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SQUAT UPDATE: WHAT SQUAT?
Following my feature on squat in the last issue I recently piloted a dredger which was fitted with extremely accurate draft meters and I was therefore able to monitor the fore, aft and mean drafts throughout the passage. With a speed of 13.5 kts the master advised me that the vessel trimmed by the head at speed and the deep water draft would be 8.5m forward. The passage involved passing over a bank which would give a
minimum under keel clearance (UKC) of around 2.5m at that draft. As a regular trader to the port the Captain advised me that the speed would only need to be reduced to around 12 kts over the shoal to prevent the pitch overload alarm from triggering. This passage therefore provided an ideal opportunity to monitor the squat passing over the shoal. As predicted once up to full speed the draft was recorded as 8.5m fwd and 7.9 aft and the mean draft displayed as 8.2m thus confirming the accuracy of the F&A sensors. Approaching
the shallows the speed was reduced to 12 kts (through the water) and once passing over the shoal with the depth registering at 2.4m it was interesting to note that the forward draft had increased to 8.9m thus
indicating a 0.4m squat. However, the aft draft had decreased to 7.5 and thus the mean draft was still recording at 8.2m. The vessel had trimmed by the head but no squat was present. This class of vessel only has a freeboard of about 60cm at this loaded draft and the deck was still above water (see photo) so the theoretical squat of around 1.5m at 12 kts is evidently incorrect. Having recently been dredged, the sea bottom is known to be solid so there is no mud layer effect and therefore, although totally unscientific, this passage once again tends to confirm that the existing squat tables are totally inaccurate and urgently need to be reviewed. JCB
6 Responses to “SQUAT UPDATE: WHAT SQUAT?”
It’s not the draught that increases, but the waterlevel around the ship that decreases.
This “SQUAT UPDATE” misses the point, completely.
JCB
Marc,
Thank you for the clarification. It is obviously a common misconception since a far more authoritative source than myself provides the following:
What in fact is Squat and why is Squat relevant for shipping?
Squat is the reduction of a vessels Keel-Clearance, caused by the relative movement of the ship’s hull through the surrounding body of water. Compared with the neutral position the hull sinks deeper into the water and at the same time will trim slightly. The algebraic sum of both, sinking and trimming is called Squat.
However, regardless of the terminolgy. the end result of a reduced Under Keel Clearance is the important factor and it is this factor where the experience of pilots contradicts the mathematics. The fact is that pilots daily transit port approach channels at speeds of 10 -12 kts with under keel clearances of less than 1 metre when in many cases the ships’ squat tables indicate that this is impossible and that the ship should go aground.
Until recently, the ships accepted a port’s experience and UKC guidelines but increasingly ship owners are requiring Masters to adhere to their ISM advice on UKC which is commonly 10% of draft + Squat. This is obviously safe but is now starting to cause major problems for a port in planning vessel arrivals and departures because vessels are being programmed with the port’s traditional UKC parameters and pilots allocated accordingly but are then finding that the programmed passage is unacceptable to the ship, thus incurring not only costly delays but also potentially preventing a ship from actually entering at all! When this latter has occurred the Master has been permitted by his company to follow the port’s guidelines but this is becoming a serious problem with enormous potential implications for trade and it is also making the important master / Pilot exchanges increasingly difficult.
Real time trials on UKC are confirming that the UKC is not reducing as dramatically as the mathematical predictions, so what I personally believe is required is not exchanges on web pages or in magazines but for the mathematicians to take note of ports UKC policies which have been proven safe for generations, take note of the data being produced from real time trials and revisit the traditional formulae in order to provide the shipping industry with the most accurate squat tables possible. You will find that most ports will willingly permit researchers to undertake passages with their pilots to make any observations that might assist improve safety.
The safety of the ship is paramount but carrying the world’s trade is also a commercial operation so it is also of the utmost importance that the information being used to determine safety parameters is as accurate as possible. Currently, evidence suggests that some squat tables are not providing the required level of accuracy.
This was the fundamental point that I was making in the above article
I think that experiences in ports and channels should not be used for determining nor checking squat by formulae. For one thing: you will probably have no direct access to actual channel depths.
(As a dredger captain, I’ll tel you: they vary more than you wish to know.)
I have a much different experience, and the squat-formulae are very exact.
I was captain of a dredger working on one of the “Palm” islands in Dubai. We had this shallow access channel in which we were tide-bound all the time; we had to reduce our load according tidal height.
Through a process of trial-without-error, narrowing UKC, and closely monitoring the depth of the access channel (my company did so with multibeam survey every forthnight) we managed to sail around with an UKC of 0.5 meter, that is with a draught of 12.5 meter. We tighthly controlled ship’s draught and especially (!) ship’s speed in the channel, by using an excel spreadsheet that dictated both parameters for the watchkeeping officers.
I just fouled around with the different formulae untill I got it right, and took it from there, relied on it without a hitch.
We also had a realtime tidal height receiver onboard.
One of the mates proved the whole point of the by overloading the vessel by 0.2 meter and overspeeding by 2 knots. He grounded. So close can you play the squat-game if you know all parameters e x a c t l y ,
especially recent data on channel depth and realtime tidal height.
Yours sincerely,
Marc
Marc Van de Velde
Some of previous experiencve is described in
http://www.theartofdredging.com.
JCB
Thanks Marc,
This is valuable practical information that tends to confirm that the mathematical squat formulae need to be reviewed.
We transit dredged channels, the depths of which are also surveyed regularly and so your experience of being able to transit narrow channels with 0.5m UKC confirms our own time proven experiences that using 0.9m ( on 300m containerships and VLCC’s) is perfectly safe at speeds of around 10 – 12 kts. Unfortunately we are confronted by mathematically derived squat tables on the ship’s passage plans that tell us that this is impossible!
Out of interest what speed can you safely transit with the dredger at 12.5m draught and .5m UKC?
Regards
John
Out of interest what speed can you safely transit with the dredger at 12.5m draught and .5m UKC?
Answer: since this ship has a high blockcoefficient, we kept 8 knots, on full throttle could’nt exceed 10 knots, for a ship with a design speed of 15 knots.
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Marc Van de Velde