Understanding Squat and Under-Keel Clearance
When you study navigation in shallow or confined waters, two ideas come up again and again: squat and under-keel clearance. They are simple to describe yet easy to underestimate, and together they help explain why a vessel that appears to have plenty of water beneath it can still find the seabed uncomfortably close.
This guide stays conceptual. It is written to support your revision and to help the ideas settle in your mind, not to give operating instructions or figures to rely on. Think of it as a starting point for wider reading rather than a source of settled numbers.
What squat actually is
Squat is the tendency of a moving vessel to sit lower in the water than it does when stopped. As a hull moves forward, it changes the flow of water around and beneath it, and the result is that the vessel settles bodily and can also change its trim. In general terms, the faster the vessel moves and the shallower the water, the more noticeable this effect tends to become.
For a student, the useful takeaway is the relationship rather than any single value. Speed and available depth both play a part, which is why the topic sits so naturally alongside pilotage and passage planning. Understanding squat helps explain why speed and available depth are so often studied side by side, and why this corner of the subject rewards a calm, unhurried approach.
Understanding under-keel clearance
Under-keel clearance is the space between the lowest point of the hull and the seabed. It is the practical measure of how much water a vessel truly has beneath it at a given moment. Because the seabed stays put while the tide rises and falls, this clearance is always changing, which is why it belongs so firmly in the same conversation as tidal height.
Squat and under-keel clearance are really two sides of one question. One describes how much a moving vessel sinks; the other describes how much room there was to begin with. Studying them together makes it clear why margins in shallow water deserve careful thought and generous allowances.
Why shallow water matters
Confined and shallow waters concentrate all of these effects. The same vessel behaves differently in open, deep water than it does in a narrow, shoaling channel, and that contrast is a recurring theme in navigation study. Tidal height, the nature of the seabed and the vessel's speed all interact, which is why the subject rewards a calm, methodical approach rather than rules of thumb learned by rote.
Bringing squat into your study
CalcDeck is a broad library of maritime calculators covering the whole ship, and its tidal tools include calculators for squat and under-keel clearance. Working through calculators like these can help you connect the concepts to the way the values move, so the ideas feel less abstract when you meet them again in your reading. CalcDeck is free to download, with an optional one-time CalcDeck Pro upgrade, and it is available on the Apple App Store for iPhone and iPad.
It is worth being clear about what a calculator is for. In CalcDeck, results are provided for training and reference. They are there to support your learning, and they must be verified against approved instruments and official tables before any operational use.
How Skipper supports the topic
If you are approaching this as part of day skipper theory, the free Maritime Master Skipper app sits alongside it well. Skipper offers interactive study of tidal heights, tidal streams and a tidal stream atlas, together with passage planning and pilotage guides, all of which give squat and under-keel clearance their wider context. It works offline, which makes it a convenient revision companion, and it is available on the Apple App Store for iPhone and iPad.
Skipper is designed as a study companion that supports a shorebased course rather than replacing one. Used together, a conceptual grasp of squat and steady practice with tidal work can make this corner of the syllabus feel far more approachable, and its results are for study and reference only.
For study only
A closing reminder. The Maritime Master apps, and this guide, are study, learning and revision aids only. They are not for real-world navigation, and they are not a substitute for approved training or properly maintained equipment. Real navigation is governed by official charts, publications and proper training, and any figure you produce while studying must be checked against approved sources before it informs a real decision. Nothing here is a promise of an exam result; it is simply material to help you learn.
