Cruise-Charter.net
Home  |   Sailing in Greece  |   Sailing instruction  |   Sailing techniques  |   Sailing history  |   Safety at sea
Sailing instruction
Boat anchor information
Understanding Winds
Clothing
Sailing Ropes
All about Headsails
Mooring and Anchoring
Sailing upwind
Mainsail techniques
The boat
Types of keel
Sailing boats by size
Buoyancy
Dinghy & Catamaran classes
Cruising dinghies
Ropes and knots >>
Tying sailing knots
Proper use of ropes and knots
Sails
The Genoa
The Spinnaker

Buoyancy and sailing



One of the basic things to study carefully and observe at all times is buoyancy of the boat. It is significant and on it depend the stability of the boat and the possibility of its capsizing or not capsizing. Turning upside-down, or as some sailors say 'turn turtle' is one of the worst things that can happen to you. It may result in fatality and it will for sure lead to loss and/or damage of property. That is why you have to take care of the buoyancy properly, thus controlling one of the most important factors preventing capsizing.
Of course there are international regulations for safety as well, designers comply with them but the crew and most importantly the skipper should do so, too.

You have to understand well the theory and calculate the measures and the forces that act upon your boat before you set sails and start even a short cruise nearby. The centre of gravity is one point somewhere in the middle of the boat (it can be calculated precisely, of course); the force of gravity acts downwards. It is important to know that the centre of gravity does not move, because it relates to the weight of the yacht, and since its weight does not change, the force of the gravity stays constant. The force of buoyancy acts in vertical direction. It depends on the size of the submerged area. When the boat is not moving and it is steady and horizontal the two forces oppose each other, the vessel is at rest. Depending on their ratio the boat shows less or more above the water. When the yacht tills, the centre of gravity remains in the same place, but the centre of buoyancy moves sideward (because the submerged area changes). At the beginning the two forces help each other because they create a kind of inertia called the Righting Moment, which is another force wanting to straighten up the boat's position to the initial one. (When the yacht is horizontal this righting moment is zero). When the imaginary line between the mast at equilibrated and the real place of the mast is 60 degrees, the righting forces reach their maximum. From then on, if the angle between this imaginary line (which is actually 90 degrees to horizontal water) continues to increase, then the forces are no longer helpful but on the contrary - they work to capsize the boat, first the hull lies down on water, then the boat capsizes. And what is worse - it stays like that and a balanced state is reached.

In this case it is very difficult to estimate properly what is the most optimal thing to do, because you are panicked, everybody and everything is in the water.

So the conclusion is that whenever the forces of gravity and buoyancy pass each other, they tend to go back in the same line. To one point they help you get back the yacht at its proper equilibrium, but BE VERY CAREFUL, as when you reach one level, they start to work against you and for the capsizing and there is no turning back. This level is different for each vessel, but it goes around 90 between the mast and the vertical straight line in the air, perpendicular to the sea level. This is called Angle of Vanishing Stability (AVS) or Limit of Positive Stability. Upon purchasing your yacht, you should ask the manufacturer for a graph or a formulae that will be handy and show you the AVS, and the righting moment. It is different for different models of yachts. Faster race yachts make compromises with it sometimes, whereas other plump-looking and not so fast boats are much more stable. However, don't judge on what you see, ask for proper documents and know everything in details.

Apart from the manufacturer's specifications, it depends entirely on you how you will manage your yacht. There is an old well known by back-packer tourists principle - the heaviest things - first. So, when you put stuff in the yacht, organize the space in such a way, that the heavy things are near the bottom. Thus you improve the stability. Be careful with storing things on the deck, because they are outside the water, so they interfere with the force of buoyancy.

Breaking waves can cause problems, that is why avoid going sideways to them. Avoid them at all in the first place, but if you get caught in such a situation, turn your bow or your stern to them. Remember that a wave no higher than the beam can have the power to overturn the whole yacht.