Why Sandwich Construction?
Sandwich construction has been well established in the composites industry for well over 40 years. Naval designers specify sandwich construction for much the same reason architects use I-BEAMS and trusses: to increase stiffness and strength while at the same time decrease weight. The honeycomb core in a sandwich laminate acts much the same as the web in the I-BEAM by connecting the load bearing skins. The increase in stiffness is directly related to the height of the web (or thickness of the core).
Due to the fact that some earlier boats used either inadequate building methods or the wrong core material for the engineered load, sandwich construction is often debated.
However these isolated incidents are not the rule and most sandwich constructed boats have performed well over the years.
Famed composites pioneer Mr. K. Brandl writes:
"The main function of the core material is to distribute local loads and stresses over large areas. Local stresses applied to one side of the sandwich have only a reduced local effect because the exposed skin and the core will distribute the loads to a larger area of sandwich. Because of this tact, a sandwich structure generally exhibits superior behavior under bending, torsion, impact and compression, parallel or perpendicular to the skins. Beside its function of a spacer and connecting unit between the skins, a core material for boat building must therefore exhibit enough resilience to absorb impact stresses. Its ability to cushion and absorb shocks in alternating stresses and torsion loads, passing from skin into the core, as they occur in a boat under practical conditions, is a necessary requirement of the core. Such dynamic stresses as well as impacts, should the core not be resilient, can result in severe damage and eventual destruction of the entire structure. A boat or ship should, with all required stability and homogenous stiffness, not be an inflexible structure. It should be a mechanically stabilized structure, which still allows movements within the elastic range of its materials. It should further exhibit the characteristic to withstand short term overloading without destruction and lasting damage. Generally, one can encounter unexpected loads and stresses by two alternatives. One is to design to such a limit that the structure will in every case be many times stronger than the unexpected loads, i.e., the structure would have to be over-engineered and be overweight. A more advanced engineering concept is to counter the unexpected loads by a structure, which, having sufficient mechanical strength and stiffness, is still in a position to withstand peak loads without damage of serious consequences to the structure. In order to realize this concept, a rigid elastic structure is a prerequisite design criteria.
The following advantages of sandwich construction are discussed in this chapter:
A sandwich construction using honeycomb core with its high degree of resiliency is more impact resistant than a single skin laminate with equal or higher weight. A customer of 3M in the Caribbean islands had a 65 foot commercial charter boat, cored entirely with 3M, polypropylene Honeycomb (less hull bottom), including hull sides, deck and superstructure, scratched and water damaged during a major hurricane on a coral reef. Insurance company agreed to write off the boat on condition that customer takes boat out of service and does not attempt to repair it. The charter boat operator agreed to dismantle the boat and discard of it. Using one of the marine's cranes, they hoisted the 50,000 # boot 50 feet in the air and dropped it onto concrete parking lot, hoping that they would so be able to collect the pieces and discard of them in the dumpster. To much of their astonishment the boat remained completely intact, with no visible structural damage. They repeated the procedure continuously for a couple of days to no avail.To accomplish their mission they had no other alternative than to rent a chain saw to cut the boat into pieces. Although, somewhat extreme of on example, it clearly indicates the superior nature of resilient cored sandwich construction in boats. The increase in impact strength, as compared to single skin laminates is better demonstrated if one sees the core as a shock absorber that evenly supports the outside skin from impacts and protects the inside skin, while still having enough elasticity to maintain the bond line between the core and the skins. Experience shows that although sandwich conduction is not completely puncture proof it significantly increases skin penetrating puncture resistance. The more brittle cross-linked PVC and SAN foams would simply crumble and shear under a severe impact, whereas polypropylene honeycomb cored structures would be locally damaged, however the core structure would be intact and cells, although elongated would still keep their structure and shape, ready to absorb or withstand more impacts and compression.
COST & WEIGHT
Weight affects different modes of transportation in a different way. For instance, in sailboats, extra weight does not automatically translate into higher operating cost, but affects performance. In full displacement type sailboats cored hulls can aid in insulation lowering cooling/heating costs. In power boats up to 20' in length, the effects of sandwich construction can be marginal, as weight advantages can be achieved by alternate means. But as the boats get bigger the weight becomes increasingly more important, as the weight translates directly into extra cost of pushing the extra mass and the initial extra cost of sandwich construction is quickly overcome. As the weight get lighter, boats will get improved range, need less power, can carry more cargo or people and need less tankage, thus improving interior space. Therefore the cost of the sandwiched boat, when compared to the single skin solid laminate boat, becomes a very cost-effective approach at over long term operating expenses.
VIBRATION DAMPING/SOUND ATTENUATION
With it's natural harmonic of 125-150 HZ polypropylene is known for it's vibration damping/noise absorption properties. Almost all of our customers, after switching from a different core material, have notified us that they have noticed a significant change in boats being quieter. Noise and vibration travel well through a single skin laminate. Boats with cored hulls are simply quieter. Balsa wood and brittle foams transfer noise energy directly through the laminate. Polypropylene and some more elastic foams dampen the noise energy due to their plastic nature.
Thermal insulation in vessels must be considered: as most often the boat sits in water much colder than the ambient temperature, consideration forms, leaving the vessel stained and allows the mildew to form. Sandwich construction significantly aids in eliminating the condensation and associated bilge water. The cored insulating layer coincidentally eliminates the need for highly flammable spray-in polyurethane.
There seems to be a general misconception that displacement and commercial boats must be solid, since weight is not the primary concern. One should not confuse solid with tough and strong. Since properly designed sandwich construction significantly aids in impact strength when compared to the single skin (solid) laminates we believe that sandwich construction would significantly aid in general safety of displacement vessels. The arguments for using composite sandwich construction are overwhelming. There are no good reasons for using single skin fiberglass construction that can not be countered with better reasons for using a tough resilient core material.
THE CLUB SANDWICH
In yachts over 60 feet club sandwich approach, where a single extra layer of fiberglass is added between the two layers of core, is often utilized. This is one of the most effective ways to utilize the properties of polypropylene honeycombs since due to its nature. honeycomb shear properties are reduced when thickness of the core is increased beyond a certain limit. By utilizing two thinner layers of honeycomb versus one thick layer of honeycomb, one will significantly increase the overall performance of the laminate. Additionally, even when the outside skin is punctured or ripped, the middle and Inside skins will remain intact, and the vessel will most certainly maintain its watertight ability.