Squat and under keel clearance

What you do on the bridge

1. Slow down

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. What is squat, and why does it happen?Often asked

   A good answer covers:

   • The ship sits lower in the water when she moves through shallow water, so the under keel clearance gets less
   • Water speeds up as it flows past and under the hull, and the pressure there falls, so she is drawn down
   • It grows with the square of the speed: double the speed and the squat is about four times as much
   • It is worst where the depth is less than about twice the draught, and worse still in a narrow channel
   • A full-bodied ship (high block coefficient) squats more, and usually by the bow; a fine-lined one less, often by the stern

   Show me on the bridgeOpen the echo sounder and watch the depth under the keel at 12 knots, then slow down.

2. How would you estimate squat, and what do you do about it?Often asked

   A good answer covers:

   • A rule of thumb: block coefficient times speed in knots squared, divided by one hundred, in metres, for open water; about double that in a confined channel
   • Use the ship's own squat figures or tables when she has them
   • Allow for it in the under keel clearance in the passage plan, with the tide
   • Reduce speed in shallow water: it is the one thing that cuts squat quickly
3. What are the signs that a ship is in shallow water and squatting?

   A good answer covers:

   • The echo sounder shows less water under the keel than the chart and the tide would give
   • She loses speed for the same engine power, and vibration may increase
   • She becomes sluggish to steer and her turning circle grows
   • The wave pattern changes: a bigger wash and stern wave

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   Raritan Bay, on a stretch charted at six metres. She draws three point nine, so at rest there are about two metres under the keel.

   The depth is less than twice the draught, and you're doing twelve knots. That's where squat bites.

2. Read the echo sounder

   At the nav station, look at the echo sounder: depth under the keel. It's about one and a half metres, not two. She's squatting about half a metre at this speed.

3. Slow down

   Bring her down to about six knots on the telegraph. Squat goes with the square of the speed, so halving the speed cuts it to about a quarter.

4. Under keel clearance

   Look at the echo sounder again: nearly two metres under the keel now, from the same water.

   In the passage plan, the under keel clearance allows for the tide, the squat at your planned speed, and a margin. Know your own figures.