Boat Sailing: Does It Work?

Denmark is a great place for sailing, with its extensive coastline, fjords, and islands. Copenhagen, in particular, offers a range of sailing opportunities for beginners and experienced sailors alike. If you're looking to get into sailing, DK offers comprehensive guides and manuals, such as the DK Complete Sailing Manual, which covers everything from basic sailing techniques to advanced sailing skills. With step-by-step instructions and visual guides, DK provides the knowledge and confidence needed to set sail and navigate the seas like a pro.

The physics of sailing

Sailing is an activity that involves several interesting physics principles. The force of the wind on the sails is the primary driving force behind sailing. However, it might be surprising to learn that sailing directly downwind is not the fastest direction. There are also aerodynamic factors at play, as sails and keels function similarly to airplane wings by providing "lift" from the fluid passing around them.

Bernoulli's Principle

Sails and keels exploit Bernoulli's principle, just like airplane wings. An airplane wing is designed so that the air moving over the top travels faster than the air moving along its undersurface, resulting in lower pressure above the wing compared to below it. This pressure difference generates lift.

In the context of sailing, the wind moving around the "upper" or downwind side of the sail is forced to take a longer path, resulting in a faster speed compared to the air passing along the "lower" or upwind side. This creates a pressure difference, with higher pressure on the upwind side and lower pressure on the downwind side.

Keel and Wing Shapes

Optimising the shape of the keel and sails is crucial for efficient sailing. While it was previously believed that a long and deep keel was necessary to prevent side-slipping, modern wing theory suggests that a keel, like a sail, provides sideways lift as water flows around it. A symmetrical keel is required for a sailboat to move to either side of the wind.

Resistance

A sailboat experiences different types of resistance, including the resistance of the hull moving through the water, creating a shear force. This resistance increases as the boat's speed increases and is affected by the total area of the wetted surface. Polishing the hull does not eliminate the shear of water molecules but can help reduce turbulence.

Another form of resistance is vortex generation at the bottom of the keel and the top of the sails. As the air or water moves along the longer path around the sail or keel, its speed increases, causing a pressure drop. This pressure difference results in the formation of vortices, which cost energy and create induced drag.

Boat Speed and Direction

The fastest point of sail is not directly downwind. Sailboats achieve their highest speeds when the wind is coming "abeam," or from the side. When sailing directly downwind, the boat cannot move faster than the wind speed since the sails would not catch the wind. However, when the boat is moving perpendicular to the wind, the wind force on the sails remains constant, and the boat's speed does not decrease.

By optimising the hull design, keels, and sails, it is possible to reduce resistance and improve sailing performance. Modern racing sailboats, such as those used in the America's Cup races, have deep and narrow keels that efficiently provide the necessary lift to prevent side-slipping.

Beginner courses in Copenhagen

Beginner Sailing Courses in Copenhagen

Sailing is a popular pastime in Denmark, with its extensive coastline and numerous islands. In Copenhagen, there are several options for beginners to learn the ropes and get out on the water. Here are some of the beginner courses available:

Sailing First

Sailing First is an RYA Recognised Training Centre in Copenhagen, offering courses in English for beginners. Their focus is on providing internationals in the city with access to sailing opportunities and internationally recognised qualifications. They offer both on-water and online courses, allowing students to gain practical skills and knowledge about sailing theory.

KDY Sailing School

The KDY (Kongelig Dansk Yacht Klub) sailing school is based at Skovshoved Harbour, just north of Copenhagen. They offer an international section with English-language instruction on Wednesday and Thursday evenings. KDY also cooperates with Sailing First, providing RYA-accredited courses for those seeking globally recognised qualifications.

Copenhagen Sailing School

The Copenhagen Sailing School (Københavns Sejlerskole) offers both navigation and practical sailing courses in English. The navigation theory course covers trip planning, position finding, and rules of the sea, while the practical course teaches boat handling in the harbour and open water. Students can take the courses separately or together, and they have the option to take the tests for the Danish sailing license, which is internationally recognised.

With these beginner courses available, anyone can start their sailing journey in Copenhagen and enjoy this captivating pastime.

The role of the wind

The wind is the very essence of sailing. Sailboats are, by definition, powered by the wind. They harness the wind for their propulsion. Sailors put up sails to capture the wind's energy, and they take them down when the wind becomes too strong.

The wind's strength and direction are crucial factors in sailing. Before setting sail, sailors need to assess these variables to ensure enjoyable and safe voyages. Gale-force winds, for instance, are unsafe, while a strong offshore breeze may be challenging for less experienced sailors.

The Beaufort scale, a British invention, helps sailors gauge wind strength by observing the activity of trees. Additionally, indicators like flags, smoke from smokestacks, and telltales (yarn tied to the boat's shrouds) aid in determining wind direction. Experienced sailors are always aware of wind direction, a vital factor in trimming sails and handling the boat.

The wind's interaction with sails is a complex interplay of aerodynamics and hydrodynamics. As the wind encounters the curved foil of the sail, it splits into two paths: the leeward (downwind) and windward (upwind) sides. The longer distance travelled by the wind on the leeward side, due to the sail's curvature, creates a low-pressure area. In contrast, the wind on the windward side reaches the aft end faster due to the shorter distance travelled. This combination generates an aerodynamic lift, "pulling" or "sucking" the boat forward.

The keel or centerboard plays a crucial role in counteracting the sideways pressure exerted by the wind on the sails, transforming it into forward momentum. This dynamic enables boats to sail upwind, towards the direction from which the wind is blowing.

While boats cannot sail directly into the wind, they can make progress towards an upwind mark by sailing at angles known as "points of sail". Close-hauled, for instance, is sailing at a rough angle of 45 degrees to the true breeze.

Understanding and anticipating the wind is a key skill for sailors, enabling them to control their boats effectively and make informed decisions about their sailing routes.

Keel and centreboard

A keel is needed to prevent a sailboat from being blown sideways at any point of sail except downwind. It also provides weight low under the water to lower the boat's centre of gravity below the waterline, which is necessary for the boat to bob back upright if knocked over by wind or waves.

A centreboard, or centerboard, is a retractable hull appendage that pivots out of a slot in the hull of a sailboat. It can be raised to operate in shallow waters and reduce drag when the full area of the centreboard is not needed. A centreboard can also be partially raised if needed to provide better boat balance.

A daggerboard is similar to a centreboard but slides vertically rather than pivoting. A daggerboard is inserted like a blade down through a slot in the hull to protrude like a thin keel below the hull. A daggerboard has the same function as a centreboard, acting as a keel to keep the boat from being blown sideways.

A fixed keel is more effective at preventing leeway (sideways movement of the boat) than a centreboard. However, a centreboard can be raised to decrease displacement, allowing the boat to enter shallower water. A centreboard can also be raised for faster downwind sailing.

A centreboard may or may not be weighted. If it is weighted, it provides weight low in the water to help keep the boat upright, although not as much weight as a fixed keel can supply. If the centreboard is not weighted, sailors must keep the boat upright by positioning their weight on the upwind side of the boat.

A centreboard has more moving parts than a fixed keel and can jam or break. It also takes up space in the boat's cockpit or cabin. However, a centreboard boat can usually be trailered and easily launched and hauled out on boat ramps, whereas a fixed-keel boat of a certain size may not fit on a trailer and will require the use of a boatyards for launching, haulout, and storage.

Sailing etiquette

Sailing is a pastime that comes with a customary code of polite behaviour, which is known as sailing etiquette. It's important to follow these rules for safety reasons, as well as to be courteous to your fellow sailors. Here are some key points of sailing etiquette to be aware of:

Packing and Onboard Behaviour

When packing for a sailing trip, only bring the minimum amount of luggage, as space is limited on boats. A soft duffle bag is a better option than a suitcase. Before you set off, discuss rules and expectations with your fellow sailors, including smoking, drinking, noise, and any other important points. Agree on how you will share the limited toilet, washing, and kitchen facilities, and be considerate of others' space. Keep the boat tidy and be mindful of water usage, as fresh water is limited.

Safety and Emergency Procedures

Understand safety and emergency procedures, such as docking/undocking, radio usage, and always wearing a life jacket. If you have a skipper, listen to their instructions at all times, as they are responsible for the safety of everyone on board.

Right of Way

Understand the rules of the road, which dictate the right of way to avoid collisions and be courteous. For example, if two boats are coming head-on, they should both turn to starboard and pass port to port. A sailboat under sail has the right of way over a powerboat, but if the sailboat is running with an engine, it's considered a powerboat. Human-powered vessels like kayaks and canoes have the right of way over any other vessel, including sailboats. If you are being overtaken, maintain your course and speed unless this puts someone in harm's way—in which case, slow down and let the other vessel pass.

Anchoring

When anchoring, be mindful of how close you are to other boats. If the wind changes, you could end up tangling lines or crashing into another boat. Follow the example of existing boats in how you tie off your lines, and leave enough space for others. Be aware of anchor drag, where your boat moves despite the anchor being down. If this happens, react quickly to avoid a collision by letting out more rope or changing the anchor.

Fueling, Dock Manners, and Boat Launch

Be efficient when launching or retrieving your boat at a ramp, so others can have their turn. At a fuel dock, refuel, pay, and then move out of the way. If you need to get supplies, relocate your boat to a quieter area. When entering an anchorage, follow the example of other boats in terms of how they tie off, how much line they use, and the distance they leave between boats. Respect your anchorage neighbours by keeping music to a minimum and being mindful of your wake, which can upset smaller boats.

Respect Other Boaters and the Environment

Do not throw trash overboard, including organic waste, which should be disposed of onshore. Offer to help catch the dock lines of a vessel coming into the dock, but respect it if they decline your help. If you see a vessel in distress, you are legally obligated to assist them as long as it doesn't put yourself or your vessel in danger.

Frequently asked questions