The ABCs of Personal Watercraft Hulls

A craft's hull determines its personality

Boat show season is kicking off shortly, and with it the opportunity to check out a lot of different makes and models of personal watercraft. And with no radically new, latest-and-greatest offering to sway you in 2013, chances are you may be taking a little closer look at each boat’s features. And if you ask me, one of the most important is one that doesn’t show up very prominently on the spec sheets — a craft’s hull design.

Almost forgotten in a sea of buzzwords like electronic throttle, cruise control, or braking, it’s a craft’s hull that truly determines its personality. That hull design dictates whether it carves corners with an aggressive bite or instead slides through in a sweeping arc, tames the bigger open waters or instead threatens to send you to the chiropractor, or even adequately hauls your family through a touring ride or towable run. So it only makes sense that you pay it a little more attention.

Not sure where to start? Here’s a primer.

To V or not to V…

The Kawasaki Jet Ski Ultra 300X features a deep V hull design, making it ideal for handling rough water.

All hulls share a similar basic shape. Viewed from the stern, it’s roughly that of the letter V. They start wide at the craft’s bond line (the link between the hull and the upper deck), and then descend downward, angling inward until the lines meet at the craft’s center. That lowest point that connects the two hull sides at its centerline is known as the keel.

Those Vs come in various varieties. A deep V, as the name implies, is relatively straight, deep, and sharp. It’s a great design in rough water, as it slices through the waves with the least amount of impact or jarring. The tradeoff? Think of balancing something with that shape atop a flat surface. It’s tippy. The same can hold true for water. With such a sharp angle, the hull can tend to roll to one side or the other.

To combat this characteristic, most hulls today sport a modified-V. The design retains that sharp V, but positions it mostly at the bow, where it will knife through the water. As you run back along the centerline, however, the angle of that V softens. The shallower angle towards the stern increases the boat’s stability by placing a flatter, less angular surface in the water. That flatter angle also improves a boat’s top speed.

Hear a salesman tossing around an angular measurement? It refers to the actual angle of the V in the hull. Obviously it differs depending upon which point of the hull it’s measured, but transom deadrise (measured at the stern) is often a good means of comparison.

Lean in aggressively…or run flat and stable?

Chines are another critical factor in how a hull behaves on the water. Look at a craft’s vertical hullside, and then note where it stops and begins to angle down toward the keel. That abrupt angle is a chine. And how sharp, or soft its shape can tell you a lot about how the PWC in question will handle.

A hard, angular chine will typically be more stable, as it presents a flatter surface on the water. A softer, more rounded chine, however, will allow the boat to literally roll more easily into a turn. Looking for the inside lean of a motorcycle? A soft chine will come closest to producing that ride. Prefer the stability of a solid cruiser? A hard chine will deliver the ride you want.

Just remember, each design’s strength is also its tradeoff. Be honest about what you want out of your ride, and then find the style that produces that result.

Weight lifters…

The last primary hull feature is a strake, a lengthwise, raised ridge that parallels the keel. When a PWC accelerates, the water its hull displaces is forced away from the keel toward both the port and starboard sides of the hull. By trapping some of this water against a strake, that water then creates lift, which pushes the hull up and onto plane. Once atop the water and running at speed, strakes also allow the craft to ride on less of the hull’s surface area, which increases top speed.

Typically you’ll see more than one strake running along each side of the keel. Theoretically this increases speed, as it allows the hull to run higher and flatter atop the water. You won’t often see more than two to three, however, as more would likely produce too much lift. With so little hull in the water, stability would suffer.

Those lengthwise ridges also help the hull to track, as they provide almost tiny little fins in the water.

Stepping it up

Occasionally, a hull will feature a step, literally an upward, step-like notch in the hull bottom. Designers notch a hull in this fashion to create more lift. Think of how water flows along the hull bottom. It creates its greatest lift at the hull’s leading edge, but the effect lessens as the water moves back toward the stern, until the point where that water is literally creating drag. A step is just a way of gaining the benefits of another leading edge in the hull surface. In theory, it should plane more quickly, run faster thanks to less wetted surface in the water, and even benefit from the turbulent, aerated water coming off the step reducing drag.

Steps, however, can be tricky, as their placement can affect handling.

Enter the sponsons

One surefire way to improve handling is through sponsons. These are the little fins, or wings, you see bolted to the vertical hull sides near the very aft corners of the hull. Sponsons can provide some lift to the back of the hull as well as enhance stability, but their primary purpose is to bite into the water when the craft is leaned over into a corner. This makes the turn more aggressive, and lessens the chance the rear of the hull will slide out.

Some sponson designs are actually user adjustable. Lowering their trailing edge typically makes the ride more aggressive, while raising it gives the craft a slightly more forgiving, less aggressive feel.

Related Reading
Understanding PWC Hull Terminology
How to Repair Personal Watercraft Hull Damage
Five Steps to Finding the Perfect PWC