Horse play
Question: In the 19th century, great Clydesdale horses were bred for sale on
the Scottish isle of Lismore. They were ferried to the mainland in a
purpose-built, flat-bottomed boat with a very low freeboard to allow easy
boarding. Walter Weyndling, the Board of Trade Inspector of Ferries, notes in
his delightful Ferry Tales of Argyll and the Isles (published by
Sutton, 1997), that the ferrymen had to induce each horse to 鈥渟tale鈥� (urinate)
before embarking 鈥渙therwise the liquid produce of four giant horses could reduce
stability to the point of capsizing鈥�.
If the urine had been retained in their mighty bladders at horse height, the
effect of this mass on stability might well be hazardous. Releasing the fluid
would obviously lower the centre of gravity鈥攜et it was this decanting
which perturbed the ferrymen. Can anyone tell me why this should make the
flat-bottomed horse ferry less stable?
Answer: A significant quantity of liquid sloshing about in a flat-bottomed
boat is a major risk to the boat鈥檚 stability, whether it is horse urine or
anything else. As the boat rolls, the liquid runs downhill and its weight makes
the boat roll further in that direction. Additionally, the momentum of the
moving water hitting the side of the boat enhances the roll even more.
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So the movement of the liquid will amplify the natural rolling that a boat
experiences in the sea to give a very rough ride. These circumstances also make
it more likely that seawater will flow in over the sides of the boat, which
could ultimately lead to the boat capsizing. For a demonstration of the effect
of liquid on a flat, moving surface, try carrying a tray of water without
spilling any.
This problem is called the free surface effect and is well known to marine
architects. When boats and ships are being designed, the effect is used in
stability calculations, which take account of the movement of fuel and water in
a boat鈥檚 tanks.
The destabilising effect of water was demonstrated clearly by the speed with
which the ferry Herald of Free Enterprise rolled over, rather than just sinking,
when water entered its car deck just outside the Belgian port of Zeebrugge in
1987.
Much of the safety work following that tragedy has been aimed at restricting
the free flow of water on car ferry decks. Boats with a rounded bottom don鈥檛
suffer from this problem, but boats with a bulwark (a low wall) around the deck
must have lots of holes in the bulwark so that any water on the deck can drain
away quickly.
So, to answer the original question, liquid held in an elastic bladder inside
a horse doesn鈥檛 have a free surface鈥� if the horses can hold on, there鈥檚 no
problem.
Doug Fenna
School of Science and Technology
University of Teesside
Middlesbrough
Answer: I think your correspondent should look to behaviour rather than
physics for the answer to his question. As a horsewoman, I can attest to the
fact that the volume of urine released by a draft horse such as a Clydesdale is
prodigious鈥攎y guess is about 9 litres per horse or possibly more鈥攁nd
quite fragrant. Thus, urinating Clydesdales might make a boat unstable by
inducing the human occupants to rush suddenly to one side in order to get away
from the odour and wetness.
Another possibility concer