Reactor Stability and the GZ curve
One hopes that no Reactor owner will ever be faced with this sort of sea. The 1998 Sydney-Hobart Yacht Race was a very clear example of weather conditions where yacht stability became an issue. This photo shows one of Graham Radford’s designs in these conditions and the large breaking waves associated with this storm.
Photo: © Richard Bennett.
One result of that particular Race has been insistence by responsible Yachting Authorities (in our case Yachting NZ) on certification that all racing yachts have inherent stability. The ROA commissioned Bakewell-White Yacht Design to produce a GZ curve of inherent stability for Reactors. From this it emerges that Reactors pass all the inherent stability requirements for all safety categories, even Category 1. Owners should note, however that many other safety factors have to be met, even for Category 4, such as flares, bailers, lifejackets and more.
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The question new sailors often ask when first experiencing a sailboat heeling is, "What stops it from going all the way over?"
The usual answer is something like: "That big lump of lead underneath us." Stability is at the very root of sailboat design, and ballast is only one of the many factors that influence it.
A boat's stability is contingent on the relationship between two key factors. One is the center of gravity (CG), through which gravity exerts a downward force equal to the weight of the boat, rig, and contents. The other is the center of buoyancy (CB), the midpoint of the underwater volume of the boat, whose upward thrust counteracts the effect of the CG (see figure to left).
When a boat is at rest, the center of gravity is positioned directly above the center of buoyancy. When the boat is forced by wind or waves to heel and the shape of the immersed section of the hull changes, the CB shifts to one side, no longer acting in the same vertical plane as the CG. The transverse horizontal distance between the CG and CB is known as the righting arm (GZ), which provides an upward-acting torque called the righting moment (RM). The RM is calculated by multiplying the GZ by the boat's displacement. The more distance between the CG and CB, the longer the righting arm and the stronger the righting moment.
The characteristics of every yacht are different. The designer (in the case of a Reactor, Paul Whiting) considered various factors and the part they would play in contributing to the Reactor’s stability characteristics:
· Beam: Wide, flat-bottomed hulls will have great initial stability at normal sailing angles but not once pushed past 90º
· Draft: Plenty of hull area beneath the waterline lowers the CB; carrying ballast as low as possible lowers the CG
· Freeboard: A good amount of freeboard would improve both the maximum righting moment and the limit of positive stability
· Deck structure: A flush-decked boat or one with a very low profile coachroof will be more stable when inverted than a similar hull with a high, narrow superstructure
· Centre of gravity: If you add extra weight above the waterline, you raise the CG.
Producing a GZ Curve is one way to learn about the stability which the Reactors have.
Understanding the GZ Curve
With heel angle increasing from left to right, and the righting arm plotted vertically, the GZ curve tells you several things about a boat's stability.
The angle of the upward leg of the curve indicates the initial stiffness of the boat. A shallow angle would indicate, for example, that the boat is initially tender; a steep angle points to a boat that has high initial stability.
The top of the curve marks the maximum righting arm; after this point is passed the boat will become progressively less stable as GZ decreases.
The intersection of the upper portion of the curve with the zero-GZ line marks the LPS (Limit of Positive Stability) or the Angle of Vanishing Stability (AVS). Below the line to the right of the LPS is the negative area, which indicates how stable the boat will be when inverted.
Just as with the positive portion of the curve, a steep angle to the negative curve as it approaches 180º of heel indicates a boat that is initially stable - but upside down. A shallow angle is desirable and indicates a boat with "poor" inverted stability. The lowest point of the curve indicates the maximum inverted moment, and the negative area inside the curve is a measure of the wave energy that would be required to right the boat. The ideal is a shallow curve with a maximum negative GZ that's less than 50% of the maximum positive GZ, with a negative area less than one-fifth that of the positive area.
Most modern production boats will have LPS angles of between 100º and 140º. On the next page is the letter received from Bakewell-White Yacht Design, and over the page is the Reactor’s GZ Curve. You will see that the Reactor’s LPS angle is 125.38º.
Bakewell-White Yacht Design Ltd.
PO Box 34-569, Birkenhead,
Auckland 1330, New Zealand
Ph +64 9 357 0606 Fax +64 9 357 0603. e-mail:
1st September 2005
Ref: “Reactor 25” 7.62m Whiting Performance Cruiser.
To Whom It May Concern:
This is an analysis of the generic class design and not for any specific vessel.
Our Office has created a computer model from original designer’s drawings, photographs, and known parameters to establish the hull form for stability analysis.
The boat has not been the subject of an inclining experiment and the vertical centre of gravity has been established using calculation from the estimated position of weights.
We have assessed the yacht’s stability by means of the calculation of a GZ Curve as attached. Accepting the estimated data this yacht achieves a Limit of Positive Stability of 125.38°.
It can be seen that the vessel passes the requirements for all safety categories as set out in the Yachting New Zealand Safety Regulations for Offshore and Coastal Racing and Cruising, Section 6.4 (K) using the GZ method. See attached Stability Curve.
Should you require any further information, please feel free to contact this office and we will be happy to provide whatever assistance we can.
Regards,
Brett Bakewell-White, B.Arch., ARINA., SNAME.
Design Principal
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Important Note to Reactor Owners
Yachting New Zealand (YNZ) requires that the owner of every racing yacht (it does NOT apply for ‘cruising only’) complete the declaration below and return it to YNZ. The ROA has forms available for every Reactor owner who races, and will collecting forms to send to Yachting NZ.