For the sailbboat to be stable we need to know that there are two forces which push in different directions. The boat will right itself when the angle doesn’t pass the Angle of Vanishing Stability, as this is the last point at which the boat can right itself. If it heels any further, the boat will capsize and even fully invert upside down.
A sailboat that can always right itself is said to have positive stability. Instructors have tried to knock the Colgate 26 flat, but the boat has always come back to an upright position without turning over because it has positive stability. Even if a freak wave flipped it over, it would turn upright again.
It is imperative to know your boat well, and additionally it is very useful to know the graph that plots the righting moment against the angle of heel. Manufacturers can provide this to you. To sail efficiently, you need to maintain the optimum drive angle of the wind on your sails. This is done by steering well and trimming your sails properly.
Stability Of The Sailboat
When the boat is upright, its center of gravity is vertical, above its center of buoyancy. The center of gravity is pushing down and the center of buoyancy is pushing up. When the boat is at rest, the buoyancy and gravity are at equilibrium. As the boat heels (leans over to one side, from the action of waves or from the centrifugal force) its center of gravity remains the same but its center of buoyancy changes, due to boat’s heel angle. At this point, these two forces are not vertically aligned and they create a rotational force, which works to try to return the boat to an upright position. A sailing boat that is over-canvassed and therefore heeling excessively, may sail less efficiently. This is caused by factors such as wind gusts, crew ability, the point of sail, or hull size & design.
We could define a rotational moment or righting moment as the overall boat weight multiplied by the distance between the centers of gravity and buoyancy. The righting moment is zero when the boat is upright, but when the boat heels the center of buoyancy moves away from the center of gravity and the righting moment rises. As the boat heels farther, the righting moment rises to maximum before decreasing again as the center of buoyancy moves back toward the center of gravity. Eventually a point is reached where the center of gravity and the center of buoyancy are again vertically aligned. This angle of heel is known as the Angle of Vanishing Stability, but is sometimes also called the Limit of Positive Stability. This is the last point at which the boat can right itself. If it heels any farther, then the righting moment will operate in the wrong direction, pulling the boat over until it is fully inverted. Finally, floating totally upside down, the righting moment becomes zero again and the boat may remain inverted if the AVS is low and there are no outside factors, such as wave impact, to help it right itself.
The best way to make sure your new sailboat will be stable is by asking a manufacturer to provide you with a graph that plots the righting moment against the angle of heel. If the manufacturer is not willing to provide such a graph be cautious, the boat may not be as stable as advertised. If the graph has the steep slope, it is more difficult for the boat to heel. If it’s shallow, the boat will heel more easily. The point on the graph at which the curve cuts the horizontal axis represents the Angle of Vanishing Stability. The greater this angle, the more effort is required to invert the boat and the more likely it is to come back upright if it should suffer a knockdown. Modern cruising yachts are more resistant to heeling than narrower boats, but only initially as it can reduce the Angle of Vanishing Stability and make them more stable in the fully inverted position. Combined, broad beam and shallow draft reduce a boat’s AVS, making it more vulnerable to a knockdown or total inversion than narrower, heavier, and deeper drafted designs.
I’ve found a similar stability diagram, showing relative positions of the center of Gravity, center of buoyancy, and metacenter.
In order to be acceptable to classification societies such as the Bureau Veritas, American Bureau of Shipping, Lloyd’s Register of Ships, Korean Register of Shipping and Det Norske Veritas, the blueprints of the ship must be provided for independent review by the classification society. Calculations must also be provided which follow a structure outlined in the regulations for the country in which the ship intends to be flagged.
Within this framework different countries establish requirements that must be met. For U.S.-flagged vessels, blueprints and stability calculations are checked against the U.S. Code of Federal Regulations and International Convention for the Safety of Life at Sea conventions (SOLAS). Ships are required to be stable in the conditions to which they are designed for, in both undamaged and damaged states. (paragraph source: Wikipedia: Ship stability)
Sometimes a keelboat can heel over until the spreaders are in the water and the keel and rudder are near the surface of the water. This is called a knockdown. Though this is a rare occurrence, you should know why it happens. The distance between center of buoyancy and center of gravity is now the greatest it has been and the righting moment is greatest too. The lever arm is longest, the sails are angled away from the wind, and the wind has lost its ability to heel the boat farther. If you take your sails down or luff them completely, the keel’s weight should eventually right the boat. But if the boat tips farther (and provided water can’t get inside) the boat will go over. First it turns turtle (goes upside down), then continues turning until it is right side up again and the center of gravity returns to its lowest point.
How weight affects balance
On a small boat the distribution of crew weight can change your boat’s balance. When a boat heels, the bow wave on the lee side becomes larger and tends to shove the bow to windward, and heeling puts the center of effort out over the water. If necessary, a small sailboat can be steered without using the rudder by shifting crew weight from one side of the boat to the other. Lee helm results when the crew hikes out—sits on the high side of the boat with as much weight outboard as is comfortable and safe. This is generally done to flatten the boat and keep it from heeling excessively. To produce weather helm, ask the crew to sit to leeward. If you are steering and you don’t feel the helm or the boat isn’t gaining any forward momentum, you should try to sit on the low side, too.
Make Sure Your Boat Will Right Itself – Stability
Avoid fitting heavy equipment high up. Fitting a radar scanner on the mast, or an in-mast or jib furling system, will raise the center of gravity and reduce stability. The higher the equipment the greater the effect on stability. Careful where you place your radar scanner.
Boats are more likely to capsize in reaction to waves than to wind. Getting caught side-on by a breaking wave is the worst case scenario. Avoid such waves by consulting charts, if possible.
How to correct too much weather helm
Weather helm is the tendency of sailing vessels to turn towards the source of wind, creating an unbalanced helm that requires pulling the tiller to windward.
- Ease mainsheet or traveler
- Reduce heeling by hiking
- Reduce mainsail area or effectiveness by carrying a slight luff, freeing the leech, or reefing
- Put up a jib (if none up) or a larger jib
- Reduce mast rake
- Move crew or equipment aft
When it comes to cruisers, there is a debate that was going for a long time regarding which cruiser configuration is better which touches on this subject as well, and it’s about monohull and multihull configurations for a cruising yacht. Monohull adherents point out that a multihull (a trimaran or catamaran) can capsize in very rough weather, whereas a monohull will usually right itself if it capsizes. Multihull proponents respond that even if a multihull capsizes, it will not sink, unlike a monohull with a weighted keel, which will sink if its hull or deck is breached by the sea. The debate will rage forever, but it is worth trying both types, perhaps on chartered vacation sails, to make up your mind.