I found myself standing on the wet and slippery fish deck of a 200-foot Russian fishing trawler staring across several hundred yards of extremely rough and breaking seas at the 180-foot black hulled Coast Guard cutter, which was my real home, and wondered if I would ever return to her warm, clean, and dry surroundings. I was in the Bering Sea, above the Arctic Circle, and wondering how in the world the crew of our rigid hull inflatable was ever going to make it across the gap between our cutter and this Russian ship.
And if they did make it across, how was I going to maneuver down the 60-foot, wet and swinging Jacob's ladder that provided the only route from the deck where I stood to the little boat that would take me home. Well, the boatswain mate driving the RIB did make it across the rough and breaking seas, and I was truly motivated to return to the warmth of real coffee and olive green bulkheads, so I somehow found a way down that Jacob's ladder and into the RIB.
Why do I tell this story that occurred many years ago? Because seas and swell conditions, i.e. sea state, are what mariners are concerned with the most. Waves control our abilities to perform, as well as the actions and stability of our vessels. Waves are the critical component of vessel design and operation. Ignore the effect of waves and you might find yourself going to sea in an unsafe vessel, or attempting passages that cannot be accomplished!
So how are waves capable of exerting such influence? For starters, know that water is 800 times more dense than air and the force generated by a wave contains tremendous energy. You cannot push a wave aside the way you can push aside an annoying sail or line whipped about by the wind.
So how do waves affect vessels stability? In calm water a boat will rarely turn over, but when waves begin pushing against a boatís hull they can exert sufficient force to a capsize the most seaworthy of vessels. Capsizing occurs when wave energy overcomes the restoring forcing inherent in a boatís design. This inherent, designed force is a combination of a vesselís buoyancy (displacement) pushing upward at a distance (called a righting arm) from a vesselís centerline, where the center of gravity is found (CG). So what am I saying? That for a vessel to return to an upright position after being rolled to one side it must be able to generate a restoring force, and this force requires a certain distance between CG to CB, or its center of buoyancy. A vessel with a high CG will develop a small righting arm and have a slow roll, while a vessel with a low CG will have a long righting arm and will have a snappy and powerful righting force.
Knowing the boat's natural roll period is an excellent indication of its stability, and there are several rules of thumb we can use to evaluate roll period. In general an acceptable, but minimum, natural roll period (seconds) should be equal to a vessels maximum waterline beam in yards. For example a boat with a 30-foot beam should have a roll period of 10 seconds (30 feet/3 feet per yard = 10 yards or 10 seconds). Cruise ships with beams of 100 feet have roll periods of approximately 30 seconds (100 feet/3 feet per yard = 33 yards or 33 seconds). Additionally, a vessel's natural roll period may be calculated using this formula:
Natural Roll Period = (.44 x Vesselís Beam)/ Square Root of GM
GM is the distance between the vessel's center of gravity (CG) and the metacenter (M), where metacenter is defined as the point through which the center of buoyancy acts when a vessel heels. GM is provided in a vesselís design and can be general defined as eight percent of a vessels maximum beam.
Waves have periods, which is the time it takes for successive crests to pass a given point. If the natural roll period of a vessel equals or is an even interval of wave period, then synchronous rolling or pitching is likely to occur. Synchronous rolling and pitching, at the least, makes vessels motion painful, and at its worst is sufficiently dangerous to capsize a vessel. What is synchronous rolling and pitching? It is wave action enhancing a boats roll such that the vessel is unable to come to an upright position and stop. It is similar to pushing a swing, where a person pushes at the outer ends of the swing, increasing the motions amplitude, and preventing the swing from slowing down and reaching equilibrium.
All vessels have stability curves that show the righting force at increasing angles of heel. There is always a maximum angle of heel, after which there is no righting force and a capsizing force comes into action. A righting force exists so long as CG and CB are separated, but once a vessel rolls to an angle where CB and CG are no longer separated but are in line, a vessel reaches the point of neutral stability. There is no restoring force and the vessel will roll over.
When synchronous rolling occurs a vessel can be pushed to this ultimate angle very quickly, and without much warning, over the vessel goes! To assist in identifying these potentially dangerous conditions the Marine Prediction Center (MPC) produces a Wave Period Charts twice each day (00Z and 12Z). These charts are found on the MPC web page and are broadcast via Coast Guard Wefax. It is important for mariners to examine both Sea State Charts (height measured in feet and meters in three-foot increments) and Wave Period Charts (wave periods in seconds) to determine if a present or projected track will provide safe and comfortable conditions for a vessel, its crew, and cargo.
A vesselís stability can be divided into two categories; initial stability and ultimate stability. Initial stability defines the angles of heel that are normal to a vessel's operation. This is usually between zero and 15 degrees of heel. At these angles a vesselís CG and CB have the greatest horizontal separation and create the greatest restoring force. When the vessel exceeds the range of initial stability, it enters the arena of ultimate stability. The separation between CG and CB begins to decrease, due to an asymmetrical shape of the immersed hull due to these extreme heeling angles. The lesson hereóin both sail and powerboat operationóis to keep a vessel within its range of initial stability. Do not allow prolonged exposure to sea conditions that roll a boat to its beam-ends.
Every effort should be made to operate a vessel inside its limits of initial of stability. Knowing its natural roll period can help you avoid extended periods of exposure to seas that match the roll period, which could lead to synchronous rolling and pitching. Keep the CG low by storing heavy gear below deck, and reduce wave impact by choosing courses that keep seas abaft the beam.