You are the officer on watch and have taken the final drafts of your vessel prior to getting underway. The vessel takes departure and proceeds outbound in a narrow channel. As the vessel increases speed and gains momentum, what will be the effect on the mean draft?
• Squat effect – how a ship’s draft changes when moving ahead in shallow or confined water • Difference between static (sailing) draft and dynamic draft when the ship is making way • Influence of speed in a narrow channel on under-keel clearance
• Think about what happens to water flow under the hull as speed increases in a restricted channel: does pressure under the keel go up or down? • If the pressure under the keel changes, what does that do to how deeply the ship sits in the water while underway versus stopped? • Compare a ship at rest alongside the pier to the same ship moving fast in shallow water: which situation risks touching bottom first?
• Identify whether increased speed in a narrow channel tends to raise or lower the ship in the water. • Distinguish clearly between sailing drafts (at rest/very slow) and drafts when the vessel is moving at higher speed in confined waters. • Verify the definition and effect of squat in relation to ship speed and water depth.
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