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Twins v Single Shoal v Deep Stability v Ballast Fishtail Rudders Space Age Cores Core Materials Unsinkability
Twins v Single Shoal v Deep Stability v Ballast Fishtail Rudders Space Age Cores Core Materials Unsinkability
Space Age Cores
Why PVC and two ‘polys’ have eclipsed wood as a coring material
by Ken Fickett
My 50-year-old, 20-foot fishing boat sits in the tall weeds behind my barn waiting patiently for its turn at restoration. It has been in the family since it was built, and not only was I the first person to drive it, as a very young kid I did demos with it at the 1966 Miami International Boat Show. It sported twin 50 hp Chrysler outboards and absolutely flew by standards of the day. Built by Raleigh Stapleton, it was one of the very first production boats designed to fish the flats. It also was one of the forerunners of a whole new breed of boats that folks were calling center consoles.
by Ken Fickett
My 50-year-old, 20-foot fishing boat sits in the tall weeds behind my barn waiting patiently for its turn at restoration. It has been in the family since it was built, and not only was I the first person to drive it, as a very young kid I did demos with it at the 1966 Miami International Boat Show. It sported twin 50 hp Chrysler outboards and absolutely flew by standards of the day. Built by Raleigh Stapleton, it was one of the very first production boats designed to fish the flats. It also was one of the forerunners of a whole new breed of boats that folks were calling center consoles.
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Not only was this a fairly revolutionary boat for the day from a design standpoint, it also set itself apart when it came to construction. The hull was hand-laminated fiberglass with the bottom and sides cored completely with half-inch thick end-grain balsa. The transom was cored with two layers of plywood and the stringers were 2" x 6" fir, encapsulated with heavy laminates of fiberglass. This boat was the proverbial brick outhouse, and, with the exception of the transom, which is rotted and needs to be replaced, still is today.
Meticulous construction was the key to this extraordinary longevity. Had my brother and I been more careful on subsequent engine, transducer and ladder installations, the transom would have likely held up too. When I get around to replacing the transom, the likelihood of my using wood will be zero. I want my son to be able to give it to his kids in another 40 years without having to rebuild yet again. Like many of today’s builders, I will use a synthetic core. |
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Synthetic cores are certainly not new and have been in use for more than 60 years. In fact, I have been using them since the mid-1960s. They do the same job as wood cores by stiffening a part, but with the added benefit of providing flotation. Boston Whaler became famous for its foam-cored hull decades ago, and other manufacturers followed suit, injecting their vessels with a relatively low density polyurethane foam between pre-laminated inner and outer shells to gain U.S. Coast Guard required flotation and improve overall stiffness.
Many people have heard of “water logged” boats, and the technique of injecting an expanding foam to fill a void has caused a lot of heartache over the years when done poorly. This is not the kind of coring we are talking about. This discussion is about sheets or blocks of core that are bedded carefully onto a laminate—usually skins of fiberglass and resin—and then skinned over on the inside to seal the core, making a sandwich in which the core is the meat.
Today the major non-wood cores used by boat builders are PVC, high-density polyurethanes, and polypropylene. The PVC cores are probably the most common of the synthetic cores and fall into two categories: cross linked and non-cross linked. While the merits of both are bantered about endlessly by builders and designers with too little to do, the short story is that the cross-linked stuff is a bit cheaper, easier to use, and thickness for thickness will build a stiffer part. The non-cross linked foam is more expensive, harder to use, and not as stiff. But when it is designed as a component of a complete laminate, a non-cross linked core can take one heck of a whack without nearly the catastrophic damage you might see in other cores.
I remember racing a J24 sailboat in the late 70s in Key West for the annual winter regatta, getting hit hard in a port/starboard incident and being shocked to see a basketball sized piece of hull knocked completely out just above the waterline. These were balsa-cored hulls and very well built. But when they got hit, it could be a bad deal. On the other hand, a good friend of mine hit a piling that was floating offshore. He was in his fully foam-cored sportfishing boat running close to 30 miles an hour at night when he hit. There was no major damage and the hull only shows a minor dent today.
Hard hits to hulls made with non-cross linked cores are more akin to impacts to a piece of metal, in that the failure tends to be what engineers call “plastic.” We might call it “more forgiving” because the material tends to bend rather then break. Probably the toughest hull structure available uses a non-cross linked core such as Airex, with the laminates on either side being a hybrid mix of fiberglass and a man-made aramid fiber such as Kevlar or Spectra. While that combo is going to be considerably more expensive than conventional structures, it’s used on many of the leading high performance racing powerboats, and by all the smugglers that have to run offshore at night.
There is a new synthetic non-rotting polypropylene core that looks like honeycomb and is making huge inroads in boat construction. It is used everywhere but mostly above the waterline. This core has some great benefits and represents the best bang for the buck in the way of a core in areas that are not highly stressed such as decks, cabin sides, and roofs. It has great sound deadening characteristics and has received a good reception amongst builders for use around and over engine compartments.
The big drawback is that it almost always has to be vacuum-bagged into place. Vacuum-bagging is a technique used to insure the proper bonding of core to its substrate. Vacuum-bagging is time-consuming and expensive, but it can make a big difference in the bond quality for all cores. The most popular of the polypropylene cores is Nida Core, which uses a resin-absorbing material that is heat-bonded to the honeycomb cells to form a nonpermeable bonding surface. Nida Core is the least expensive core with the exception of the polyurethanes.
Polyurethanes are typically high-density and are the heaviest of all cores. In many ways they most closely resemble wood, except they will not rot. They have the highest resistance to compression (being squished) and can be an excellent choice in isolated areas that have bolts holding components on to the boat. A good example? Transoms that have outboards bolted on. Many transoms today have pre-molded, high-density polyurethane transoms bonded into their structure to avoid just the problem that I have with my old boat.
Many people have heard of “water logged” boats, and the technique of injecting an expanding foam to fill a void has caused a lot of heartache over the years when done poorly. This is not the kind of coring we are talking about. This discussion is about sheets or blocks of core that are bedded carefully onto a laminate—usually skins of fiberglass and resin—and then skinned over on the inside to seal the core, making a sandwich in which the core is the meat.
Today the major non-wood cores used by boat builders are PVC, high-density polyurethanes, and polypropylene. The PVC cores are probably the most common of the synthetic cores and fall into two categories: cross linked and non-cross linked. While the merits of both are bantered about endlessly by builders and designers with too little to do, the short story is that the cross-linked stuff is a bit cheaper, easier to use, and thickness for thickness will build a stiffer part. The non-cross linked foam is more expensive, harder to use, and not as stiff. But when it is designed as a component of a complete laminate, a non-cross linked core can take one heck of a whack without nearly the catastrophic damage you might see in other cores.
I remember racing a J24 sailboat in the late 70s in Key West for the annual winter regatta, getting hit hard in a port/starboard incident and being shocked to see a basketball sized piece of hull knocked completely out just above the waterline. These were balsa-cored hulls and very well built. But when they got hit, it could be a bad deal. On the other hand, a good friend of mine hit a piling that was floating offshore. He was in his fully foam-cored sportfishing boat running close to 30 miles an hour at night when he hit. There was no major damage and the hull only shows a minor dent today.
Hard hits to hulls made with non-cross linked cores are more akin to impacts to a piece of metal, in that the failure tends to be what engineers call “plastic.” We might call it “more forgiving” because the material tends to bend rather then break. Probably the toughest hull structure available uses a non-cross linked core such as Airex, with the laminates on either side being a hybrid mix of fiberglass and a man-made aramid fiber such as Kevlar or Spectra. While that combo is going to be considerably more expensive than conventional structures, it’s used on many of the leading high performance racing powerboats, and by all the smugglers that have to run offshore at night.
There is a new synthetic non-rotting polypropylene core that looks like honeycomb and is making huge inroads in boat construction. It is used everywhere but mostly above the waterline. This core has some great benefits and represents the best bang for the buck in the way of a core in areas that are not highly stressed such as decks, cabin sides, and roofs. It has great sound deadening characteristics and has received a good reception amongst builders for use around and over engine compartments.
The big drawback is that it almost always has to be vacuum-bagged into place. Vacuum-bagging is a technique used to insure the proper bonding of core to its substrate. Vacuum-bagging is time-consuming and expensive, but it can make a big difference in the bond quality for all cores. The most popular of the polypropylene cores is Nida Core, which uses a resin-absorbing material that is heat-bonded to the honeycomb cells to form a nonpermeable bonding surface. Nida Core is the least expensive core with the exception of the polyurethanes.
Polyurethanes are typically high-density and are the heaviest of all cores. In many ways they most closely resemble wood, except they will not rot. They have the highest resistance to compression (being squished) and can be an excellent choice in isolated areas that have bolts holding components on to the boat. A good example? Transoms that have outboards bolted on. Many transoms today have pre-molded, high-density polyurethane transoms bonded into their structure to avoid just the problem that I have with my old boat.
