Seaworthiness would be defined as the ability to stay afloat in the worst conditions of wave and weather. But, if we were to solely base ourselves off this definition then we could qualify a piece of plastic foam as : seaworthy. So, on the practical aspect it isn't exactly correct. As John Vigor explains in his book, The seaworthy offshore sailboat, seaworthiness has more to do with design and construction than size.
The main characteristics of a seaworthy boat are the following, firstly the ability to steer clear in heavy (storms) and dangerous conditions (rocks,reefs). Secondly, it needs the possibility of accomodating humans. And last but most important, the ability to move from the upside down position to continue her voyage. This last factor, capsizing, is very important to take into account during the design processes of a ship.
One has to understand that the most essential aspects of the ship design affecting seaworthiness are the keel and beam. To illustrate, a wide beam gives initial stability and the keel weight gives ultimate stability.
In order to have a size reference for the beam, we need to understand what is the correct "reasonable/normal" beam size. The dimension is related directly to the lenght-on-the-waterline ( noted LWL ). Still today, the typical ratio is the following, 3:1. This means the ships are about 3 times as long as they were broad. In addition, this ratio is fairly standard in deep sea ship design. We can take as examples the Pearson Triton 28 which was constructed by Alberg, or we can also take the Valiant 40 constructed by Perry. This two vessels define the 3:1 model ratio.
Earlier was mentioned the most important aspect of seaworthiness : capsizing. The root cause of capsizing is in relation to the wave action. To illustrate, let's say that your ship is designed with the following traits : narrow beam, long keel (deep in the water). Once capsized, these two aspects will simplify the process for the tilting of your boat. However, if your vessel has a wide beam, then there will have to be the need of more incoming waves to tilt the inverted hull. Thus, designing a ship is a lot about trade offs. Therefore, it really depends on the environment in which the vessel will travel and the characteristics the shipowner needs in order to answer his demands.
Here are examples of different characteristics to enable a ship to be more or less seaworthy.
Firstly, the beam size. The wider the beam, the harder it is to recover from a 180 degree capsize. Now, depending on what type of sea the vessel is navigating on, it will be more or less of a key component. If you are to navigate on typically calm oceans, then a wider beam can allow more comfort on the interior or even a wider deck. The wider beam will also give it greater initial stability.
If we compare to a narrow beam, the narrowness will give less initial stability. And more often than not, a narrow beam results in more performance. To illustrate, it slips through the water more easily.
Since the beam is in direct correlation with the hull shapes, we must note that hard bilges contribute to initial stability, or even form stability. The softer, slacker bilges have less wetted area to cause friction and resistance. They also provide more head room and stowage space.
Secondly, the keel has 2 main functions : 1) stop the boat from being blown sideways by the force of the wind in the sails, and 2) counterbalance the overturning force of the sails.
Third, but less prominent, is to provide resistance to sudden overturning forces from a breaking wave.
There are several types of keels and they all have their differences. In example, a narrow, deep and fin keel is more hydrodynamically efficient than a wide and shallow keel of the same area. In order for the fin keel to be efficient it must be moving forward in the water, and with time will gain hydrodynamic lift. However if it is stopped, the vessel will easily turn sideways.
The traditional or cruising keel depends more on lateral area efficiency than hydrodynamic lift. For cruising vessels a traditional keel is good because it keeps the boat on course for extended periods without necessary attention on the helm. And it provides stronger support for the rudder.
If we compare it to a fin keel (mainly used on modern vessels), it is proven to be very performant but needs more attention on the helm. The reason it is more performant is because it has less surface area and thus give less drag while sailing.
We can conclude that seaworthiness of a ship has less to do with actual size but with how it is constructed. Therefore, it is important to know what the shipowners' intentions are. From there, it can lead to a statement of requirements and even to a ship design. For any vessel, especially sailboats, it is important to always take into account capsizing as a characteristic of seaworthiness. Additionally, there many more aspects that provides a certain trade off to the vessel. As we have seen, a wider beam can provide more space for crew members, but it provides less efficiency. These are aspects that change alongside the shipowners goals.
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