A boat hull is one of the most important parts of a boat, in fact, if we are totally honest, it is the most important part of the boat. Without it, a boat wouldn’t perform properly, or even float.
The hull is significant in performance, the comfort of the ride, and fuel economy, and each type of hull has different characteristics that make them unique and ideal for different types of boating.
Deadrise is what we will discuss today, and it is one of the more critical metrics with respect to the design of the hull. It is commonly used to determine how well a boat can cut through seas and waves, and how a boat will ride.
We will talk through every aspect of deadrise today, so you go away with a full understanding of what deadrise is, why it is so significant in those who enjoy taking to the open waters, and the intricate truth about what it means.
What Is A Deadrise? The Basics
Let’s start off with the most basic factors about deadrise, shall we?
You have surely read a boat review or two in your day, or perhaps you have read manufacturers brochures and within the data that covered a specific model of planing hull, you will usually see a specification, as well as the boat length, beam, and weight, you will see ‘deadrise’.
Deadrise is a number quoted in degrees. The most basic way we can explain this is to say that it refers to the angle of the ‘V’ at the hull’s bottom. Or, at least, it is typically measured in reference to the hull bottom, but not always.
Imagine the boat was resting perfectly on level ground, upright on its keel. If the boat were in this position, then the deadrise would be the angle between the ground it is resting on and the bottom of the hull on either side. This measurement is taken in degrees and is equal on both sides of the hull bottom.
The angle of this ‘V’ which is the deadrise of the hull at the transom, can even be close to 0 on a flat-bottomed boat such as a jon boat or punt, and it can even be as high as 25 degrees which is often found in most offshore high performance boats.
Naval architects and boating industry professionals will generally view a ‘Deep-V’ as being a hull that is generally 20 to 21 degrees or more. A ‘Modified V’ would be between 16 and 20 degrees, and a ‘shallow V would be less than 16 degrees. Finally, a flat bottom is any boat that has a deadrise of 0 to 2 degrees.
The Definition Of A Deadrise
The Deadrise of a boat is the angle measured between the bottom of the boat and the horizontal plane on either side of the center keel. The deadrise of a flat bottom boat is very low and can even be zero, compared to a vessel with a deep -C hull which could be 25 degrees.
You will often hear that deeper or sharper V-shaped hulls have a lot of deadrise. This is simply because these boats are built to run offshore, and boats that are made for this will likely have a deadrise of 20 degrees at the transom, and 30 to 50 degrees at the bow.
Flat bottom boats can have anything less than 10 degrees of deadrise. Boats with such little deadrise are built to be able to navigate shallow waters, but to take them offshore in any sort of oceans or seas would be a big mistake as they would be a nightmare to sail.
In oceanic conditions a boat with a flatter hull is unable to channel water away, and therefore as the hull meets the water it results in a ‘slam’ which can make the ride very bumpy, jolty, and downright unpleasant.
Why Does Deadrise Matter?
How much deadrise a boat has is important to know. Why? Well, because it gives the owner, or prospective buyer of the boat an idea on how well the boat will be able to cut through choppy rough seas.
A larger deadrise value will fare better in rougher seas, which will provide the owner with a smoother ride, whereas a boat with a lower deadrise will likely have a more unpleasant and maybe even scary ride if they took their boat into choppy seas.
It is not all sunshine and rainbows to have a boat with a higher deadrise value, though The tradeoff that comes with cutting through choppy waters with ease while the boat is running does mean that the boat is not quite as stable when it is slow or at rest.
While a deep-V hull can cut through water better than others while it is going at speed, when it is slow or stopped it is not so pleasant anymore. Boats with a flatter bottom will be more stable at lower speeds, or when they are not moving.
Of course, do not choose a boat just based on its deadrise, as deadrise is not the only factor that contributes to stability on the water. Beam width can also have an effect on the ride and the stability.
A wider boat will begin to pound on the water sooner than a more narrow boat, even if they both have the same deadrise, and wider beamed boats will have more stability at rest too!
As there are many factors that contribute to the pleasantness of the ride and performance of the boat, the importance of trialing boats at sea is imperative. Getting out on the open waters in a boat is the best way to see how different vessels in various conditions compare.
Experienced boaters will know that while specifications on each boat are equally important, assumptions based on such numbers can be mistaken once a boater gets a feel for the boat out on the water.
A Little More Info About Deadrise
By now, we expect you probably feel like you have learned a lot about deadrise, and you would probably feel comfortable looking at boats and understanding their deadrise values. And how this affects how they will deal with open waters and rough conditions.
Lets, give you a little more information on deadrise before we leave you to go out and start looking at dead rises yourself. You see, there are two parts, the first part to consider is the longitudinal along the hull bottom from the transom to the bow.
Since you are mainly considering planing hull boats, you should think that at planing speed a majority of family-oriented boats will ride on the aft at about 50-70% of the hull length, depending on the loading of the boat and the speed you travel at.
A high performance deep-V may only run at about 20-30% of its hull length at full speed.
Considering this, and depending on the use of the boat if it is a bow rider, express cruiser, or offshore fishing boat, a full designer will decide at what point along the bottom that the deadrise should increase substantially to provide the boat with a ‘pointy’ bow for a smooth entry into the waves.
Why are we telling you all of this? Well, it is for this reason that only if the deadrise is measured at the transom is there any true way to actually compare one hull bottom to another hull bottom
The second issue is lateral, in that the deadrise might not actually be constant from the keel to the chine. What is the chine? Well, if you do not know, it is the longitudinal line of demarcation on the side of the hull where the bottom turns upward suddenly to form for sides of the boat,
Between the keel and the chine at the transom, it is possible that the deadrise might vary, and therefore it would be labelled as being a variable deadrise hull. The designer of this heel might want to design a deeper deadrise closer to the keel for a more smooth running, and a flatter deadrise toward the chines.
This would assist in reaching planing speeds, or it may also assist in achieving better stability at slower speeds and at rest.
The deeper the V and the more narrow the hull is the less stable your boat will be at slower speeds or rest. This is known as being ‘tender’, which basically means that the boat will feel ‘tippy’.
This is the primary reason why around 24 degrees of transom deadrise has been found to be the greatest practically achievable. And this is also why jon boats, and barges and so on are flat-bottomed, and have such great stability.
The deeper the V the greater the potential you have for a smoother ride through rough waters, and the flatter the hull bottom the rougher the ride will be in even the most minor adverse conditions.