The Importance of Bottom Deadrise: Is Sharper Always Better?
By Thomas P. Courtesy of e-Ribbing.com
By the term deadrise, we mean the angle that is formed between the horizontal plane and the side of the hull, at 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.
Reader Caveat
The information in this article is specifically intended for owners of RIBs and many of the comments are not appropriate to other types of boats. For example, other types of boats do not generally ride just on the aft sections of the bottom when running fast. Nevertheless, the information contained here is important for large RIB applications.
Location, Location
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. Just as there is no the ideal boat or RIB, there is no the perfect deadrise, too. It’s all about how the boat is going to be used.
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 Bottom:
- Cuts more efficiently through 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 shallower V needs less horsepower to push through the water, is faster with better fuel economy, is quicker to 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 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 traveling in areas such as the Aegean Sea where conditions are often rough and we plan to cover long distances, we inevitably choose a deep-V bottom. In this case, it’s preferable to focus on the most comfortable ride and sacrifice the advantages a shallower design has to offer.
If we must choose between two V hulls when one has 22 degrees of deadrise at the transom and the other has 24, we must be more careful in our final choice because beyond the deadrise at the transom there are other factors that play an important role and affect the degree of soft ride we’re looking for. The spray rails and the chines, their width and inclination, the hull’s beam, the existence of a pad on the keel and other minor design interventions at the aft section of the hull assert their roles in reducing pounding.
When we have two deep-V hulls, it doesn’t absolutely mean that the one with the deepest V has a softer, more comfortable ride. A hull with 22 degrees of transom deadrise can achieve a softer ride than one with 24 degrees.