“I believe that boat performance is as important as her appearance.” -Yoh Aoki, designer of the Zen 24

Performance characteristics of a sailboat is defined by four properties:

1. Speed of the boat – Property 1 (P1)
2. Ease of steering – Property 2 (P2)
3. Degree of comfort – Property 3 (P3)
4. Seaworthiness – Property 4 (P4)

P1, Speed of the boat. Some sailors might say, “I’m not interested much in boat speed.” However, properties 2, 3 and 4 depend on property 1. Naval architects try to design boats to sail as fast as possible, because they are aware of this fact. Potential boat speed is like money in your pocket. You have to give up many things if your budget is not very big. Boat speed is a function of 1. Sail area/Stability, 2. Displacement/Water line length and 3. hull form. Naval architects always try to establish a balance between those factors. The hull form is designed to have the least wave making hull section as the boat moves through the water, because wave making resistance accounts for 95% of the drag on the boat going through the water. Also to consider is that if your boat is heavier you need more sail area. This requires heavier spars, rigging and other gear which makes the boat heavier and starts a vicious cycle that slows down the boat.

P2, Ease of steering. If you don’t have enough speed, it will be difficult to control and steer the boat. This is because the rudder does not work as well under slow speed. The rudder will lose 19% efficiency when there is a decrease of 10% in speed. When a boat is travelling slowly, the helmsman will have to work harder physically and mentally. Imagine that you are short handed or with family and the conditions are bad. You will need to be able to maintain control in these conditions. Another reason that you need ease of of steering is to reduce weather helm even when the boat is heeling. When the center of lateral resistance (CLR) is balanced with the center of effort (CE) the boat’s helm is balanced. Many boats can not maintain a balanced helm when heeled but the Zen 24’s hull form creates hydrodynamic lift which can offset weather helm.

P3, Degree of comfort. Comfort under way is another important property for boats and her skippers. This property goes hand in hand with the previous two. Comfort is partly a function of ease of steering and boat motion. If the boat has a gentle motion, the crew can steer without too much effort. Since the crew is not struggling to steer the boat, they can enjoy the ride and cruise in comfort.

P4, Seaworthiness. Is a typical traditional boat that has a heavy displacement and a wine glass hull section more seaworthy than a modern light weight fiber glass boat? Not necessarily. The Volvo Ocean racing boats, as well as traditional designed boats have been sailing around the world and in places as treacherous as Cape Horn. Racing boats sometimes lose spars, keels or are capsized. These problems are usually because of engineering factors. Traditional boats also encounter the same problems. “Tzu-Han” a 50 foot traditional design boat even lost her deck after recovering from capsize in the Southern Ocean. Traditional design boats often have to heave to under heavy condtions, because they will be out of control due to lack of speed and thereby also lacking steering control. They also get submerged in big waves because of their limited reserve buoyancy. The Volvo Ocean race boats also might lack properties 2 and 3 but their seaworthiness is compensated with well trained crewmembers.
The Zen 24 has the potential to sail under the Force 10 conditions without much stress because she has been designed with all four properties in mind.

Summary of the characteristics of boat types on the ocean

Weather helm is the term used to describe a sailboat’s tendency to head up into the weather side when the tiller is a mid-ship. Too much weather helm makes the boat difficult to handle and reduces speed. Most sailboat hull forms have not been considered in reducing the weather helm when the boat is heeled over by the wind.
However, the hull of the Zen 24 has been especially designed so that weather helm is reduced and neutral helm is maintained when the boat is heeling.

The hydrodynamic advantage of the Zen 24 hull form comes from the interaction between the centers of effort and lateral resistance, as the diagrams below show. Because of the unique shape of the hull, the Zen 24 is able to maintain a balanced and neutral helm, even when heeled by the wind. This makes the Zen 24 a very stable and comfortable sailing boat.

Characteristics of sailboat stability with same L.W.L., Beam, Displacement and Keel weight

Types of the hull sections and the characteristics of stability

The stability curve by the type of above hull section