The Easiest Way of Understanding Ship Squat Effect

Understanding ship squat effect helps reduce the risk of grounding ships. How does international shipping deal with ship squat effect?

With the fast rising economy, industrialized country has continually created large ships navigating in different major ports and channels. Seldom do senior officers on duty know how important “Ship Squat” in shallow waters. Controlling the problems and dangers of ship squat in shallow waters would reduce the risk of getting aground or to a more serious situation.

What is Ship Squat?

While the ship is making way through the water, she pushes large amount of water ahead of her. This water returns to the side and under the ships bottom. Imagine this cycle of water building up speed under the ship causing a drop in pressure. This later causes the ship to vertically drop in the water.

As the ship vertically drops, we can say the ship trims forward or aft. So we can say that, the decrease in the under keel clearance of the ship, either forward or aft is known as Ship Squat. Remember that trim is different from Squat. Trim is the difference of the forward and aft draft while the ship is stationary, while in squat the ship is moving.

Factors that Affects Ship Squat

Bodily sinkage plus Trimming effect comprise ship squat. If the ship is on even keel and not moving, the trimming effect depends on the type of ship and its block coefficient (CB). Let’s give an example on this:

The ship moves forward and gains speed while in shallow water, considering that she has an under keel clearance of 1.0 to 1.5 meters. If she increases her speed, she will most likely have an excessive squat on the forward or the aft.

  • If the ship’s CB is greater than 0.700, the maximum squat would likely occur on the forward part.
  • If its CB is less than 0.700, the maximum squat would most likely occur on the aft part.
  • If CB is almost equal to 0.700, the maximum squat would most likely occur on the forward, amidships, and aft part.

“the decrease in the under keel clearance of the ship, either forward or aft is known as Ship Squat”

The hull shape of ships also affects ship squat. Large ships like OBO and Super tankers commonly squat forward. On the other hand Passenger ships will likely squat on its stern. Assuming they are even keel when non-moving. Let’s consider another example:

The Ship is trimmed by the stern and non-moving. Then she starts moving and creates further trim at her stern. The dynamic trim (while the ship moves) will add to the static trim (while non-moving). Then we can say the maximum squat will occur at the stern.

Open and Confined Channels

Most of the time, this could lead to an open debate between the Master (the captain of the ship) and the Navigating Officer. Depending on how they understand it according to personal interpretation or how it was presented to them. Let’s try to clarify some things about Open and Confined Channels:

  • When a vessel navigates on a channel with no breadth (the width of the channel) restriction, like riverbanks and canals. The ship navigates in an Open Channel.
  • When a vessel navigates on a channel with breadth restriction. The ship navigates in a Confined Channel.

Summarizing all that you’ve learn, would give you an idea of finding the best way in finding out how much your ship squat. Keep in mind that safety of navigation plays an important role in our goal as a watch officer. This series of articles about ship squat effect will be posted here so it is better that you subscribe to my RSS Feed for updates or share some link-love and mention me on your site or bookmark this article.

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