# Soft Science: RIB vs V Hull

If you wanted to design a boat to have the softest possible ride, what hull shape would you choose -- vee hull or rigid inflatable? Boating's Tech Team decided to find out.
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G Whiz

Technical Editor Dan Long and I are experienced with accelerometers to test for ride quality. We've used them before to compare catamarans to deep-Vs. We depend on our trusty Vernier Logger Pro software loaded onto notebook computers aboard each boat to collect and analyze the data. Each computer is fed data from a sensor installed on the centerline just behind the consoles where you'd normally stand. The sensor compiles data from three directions, but we're only interested in one, the vertical. It's this motion in which slamming or pounding occurs and makes you feel whether a boat is an old softie or a hard-riding beast.

When a boat moves through the water, it accelerates upward and downward as it encounters waves. This acceleration is measured in Gs. One G is equal to 32' per second squared, the normal acceleration of a free-falling object. Although we normally think of acceleration as speeding up, the term actually refers to a change in speed. Accelerations can be positive (speeding up) or negative (slowing down). And it's the negative acceleration expressed in G force that we're concerned with. A boat that slows down faster as it hits a wave rides harder and will post a higher G force value than one that slows down slower-easing itself into the water.

Our test procedure consisted of running the boats next to each other several yards apart at 25, 30, and 40 mph. By doing so we tried to ensure that both boats hit the same waves at the same time. We ran these speeds in three different directions: into the waves, with the waves, and across the waves. We did so for 60 seconds at every speed, giving us enough time for the accelerometers to deliver tens of thousands of data points and a good picture of how each boat performed.

Before revealing the results, let me say that the ride quality of these two boats was so close that at 25 mph the graphs showed no noticeable difference. Therefore, we only used the data collected at 30 and 40 mph. For it was only at these higher speeds that we could begin to make a useful comparison.

Into, Across, and With

Sea conditions in the Gulf of Mexico on test day were 10-knot winds with a gentle 2' swell. Not exactly ideal. The average boater would be hard-pressed to feel any differences between the two boats as the impacts are slight. But our equipment is so sensitive and accurate that it had plenty to work with. Unfortunately, these conditions wouldn't allow us to compare "off-axis" wave impacts, which is purportedly one of the key benefits of a RIB.

According to John Hale, Zodiac's director of engineering, "In rough water, it's likely that a boat won't always hit with the hull perpendicular to the wave's surface." In other words, the keel isn't going to be slicing the wave at its most ideal angle, which is head on. Hale goes on to explain that "a boat can easily hit a wave at an oblique angle, thereby reducing the effectiveness of its deadrise. For example: If a boat has a deadrise of 24 degrees and lands on a wave heeled over by 10 degrees, the resultant effective deadrise [what the wave meets] is only 14 degrees." And naturally landing on that 14-degree bottom will feel a lot harsher than landing on a 24-degree bottom.

This is a problem most of us have experienced, regardless of the type of boat-unless it was a RIB . Hale asserts that a RIB's inflatable collar absorbs a lot of the off-keel impact. This notion rings true to those of us who have ridden in RIBs. But it's a ride quality that today's calm conditions can't confirm.

We started by running into the waves at 30 mph. Here the deep-V's ride proved softer, posting only a couple of small G values that averaged 0.14 and maxed at 0.16. The RIB 's impacts averaged 0.32 and peaked at 1.03. So the deep-V hull rode better at this speed.

However, things changed at 40 mph riding into the waves, with the RIB being softer. Peak values for the deep-V were 0.56, compared to 0.48 for the RIB-a negligible difference. But the RIB's average G value was only 0.11, or one-third the impact felt by the deep-V, which averaged 0.33.

Running in following seas at 30 mph, the deep-V hull won handily, posting only one G value of 0.11. The RIB peaked at 1.2, and scored several values, averaging 0.51.

After throttling up to 40 mph in following seas, the deep-V won again, posting G impacts on only two waves (0.06 and 0.09). The RIB regularly posted G force values, which averaged out to 0.68, peaking at 1.2 for one wave.

In beam seas at 30 mph the deep-V posted one slamming 0.81 impact. Although the RIB banged several times, it averaged only 0.67, which was less than the single bumpy hit by the deep-V.

At 40 mph with waves abeam the story was about the same. While the deep-V posted a total of four values-the peak one being 0.21-it averaged 0.09. The RIB posted 17 values during the same period, with a maximum 2.1 and averaged 0.76.