Sailing Yacht Design: a Guide for Boat Owners, Crew and Buyers
By Kim Klaka
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About this ebook
Do you want to understand what makes a sailing yacht perform the way it does, but are sometimes baffled by the science?
What stops a yacht from capsizing?
How smooth should the antifouling paint be?
What is the best shape for a rudder?
How can I keep a boat safe during a storm at sea?
Are catamarans and monohulls really that different?
The answers can be found in the pages of this book. It explains things in ways that can be understood by the lay person, using the minimum amount of science. The book comprises 5 chapters, 40,000 words and 75 diagrams. The diagrams are simple line drawings that can be read easily on all e-platforms and screen sizes.
The author is a naval architect with more than 40 years' experience in yacht design. He holds a Masters degree and a PhD in sailing yacht performance, and was the director of a university marine research centre. He has sailed over 30,000 miles, including singlehanded offshore passages and world championships. In 2015 he was awarded the David Walters Memorial Medallion for services to yachting safety.
If you want to improve your understanding of sailing yacht design, this is the book for you.
Kim Klaka
Kim Klaka is a naval architect who established his first yacht design consultancy in 1970. He holds a Masters degree and a doctorate in sailing yacht performance, and was the Director of the Centre for Marine Science and Technology at Curtin University, Australia. He has conducted research for several America’s Cup syndicates and was the leader of Australia’ only national yacht research programme. He has also been the director of a yacht motion stabilisation company, and worked as a professional yacht designer and yacht builder for many years. He has published thirty scientific papers and many more technical reports on naval architecture and maritime engineering. He has sailed over 30,000 miles, including singlehanded offshore passages and world championships. He has advised government bodies and sailing organisations on technical issues, and holds the David Walters Memorial Medallion for services to yachting safety - only the fifth person ever to receive this award. Kim also holds postgraduate qualifications in adult education and has lectured in yacht design and naval architecture since 1978.
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Sailing Yacht Design - Kim Klaka
Nomenclature
A Area (usually sail area) (m²)
Ab Profile area of dagger board (m²)
AWA Apparent wind angle
AVS Angle of Vanishing Stability
AWS Apparent wind speed (kn)
Bmax Maximum beam of hull (m)
BML Longitudinal metacentric height above centre of buoyancy (m)
BMT Transverse metacentric height above centre of buoyancy (m)
BOA Beam overall (across both hulls of a catamaran) (m)
Btrans Beam at transom (m)
BWL Beam at waterline (m)
c Chord (m)
e Foil efficiency factor
eb Board efficiency factor
es Stub keel efficiency factor
FBD Freeboard at mast (m)
GM Metacentric height (m)
GMT Transverse metacentric height above centre of gravity (m)
KKstiffness Stiffness metric (monohulls)
KKroom Roominess metric
KKtip Tippiness metric (catamarans)
KK vol value Volume value metric (Euros per m³)
LOA Length overall (m)
LWL Waterline length (m)
m Mass (usually kg)
s Span (m)
SA Sail area (m2)
T Draft (m)
Th Hull draft excluding keel or dagger boards (m)
TWA True wind angle (°)
TWS True wind speed (kn)
Vfd Downwind fresh breeze speed metric
Vfu Upwind fresh breeze speed metric
Vld Downwind light breeze speed metric
Vlu Upwind light breeze speed metric
V Flow speed (kn)
Vmax Maximum hull speed (kn)
1 Stability: How far does a yacht lean over?
1.1 Summary
This chapter firstly shows how a yacht generates stability to carry sail, then describes the design characteristics affecting the likelihood of capsize. The free surface effect of tanks on stability is explained, with guidelines for minimising its effects. Advice is given for assessing the stability qualities of monohulls and cruising catamarans.
1.2 Background
Stability is the tendency of a yacht to remain upright or the ability to return to the upright when heeled over by the action of waves, wind, etc.
1.2.1 Centre of gravity
The centre of gravity is the most important stability feature of a yacht. It is the point through which the weight of the yacht acts. Every object has a centre of gravity and you can find it very easily, right now... take a pencil or pen and try to balance it across a finger so that it does not fall off.
Figure 1 Centre of gravity of a pencil
A close up of a logo Description automatically generatedThe point you are supporting it at when it is balanced, is directly under the centre of gravity. The principle is the same for a yacht; imagine taking the yacht out of the water and chocking it up on just one narrow block of timber as shown in Figure 2 (do not try this!). When the timber is in just the right place for the yacht to sit precariously in level trim on top of it, the centre of gravity is directly above the chock, with the weight of the yacht acting directly on the chock, not ahead of it nor behind it.
Figure 2 Longitudinal centre of gravity
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Clearly this is not a practical way of finding the centre of gravity, so yacht designers adopt another approach whereby they estimate the mass of each item on the yacht (including the hull, keel, engine, etc.) and its location, then take a weighted average. It is a tedious process and one that is prone to error, which is why some yachts do not float level when launched – the designer’s estimated position is a bit out. It is important to recognise that the centre of gravity of the yacht is in the same position regardless of whether the yacht is ashore or afloat; it has nothing to do with the water. So the weight of the yacht acts downwards at the centre of gravity.
1.2.2 Centre of buoyancy
Now put the yacht back in the water, but only for a moment. Imagine the yacht is not in normal water, but some kind of gloopy stuff (soft sand maybe?). Pull the yacht out of the sand/water and look at the shape and size of the hole that has been left behind in the sand/water (Figure 3).
Figure 3 Centre of buoyancy
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This shape will have its own centre. What we have here is the centroid of the water displaced by the yacht. It is not the centre of gravity of the yacht - that is a characteristic of the entire yacht both above and below the waterline - this is the centre of only the in-water shape created by the gloopy-stuff exercise. It is called the centre of buoyancy because it is the point at which the upward buoyancy force acts (Archimedes’ principle[1]).
Figure 4 Vertical centres of buoyancy and gravity for an upright yacht
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The two principal forces acting upon a floating yacht are its weight and buoyancy. The weight and buoyancy are both vertical forces and equal in magnitude. The weight acts downwards through the yacht’s centre of gravity (G). The buoyancy acts upwards through the centre of buoyancy (B). A yacht that is upright has its centre of gravity and centre of buoyancy in the same vertical line (Figure 4).
1.3 Stability at small heel angles
Figure 5 Righting lever GZ
A close up of text on a black background Description automatically generatedWhen a yacht heels a wedge of buoyancy is lost from one side and another wedge of buoyancy is gained on the other side (Figure 5). The wedge volumes must be equal since the mass of the yacht (and from Archimedes’ principle, displaced volume) remains constant. This relocation of part of the buoyancy results in a transverse shift in the overall centre of buoyancy – there is now more volume on the right than on the left, compared with the upright immersed shape. The buoyancy force acts through this new position of the centre of buoyancy (B), while the gravity force still acts through the unchanged centre of gravity (G). The transverse separation of the two centres means that the two forces are now trying to turn the boat back to its upright position. The amount of righting effect they have is governed by their horizontal separation, which is the distance from G to Z in Figure 5. The distance GZ is called the righting lever, and is a fundamental concept in the stability of the yacht.
Figure 6 The position of the metacentre M
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There is another very important point in the geometry of a heeled yacht. The point where the force acting through the centre of buoyancy (B) intersects the yacht centreline is known as the transverse metacentre (M) (Figure 6). The distance GM is thus a measure of the stability of the yacht. For a yacht to be stable, the transverse metacentre (M) must be above the centre of gravity (G). The height of M above G is the metacentric height (GM), often called simply GM
. Consider the three possible locations of M in Figure 7:
Figure 7 The three different types of stability
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If the centre of gravity of the yacht is below the transverse metacentre (the left-hand picture of Figure 7) then the turning effect of the lever GZ is to bring the yacht back upright; the yacht is positively stable. The yacht has a positive GM
.
If the centre of gravity is above the transverse metacentre (the middle picture) then the turning effect of the lever GZ is to increase the heel angle further; the yacht is unstable, or negatively stable. The yacht has a negative GM
.
If the centre of gravity is located precisely at the metacentre (the right-hand picture), there is no turning effect (GZ is zero) so the yacht sits poised at its heel angle, and could tip either way; the yacht is neutrally stable. The GM of the yacht is zero.
There is a popular misunderstanding that presumes that the centre of gravity has to lie below the centre of buoyancy for a yacht to be stable. The above explanation shows this to be false. A yacht can still be stable if the centre of buoyancy is above the centre of gravity. Indeed, most yachts have their centre of buoyancy above their centre of gravity. The correct and sole criterion for positive stability is that the metacentre M must be above the centre of gravity G.
1.4 Stability at large heel angles
At small heel angles, the righting lever (GZ) can be calculated from the GM.[2] However, at angles of heel larger than about 5° or 6°, the metacentre M moves up and down as the heel angle changes, so the metacentric height GM also changes with heel angle. Therefore stability at large angles must be described by the righting lever GZ rather than the upright metacentric height GM.
Figure 8 Stability at large heel angles
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The graph of how the righting lever GZ varies with heel angle is called a stability curve, or a GZ curve. An example is shown in Figure 9.
Figure 9 A typical stability (GZ) curve
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This curve provides a lot of information about the stability of the yacht:
The slope of the curve at the origin (at heel angle of 0°) is proportional to the metacentric height GM. The greater the slope, the higher the value of GM.
The peak of the curve, at about 60° heel in this example, is the angle at which the greatest resistance to heel (righting moment
) is generated.
The point at which the curve crosses the horizontal axis, about 130° heel angle in this example, is especially important. It is the point at which the centre of buoyancy B starts to switch sides
(see Figure 8) changing the lever GZ from a righting lever to a capsizing lever. If the yacht is heeled over to slightly less than 130° it will come back upright. However, if it is heeled over slightly more, it will continue to heel over of its own accord and the yacht will capsize. This critical angle between these two very different outcomes is called the Angle of Vanishing Stability (AVS).
The area under the curve above the horizontal axis (i.e. from the origin at zero heel rightwards to the AVS) represents the amount of energy required from a wave to capsize the yacht. The area under the curve below the horizontal axis (i.e. from the AVS all the way to fully inverted at 180° heel) represents the amount of energy required from a wave to re-right a capsized yacht. Consequently the ratio of these two areas is a useful measure of the capsizability of a yacht.
The calculated stability at angles greater than about 90° becomes a bit questionable because there are likely to be hatches and other openings that will submerge at some angle, causing the yacht to flood and possibly sink. The angle at which this happens – the downflooding angle – is very important when considering what happens in a knockdown. If the downflooding angle is exceeded the yacht will start to fill with water and possibly sink, making the stability curve somewhat irrelevant. The downflooding angle can be dangerously low if there are off-centre hatches, ventilators, or cockpit locker lids if they cannot be sealed watertight. One popular class of one-design racing yacht used to sink very easily if it broached, because the cockpit locker lid went under water at quite moderate heel angles. Their solution was to change the class rules so that the locker itself had to be fully sealed from the rest of the yacht, thereby restricting the amount of water entering below.
1.5 Some results
1.5.1 Wide versus narrow yachts
Figure 10 Effect of beam on stability
A dark room Description automatically generatedThe centre of buoyancy on a wide yacht will shift a long way off the centreline at moderate heel angles, making it very stiff (i.e. it will have a large righting lever GZ) compared with a narrow yacht. However, once the wider