Building a “Bullet Proof” Structure
Our goal is to build a “bullet proof” structure – one that will give the owner and crew total peace of mind and confidence in the boat when the conditions get marginal. The last thing we want is you worrying about the strength of the boat when you should be concerned with managing the conditions. We do this by building a structure with extremely high safety factors. As stated in the design brief, our boats are designed in the medium displacement range and, as such, they need to have a certain mass to stay within the design parameters. We start by taking a well engineered, high tech, reasonably light weight cored laminate and increase the skin thickness until we have obtained our required safety factors. Indeed, if you took our skin thickness from either side of our core and put them together to form a single skin laminate, the resulting thickness (and strength) would rival what most production builders consider their standard laminate. This results in a laminate (and structure) that is easily in the top percentile available in this size range. While you may have read this sort of thing before let’s look at a few of the details to back this statement up.
Fully Cored Hulls and Decks
Over the years we have had a good deal of experience with the different construction approaches. Back in Ted’s Bayfield Boat Yard days the high volume requirements were a natural match for the hulls constructed using a solid laminate (single skin). Although these hulls were quite robust (very thick), the single skin process allowed a hull to be completed in just 2 1/2 days which is good for production levels. A current production Gozzard 37 hull takes just under 3 weeks to complete – so, despite what you might be led to believe, the main reason you would use a solid laminate is to increase production not to build a better quality boat.
What about durability and toughness of cored verses solid laminates?
Today most manufacturers take advantage of a cored laminate in their decks. It offers stiffness at a lighter weight but there are still some people who claim that a solid laminate hull is tougher and more robust against impact than a cored hull… unfortunately, this is not a true statement. In actual fact, independent labs studies have unequivocally proven that cored laminates of equal weight to their solid laminate counterparts outperform a solid laminate in impact (puncture) tests.
Advantages of Cored Construction using Core-Cell SAN coring materials
Cored hulls (and decks) offer a degree of safety that is analogous to the “double hull” or “double bottom” construction found in all new super tankers. If the outer skin is punctured, the core isolates the inner skin from further damage and maintains the watertight integrity of the hull better than a single skin laminate. Core-Cell performs this function better than other cores because of its linear nature and high elongation. Core-Cell can absorb the energy of any impact and dissipate it into the laminate without either skin damage or core damage. Cored construction alone provides a higher safety factor, and the use of Core-Cell increases this safety factor substantially.
Other advantages to a cored laminate include:
- Noise and Vibration Isolation – Cored laminates are quieter than single skin laminates because they absorb rather than transmit noise and vibration.
- Thermal Insulation – Cored laminates insulate the interior of the hull from both excessive cold or heat and will have less interior condensation. Increased Panel Strength and Stiffness – Cored laminates provide a substantial increase in panel stiffness over single skin laminates.
- Increased Safety Factor – The advantages of Core-Cell tend to deal with the higher safety factors resulting from higher impact strength properties of this type of core even over other coring materials such as balsa, other foams and syntactic cores on the market. For this reason, ABS (American Bureau of Shipping) requires a 3.33 safety factor on balsa, 2.5 SF on cross linked PVC cores but only 1.8 SF on Core-Cell. We couldn’t find the SF for syntactic core but is considerably higher than balsa (read not as good).
- Better Damage Tolerance – Core-Cell’s linear properties will absorb, not transmit, impact loads and then will recover to original shape. It will not rot or absorb water.
- Better Resale Value – Core-Cell is a premium core. Better boats are built with better cores.
What about Syntactic Cores?
This type of core is utilized by at least one popular sailboat manufacturer and a number of small (Bass) boat manufacturers. It is essentially a filled resin system which is sprayed (pumped) on to a laminate to bulk it up (without a substantial weight increase). It is then covered (encapsulated) by another laminate (skin). It uses the same principle as a cored or sandwich laminate in that stiffness increases as a function of thickness. This type of core has a distinct production advantage in that it can be applied during the laminating process straight onto a wet laminate without interrupting laminating process. They also claim it is less likely to delaminate because it is chemically bonded to the laminate. Conventional cores like foam or balsa core require a completely separate step where the core is bedded (usually under vacuum) onto a dry laminate (which is known as a secondary bond). This step adds days, not hours, to the laminating process and is obviously not “high production” friendly. While there is a production advantage, it is not without a downside. Syntactic cores have very poor mechanical properties compared to Core-Cell and indeed most other available foam cores, including balsa, and is easily damaged (actually destroyed) when subjected to heavy impact or moderate flexing. We choose not to use syntactic cores anywhere in our boats.
When you compare laminates of similar weight, a cored laminate (and especially Core-Cell) has a substantial advantage over a solid laminate. Further, as you increase the safety factor (thickness of the skin(s) (on both types), the advantage continues to increase in favor of the cored laminate. With all the testing data available today it is hard to understand why anyone would make the claim that solid glass is better than cored glass… unless that favoritism was driven by cost consideration. Cost aside, if quality is your primary concern there is little question which laminate and core should be utilized.