By: Richard Crowder
We’ll start off with the basics and ease into this complicated and often confusing issue. I am sure you have read boat reviews and manufacturer’s brochures and within the data on a specific model of a planing hull, there is often a specification along with boat length, beam, and weight that is labelled “deadrise.”
The number quoted is in “degrees” and refers to the angle of “V” in the hull bottom of the boat as measured most often, but not always, at the transom. In other words, if the boat were resting on level ground and perfectly upright on its keel, then the deadrise would be the angle between the ground and bottom of the hull on either side. This measurement in degrees would be equal on both sides of the hull bottom.
This angle of “V", the deadrise of the hull at the transom, can be close to zero for a flat-bottomed boat such as a punt or jon boat and as much as 24 or even in some rare cases 25 degrees as found mostly in offshore high performance boats. Most naval architects and the boating industry in general consider a “Deep-Vee” hull to be generally 20 or 21 degrees or more, a “Modified Vee” to be roughly between 16 and 20 degrees, a “Shallow Vee” to be less than 16 degrees, and a “Flat Bottom” boat having 0 to 2 degrees.
That was the easy part. Now the complications start and there are two parts. The first part is longitudinal along the hull bottom from the transom to the bow. Since we are dealing with only planing hull boats here, consider that at planing speed, most family-oriented boats generally ride on the aft 50-70 percent of the hull length depending on speed and loading. A high performance deep-vee may run on only 20-30 percent of its hull length at speed.
Given the above, and depending on the type and intended use of a boat being a bowrider or express cruiser or offshore fishing boat, etc., a hull designer will decide at what point along the bottom that the deadrise will increase substantially to provide a “pointy” bow for smooth entry into the waves. For this reason, only if the deadrise is measured at the transom is there any way to compare one hull bottom to another.
The second complication is lateral in that the deadrise may not be constant from the keel to the chine. The chine is that longitudinal line of demarcation on the side of the hull where the bottom turns abruptly upward to form the sides of the boat. Between the keel and the chine at the transom, the deadrise may vary and thus be labelled a variable deadrise hull. The hull designer may want a deeper deadrise closer to the keel for smoother running and a flatter deadrise toward the chines to assist in reaching planing speeds or to achieve better stability at slower speeds or at rest.
At this point, some added explanation is required. The deeper the vee and the narrower the hull, with all else being equal (which it never is in boat design), the less stable the boat will be at slow speeds and at rest. In nautical terms, this is called being “tender” or in other words, the boat feels “tippy.” This is the main reason why 24 or so degrees of transom deadrise has been found to be the greatest practically achievable. That is why jon boats and barges are flat bottomed and have such great lateral stability.
In addition, the deeper the vee (meaning the higher the deadrise number), the greater the potential for a smoother ride in rough water. Conversely, the flatter the hull bottom (i.e., the lower the deadrise number), the rougher the ride in even slightly adverse conditions. To summarize, the deeper the vee the smoother the ride at speed and in rough water but less stable at rest. The shallower the vee the more stable at rest but the rougher the ride at speed and in rough water. Also, the shallower the deadrise the shallower the draft thus allowing the boat into shallower water.
Because of the above compromises, boat designers have created ways to deal with some of these issues in V-bottom boats. You will notice running longitudinally on most modified and deep-vee hulls two or three full length or partial length “strakes” on each side of the hull bottom. These strakes have a flat bottom surface of 2-3 inches in width. You will also notice that at the chine, the hull bottom flattens out roughly 3-8 inches wide and may be horizontal or even reverse in direction or even be concave in shape.
All of the above are designed to give the V-bottom hull increased lateral stability at slower speeds and at rest and have the added benefits of assisting the boat onto plane and deflecting the water away to assist in a drier ride. In addition, the strakes and chine design can assist in increased cornering control at speed as well as “cushioning” the hull as it comes down off a wave. This “cushioning” effect may also be achieved by some designers by very slightly rounding out the hull bottom from keel to chine.
Let’s add another wrinkle to all of this. Given the same length, beam, and weight, a flat bottom boat will be much faster onto plane and at top speed than a deep-vee boat given equal horsepower and type of propulsion (outboard, inboard, sterndrive, etc.). For example, given two nineteen foot bowriders of equal weight and waterline beam, the one with a 16 degree deadrise could get away with a much lower horsepower (and therefore less cost) outboard motor than the boat with a 22 degree deadrise and still be as fast.
Given the above, a boat designer has to know the intended purpose and use of the boat and where it is likely to be used in terms of type and size of water in order to design the hull to accommodate these objectives. And as you can now see, there is no right answer but a raft of compromises. Every factor in the hull design involves a trade-off.
I hope you now see why deadrise and all the factors that accompany a single number on a specification sheet is so important in the boat you choose. The problem is that you only see that single number and not any of factors that accompany it as outlined above and therefore you are really flying in the dark.
The only real solution is to test drive the boat of your choice in the type of water and conditions you would expect to encounter most of the time. This is the acid test of how well the boat will perform and live up to your expectations in the months and years ahead in terms of performance, stability, ride comfort, and even efficiency. Good luck.
-
Comentários