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The Definitive Guide to Stepped Hulls

By: Captain Bill Jennings

Stepped hull boat hitting wave

Let's get to the bottom of the pros and cons of 'steps' in your hull


What is a "Stepped Hull"?


When a regular boat is running on plane, water runs along the bottom wetting the complete underside of the hull. Through experimentation with hull bottoms, it has been discovered that by placing notches or indentations from one side of a boat bottom to the other and perpendicular to the 'chines,' you can force the water flow to separate from the hull, thereby reducing the bottom surface area that comes in contact with the water. Less water contact equals less drag. Boats with these step like indentations in their bottoms are called stepped hulls.


History of Usage


The concept of stepping a hull was originated by Englishman Rev. Ramus in 1872. So like many boating ideas today, the concept is not new. Advances in performance boats brought the use of steps back into vogue in the 80's. Today, steps are a common feature of performance powerboats that are either outboards, sterndrives, or surface drives.


What Do Steps Do?


Notching a boat hull makes a difference in the way a boat interacts with the water flow. Firstly, it reduces the bottom area that comes in contact with the water and mixes some of the water that does flow under the bottom with air, thereby reducing drag. Secondly, it sets up additional pressure points to the ones found at the transom on most boats. Pressure points are what supports and lift the boat when it is running and with more than one such area, the lift from multiple steps provide more lift and a more regulated trim angle for the boat. These combined actions, when correctly engineered, will improve boat performance.


How Exactly Do Steps Work?


Molded into a flat or deadrise hull are one or more notches that run from chine to chine (side to side). At speed, much of the water flow along the bottom of the boat partially separates from the hull when it runs over the step. The separation behind the step creates a vacuum that sucks in air. The air sucked into the step mixes with the water, creating an air vented mixture of water that continues to flow aft of the step. While at slow speeds the entire hull bottom is in the water, at higher speeds the water to hull adhesion is broken by the step, and the aerated water aft of the step reduces hull drag. Of course, less drag in a boat translates to more speed.

Photo- Aero Marine Research

But there is more than drag reduction to steps. On a standard V-bottom boat hulls, there is a "V" shaped area at the stern that provides the majority of lift for the boat. It is a major "pressure point." On a hull with one step, a second pressure point is created ahead of the step, because the break in the water flow at the step acts like the water flow break at the transom. This second pressure point being more midships moves the center of lift forward. If there are two steps, there are two added pressure points that are ahead of the transom pressure point, making the boat lift with even less angle relative to the surface of the water. With the steps at different angles to each other, the boat, using these added pressure points, can ride flatter to the surface of the water. This lower angle of attack will provide more efficient attitude for improved ride and stability. Where a boat builder places steps is critical because too much lift at a midship pressure-point can destabilize the hull and too much lift at the bow can induce stuffing. The optimum pressure of each step differs with each boat design. Most builders say that the forward step should be have minimal lift, the midship step a little more and the aft step the largest. Correctly calculated steps should result in a an inherently stable trim angle, without the need for excessive drive trim to raise more hull out of the water.


What are the Pros and Cons of Steps?


Pros:


Steps can reduce the amount of wetted hull surface, thereby reducing drag.

Photo- Aero Marine Research

Steps can reduce bow lift when accelerating onto plane and when cruising through chop. A single step will provide two planing surfaces, or pressure points on a boat. It can maintain an optimum angle of attack while coming onto plane and running. In the case of a two stepped hull design, the lifting surfaces should be even more more efficient, and certainly more so than the single surface that a non-stepped hull will deliver. Two step hulls that I have tested run exceptionally stable in two to three foot seas and can execute comfortable turns. This is not always the case with no step hulls. This efficient onto plane action and flatter trim angle will result in lower fuel consumption.


Most boats with steps claim higher top speeds. The claimed speed increases that I have seen vary from 5% to 15% and since most offshore race boats have steps you have to believe that they do indeed improve speed.


A stepped boat can be more fuel efficient. In terms of miles per gallon, if a stepped boat is running faster than a non-stepped boat, both running at the same RPM, it is logical that in most cases you would enjoy an improvement in fuel consumption.


Cons:

A stepped hull does not produce performance benefits at all speeds. In fact, at lower speeds, steps can add drag to a hull. Looking for the speed sweet spot can be difficult and it is usually only a narrow velocity band. Some manufacturers are on record as saying that while there are small advantages in the 40 mph range, it is only when running over 60 mph that the speed advantage becomes quite evident.


Well engineered stepped boats are expensive. It requires advanced engineering to design steps that improve performance through a worthwhile speed range. For each hull, designers must determine the length of the planing surface at each step and the depth of the step. These engineering costs add to manufacturing costs.


In addition to higher design costs, the manufacturing of a boat with steps is more costly because the hulls themselves are more complex to mold and laminate.


It is hard to justify steps in boats under 25'. Boats 26' to 30' can benefit, but it seems the best results are reserved for boats over 30'.


Improperly designed and placed steps in a boat can result in bad handling and even dangerous running characteristics.

When loading and unloading a stepped boat from a trailer, the steps can catch on the trailer. A bunk trailer can usually resolve this disadvantage.


Further Explanations:


The planing surface on a boat is an area of the hull that adds to lift when it is running. The objective is to have a planing surface, or surfaces, that can effectively lift portions of your boat free of surface drag, while still providing stability. In a normal, non-stepped boat, we change the planning surface using the trim button to move the drive. This 'levers' the boat at the transom, raising the bow in the process. The hydrodynamic lift is produced by the surface area, aspect ratio, and trim angle of the wetted pressure area. Using only a pressure point at the transom wastes considerable energy just to lift the bow. A prop is not maximizing its energy when its thrust is being used to lift the bow. This waste of energy is continued as you trim the boat while cruising. With one or two additional pressure points more centrally located on the hull, the boat will lift with minimal bow rise, allowing the propeller thrust to move the boat forward more efficiently.

Photo- Aero Marine Research

Placing a step in a boat hull creates a planing surface just ahead of the step (in the same way the water flow break creates a pressure point at the transom). With a second pressure area further forward on the hull, the hull will lift while remaining in a more level position. With a step, or two, ahead of the aft planing surface, a boat will more easily ride at an optimum trim angle at all speeds. This reduces the need to continually work the trim tabs to set the boat at its optimum attitude. I should note that any change to step height and step angle will affect boat performance, making it tricky to design a step.


Logically, you need to know what is considered to be an optimum step angle for a particular boat. To do this, start with the question -- what is the optimum trim angle for the entire boat? Generally, it is between 3.5 and 4.5 degrees. This is where we find the best balance between trim and drag. From existing studies we are told that the angle of a step should be less than half of the boat's trim angle. If for example, a boat has an optimum running trim angle of 4 degrees, the maximum step angle should be 2 degrees. A properly calculated step angle will help maintain a minimum angle of attack for the entire stepped hull. From observation, two steps work well for boats over 30', while for boats 29' and under one can be enough.


By this point you have read enough to understand that the performance analysis of a step design is quite complex. If you are interested in knowing more about steps, or would like an evaluation of a stepped boat that you currently own, I would suggest that you speak with my friend Jim Russell. Jim is an engineer with an extensive hydrodynamics background and has developed step design formulas for various boat hulls. You can learn more on his website, AeroMarineResearch.com.


Today, a wide range of boat companies offer stepped hull models. To decide if a stepped hull is best for you, consider the above pros and cons, as well as your boat size, engine power, budget, and need for speed.


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