The forces and moments affecting a sailing boat vary a lot depending on points of sail, but also on wind strength and waves. The same hull, foils, and sails cannot be optimal for all circumstances, and compromises and different weighting of performance are needed. For example, the keel profile (foil section) of a yacht designed for upwind-downwind courses is not optimal for round the world races, which include much less close-hauled sailing.
1. Downwind sailing
1.1. Heavy weather
Sailing downwind in heavy weather is an easy situation in that the sail force is affecting more or less to the same direction as the boat is traveling, and there is also lots of sail force available.
– Dead run
The centerboard can be lifted up in dinghies, or the keel profile can be such that it produces a minimum drag at very small leeway angles, as generating lift is almost not needed. Additionally, as the lift force is proportional to the speed squared, it is an easy task for the keel or centerboard to produce that with a small leeway angle.
The same applies to rudders if the boat is in optimal balance; however, this is seldom the case, especially in waves, and large steering moments are occasionally needed to keep the boat under control. This means that the planform area and profile shape of a rudder need to be designed to generate enough lift force when required.
A long waterline to reduce the wave-making resistance of the hull is preferable. The hull can be narrow because not much stability is needed. The rig can be of a low aspect ratio, and there are not too many requirements for the sail profile shape.
– From broad reach to beam reach
From a broad reach to beam reach, the aerodynamic side force increases. This sets requirements for stability and for producing hydrodynamic side force. Still, as the speed is high, even higher than on dead run (because the apparent wind speed increases, and typically the sails are more aerodynamically efficient), the keel or centerboard easily create the required lift force. The need for good stability becomes more obvious, especially on high-speed crafts, and there may be a need to reduce the heeling force by flattening the sail shape or reducing the sail area.
1.2. Low wind speed
If the wind speed is low, there is a lack of sail force and thus driving force. Running is especially demanding, as the boat speed further reduces the apparent wind speed.
Normally, it makes sense to luff to broad reach to increase the apparent wind and zigzag downwind. There is thus some amount of aerodynamic side force that a hydrodynamic side force must compensate. As the boat speed is now quite low, the keel or centerboard cannot create enough lift force without a bit larger leeway angle than in the case of heavy weather. A good lift-to-drag ratio of keel or centerboard becomes now of some significance.
Concerning the hull, the viscous resistance becomes very important. Reducing the wetted area is then the major thing to do. Extra stability usually is not needed.
As light air does not provide too much energy, maximizing sail area and sail forces is vital. Deep sail shape, i.e., much camber, is favorable, even though the lift-to-drag ratio of such a shape is smaller. However, the larger total force and the fact that the direction of the drag force is not too bad, compensate for the negative effects of the increased drag.
2. Upwind sailing
The side force needs to be compensated by a hydrodynamic side force with as low induced drag as possible. The planform area of the keel or centerboard should be increased to keep the leeway angle reasonably small. Some side force may also be created by the rudder and the lateral area of the hull.
Normally a deep draft, high aspect ratio keel or centerboard, and high aspect ratio sails, all with a section shape having a small thickness ratio and camber, are beneficial for high lift-to-drag ratio and upwind courses in general.
2.1. Heavy weather
As the wind speed increases, lots of sail force becomes available.
On close-hauled or close reach courses, this also means lots of aerodynamic side force, which heels the boat. High righting moment is now essential to be able to utilize the driving force component of the high sail force.
High aspect ratio rigs offer a good lift-to-drag ratio, but they also induce a large heeling moment as the center of effort of the sail force is high from the waterline. Flattening the sails helps keep the heeling force moderate, and fortunately, that also gives a better lift-to-drag ratio. Keeping the heel angle small enough may require a reduction of the sail area.
As the boat speed is now moderate, the keel or centerboard can produce the required side force with a quite small leeway angle if the planform area is adequate. Quite small section thickness is favorable for these circumstances, especially on flat water.
2.2. Low wind speed
In very light air, the low boat speed and low sail force available may again favor a larger thickness ratio that gives more lift force, although it must be paid by more drag also. The same applies to sails: slightly deeper sails (not as pronounced as in light air downwind courses) create more total force and thus more driving force than flat sails, although their lift-to-drag ratio is smaller. The boat’s pointing ability suffers from this, but the increased speed results in a better VMG (velocity made good).
The same comments as in light air downwind course apply to the hull shape. Some stability is needed, but typically it is not a problem until the wind increases to a moderate breeze.
3. Effect of waves
3.1. Downwind sailing
On downwind courses, the added resistance due to waves is much smaller than upwind because the frequency of encounter of the boat with the waves is smaller.
On the other hand, the wave energy can be utilized by surfing along the wave crest. Depending on the length-displacement ratio and the underwater shape, especially in the aft part of the hull, a yacht can achieve a semi-planing mode and occasionally very high speeds compared to normal displacement mode speeds.
3.2. Upwind sailing
Surface waves clearly affect the performance of a sailing boat. Especially on upwind courses in light winds, the drop in speed and pointing ability may be dramatic. This is due to added resistance, which can be remarkable compared to other resistance components in that situation. The added resistance due to waves is affected by the wave spectrum, the boat displacement, her longitudinal gyradius, her speed, and her hull shape.
– Heavy weather
In heavy weather, the added resistance induced by waves is also remarkable, but there is enough sail force available now.
A good lift-to-drag ratio of both sails and underwater appendages is required, and thus a small section thickness is preferred. However, there is a difference between the optimal section shape in waves and in flat water because of the yacht motions in waves:
- In flat water sailing, the sails and the appendages are working at quite a constant angle of attack.
- In waves, this is difficult to maintain, and certain tolerance must be accepted. A section shape with a more blunt nose is not so sensitive to occasional large angles of attack and is, therefore, more effective in wavy conditions.
– Light winds
In light winds, the sail force should be maximized with deeper sails, again with a cost of increased drag. This leads to a bit lower course but is necessary to achieve enough driving force.
- Challenges in sailboat design.
- Considerations about the design of a sailboat.
- Points of sail.
- Wind triangle.
- Surface waves.
- Wind-generated sea waves.
- Sea state and wave forecasting.
- Introducing the hull.
- Hull appendages, planforms, and wing sections: what are they?
- What the hydrodynamic resistance is and why it matters.
- Added resistance in waves knowing yacht’s radius of gyration (calculation).