Shannon Defiance 46 Design Comment

by Walt Schulz

The New 21st Century Boating Reality: Defiance 46

I was pleasantly surprised by the quality and quantity of the responses I received from the questionnaire we sent out to experienced boaters concerning the next powerboat Shannon should build. I was impressed enough with this new input to put aside my designs for the 42’ and 48’ and come up with a totally new design: the Defiance 46. We decided to call the new model “Defiance” because the design is in defiance of conventional wisdom that a powerboat should be using mid- 20th century hull designs and power plant engineering for 21st century boating realities. Back in 2003, I came up with a totally new underwater hull design we called the SRD and we built a 38-foot prototype that I sea trialed for over 1200 miles from Maine to the Bahamas in every possible wind and sea conditions over a 13-month period. In 2005, I received a US patent on the hull design, the first patent on a powerboat hull design awarded since 1945.

There is a detailed description of the SRD attributes elsewhere on Shannon’s website but the goal for the hull design was simple. There are three different hull designs that have been around since the 1950’s and I was not satisfied with any of them. Displacement trawlers are essentially slow sailboats that get only 2 miles to the gallon and are seriously unsafe without working stabilizers. Semi-displacement lobster yachts and trawlers also roll in a seaway and offer no real fuel efficiency, getting about 1.1 miles to the gallon at 10 kts/11.5mph. And, deep vee planning hulls are stable but burn 22 gph at 20 kts/23 mph or .9 miles per gallon. My goal with the SRD was to come up with a design between semi-displacement and deep vee planning hull designs that was stable, fast, seaworthy and fuel efficient. We have a short video on our website showing the 38’s seaworthiness in an ocean storm. The 38 prototype was independently tested and obtained 1.4 miles to the gallon at 15kts/17.25mph, or 37% less fuel burned. While the SRD hull is capable of higher speeds, it has been my experience in 40 years on the water, along with countless others, that speeds in excess of 20kts/23mph, even in moderate seas, is both uncomfortable and impractical regardless of the boat’s size.

The questionnaire also highlighted some of the confusion concerning Zeus and IPS pod drives that are being heavily marketed as fuel-efficient power solutions for conventional planning hulls. If you go to their respective websites you will quickly see that pod drives only start to become fuel efficient at speeds in excess of 25kts/30mph and do not reach peak efficiency until 35kts/42mph as compared to conventional propellers. The advertising claims made by boat companies that say the engine throttle can be reduced and the boat can run at 10kts/12mph speeds for long periods of time to save fuel and increase range are pure nonsense. Everyone involved with diesel engines knows that a diesel motor must be run at 70% of maximum RPM a minimum of 70% running time or serious carbon buildup will occur. Running a diesel engine at 30% of maximum RPM for long periods will negatively affect the life of the engine. On the positive, pod drives do have a neat docking feature that uses a joystick control coupled to a bow thruster and the engines. Now with all that being said, folks that have a smooth water place where they can operate a boat at speeds above 25kts/30mph 70% of the time, the Defiance 46 hull can easily accommodate a pod drive system.

The results of our questionnaire about the ideal powerboat also proved that many people are interested in Intercostal Waterway (ICW), “Great Loop”, and European canal cruising. There are thousands of miles of beautiful cruising grounds in the US and Europe, but there are also some interesting challenges for boaters in getting to and from these areas. For instance, there are very long stretches where speeds of 8kts/10mph or less must be maintained. There are also areas where running in potentially rough open water such as the area off the New Jersey shore, the Great Lakes or the Baltic Sea is an important part of the adventure and makes the ability to out-run bad weather at fast speeds a requirement. This is in addition to the need to complete travel in daylight hours once entering the ICW or one of the European canals, since many of them can only be traversed during the day. The Defiance 46’s speed has the ability to overcome a river with currents of three knots while trying to reach a destination before dark.

Added to this equation are the issues of shoal draft, especially in the ICW, so the 46 only draws 3’-0”. “Air draft” or bridge clearance is another important issue. I designed the Defiance 46 to be well under the 10’-6” maximum height for European canals. Also, there are wide side decks that run unobstructed from the bow to the stern since daily docking and lock tending require ease of line handling. The side door for the helmsperson to quickly exit is part of the docking issue when there are only two people aboard. Naturally, there are 30” high s/s bow and side deck rails for safety.

The Defiance 46 is offered with conventional twin 330hp Cummings diesels, pods or a single 600hp engine and there is also the Shannon exclusive “3 prop” engine option. We have built hull #1 for an experienced boating couple that want to cruise extensively on the ICW and in the Bahamas. They chose the “3 prop” engine combination for fuel efficiency, range and speed. The “3 prop” utilizes a 600 hp installed in the single screw location and a 220hp diesel installed to the side. The 220hp engine is hydraulically coupled to a pair of sail drive units with folding propellers. When running at 20kts/23mph just the 600hp is used. When operating the vessel at 8kt/10mph or docking the 600hp is shut off and the twin sail drives are used just like a twin-screw powerboat. Thus, both engines are always run at 70% of maximum RPM for optimum efficiency and longevity. An added feature of this engine combination is the ability to operate the twin sail drives and bow thruster giving the 46 incredible maneuvering and docking ability much like pods.

This triple “3 prop” engine setup is not as radical as it might seem at first glance. Many of the bigger boats using pod drives have triple and even quad engine setups. And during World War II, the famous “PT” boats had three engines to obtain fast top speeds. The PT boats were mostly run on only the single center engine to reduce fuel burn and extend range, with all three engines used only in high speed attack mode. Even Ernest Hemingway’s 38-foot sport fishing boat “Pilar” built in 1934 had one big primary engine for speed and a second small inboard engine for slow speed trolling. Let’s not forget, the Defiance 46 is designed by the same guy that had the “radical” idea of putting bow thrusters on a 50-foot sailboat in 1981 (we had to build our own because none were available for boats under 80 feet at the time). Today, it’s hard to find a single inboard engine boat, sail or power, over 30 feet without bow thrusters. Obviously, since every Shannon is built custom-to-order for a specific owner, the number of engines, horsepower and top speeds are an owner’s choice on the Defiance 46. I will discuss choices in-depth with every owner so they get a yacht that exactly suits their needs and requirements.

Please let me know if you think the new Shannon Defiance 46 is a solution to your vision of the perfect powerboat.

- W.S.

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