34z Structural Design Philosophy

by Steve Burke

ISO Standards and CE Mark Certification

The ISO (International Organization for Standardization) small craft standards are the emerging worldwide regulations governing the design and construction of vessels, both power and sail, of up to 24 meters (79 feet) in length. In order for boats to be sold or imported into Europe, it is currently mandatory that each model obtain CE mark certification. This certification is based on the ISO standards, which cover the entire vessel and its systems. They include the boat's designated type of use, stability and buoyancy, structures, cockpit drainage, visibility from the helm, and the fuel, electrical, steering, and fire protection systems. Onboard equipment, including engines, electrical equipment, pumps, tanks, hoses, hatches, portlights, navigation lights, etc., must also be CE mark certified.

New structural requirements are now in the process of being "harmonized", or incorporated, into the ISO standards. Based on the vessel dimensions, type of use, speed, and structural arrangement, these new standards require that rigorous calculations be performed in order to determine the required scantlings and laminates for the various parts of the boat.

34z Structural Design

The structure of the 34z powerboat is designed to exceed the 2003 draft ISO small craft structural requirements for design category A, "ocean". This is the international standard's toughest design category, and is defined as the "category of boats considered suitable for seas of up to 7 meters (23 feet) significant wave height and winds of Beaufort Force 9 (41-47 knots) or less." While we certainly don't expect the majority of 34z owners to venture out in conditions like these, it's nice to know that the boat is designed and built to take it?

The ISO calculations were carried out using the 34z's top design speed of 50 knots. Design pressures were calculated for the hull bottom, hull sides, decks, bulkheads, structural grid, and superstructure.

ISO then proceeds to specify material properties of the required laminates. These material properties are based on E-glass fabric with polyester resin, which are by far the most common materials used in the boatbuilding industry today. In addition, the standards specify, for the type of glass fabrics used in our construction, a glass/resin ratio of 50%.

For several reasons, the laminates used in the construction of the 34z exceed the ISO material properties, by a considerable margin:

1. The 34z is built entirely with epoxy resin, not the more commonly found, and much less expensive, polyester resin. Epoxy resin produces a superior laminate to polyester for a number of reasons:

• Higher mechanical properties- Epoxy resin is stronger than polyester resin.
  
• Improved fatigue characteristics- Epoxy resin is more durable than polyester resin. This is principally due to the fact that epoxy has higher elongation properties than polyester. The result is that epoxy resin is able to "stretch", and "rebound" with the encapsulated fabric. Polyester resin, on the other hand, with its lower elongation properties, tends to develop microcracks over the service life of the vessel, gradually weakening the laminate (and the boat).
  
• Superior resistance to water degradation.
  
• Superior adhesion properties- Epoxy resin is much better "glue" than polyester resin. It is this "glue" that is binding the resin to the encapsulated fabric- and holding the entire boat together!

2. The 34z has a higher glass/resin ratio than that specified by the ISO standard. The high-tech construction methods and process controls employed by Boston BoatWorks result in a glass/resin ratio exceeding 60%. This significantly exceeds the ISO standard of 50%. The higher the glass/resin ratio, the stronger the boat.

3. Kevlar, used in the outer skin of the 34z, is significantly stronger, and more abrasion resistant, than the E-glass fabric used in the ISO standard.

High Strength and Light Weight

Since we've established that the 34z laminates exceed ISO standards' toughest requirements, what about the weight? Aren't there manufacturers saying the more weight, the better? Yes, there are, but the claim doesn't make much sense. The problem with weight in a powerboat is that it starts what seems to be an endless, unproductive cycle. More weight demands larger engines to get the boat up on a plane, which demand more fuel. Heavier engines and more fuel demand more buoyancy to keep the boat floating, which for a given length, means wider boats, creating more frictional and wave-making resistance. More horsepower, still, is needed to overcome this drag, etc. etc. It's not hard to imagine why some 35 footers trying to attain cruising speeds in excess of 20 knots, weigh twice as much as the 34z.

A strong, high-tech lighter boat provides its owner with significant advantages:

• It is more fun to drive, offering higher speed and increased maneuverability with the same horsepower. Think of the Mack Truck vs. Porsche analogy in vehicles.
  
• A smaller engine with less horsepower is required for equal speed, reducing fuel consumption and operating costs.
  
• Lighter weight makes a narrow beam possible, which in turn makes trailering possible. Cored construction with high strength skins enables the 34z to be dramatically stiffer than its heavier competitors built of solid glass. This is due to the fact that the stiffness of a given panel increases with the cube of its thickness. It is for this very same reason that I-beams are employed in steel construction, rather than using solid steel plates.

The principal core material employed in the construction of the 34z is Core-Cell foam. Varying densities of Core-Cell are used in the hull and bulkheads of the boat. Slightly higher density foam is used in the hull bottom, due to the impact pressure of the waves at speed, while lower density foam is found in the topsides and bulkheads. The advantages of Core-Cell are:

• The closed-cell foam core is impervious to water in areas damaged by collision.
  
• Core-Cell foam is more tolerant than other core materials to large impacts, resulting in smaller areas of damage and potential delamination following a collision.

Superlite Baltek Core material is used in the decks of the 34z for its higher compression strength for localized impact, so resists small denting events, like dropping the bottom fin of an outboard engine or an anchor on the cockpit sole, better than the foam core used in the hull and bulkheads.

Rugged Where it Needs to be...

Although light, the 34z is overbuilt where it really matters. The hull bottom includes a reinforced structural grid, that incorporates the (single or twin) engine foundations and the hull stiffeners into one, very strong, unit. The grid exceeds the ISO structural requirements for this part of the boat, and provides peace of mind when driving into a seaway at high speed.

For extra protection against possible grounding or impact, and in keeping with proposed 2003 ISO standards, the 34z has a solid fiberglass keel and chines running from bow to stern.

Summary

The structure and laminates of the 34z are significantly stronger and more durable than the new ISO structural requirements for Design Category A 50 knot Ocean-going Powerboat because of:

• Epoxy instead of polyester resin laminates
  
• 60% vs. the 50% glass/resin ratio of the ISO Standard
  
• Use of Kevlar in addition to E-Glass in hull outer skin
  
• Thicker, structurally stiffer, cored composite hull, deck and bulkheads.
  
• Overbuilt structural grid in hull bottom
  
• Solid epoxy glass on bottom centerline and chines for protection against grounding