A well-designed stern is designed outside in. It is the waves and sea state that define how the stern needs to be constructed. It’s not the inside out designed overstated owners cabin that dictates how the stern will look. At least not in our design approach. So, having said that, what constitutes a well designed stern section? Basically three things, as I learned from talking to many naval architects over the last two years. A well-designed stern is:
- Relatively narrow;
- Shallow draft.
Functionally, the most important responsibility the ship’s stern has is to deal with waves that hit from behind. A well-designed ship manages waves well. Waves can hit the ship forward (at the bow), from one of the sides (beam waves) or at the back. Waves that hit the ship from the back, that sorta roll-up from behind, that’s what a ship’s stern needs to deal with.
A large wave from behind lifts the aft section of the ship. As the aft section lifts, the boat accelerates downwards, as if downhill. As the ship gains speed, and accelerates down the wave, it runs the risk of burying its bow into the wave in front. As the bow submerges in the wave in front, the ship quickly decelerates. The original wave, coming from behind, now pushes the slowing down ship harder and harder, so that it may broach. Broaching is where the ship is pivoted to the left or right. This is extremely dangerous, because the ship now faces the next big wave with its beam, increasing the risk of being capsized.
A wave from behind can lift even the biggest ships …
A well-designed stern mitigates both the lift-energy and the push-energy of a wave that hits the ship from behind. A round-bilged stern is best: it dissipates a lot of energy by guiding the upcoming water, that tries to push the ship upwards, sideways, around the ship. Hence its name: round-bilged stern.
Round-bilged stern: to the right bottom …
The picture above shows four basic stern designs. Imagine a wave hitting the stern from underneath, pushing the ship upwards. If you look at the box-design stern to the left top, it is easy to imagine that the wave will push this ship upwards easily. All the wave’s energy is translated into the ship’s aft section being lifted, resulting in a quicker acceleration down the wave, and an increased risk of the bow being buried in the next wave, slowing the boat down, at the risk of it broaching and capsizing.
The design to the left bottom shows a single-chine design. Relative to the box-design discussed above, it is easy to imagine that part of the wave energy dissipates sideways. As a result, this stern design will result in slightly less aft section rise, a slightly slower acceleration forward, and a smaller risk of the ship broaching and capsizing.
The stern design to the top right is called “multi-chine”. It is easy to imagine the more rounded shape being able to shake off even more of the wave energy, resulting in less down-hill acceleration, a lower risk of the bow being buried in the wave in front, and a lower overall risk of broaching and capsizing.
Finally, to the right bottom of the picture above, we see the round-bilged stern design. This design is best at dealing with waves that hit from behind, because it dissipates more energy than the other stern designs. As a result, the round-bilged stern shape will experience less total lift. Less lift means less acceleration. Less acceleration translates into a lower risk of the bow ploughing into the wave in front of the ship. The good news? This design really limits the risk of broaching and capsizing!
A well-designed stern is round-bilged. I hope the above paragraphs made that clear. A well-designed stern is also relatively narrow. A wave from behind not only lifts the aft section of the ship, it also pushes the complete ship forward. The wider design stern offers a bigger surface area for the wave to push the ship forward. As the ship accelerates forward, the risk of running into the wave in front of the ship increases. As before, burying the bow into the wave in front of the ship, will result in your ship now suddenly decelerating, giving the wave from behind a chance to broach and capsize you. The picture underneath can help as a further clarification:
It is easy to see how the ship to the left has a stern that dissipates more wave energy than the ship on the right. The ship to the right offers a wave coming from behind a much bigger surface area to accelerate the ship forward. Also, with a wider stern, the total volume of the aft ship is bigger, increasing the buoyancy aft, further increasing the ships lift angle.
A narrow stern offers another advantage. Not only does it help fend off waves hitting the ship from behind. A relatively narrow stern, in calmer conditions, does a better job at guiding the water around the ship, as it sails forward. It makes for a more efficient ship. The efficiency gains of round-bilged and multi-chine, narrow stern-designs can be as much as 56% (Van Oossanen et al.)!
Here is a bird-view drawing of the Hammerhead stern. Not only is it round-bilged, it is also relatively narrow, for better wave-management and improved overall efficiency:
The final characteristic of a well-designed stern is that it is shallow, meaning it doesn’t sit deep in the water. The reasons, I hope, are by now clear. A shallow draft stern offers waves, that come from behind, little surface area to push the ship forward. It also limits the total buoyancy of the aft section, further limiting the lift angle a wave from behind can create. An added benefit of a shallow stern design is that, in normal sea conditions, it better guides the water flow under the ship, resulting in improved efficiency. Here’s a centerline section drawing of the Hammerhead shallow draft stern:
The horizontal line at the left is the water line. Do you see how shallow Hammerhead’s stern sits in the water? It results in better wave management and improved efficiency.