Shannon 38 SRD
Exclusive: Shannon 38 SRD — By George L. Petrie — January 2005
Four in One
A quartet of shapes produces a unique hull form with startling performance.
When I first asked designer/builder Walt Schulz what his motivation was in designing the radically different hull form for the new Shannon 38 SRD, his answer was disarmingly candid: bad knees.
Apparently sensing my perplexed reaction, he launched into a more complete reply. His company, Shannon Yachts, has been building sailboats for some 30 years and powerboats for almost 20 years. His knees, and those of many of his customers, long for a hull that can deliver planing-boat speed but with a softer ride than any existing hard-chine, V-bottom form. Having heard my wife voice the same yearning over the years, my curiosity was instantly aroused.
As Schulz continued his discourse, he conceded that there was also a second, much more profound objective: developing a hull that would be optimally suited for use with surface-drive systems. And not the surface drives that are presently available, which protrude several feet aft of the transom, but a surface drive that can be mounted under the hull and not require any special equipment—just a prop and a shaft, in a suitably designed cavity beneath the hull. Now my curiosity was really piqued.
According to Schulz, the main advantage of a surface drive is that it can reduce appendage drag by nearly 80 percent, because the shaft and struts are up and out of the flow of water beneath the hull, while just the bottom half of the propeller is immersed. Hull No. 3 is presently under construction in a single-screw surface-drive configuration. (See “The Driving Force,” this story, for more details.)
The drivetrain installed in our test boat, Hull No. 1, was a conventional twin-screw inboard configuration powered by a pair of 160-hp Yanmar diesels. But I was no less anxious to see how the novel hull form would perform. Her bottom shape was unlike any other fast powerboat I had ever seen.
Schulz describes the hull as a combination of four shapes. The waterline sections in the forward-most four feet of the hull are “hollow” (concave in plan view) to “push aside water at the bow that you don’t want.” The next four to six feet aft are “round” sections (convex in the transverse plane), “similar to the shape of a sailboat hull, to give a soft ride.” (Remember, Schulz has been building sailboats for 30 years.) The midlength has an undulating cross-section, which Schulz refers to as a “vortex generator” that redirects flow upward toward the stern to provide lift and a smooth flow of water into the props. And finally, the aftermost ten feet is a gently cambered section that forms a reverse deadrise, providing the lift necessary to combat stern squat when the boat exceeds her theoretical hull speed of about 8 knots.
They say seeing is believing, and although Schulz has a convincing manner, I wanted to see for myself whether he was really onto something, or if he just had an impressive-sounding sales pitch. What puzzled me most was that the hull had a hard chine only over the last few feet of its length. How could flow separate at the chine (allowing the hull to plane) if there was none, but only a large-radius, round bilge over most of her length?
As I eased the boat out of the harbor into Rhode Island’s Narragansett Bay and pushed the throttles forward, my misgivings were quickly put to rest. The twin Yanmars spooled up, and the readout on my radar gun climbed steadily, topping out at nearly 25 mph. We made a few more runs to check trim readings—I figured my trim gauge had developed a snafu, because the trim angle was pretty much a constant 2.5 degrees over most of the speed range. There was no bow rise at all, and in our path we left virtually no wake. Seemed to me Schulz really was onto something.
This article originally appeared in the December 2004 issue of Power & Motoryacht magazine.