Letter: Squat

June 29th, 2008 by JCB
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I am a retired Humber pilot and was interested to read the two articles in the January issue on Bridge Visibility and Squat. On Bridge Visibility I do understand the Southampton Pilots’ problem I was a member of the UKPA technical committee some years ago and was invited to write a chapter on Bridge design for inclusion in the Nautical Institute’s Pilotage and ship handling book (1990). In the first paragraph of my piece I made the point that the primary reason for having a bridge is so that a lookout can be kept. What a pity no one seems to have read it, or if they did what a pity they took no notice. My co writer LJ Harrison made the same point in his submission as it relates to off shore supply vessels. On the subject of squat I would offer the following thoughts, Dr Barrass and most of the other writers on the subject use towed tank models. This may well reveal some aspects of squat and verify Bernoulli’s equations but a towed model has no propeller and I (simple sailor boy that I am) think that the propeller draws water from not only the sides of the ship in way of the stern run in but also from underneath the  vessel and I think that drawing water from
under a ship which is in shallow water will tend to make the after draft increase a bit. The tank towed model does not usually have any ballast water, cargo etc either. When a vessel suddenly runs into shallows, the first

thing that usually happens is that the vessel drops by the head. I think that when that happens any water in slack ballast tank to say nothing of a tanker cargo with a reasonable ullage will tend to surge forward adding to the trim by the head effect and also I seem to dimly remember some ship stability which says something about the centre of floatation moving aft under these sort of conditions which would also add to the effect. However the fact that the forefoot may not touch bottom may be because the water cannot get away from under the bow fast enough in a very restricted dredged channel for example and because liquids
cannot be compressed it may be that the bow is riding a water cushion. The last vessel of which I was Master before joining the pilot service was fitted with a bulbous bow and even in very deep water (Norwegian fjords)
when bottom proximity was clearly not a factor my observations seemed to suggest that as soon as we had a reasonable bit of way on, the bow dropped about 18” or so. On a sea passage as stores and water and
bunkers were consumed from aft the effect was very noticeable at an arrival port and I think bulbous bows have a downward thrust when the vessel is loaded. For hundreds of years sailors told of experiencing huge waves yet no one believed them because the maths people said the equations proved it was not possible. Then when waves were measured and observed by satellite and radar it was found that they did indeed exist. WAS THERE AN APOLOGY FROM THE DOUBTERS? NO! And I do not think the good Doctor will be saying sorry to the Houston and other pilots either.Mike Barratt PS Just to make my point about the propeller taking water from under the vessel I can tell you that some time ago the regular line running to Grimsby built a ship called Dana Maxima so named because she was built to  JUST fit Grimsby Lock and the vessel always
sailed through the open lock when the river and dock were level. One day the vessel arrived a bit deeper than usual but still within limits. However, as she got to a position head and shoulders in the lock she squatted down and sat on the lock sill. At this juncture the assistant dock master threw his hat on the dock side and began to dance on it. The pilot ordered the main engines stopped, ran some headlines out and hove the vessel ahead, she having refloated when the engines stopped. The bystanders may have missed this stage of
the proceedings as they were enthralled by the dance of the demented dock master! Once the stern of the vessel was clear of the outer gates they were closed and the ship’s engines were restarted and the vessel duly proceeded to her berth. Associated British Ports now make the dock masters wear hard hats to discourage hat dancing as it is considered unseemly!