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Why do some boats eject water?

For anyone who has spent time on a boat or has witnessed boats passing by, the sight of water being ejected from the back of certain vessels might seem odd or out of place.?

The answer lies in the mechanics of how boats move through the water. When a boat is in motion, it creates a wave of low pressure behind it. Water rushes into this low pressure zone, essentially filling the void created by the boat’s forward movement. This results in a raised water level at the stern of the boat.

To prevent the increased water level from causing issues, boats are designed with a feature known as a ‘transom’, which essentially serves as the back wall of the boat. This transom is typically curved outward and upward, helping to deflect the water to either side instead of allowing it to pool in the stern.

But as boats gain speed, the pressure difference between the water levels at the front and back of the boat increases. This difference in pressure causes the increased water level at the stern to push back against the boat. In order to maintain efficiency, boats will often have a ‘relief valve’ in the form of small holes near the waterline on the transom. These holes allow some of the water to escape, equalizing the pressure at the stern and reducing drag.

So, the next time you see water being ejected from the back of a boat, know that it’s just the mechanics of moving through water at high speeds. While it might seem strange or even concerning, it’s actually a necessary part of keeping boats running smoothly and efficiently.

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