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Buzzards Bay Blog No 2: The Ride Advantage 12/4/10 RBH
For a while we‘ve been telling folks about the ride comfort advantages our Buzzards Bay 34 enjoys. Within this blog, we wanted to share the technical features that produce these advantages. After all, there is nothing magical about naval architecture. It is something that can be understood, compared, and judged
So how does a 34 foot, 12,000 lb vessel offer the ride comfort of 40+ foot, 30,000 lb deep vee’s? The answer is straight forward physics. Force is equal to Pressure x Area (F=PA). Water has a given pressure at 30 MPH and the more hull area that is impacting the water, the greater the force produced. As a result, naval architects are constantly campaigning for narrower mono-hulls in order to present less surface area to the waves, decreasing slamming forces. However, important considerations such as stability and cabin space require a minimum amount of beam, working in direct opposition to these slimming efforts. Mono-hulls can only be so skinny before they become un-livable and unstable. This practically limits mono-hulls to a length to beam ratio of 3.5 to 1, but normally are around 2.75: 1. In comparison, the Buzzards Bay 34’s hulls are each over 9:1. A catamaran’s architecture combines very narrow hulls that slice through seas instead of pounding their way through, with wide overall beam that affords the kind of space that boaters love. Just as important, the wide spacing of the hulls from the vessel centerline greatly increases stability, for a drastic reduction in roll compared to a mono-hull. Consider the advantages presented thus far. Even if a catamaran did not have a softer ride than a mono-hull, the stability advantages would be merit enough. To combine a softer ride with much greater stability is simply yacht design nirvana, and is something that physics just won’t let mono-hulls do.
With a mono’s broad hull, you’ve got but one choice if you don’t want to be limited to hull speed. Hull speed (in knot’s) is equal to √(1.34 multiplied by the vessels water line length). Beyond this speed, the mono-hull is pushing so much water that the drag goes up exponentially, practically limiting speed. For the average 34 foot mono-hull, this hull speed limit is around 6 knot’s. To go faster than this with any degree of efficiency, a mono-hull must climb over its bow wave and travel on top of the water, much as an airplane transitions from runway travel to flight. There is allot of energy used in the process of climbing onto plane, but once there, drag is reduced, and the hull is much happier. Just as an airplane has lift producing surfaces (i.e. wings), the planing mono-hull must also have lift producing surfaces. The hull bottom itself is the lifting surface, with the amount of lift dependant on deadrise angle and surface area. Designers create lift with relatively flat and broad hull sections and wide chine flats. Again, compromise limits what mono-hulls can achieve, because the more surface area and flatter sections there are, the greater the lifting force produced, which is good for efficiency and speed (and stability for that matter), but bad for ride comfort. Those broad and flat planing surfaces really bang in a sea-way, making you pay for every percent of efficiency gained.
The Buzzards Bay 34 takes a different approach to mono-hulls, and even most catamarans, because our hulls have a displacement hull shape. A Buzzards Bay does not need to climb up on top of the waters surface in order to travel faster than hull speed, because our very slim hulls produce so little drag, the hull just glides right through this barrier. So here again, physics allows the catamaran to avoid the compromise that mono-hulls are stuck in. Chris White, the designer of the Buzzards Bay 34, was free to design a hull shape that is soley focused on producing as little drag as possible traveling through the water, with rounded sections, free of the broad expanses of flat areas that would slam into waves. The same attributes in displacement catamaran design that make for smoother slicing through waves also make for lower drag and higher efficiency. The net affect is a much smoother ride in waves, while still being able to cruise over 30 mph. Though not optimized for top speeds, the Buzzards Bay’s are still fast enough. Recently, Doubletake has been clocked at 43 mph by Boating Magazine. Perhaps more importantly, the Buzzards Bay 34, even with base power, can run 20 mph on one engine!
Catamarans do have an Achilles heel, and that is the tunnel roof, or bridge deck. This broad panel can, when slammed into waves, create a great amount of force, resulting in pounding. The trick then, is to avoid the waves by keeping it high and gently sloped, especially forward. Chris White, when asked “what is the right amount of tunnel clearance” will say he’s never met a catamaran that had too much tunnel clearance. And I think he’s probably right. There are practical limitations of course, but in general, you want a lot of clearance. The Buzzards Bay 34 has about 20 inches of clearance minimum at rest. I’ve kayaked underneath one, using a sit on top kayak to boot. This amount of tunnel clearance is often two to three times what is normally seen in power cats in this size range. The Buzzards Bay 34 has this design for a reason and the results speak for themselves.
In general, these are the biggest contributors to the Buzzards Bay 34 exceptional performance. It’s hard to beat a well done displacement hulled power cat when it comes to all around performance and comfort. In fact, I’m not sure you can.
If you have any questions about these issues and wish to discuss them, please contact me any time. Thanks for reading.
Blog No 3 will be dealing with interior space, how the catamaran hull shape affects this, and the cruising aspects of the Buzzards Bay 34's interior design.
Russell Hunt, President
Buzzards Bay Cats