Ship Squat

[Capt Bill Davies]

This is an interesting topic and I have purposefully placed it in the general Forum as I am sure it will be of interest to all department members. To grab your attention the QE2 grounding off Martha's Vineyard (USA) back in the early 90s was caused by this phenomenon. In a nutshell squat describes the behaviour of a ship when underway and particularly in shallow water. In essence, squat describes the way a ship when underway sinks bodily in the water (increases her draught) and trims according to her Cb (Block Coefficient). Example, a large 'bluff hulled' bulk carrier sinks bodily in the water and trims by the head. A 'fine hulled' passenger /container ship will sink bodily in the water and trim by the stern. The increase in draught can be expected to be in region of 10% (rule of thumb') but is proportional to speed. There is of course mathematical formula. Some of the vessels I commanded were calculated to have a squat inxs of 3mtrs. Any experiences of this or ancillary topics like 'bank effects etc??

Brgds

Bill

[Captain Kong]

Some of the VLCCs, 250-300K, I was on could increase their draughs by as much as ten feet and obviously this had to be applied when navigating in shallower water and channels.
I remember the incident with the QE2 off Marthas Vineyard, and was surprised they never compensated for it in their passage planning.
Cheers
Brian.

[Trader]

When I was running up to Paris years ago on a small coaster when approaching a low bridge the pilot would increase speed to make sure we got under the bridge. I could never understand this until it was explained to me. This happened mostly in the winter when the river was running high due to heavy rain etc.

There was never any danger of us not getting under the bridge but it was done just to make sure.

Would this be classed as squat Bill.?

Alec.

[Capt Bill Davies]

When I was running up to Paris years ago on a small coaster when approaching a low bridge the pilot would increase speed to make sure we got under the bridge. I could never understand this until it was explained to me. This happened mostly in the winter when the river was running high due to heavy rain etc.

There was never any danger of us not getting under the bridge but it was done just to make sure.

Would this be classed as squat Bill.?

Alec.

Good morning Alec,

This would indeed be classed as squat. Your mention of the increase in speed is important.

Brgds

Bill

[Captain Gordon Whittaker]

I took a ship into Harlingen, Netherlands, which meant a passage through the Terscelling sand banks. The passage was marked only by broomsticks and was timed to coincide with high water. As we were maximum size for this passage, we proceeded at dead-slow which made steering sluggish. Luckily we had a Becker rudder. Any increase of speed took us very close to the bottom and increased vibration and caused the Engineers panic as we took in a lot of sand and mud in the cooling water. When passing an outbound ship, we experienced bank suction effect which brought us very close in the narrow channel, (lots of helm and engine movement jiggling) All in all it was a very "Hairy" experience with the ship not wanting to do as she was told due to the conditions imposed by the nature of the shallow channel. A reference to Harlingen in Google maps will illustrate the tortuous channel invoved.

[John Cassels]

Morning Bill,

My understanding ( theoretically only ) is that squat is nearly always experienced only in shallow waters
and caused by the creation of a low pressure area between the ground and ships bottom.

Have no experience of this - possibly because I never sailed on ships fast enough.

When working ashore , one of our conbulkers on sailing , failed to make a turn on the Schelde
and opened up No.5 stbd DB to sea.
This was put down to interaction.

[Capt Bill Davies]

Morning Bill,

My understanding ( theoretically only ) is that squat is nearly always experienced only in shallow waters
and caused by the creation of a low pressure area between the ground and ships bottom.

.

Morning John,

Absolutely correct. Bernoulli and all that jazz.

Brgds

Bill

[Captain Kong]

Coming up from the Gulf fully loaded VLCC, bound for Rotterdam, with a 65 foot draught increased to around 75 feet with squat, we always had a tremendous vibration around the Varne in the Dover Strait, with sand being churned up astern of us, "Ploughing Sand". caused by the compression of the water between the bottom of the sea and the bottom of the ship.

[Tony Wilding]

I had my first motor boat at southend on sea , essex, tide goes out over a mile, if you were out in the deep water, non drying area and comeng back to moorings as the flood covered the mudflats you could go full ahead feel her squat and build up a big stern wave, ease the throttle and ride in on the stern wave, when other boats were still aground at your moorings, but she would be a pig to steer, could allways tell when enrtering shallow water by your stern wave, no depth sounders then,

[Gulliver]

Some of the VLCCs, 250-300K, I was on could increase their draughs by as much as ten feet and obviously this had to be applied when navigating in shallower water and channels.
I remember the incident with the QE2 off Marthas Vineyard, and was surprised they never compensated for it in their passage planning.
Cheers
Brian.

Hi Brian,I meant to reply earlier to this post.I found an interesting article in The Independent about that.
Apparently the Master and Pilot on QE2 had allowed for squat ,but not for the greatly increased squat caused by her speed.(24 knots). The tide level was also 6 inches less above charted level .All could have still been well though....until they struck those uncharted rocks.. Damn!

READ THE ARTICLE


Cheers!
Gulliver