The unique element of the hull design is reflected in the acronym SRD, for Schulz Reverse Deadrise. Deadrise is a yacht design term that describes the angle of the hull relative to the horizontal plane. The hull sections of the SRD are radically different from all other powerboats that have convex or positive deadrise hull shapes which has been the convention since the Phoenicians. By inverting or reversing these aft sections on the Shannon 38 SRD, tremendous lift is produced, while the forward sections have a deep “Vee” to prevent pounding the hull shape is then “twisted” amidships to provide unusual laminar flow that prevents suction at the concave sections at the stern. It is this aspect of the hull shape that makes the SRD hull shape so revolutionary and is the basis of the Shannon 38 SRD hull’s U.S. Patent filing. Anyone who looks at the absence of a wake produced by the Shannon 38 SRD at high speeds will know that this is a very unique hull with extraordinary performance characteristics.
How does the hull shape of the Shannon 38 SRD compare with existing powerboat hull shapes?
Before the SRD hull design, there were three types of powerboat hull shapes – displacement-like long distance trawlers, semi-displacement-like lobster boats, and planing boats that reach speeds from 15 knots to 100 knots depending on engine size. Displacement hulls are used for long distance powerboats, and while are they very fuel efficient, their speed is limited by the laws of physics to a speed measured in knots of no more than the square root of the vessel’s waterline multiplied by 1.4. A 38' displacement hull will have a top speed of 8.6 knots. The hull sections of displacement hulls are a relatively similar U shape from the bow to the stern. Displacement hulls are very uncomfortable in even moderate conditions as they roll excessively in beam seas requiring complicated and expensive stabilizing devices
Semi-displacement hulls are best known as lobster boats and coastal trawlers and exhibit slightly better speeds (a top speed in knots of no more than the square root of the vessel’s waterline multiplied by 2.0.) A 38' semi-displacement hull will have a top speed of 14 knots. Semi-displacement hulls are faster than displacement hulls because the flat aft sections of the hull provide lift allowing the forward V sections to rise out of the water. Besides providing higher speed, the semi-displacement hull is more sea kindly in rough water than a displacement hull without stabilizers because of the form stability of the semi-displacement hull shape. Unfortunately, the gains from the semi-displacement hull are offset by a higher fuel burn caused by the large wall of water that is pushed in front of a semi-displacement hull.
The deep Vee planing hull shape took advantage of increases in marine engine horsepower after World War II to produce very high speeds. The deep V hull shape continued the V forward sections all the way to the transom, allowing the hull to “get up on plane”, or to rise out of the water to reduce wetted surface thereby allowing higher speeds than possible with displacement or semi-displacement hulls. But to achieve these speeds, larger engines that burn significant quantities of fuel are required. And while stability is gained by the forward thrust of a deep V hull, the hull shape causes a pounding motion in head sea conditions.
By contrast, the SRD hull shape combines the best elements of the other three hull shapes while eliminating their worst qualities. The Shannon 38 SRD can maintain higher speeds than the other three types of hulls in ocean storm conditions. The fuel burn is much better than the high consumption of both semi-displacement and deep V hulls because no energy is wasted producing a wake. If I didn’t say that I was pleasantly surprised how well the Shannon 38 SRD has done during rigorous sea trials by me in the worst offshore conditions that I could find, then I would be lying
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Didn’t you design the Shannon 38 SRD for use in shoal draft waters and to able to be run right up on the beach?
Absolutely. One of my criteria was to have a boat that could be enjoyed in shallow waters. The reverse deadrise tucks the propellers and rudders up under the stern, enabling the Shannon 38 SRD to be run right up on the beach. The hull laminate schedule was specifically engineered for beaching. The second layer of laminate is a Kevlar/S glass hybrid that provides tremendous resistance to penetration and abrasion. The 3/4" PVC linear semi-rigid foam core extends along the bottom of the hull to stiffen the entire structure without adding a lot of weight. The triaxial laminates on either side of the core provide amazing strength. While this hull will be impenetrable in normal beaching situations, in case of a high speed grounding on rocks, there is a watertight crash compartment under the V berth to keep the Shannon 38 SRD afloat in a worst case scenario.
Does the hull shape of the Shannon 38 SRD lend itself to water jet power or surface piercing propellers?
While water jet propulsion has become a popular trend in the last decade, it is not a suitable system for a long-range offshore powerboat like the Shannon 38 SRD. Obviously, it is very inefficient to pump water to produce thrust, and jet boats always burn more fuel. And such large pumps are very complicated, making them difficult and costly to maintain. I expect the owners of the Shannon 38 SRD to go to faraway places, and not just to go on picnics. I also think that these places will not be anywhere close to factory authorized service representatives, so jet power doesn’t make any sense. Surface piercing propellers, on the other hand, are an intriguing possibility for the Shannon 38 SRD hull shape, and I anticipate speed gains and fuel savings of at least 25% from any of the units currently on the market. I designed the hull of the Shannon 38 SRD to accommodate surface piercing propeller units in either single or twin engine configurations, and would welcome the opportunity to try one.
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