The Mark 12 US

As of 2003, every Winner hull sold in the United States is a MK 12 US. What does that mean? Well, the short answer is that Winner currently builds two types of Optimist Hulls. The MK 11 is the standard Winner Optimist hull, a multiple World and Continental Champion, it is the benchmark by which all Optimists, worldwide, are compared. The MK 12 is Winner's ultimate hull, the very best of the best, from the worlds most respected Optimist builder. The "US" designation denotes the special rigging and equipment specifications chosen for the United States market. In the MK 12 US, no limits were put on equipment, the very best was used with out concern for cost. The MK 12 US is rigged with padded/adjustable straps, a Fredricksen ratchet block with a milled aluminum sheave, Harken Carbo Air Blocks and stainless steal snap shackle mainsheet lanyard.

So what makes the MK 12 better than MK 11? Two things, time in the mold and biaxial fiberglass. Time in the mold is important because fiberglass composites take time cure, or harden, to full strength. The better the structure is supported during the curing process, the better final structure will be. After 12 hours, a laminate the weight and thickness of an Optimist hull is 98% cured but, a MK 12 stays in the mold at least 4 times that long.

Biaxial fiberglass is the other reason for the improved performance of the MK 12. Most Optimist are made from woven cloth, which weaves bundles of fibers over and under one another. This is the same weave you typically see if in a table cloth, or wicker basket. Biaxial fiberglass is laid down straight, with no tight weave. On the other hand, a woven cloth relies on fibers that weave over and under each other, creating crimps that tend be fracture points in the fibers, and in the case that they do not fracture, they will stretch far more as the loaded fibers seek straighten out. It's also import to note that in all hull laminates where biaxial is allowed, Winner uses a 100% biaxial cloth.

The straight, flat paths of fiber bundles in biaxial leads to greater strength and stiffness. By putting down straight, flat fibers, directly in line with loading forces, the fibers will resist stretching and breaking immediately and with 100% of their strength. Biaxial also tends to have a higher fiber count than woven. This has three advantages. First, more fibers means greater strength. Second, higher fiber concentration means less matrix (resin) and therefore far less brittle laminate. Third, less matrix (resin) means less weight. So in addition to the fundamental physical advantages of biaxial, we get the secondary advantages greater fiber content, greater impact resistance and lower weight.

By contrast, the crimping of the fiber bundles in woven cloth leads to stress differentials in the fiber bundles and a lower overall strength. When fibers are bent (as they are in woven cloth) they tend to fracture, weakening the composite. The crimp in a woven fabric also has a tendency to straighten itself out under load, reducing the stiffness or modulus. Another disadvantage of woven cloth is bumps, or high point that naturally come from the weave. When the laminate is laid up in the mold and the resin is rolled out or squeegeed, these high point are damaged and fibers are broken. Further more, when the surface is finished, it has to be sanded, which again damages these high points and fiber bundles. Biaxial tends to have fewer high points and therefore is damaged less in building process.

The biaxial we use in the Winner MK 12 especially well suited to the Optimist. Our cloth is what's know as a double bias +45°/-45°. What that means is that the fibers are rotated on a 45° angle from vertical and horizontal. This has several effects:

First, it lines up the fibers with the stresses greatest stresses on the boat. An Optimist is constantly be twisted by two great levers, the mast and the sailor, at opposite ends of the boat. The harder the wind blows, the harder the sailor hikes in the opposite direction. The result is a huge twisting force on the boat. Every millimeter of hull twist that can be prevented is energy that's transmitted directly into forward motion instead of hull flex. The orientation of the biaxial puts the fibers directly in line to resist that twisting. In contrast, a woven +90°/-90° , which most builders use, has NO fibers in line with these twisting forces and presents it's weakest axis to the most important forces.

Second, a double bias +45°/-45° is oriented so that all the fiber bundles cross the floor/side joint in an Optimist. A woven +90°/-90° is oriented so that half the fibers are running the length of the hull and do not cross the hull/floor joint at all. Further more, the +45°/-45° orientation of Winner's biaxial creates a greater effective radius because the fiber bundles cross the hull/floor joint at 45° angles. This means that the loads are dispersed over a greater length of fiber which leads to a 41% joint over any given radius. More than twice the fibers crossing the joint, plus 41% greater strength per fiber bundle equals a 282% stronger joint. Materials do make a difference!

Strength of Woven and Biaxial Composites	Modulus of Woven and Biaxial Composites