results have been deduced from experiments on model JR, representing "Ark Royal" to a scale of 1/36th full size, and have been interpreted for "Hood", assuming a scale of 1/42.2 full size. At this scale the model closely approximates to "Hood" at 32 ft. draught forward and aft, the condition of the ship when the recent trouble was experienced due to shell, sand and small shingle being drawn into the main inlet when ship was steaming at 28 knots in shallow water.

It will be noted that the information comprises depths of 12 fathoms and speed of 28 knots, but dies not include the depths of 10 and 8 fathoms at this speed, concerning which information was requested in the reference, but particulars are given for 21 knots speed at these lower depths. This was the maximum speed at which experiments were made at the lower depths as the increase in resistance and wave formation were prohibitive at higher speeds for these small depths. Amidships the sinkage appears to be 11, 4, and 3 ft. at 8, 10 and 12 fathoms respectively at 21 knots, and approximately 8 ft. in 12 fathoms at 28 knots. Sinkage results for 8 and 10 fathoms for 28 knots cannot be give as they are beyond the range of the experiments. It is inferred that the distance between the main inlet and the bottom would be very small at 8 fathoms, presuming a speed of 28 knots was obtained. From the curves of resistance given in the Tracing, it appears doubtful whether such a speed could be obtained in 8 fathoms of water, although it is understood that the ship reports suggest that a depth of 6½ fathoms was steamed over at the speed in question. This tends to confirm previous experience that the model results exaggerate the phenomena.


D.N.C.'s reference also requested that depth of water be given at which bottom effect becomes appreciable at 28 - 32 knots. Sinkage and trim begin to increase rapidly in depths less than 20 and 25 fathoms at 28 and 32 knots respectively. Observation of the false bottom during the experiments shewed that there was appreciable upward movement of the bottom as the model passed. Although this largely occurred immediately subsequent to the passage of the stern of the model, it suggests that there is a suction on the bottom which would serve to stir up the sand and shingle, and so render it more liable to be drawn up into the main inlet when it approaches the bottom, due to the sinkage of the ship.

Having regard to the close approach of the main inlet and still more of the stern to the sea bed, with the consequent risk of structural damage, particularly if pitching occurs, it is suggested that ship should not steam at speeds in excess of 21 knots in depth of 8 fathoms and less. Such a restriction is considered all the more important because of the possibility of isolated areas of still less depth in a locality of a general depth of 8 fathoms. There would appear to be little or no objection to speed of 28 knots in 10 fathoms, or full speed of 32 knots in 12 fathoms of water, but it is not considered necessary to impose restrictions in these depths, as the resistance results suggest that it is doubtful if the ship could obtain these speeds, and the risk of damage is slight.

Signed (M.P. Payne)

Enclosure: 1 Tracing
                 Sinkage, Trim and Resistance Curves

Note from website editors: We saw no tracing with this document. It may be elsewhere in the volume. We will endeavour to locate it at a future date.

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