Here’s what that deadrise number really says about your boat and its performance potential.
By Eric Sorensen | SOUNDINGS Magazine
We’ve all heard it: the typical conversation between a customer and a salesperson at a boat show that invariably turns to deadrise.
“How much deadrise does she have?” the buyer asks, innocently enough.
“Twenty degrees,” the salesperson answers reflexively, referring to the deadrise at the transom.
Having exhausted his knowledge of the subject, the seller quickly moves along with the buyer to something they’re both more comfortable with: “How about those cupholders?”
It’s a shame because deadrise is important to understand how a boat will perform at sea. The right amount of deadrise, which generally varies from bow to stern, matters. Deadrise distribution is the key to making a planing hull perform well.
What’s Deadrise?
Deadrise is the angle between the bottom of the hull in relation to a horizontal plane as you go from the keel towards the chines. Unless it’s a flat-bottomed barge, if you imagine the boat sliced like a loaf of bread from bow to stern, the deadrise should vary.
In most boats, the deadrise in the bow of the boat is greater than at the stern. That’s because the bow is the first part of the hull to impact waves as the boat moves through the water. When a boat moves through the water at low speed, say up to 8 knots, the hull is displacing water rather than climbing up and planing on top of it. At low speed, the hull is at the same level as when it’s stopped, so the waves meet the bow all the way forward.
The boat is supported by buoyancy, which is a function of the weight of the water displaced by the hull in the water. The displaced water exactly equals the weight of the boat and all its contents. As power is applied and the boat starts to speed up, the boat starts to rise up on the surface of the water, increasingly supported by the dynamic pressure of water moving rapidly below the boat.
Now, the boat’s hull has to be shaped to create lift as its speed picks up. The after half of the hull should be nearly level with the surface of the water when at rest, so the stern doesn’t sink as power is applied.
This, in turn, means the transom must be immersed underwater when the boat is at rest, rather than swooping up like a ship or sailboat’s stern. It also helps that the hull has a sharp corner, or edge, at the chines, where the bottom meets the hull sides, and at the transom. The hard corners allow the water to break free of the hull when running at speed, creating flow separation and reducing resistance.
Variable Deadrise
The amount of deadrise at any given point in the hull changes as we move from the bow to the stern. More deadrise forward allows the hull to slice through waves with little or no pounding. Less deadrise aft allows the boat to climb up on plane with less energy and at lower speeds as power is applied.
Let’s say the deadrise is 45 degrees at the bow. If that deadrise continued all the way to the transom, the boat would not be able to get on plane because there wouldn’t be enough dynamic lift generated by water flow. The boat would also roll easily and deeply in even a light chop, and it would list to one side excessively when a person or heavy object moves from one side of the boat to the other.
On the other hand, if the hull is too flat in the bow, it will pound hard and incessantly, making the boat uncomfortable and even unusable unless it’s in flat water. If it’s too flat in the stern, the stern will just as readily move sideways as ahead, and it will be difficult to keep on course, especially when running downsea. Moderate deadrise aft helps keep the boat going straight, like a keel does on a lobster boat.
Deadrise distribution has to be sharp enough forward in a planing hull to create a smooth entry and part the waves to the side gently upon impact. The faster a boat can go, the more deadrise it needs farther aft in the hull, because higher speeds cause the hull to rise farther out of the water vertically. When this happens, wave impact shifts aft.
Faster hulls need more deadrise farther aft and in the middle of the hull. Go-fast boats can reach speeds of 100 mph or more. They jump from wave to wave, landing on the last few feet of the hull, so they need 24 or even 25 degrees of deadrise at the transom to absorb the shock of wave impact at those speeds. The center of gravity is also well aft in these boats, so the stern lands before the bow.
A slower boat cruising at 20 to 30 knots doesn’t need as much deadrise aft, since waves impact mostly at the middle of the hull, not the stern. Too much deadrise aft makes the boat slower and causes it to roll excessively. But even a slower planing hull needs some deadrise at the stern to give it directional stability. For boats that cruise at 25 to 35 knots, 20 degrees of deadrise at the transom creates plenty of lift, and it resists sideways movement aft when the boat is on plane. This helps keep the boat on course without excessive helm input.
If a bottom aft is flat like a shoebox, it will easily veer off course. Deadrise forward creates a smooth ride, and deadrise aft keeps the boat on course with less attention and effort at the wheel. This course-keeping attribute is especially important when running downsea, with the seas on the stern or on the quarter throwing the stern around.
A bad combination is a hull with a very sharp bow that turns into a rudder when running downsea, and a flat stern that has no resistance to sideways movement. This can make a boat uncontrollable when running downsea and can create a course-keeping circus. This can be dangerous when taken to extremes, since a yaw downsea can be followed by a roll and subsequently lead to a broach.
Deadrise Effect on Trim
Boats with monohedron hulls have the same deadrise from the middle of the hull back to the transom. These hulls can tend to run bow high and need oversize trim tabs to bring the bow down. If the chine is at the same depth from the middle of the hull to the stern sitting at the dock, it’s a monohedron.
A boat with chines that are deeper—that run downhill—as you move aft are said to have warp, or twist back aft. This shape shifts the center of dynamic lift aft as the boat gets on plane, so the boat will tend to run at the optimal trim without resorting to drag- inducing trim tabs or interceptors to get the bow down.
With the hull running naturally at a 3- to 4-degree trim angle, the engines on an outboard-powered boat can be trimmed up to the get the bow out of the water to reduce drag. A well-designedhull rarely needs trim tabs to correct running trim, only to correct for side-to-side imbalance caused by wind or offset weight.
Keeping the Bow Where it Belongs
A boat that runs bow high creates more drag, with a more deeply immersed transom displacing more water than it should. This adds resistance and increases fuel burn. It also produces a bigger wake, Bow-high trim also obscures the view forward from the helm, creating a dangerous operating condition. There is also a near-linear relationship between trim and slamming, so the more the bow goes up, the more it will pound.
Deadrise distribution from bow to stern is essential to a good-running boat. All planing hulls need 18 to 20 degrees of deadrise at the transom to keep the hull going straight, especially when running downsea.
Deadrise vs. Keels
With deadrise aft resisting sideways movement, the hull shape itself acts as a keel, but unlike a boat with a keel, a planing hull without a keel will heel, or lean, into a turn when running at speed. On the other hand, a boat with a keel will heel away from a turn, which has several negative effects.
A passenger on a planing boat in a turn that heels just the right amount will not know the boat is in a turn, at least in calm water. The boat will heel like an airplane or bicycle in a turn, cancelling out centrifugal force. This is very important for passenger safety reasons,. A boat that turns flat can throw people outboard, possibly causing injury or even ejecting them overboard. A boat with 20 degrees transom deadrise will lean into a turn and send centrifugal force down through your feet, keeping you stable. A boat with a full keel heels outboard in a turn, reducing stability for its passengers. Performance wise, keels are not appropriate for planing hulls that cruise much above 18 to 20 knots because they also add a lot of frictional resistance, absorbing propulsion energy. Although a keel is there to add stability, deadrise allows a boat to heel into a turn, thus making a boat safer for its passengers under this circumstance.
What It Means
A good-running boat is much more enjoyable, kinder to your body and helps prevent injuries. Your family and friends will want to go out on the boat. You will be able to go faster and farther in a given sea state. And you’ll be able to go out more often, when lesser boats stay tied to the dock.
Eric Sorensen is a consultant to boatbuilders and boat buyers. He is the author of Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.
April 2025
