By the term of deadrise we mean the angle which is formed between the horizontal plane and the side of the hull, at its any point.
As it can easily be understood, this angle is not constant but varies gradually over the whole length of the hull. In other words, the deadrise angle starts with its lower value at the transom and gradually increases as the hull goes forward until it takes its maximum value at the bow, where the hull cuts the wave.
In practice, the deadrise angle defines the size of the dihedral angle of a hull, which is the angle that is formed between the two sides of the hull.
The larger the deadrise angle, the lower the dihedral angle is and the less flat the hull gets.
At this point we should note that usually when we refer to the hull’s deadrise we focus our attention on the angle at the transom and certainly not by chance, since at the cruising speeds in which we are mainly interested in, only the aft section of the hull participates in our ride. Therefore the deadrise angle that corresponds at the transom is mainly responsible for the behavior of our rib and for the quality of riding too.
We can therefore claim that the deadrise at the transom is the most important factor which shows us the behavior of our hull.
Still, most of us finally wonder whether the best choice is a boat with deep or swallow V hull.
Of course, the answer is: It depends ...
As there is no the ideal boat or rib, there is no the perfect deadrise too.
All is about Use!
Both the deep V hull (with 22 to 24 degrees of deadrise at transom) and the shallow V hull (less than 20 degrees of deadrise at transom) have advantages and disadvantages.
Generally a deep V hull:
- cuts more efficiently the wave as it penetrates the water
- offers a softer ride with smoother landing on the crest of the waves and so we travel more quickly, comfortably and pleasantly in rough seas
- is heavier, has more wetted surfaces which create greater friction and therefore needs more horsepower to move forward through the water
- is slower and gets more difficult on plane
- has worse fuel economy
- has less stability when we are anchored
On the other hand, a hull with a shallow V needs less horsepower to push through the water, is faster with better fuel economy, gets easier and faster on plane, usually offers drier ride and is more stable when we are anchored. In other words, a shallow deadrise hull is more preferable in calm seas but when the sea conditions get worse and the waves grow up, it pounds very violently which is too hard for both the crew and equipment.
From all the mentioned above it can be concluded easily that a shallow deadrise hull is preferable when we use our boat on lakes or on sea areas in which we rarely meet rough water conditions.
Our choice will then be based on the use our boat is intended for, but having seriously in our mind the sea area in which we intend to move.
If of course we are used to travel in sea areas such as the Aegean Sea, where conditions are often very difficult, and we intend to cover long distances then we inevitably have to choose a deep V hull.
In this case it is preferable to focus on comfortable and most pleasant ride sacrificing all the advantages a shallow V hull offers to us.
But what about if we have to choose between two V hulls?
When the one has 22 degrees of deadrise at transom and the other 24;
If the only selection criterion was the most comfortable ride will we then choose the boat with the higher degrees of deadrise?
The answer is that in this case we must be more careful in our final choice because beyond the deadrise at the transom there are other additional factors that play an important role and affect the degree of soft riding we're looking for. The spray rails and the chine, their width and inclination, the width of the hull (beam), the existence of pad and other minor design interventions at the aft section of the hull assert their role in reducing the pounding.
Therefore when we have two deep V hulls it doesn’t absolutely mean that the one with the deepest V has softer and more comfortable ride. A hull with 22 degrees of deadrise is possible to achieve softer ride than a hull with 24 degrees.