Power seafaring defined

Feb 28, 2011

Half Niagara Falls barrel, half high-performance motor boat, the U.S. Coast Guard’s 47-foot motor lifeboat is anything but your average power cruiser. A look at these hard-nosed rescue craft provides us with a baseline for a dependable, seaworthy power voyaging craft.

One of the best ways to peel through sales rhetoric and get a feel for the seaworthiness of any motor boat is to compare it with a vessel designed and built for the extreme end of wind and sea interaction. And when it comes to this kind of platform, the 47-foot motor lifeboat — better known as the 47 MLB — is in a class all its own. A first responder that goes to sea when conditions are approaching their worst and other mariners are in real danger. The vessel sprang from the USCG’s experience with prior rescue craft and through the insight of their own gifted in-house naval architects.

Staying afloat, staying right-side-up and staying the course may seem a simple enough set of design goals, and in calm weather and flat seas indeed it is. But as the wind velocity and the sea state builds, so does the challenge to stability and structure. Pressure loads on hull plating increase exponentially with sea state, as does the load on hatches, doors, windows and the frames that hold them in place. The best way to get a feel for what the ocean crossing power cruiser may face in a storm at sea is spelled out in the scantlings that define the 47 MLB. Here’s a vessel that’s structurally a cut above the rest of the fleet. When it comes to hull and deck scantlings and the ability of the 47 MLB to handle abuse, most of the recreational fleet takes a back row seat. The two big questions worth delving into are exactly what factors make the rescue craft stronger than even the upper end of today’s recreational power cruiser fleet, and are such structural and stability enhancements really necessary?

For the Coast Guard, the design process began with a parametric analysis of other similar-sized oceangoing craft designed to handle heavy weather rescue work. Computer engineering and finite element analysis defined the pressure imposed upon hull and deck surfaces, and profiled how they increased as conditions deteriorated. Impacts from breaking waves and the loads associated with jamming the cabin side into non-compressible green water were analyzed. Tank testing and capsize recovery experiments were conducted using scale models and the tow tank at the U.S. Naval Academy. Along with hull shape, structure and stability were key factors in the design development of the 47 MLB.

Stability
Static stability calculations are a valid reference point calculated with an assumed level ocean surface. Unfortunately, at sea the inclination caused by wave faces can change the geometry linked to form stability. In a nutshell, this means that when the level playing field like sea surface is tilted, the status quo assumption that gravity is a force that acts perpendicular to a the sea surface is no longer in play. Sloping wave faces alter the role of form stability and a vessel can be tilted 10 to 20 degrees or more with very little righting moments being created. What happens is the water surface is momentarily sloped and the vessels buoyancy responds to the new water plane. Gravity’s influence on the center of gravity (CG) lends a bit of secondary righting moment, but the vessel moves toward a capsize situation without a buoyancy shift. The reason is that there hasn’t been any significant heel and a shift in the center of buoyancy. In flat water, heeling is initially countered by a form stability-derived righting moment. On a wave face, there’s an incline of the water surface caused by the passing wave energy, and the vessel is momentarily inclined due to the shifting sea surface.

Other contributing causes of capsize are wind pressure and breaking wave energy, forces that can team up with wave face induced incline and significantly increase the potential of reaching the vanishing point of positive stability. This limit of positive stability can be given as an angular measurement that defines boundary between capsize and recovery, much like a coin balanced on its edge. Sailboats with deep ballast keels have a lower CG, and this adds a powerful secondary righting moment. A low CG in a power cruiser can cause a whiplash-like roll to occur and convince the crew to take up golf. So the big challenge with oceangoing power boat design is how to keep a deeply heeled vessel, with a fairly high CG, from capsizing.

One answer is the addition of a different source of secondary righting moment, and in the case of the 47 MLB, it’s an elevated, high-volume cabin house. Not only does this structure add lots of room for systems, cabin space and an inside steering station, but it also becomes a source of buoyancy that can arrest the rotation toward capsize. In cases where the rotational energy is so strong that the heeling moment overwhelms the righting moment, and the vessel is inverted, the added buoyancy of the superstructure causes a recovery from even an inverted position. All this is contingent upon maintaining the watertight integrity of the superstructure, and this is where we see another night and day difference between the recreational mainstream and special-purpose seagoing vessels.

The Coast Guard’s tradition of incorporating the Navy’s small craft stability criteria in their design efforts complicated the project. The Navy guidelines call for high initial stability in the 0 to 45-degree range. The weight and windage of a high-sided deckhouse, however, caused both wind heel and the elevation of the vertical center of gravity to play an antagonistic role in the MLB’s initial static stability. Fortunately, the moderate beam, deep-V hull, and hard chine helped to dampen roll, and careful control of material weigh and placing machinery as low in the bilge as possible, allowed the design to meet both the Navy guidelines for initial stability and the Coast Guard’s goal of a 180-degree self-righting vessel.

Structure
A quick look at power voyagers reveals a wide range of approaches to watertight integrity. For some, it’s enough to cope with inshore wind and sea conditions. For many, the term well-sealed equates to being able to shed the effects of a summer down pour, and large openings sport patio-like doors and sliding windows. Others have smaller, more point load resistant doors and ports. Keeping water out of the interior and preventing down flooding grows both more difficult and more important as conditions deteriorate.

It’s not too difficult to build a boat like a tank if weight and performance can be disregarded. But the Coast Guard needed a 25-knot-plus performer that weighed less than 20 tons (station lift constraint). Targeting both performance and heavy weather survivability, the design team found weight and cost savings linked to alloy construction. Effective engineering allowed the builder to vary hull and deck plate thickness according to the pressure loads that would have to be endured. As might be expected, in regions where intense slamming loads would be focused, the alloy plate was thicker and the supporting transverse and longitudinal framing was increased. Ring frames, bulkheads and gusseted supports tied the deck, hull and house together. One of the most obvious departures from recreational craft was the avoidance of “picture windows.” There’s a very noticeable up-tick in how port lenses are attached to the cabin house. There was a clear understanding that down flooding a port had to be avoided at all costs, and big sliding windows and poorly reinforced doors had no place on a boat designed to survive breaking seas and storm-force conditions.

Construction
The main alloy used in the construction was 5456 aluminum because of its higher yield strength in a welded structure (26,000 psi. vs. 22,000 psi.). There’s also significant use of 5086 in lower stress areas, and where tube and other shapes were not available in the 5456 alloy. Bottom plating thickness remained 5/16-inch throughout the vessel and all mating surfaces were continuously welded. The sides and deck plating were kept to a quarter of an inch, while the deckhouse sides trimmed down to 3/16-inch plate and the house top thinned to 1/8-inch material. The preliminary design was very specific about the transverse and longitudinal members and other stiffeners incorporated into the hull and deck. From a close look, but not using a steel tape and micrometer, it seemed as if the “as built” vessel was close to a clone of the structural integrity spelled out by the original spec.

The coatings issue has been handled in functional workboat style. Rather than bending to cosmetic concerns, aluminum’s self oxidizing characteristic has been harnessed to protect the skin, and only bottom paint and a bold USCG stripe interfere with the utilitarian and cost-effective finish. The Coast Guard clearly understood what every alloy yacht painter learns the hard way: aluminum, stainless steel fasteners and a vessel’s DC electrical system are antagonistic to every coating system.

As with most commercial vessels, the welds stand proud, and thanks to the two-module approach to construction (hull and deck) there was an opportunity to incorporate a rotary jig. This allowed the hull to be “rolled” giving welders the chance to apply their bead in a “hand down” position.

Numerous CNC cut panels had to be attached to the armature as well as each other, greatly increasing strength and reliance upon skilled welders. The resulting rugged look of the un-faired, un-primed and un-painted craft is pure unvarnished form and function, and the furthest from any semblance of yacht appeal. Nothing speaks of this vessel’s ruggedness as loudly as her deckhouse windows and frames. The need for inside steering and superior visibility necessitated a row of ports that can withstand the load of breaking seas, and nothing separates the average trawler/power cruiser from the motor lifeboat as much as the structure of these windows.

The big question is how much of a 47 MLB pedigree should be transplanted into the average power cruiser, and the answer has been debated by designers and builders for decades. The bottom line is it all depends upon how a boat is going to be used. Those planning to cross the Gulf of Alaska or have an interest in a gunkholing the south coast of Greenland, might very well consider cloning a 47 MLB. But if you’re content with coastal voyaging and familiar with picking sensible weather windows, you don’t need a Niagara Falls barrel to cope with average inshore conditions.

It doesn’t hurt, however, to take a close look at the vulnerability of big windows, recognizing that in heavy weather it’s not just the windows facing forward in the pilothouse that take a beating. Breaking waves and the effect of being dumped into the trough of a steep sea can put plenty of pressure on windows on the cabin sides. Well-engineered flanges with thick safety plate glass or lexan lenses can be quite strong. Lightly framed sliding ports are much more vulnerable.

Those headed offshore on a lengthy passage should at least purchase or fabricate emergency cover plates to “board up” less well-structured windows, preferably before rather than after a wave enters the main cabin.


Ralph Naranjo is a sailor, author, marine technical writer who has been the director of Navy Sailing and has managed boatyards. His account of his circumnavigation is called Wind Shadow West. He is also the author of Boatyards and Marinas.

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