The Question of Vessel Squat
I use the grounding of the Queen Elizabeth 2 here for demonstration purposes only
At the trial of the grounding of the QE 2 in New York District court there were two
expert witnesses who testified regarding vessel squat. The expert witness hired
by the American Government testified that the QE 2 could have been
experiencing as much as 8 feet of squat. The expert witness hired by the plaintiff,
and who was from England, thought that she may have been experiencing less
than half of that.
As managers and surveyors of hydrographic vessel, how do you compensate
your depth measurements for the squat factor?
The following will give you a quantitative measure of the absolute value for the
squat that this vessel was experiencing while traveling at a speed of over 20
knots in depths ranging from 36 feet to 50 feet.
I will use, for demonstrative purposes, two illustrations provided by the Captain of
the NOAA vessel “Rude”, and who conducted the investigation into the QE 2
grounding.
This echo sounding trace was taken from the sounder of the QE 2 on August 7th 1992
At the time of the grounding there was a tidal height of 1.5 feet and the QE 2 was drawing 32.4
feet of water.
On August 7, 1992 at 21:44 hrs the sounding trace above shows that the ship
came extremely close to the bottom near the NA buoy on the chart. (See attached
illustration for a copy of the chart and the ships position). The 1966 NOAA measured depth at
this position was 36 feet and was shown on the chart. Therefore, we have the
following:

Charted depth, NOAA 1966…….36 feet


Therefore water over the bottom was ……37.5 feet


Height of tide…… 1.5 feet
The ship was drawing…… 32.4 feet
Let’s say that she cleared the bottom by…… 0.5 feet as the graph shows

Therefore her squat was……. 4.6 feet
Let’s look at the depth of water she was traveling over at this time. For some
length of time, 2 to 3 minutes, leading up to this near grounding she was in water
of 40 feet and less. Experts know that the shallower the water and the greater the
speed the more squat will be experienced.
Fourteen minutes later she grazed a boulder that was described as a glacial
erratic standing about 10 to 20 feet off the bottom. The depth of this boulder was
measured to be 34.5 feet and the surrounding water was in excess of 40 feet
with most of it being 40 to 50 feet. The ship was within the 50-foot surrounding
contour, as shown on the 1939 smooth sheet. This would place the ship in water
deeper than the “almost grounding” 14 minutes earlier. Let’s look at the squat
numbers.

Charted depth “Rude” 1997……….. 34.5 feet (grazed)


Height of tide……….. 1.5 feet
Therefore water over the grazed site……….. 36.0 feet


The ship was drawing…….. 32.4 feet
Therefore the squat was………. 3.6 feet
For the most part, and except for the odd isolated boulder, the ship was traveling
in excess of 20 knots across an area that was slightly less than 50 feet in depth
with isolated boulders having water over them of less than 31 feet but widely
spaced. I doubt very much that boulders widely spaced could be considered
shallow water as the ship would be over any one of them for such a short time
and therefore would not accentuate or contribute to the squat factor.
Please refer to this illustration for points referred to in the body of this article
So what does this exercise tell us about vessel squat? In the first “near
grounding” the ship had not reached her intended speed of 20 knots plus but had
almost grounded with a squat of 4.6 feet. After reaching her speed of 20 knots
plus and in deeper water she grounded with 3.6 feet of squat.
Therefore, I believe the evidence is clear and indicates that this vessels hull
squat characteristic are in the range of 3.6 to 4.6 feet when she travels across
water depths of 40 to 50 feet at speeds of 20 plus knots. In deeper water squat
would be less and in shallower water it would be more as shown by the amount
of squat at the NA buoy.
If your survey vessel had the same hull characteristics as the QE 2 and was
conducting hydrographic survey depth measurements in water such as these and
at speed such as these then you would be required to apply a depth
measurement correction of 3.6 feet. Are you applying a squat correction and
have you determined the squat characteristics of all your vessels?