Thread: Stepped Hull
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Old 08 October 2005, 16:47   #8
Country: Canada
Join Date: Jan 2003
Posts: 15
Stepped Hull Design

We have had many questions regarding the design and implementation of steps in performance hulls.

Like airplane wings, most planing boats tend to generate their best lift/drag numbers at about a 3 to 4 degree angle of attack, and tend to generate most of that lift along the leading edge of the wetted surface. An issue with planing performance hulls is that the faster one goes the more the hull lifts out of the water and the farther aft this leading edge moves, altering the trim angle of attack. A boat without a step relies mostly on the longitudinal CofG (distance of the center of gravity forward of the transom) – helped by trim tabs or trimmable outdrive/lower units - to maintain its proper planing angle. For every hull shape and speed there's an optimum LCofG, but as speed increases the optimum LCofG moves aft.

Stepped hulls have two advantages – 1) they can maintain near optimum angle of attack throughout a wider speed range, and 2) they can reduce the amount of wetted surface that is not near the leading edge (and is therefore not producing efficient lift). While there are many challenges in designing a hull with an efficient step, the simplest design (and thus, I suggest, the most reliable approach) is a single step forward of the LCofG, so that the running trim angle is more dependent on the size of the step than on the exact location of the LCofG. This is important because the LCofG is constantly changing with velocity, and thus makes it difficult to optimize for.

Step Design…
The design of an effectively performing step is VERY difficult - and will most always achieve optimum 'benefit' (more than the losses) at only one planing velocity. A step design is only good for a single angle of attack with a single center of gravity (CofG). That is why it is so complicated to find a step design that can "help" the performance throughout the speed range of a performance boat.

For example, the design and manufacturing tolerances are far more critical in stepped hulls than in non-stepped hulls. A slight change in plane angles, particularly the angles of the aftermost plane, has a marked effect on the running of the boat. Changing the after step dimension by only one-eighth of an inch can change the boat performance from one that runs smoothly to a porpoising hull.

Even the change in weight of passengers, or fuel weight can be enough to throw off the CofG so that the step design no longer works.

It gets worse, too! When a stepped hull turns, the wetted surface of the stepped portion can change, which changes the center of pressure of the lifting surface, which changes the dynamic CofG of the hull - stymied again!

The theory...
On a planing hull, the highest-pressure water is just aft of the leading edge, so we want to take advantage of as much of that pressure (lift) as possible without the drag penalty of the low-pressure water farther aft. The efficiency of a planing surface is a strong function of its aspect ratio, (the relation of the width to length). The most efficient planing hull is one that is very wide, but very short. (The aspect ratio of a fast prop-driven airplane is perhaps 8:1, while the aspect ratio of a non-stepped planing hull is on the order of 0.5:1)

Now, the design of a simple non-stepped hull, we must select a built-in angle of attack and the center of gravity. We can control the angle of attack by specifying keel camber, deadrise, and chine warp (all of which may vary over the hull length). Stepped hull design includes all these considerations, but now we ALSO have to balance the angle of attack of each step section with the distance between the step and the transom or between multiple steps!

At speeds that are different than the speed that the step is designed for, the steps are often entirely immersed, so each step actually adds drag to the hull.

Air Bubbles and Pad Design...Another suggestion that is sometimes presented…Introducing "air bubbles" to the running pad surfaces? Most experimental data leads designers to say that this is a misnomer. A myth about stepped hulls is that the introduction of air into the water that flows under the hull reduces the viscous drag and makes a stepped hull go faster than a non-stepped hull. But in reality, running on air bubbles doesn't reduce the frictional resistance at all. The hull lifts on the water, not the bubbles. So bubbles or "two-phase flow" (water and air) will actually increase the drag. The "venting" of steps is usually added to designs in an effort to reduce the tendancy to "trip" in cornering or heavy waves.

Multiple Steps... And finally, there is the question of "multiple steps". There are two problems with multiple steps.
1) If the steps are located too close to each other, the water attaching to the second step is "contaminated" by the aerated low-density water from the first step (as per my explanation above), so the aft step does not produce the high lift forces desired.
2) Where do we locate the center of weight (CofG) so that the weight is balanced across the steps? Remember, the running trim angle of your boat changes dramatically as you go from zero to full speed, and this makes a huge difference in the lift-force distribution on your steps. It takes only a small change in the relative locations of the dynamic CofG (and the center of pressure) to change your boat from a stable, efficient boat to one that porpoises at several velocities.

OK...I've already written too much on this. Steps is really complicated design issue. They are difficult to design effectively, and thus, most don't work very well...and it should be easy to see why. When you read about, or experience the many boats that behave in a really nasty way with steps, we can appreciate how they got there - the steps were probably not 'designed' at all!
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