Considerations about the design of a sailboat

Catamaran with foils

Although the borders of different hull concepts are not always clear, the main governing parameters when designing a sailboat in terms of performance are:

  • Keelboats and yachts with relatively heavy displacement hulls: waterline length, stability (i.e., righting moment), sail area;
  • Dinghies and other planing or semi-planing hulls: (low) weight, stability, sail area;
  • Multihulls (catamarans, trimarans, proas): distance between hulls, (small) waterline breadth, (low) weight, sail area
  • Sailing hydrofoils: (very low) weight, stability, foil size.

The importance of different parameters is affected by the emphasis of different points of sailing and wind strength. Naturally, if the main use of the boat is in running dead downwind, stability has less importance, but in upwind courses, a high righting moment is essential for speed.

Interestingly, the high-speed sailing boats need good stability also in downwind courses. This is because the optimal downwind angle and thus the apparent wind angle decrease as the boat speed increases, meaning that the heeling force increases. Sufficient righting moment can be achieved by different means: breadth of the hull, low center of gravity, movable ballast (crew weight, water tanks, canting keel), or vertical forces of hydrofoils.

As the wave-making resistance of heavy displacement hulls increases exponentially with speed and waterline length is an important parameter for them, slim multihulls have a more linear resistance curve and can thus achieve high speeds compared to their size. The same applies to lightweight planing hulls and sailing hydrofoils after take-off.

It can be said that the main dimensions dictate the speed potential of a yacht, and this potential is then reached with effective foils (keel/centerboard and rudder) and sails. The need for efficient foils and sails that have a high lift-to-drag ratio is emphasized in upwind legs with all boats. High-speed sailing boats need such foils at all legs.

Ratios

Ratios of main dimensions are very useful for initial comparisons of the character of different boats or the effect of different parameter changes.

The earliest handicap systems were also based on simple ratios of length, sail area, and displacement. New ways to measure the values of those parameters have evolved, and lots of details have been added to the handicap rules, but still, the basic ratios give an idea of the performance of different boats. They are also easy to gather from the measurement certificates of existing boats.

However, ratios are very rough numbers; for example, the hull hydrodynamic resistance for a given waterline length also depends on the prismatic coefficient and the longitudinal center of buoyancy, among other parameters.

The most common dimensionless ratios are:

  • Sail area vs. wetted surface area SA/AW for light wind performance.
  • Sail area vs. displacement  SA/∇ 2/3 for medium wind performance.
  • Length vs. displacement L/∇ 2/3 for hard and medium downwind performance.
  • Righting moment vs. heeling moment RM/HM for sensitiveness, or need for reefing.

If the size of the boats to be compared are close to each other, ratios with dimensions can also be used. For example, righting moment vs. displacement, RM/∇, gives a good indication about power and the boat’s upwind performance.

The example below shows a design having a 4-ton weight (with the crew, ready to sail) and a 50 m2 upwind sail area. Adding fuel, water, and provisions for an offshore race, has about the same effect on SA/ 2/3 as taking a 20 cm “reef” in the mainsail. Or, if we try to get a better handicap rating by reducing the sail area by the same amount as the first reef, we need to reduce 350 kg (four persons of the crew) to get the same performance in terms of SA/ 2/3 as in the initial condition.

SA [ m2 ] [ m3 ] SA/∇ 2/3
50419.84Initial situation
504.119.52Fuel 45 l, food and cutlery 10 kg
49.25419.54“Reef” about 20 cm in mainsail
473.6519.831st reef and 4 persons less

The Design Spiral

Conventionally, naval architects have used the design spiral to determine the main parameters like weight, volume, stability, and performance characteristics one at a time. After each round of the spiral, the results are closer to optimal dimensions or solutions for the given task.

Design spiral
The Design Spiral

As the calculation methods, computers and databases have evolved, more and more calculations can be done simultaneously. However, in sailboat design, the thinking behind the design spiral is still very useful: which characteristics of the boat are the most important? How do they affect other characteristics? When are some boundary conditions restricting the design?

The design spiral can also be utilized to visualize where to put the most effort in a particular design case. For example, if the draft is limited, many design rounds are probably needed to optimize keel weight and shape and canoe hull depth.

Improvement, modification, or newbuilding?

Often the most important boundary conditions are coming out from the object of the design task: is it a one-off newbuilding, a modification of an existing boat or boat type, or an improvement of an existing boat?

In an improvement, the design task is more or less determining how to get the best out of the given rig, foils, or hull, and how much can be achieved by, for example, using new high-tech sails, optimizing the keel profile, or just emptying the boat from all extra weight before a regatta.

A modification may include, for example, adding a bowsprit, modifications to the rig dimensions, a new keel or rudder, or even hull shape modifications. Many racing yachts have got an extended competitive life after a comprehensive refit. Compared with just improvements, modifications demand a more holistic approach, as the interaction of different actions has to be taken into account. In fact, the design spiral thinking is useful here: for example, if a new keel has a lower center of gravity, this increases the stability and thus allows more sail area, but also requires a stronger rig, which may increase its weight and decrease the stability of the boat.

The designer has, of course, more freedom (and work) in the case of a newbuilding. However, the design task is almost always somehow limited. At least there is a certain concept (e.g., monohull, multihull) or size to work with. Designs for speed record attempts, transatlantic or round the world records may give the most freedom for the designer. Typically, the budget and size of a sailboat design are limited, the designer has to play with racing class or handicap rules, or simply the depth of harbors or waterways put their restrictions on a cruising yacht. This does not mean that the design task is less attractive. It can be vice versa, as the designer needs to find the best solutions against a racing rule, for example.

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Related

Challenges in sailboat design

Using wind power and sails as the primary means of propulsion brings additional dimensions to the design of a sailboat compared to ship or powerboat design. The designer’s challenge is to balance the different parameters to achieve the best overall performance and characteristics, rather than optimize the boat to one operating situation.

Wind triangle

The wind experienced by a fixed observer (true wind) is not the same wind an observer on a moving yacht will feel (apparent wind). The wind triangle helps us understand what the true and apparent winds are and how they are related.

Introducing the hull

The hull provides a volume to house accommodation, machinery, supplies, and cargo. It has to be seaworthy enough for the routes the yacht will sail and provide the lowest possible resistance to forward movement. It has to resist the heeling forces generated by the wind’s interaction with the sails and have a large resistance to sideways movement to reduce the sideways drift to leeward of the desired course. The hull determines most of the yachts’ main attributes: stability, resistance, seaworthiness, maneuverability, and load-carrying capacity.

What the hydrodynamic resistance is and why it matters

The motion of a sailing yacht through water requires energy to overcome resistance. It is essential to know the mechanisms behind the generation of this force that works against the movement so that we can make the resistance of a new design match the project’s overall performance goals.

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