Why No Steps?

May 1, 2024

Why The Catch 27 Crossover Opts Against a Stepped Hull Design.

The beauty of crafting a boat from scratch lies in the freedom from constraints, allowing for thorough consideration of all options to select the most suitable design. While exploring various possibilities for the Catch 27 Crossover, we deliberated extensively on incorporating a stepped hull design but ultimately concluded that it wasn’t the optimal choice for our application. Here’s our reasoning:

Stepped hulls have witnessed a surge in popularity over the past decade, yet their origins trace back much further. In 1872, Englishman Rev. Ramus pioneered the concept, which gained traction in the early 1900s with the rise of more powerful gasoline engines. Over the last three decades, significant advancements by astute engineers have further refined this technology.

The primary allure of a stepped hull lies in its ability to reduce the wetted surface area of the hull in contact with the water, consequently diminishing drag.

This phenomenon occurs as negative pressure forms behind the steps, drawing in air from the sides.

As boat speed increases, more air is drawn in, resulting in less hull drag through the water.

Additionally, stepped hulls tend to ride flatter than their non-stepped counterparts, with pressure points on the bottom shifted aft, causing the stern to rise and the bow to lower, thereby reducing the angle of attack.
For certain vessels, such as large high-speed center consoles or offshore race boats characterized by their deep vee hulls, stepped designs offer distinct advantages.

However, for the Catch 27 Crossover—a mid-sized, mid-speed, shallow draft, family fishing boat— a stepped hull presents several disadvantages, even safety concerns, for the following reasons:

  1. Operational Speed: Stepped hulls require considerable speed to outperform non-stepped hulls efficiently. The negative pressure behind the steps induces drag, necessitating sufficient airflow beneath the hull to offset this drag. Research indicates that the efficiency threshold for stepped hulls typically begins around 40 mph, with some high-performance hulls requiring speeds upwards of 70 mph. However, the Catch 27 Crossover is expected to operate within the speed range of 25 to 45 mph, wherein a stepped hull would either increase drag or offer negligible drag reduction.
  2. Risk of Spinouts: Stepped hulls, particularly on shallow draft vessels, can be prone to spinning out at high speeds. As speed increases, the aerated water passing under the hull can render the back end slippery, leading to loss of control and potential spinouts. Given the Catch 27 Crossover’s shallow draft of only 12 to 13 inches, the lack of substantial hull depth to grip the water would heighten the risk of high-speed spinouts with a stepped design.
  3. Limited Bow Control: Stepped hulls tend to maintain a flat ride on plane, even at lower planing speeds, with pressure points shifted aft. While this characteristic suits offshore race boats, it may pose challenges for a family fishing boat. Instances requiring the ability to raise the bow—such as navigating large boat wakes or rough inlets—could prove difficult with a stepped hull. To address this, we designed the Catch 27 hull to accommodate ZipWake interceptors instead of traditional trim tabs. These interceptors afford the captain the flexibility to adjust water pressure at the aft end of the hull, allowing for precise control over the running angle in various conditions. At the anticipated operating speeds of the Catch 27, properly adjusted ZipWake interceptors enhance hull efficiency more effectively than a stepped design.