Is glass good enough for class?
Superyacht designs are featuring more glass, but what do the classification societies think of the material?
With superyacht projects increasing in size, the pressure is on for designers to maintain a connection with the marine environment. The use of glass has been a popular solution as it allows for more natural light and a visual link to the ocean. While glass can cause concerns over the strength of the vessel, class societies are starting to recognise it more and more as a structural material.
“Glass has been a part of ship construction for decades and over time the size of glass elements has increased considerably with new applications such as glazed bulwarks and atriums being introduced,” says Martin Richter, ship type expert yachts at DNV GL. DNV GL has worked extensively with large glass elements on yachts, dating back to Lürssen’s Rising Sun, with its slim supports and full height glass panels.
“With the development of laminated glass we are beginning to see a greater interest in using glass even as a structural element,” Richter continues. “Especially over the last year or two as several designers and shipyards have approached us showing interest in making greater use of glass as a structural element.”
For Lloyd’s Register, yacht design very much follows the progress of the architectural world in terms of how it has adapted to the use of glass. “Eighty per cent of the flat glass produced in the world is used in architectural applications,” explains Frans Verbaas, principal specialist at Lloyd's Rotterdam technical support office. “Therefore, the glass that the maritime industry can procure is architectural glass, manufactured to architectural standards. The research and development done on glass constructions is for architectural applications.”
According to Verbaas, the maritime industry as a whole, together with other mobile, non automotive applications such as trains, absorbs 0.8 per cent of the glass produced and is, therefore, too small to afford its own production control system. “The architectural world is just beginning to recognise glass as a structural material – we follow on their heels but we would be foolish to run ahead,” he cautions.
The concerns and considerations
Glass is very different from other maritime construction materials and, as such, the main consideration when using glass in yacht design is to remember that strength is not a substitute or equivalent for safety. “Failure can occur instantly and at any time at load levels much less than the design load under the impact of a sharp object, faulty mounting, or cracks growing into the critical size,” explains Verbaas. “This makes safe failure and risk analysis key to the design.”
Verbaas explains that one design solution is to install laminated glass consisting of two or more panes glued together, which provides a certain amount of residual strength and protection from flying shards of glass should one of the panes become damaged.
“The design of glass structures should also be linked to the consequence of failure, for example a large underwater viewing panel needs to provide a higher level of protection than an aft-facing glass wall fitted high up in the superstructure,” continues Verbaas. “The key question that needs to be answered is how much strength the glass construction needs to have after failure. What happens then? What is the scenario? What performance is expected and for how long would that be? Considering post-failure scenarios may prove that it is better to use glass material that has less strength when intact but has much better strength properties after failure.”
A classification society’s main focus is to ensure the safety and structural integrity of a vessel. For DNV GL, this means finding a way to apply glass elements that complies with statutory and class requirements – most importantly in terms of structural strength, watertight integrity and freeboard requirements.
“How any particular element will be integrated into the structure of a yacht demands a close and intense cooperation, not just between the designer, yard, flag state and class, but also with the suppliers and manufacturers,” advises Richter. “This is of vital importance due to the specific characteristics of the glass intended to be used, its bonds and how elements are connected. This together with an open mindset is essential in fulfilling the vision of the designer and the wishes of the customer without reducing safety and structural integrity.”
Another concern with the use of glass is the integrity of the structure in case of fire. “If being part of a fire rated division, glass elements have to be fire tested,” adds Richter. “As, in general, the largest glass element in the structure should be fire tested, it might be challenging to find a fire test laboratory that is able to perform testing on such a large glass element.”
While glass has design advantages in terms of creating open spaces and natural light, there are some important tradeoffs. “Glass has a comparatively low tensile strength, which might result in increased weight compared to a similar structure made of traditional materials such as steel,” explains Richter. “The stiffness of glass elements when surrounded by the comparatively flexible hull structures of the rest of a yacht also means that increased attention needs to be given to the structural response and bond design when designing and constructing the yacht.”
Verbaas adds that a major concern with glass is that the results of a strength test can be misleading. “The failure strength of a piece of glass is determined by surface flaws, which have a random distribution,” he says. “Therefore, the strength observed on one piece of glass says very little about what to expect for the next identical piece. Test results can be used only to validate computations of the deformation.”
“DNV GL sees an increasing interest in realising large glass constructions as structural elements in new yacht designs, which is confirmed by the number of enquiries we have received over the last few years,” says Richter. “There is a clear intention by designers to push the envelope on yacht designs.”
The most obvious trends are increased size and new materials. “Glass elements are growing, something that is also apparent in the cruise industry where large open areas create amazing views for guests,” Richter adds. “Designers are looking to bring natural light in – into cabins and the superstructure of the yacht especially. They are also using curved glass elements to reinforce the design of the yacht, something that structural glass will enhance.”
In terms of the material itself, Richter also observes an interest in using products commonly referred to as ‘smart glass’, with sensors embedded within to alert of damage, or to change colour to reduce or change the light coming into the vessel. He believes that stronger and lighter types of glass could also enable a re-examination of how glass can be used in the structure.
“We have had extensive discussions on how we as a classification society are able to support designers and manufacturers on this front, especially in getting lighter and safer glass structures accepted as being in compliance with the respective requirements,” he concludes.
For Verbaas, however, a living space with lots of glass may look good on paper, but in reality there are factors such as sun radiation, privacy, acoustics and cleaning that need to be taken into account. “Protection is a fundamental human need,” he explains. “There are limits to the ‘sea experience’ through glass hull sides that someone can appreciate. I expect that the ‘glass hull’ gimmick will wear out quickly and glass will be used mainly in the superstructure volume.”
Image of CF8, concept by Sea Level Yacht Design & Engineering
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