By Bob Currie, Recreational Boating Safety Specialist
U. S. Coast Guard Auxiliary Station Galveston Flotilla
What is Vessel Stability
Stability is the ability of a vessel to return to its upright position after being heeled over by any combination of wind, waves, or other forces. An originally stable boat may become unstable at any time due to changes in the weather (wind or waves), changes in loading, or changes in overall stability.
One of the most important concepts boaters must understand in order to safely operate their boat is the concept of stability. Driving a boat is not like driving a car, which has four corner stability. It is not like driving a motorcycle, which has front to rear stability. Instead, driving a boater is closer to riding a unicycle, which has neither side to side or front to rear stability unless properly balanced. This column will discuss issues that affect your boat’s stability. Failure to correct for these issues may result in your boat being capsized. I will discuss the following issues which can lead to boat instability:
- Weather
- Overloading
- Initial Stability vs. Overall Stability
- Weight Creep
- Free Surface: Water on Deck
Weather
One of the greatest hazards to boaters is when they operate in heavy weather conditions. The waves, seas and wind can present the greatest challenges to seamanship and greatly affect survivability whenever there is an accident. The greatest dangers from heavy weather are that the boat will swamp, capsize, or sink. In any case, the boat occupants will need to be rescued. A boater’s chances of getting home safely depend on knowing when not to go out, knowing when to come in before the weather gets too bad when the weather changes, and knowing when to call for help when the situation becomes dangerous.
Although strong winds can and do act directly on the stability of your boat, winds act indirectly by increasing the size of the waves. One danger from the waves is that they can swamp the boat when they are higher than the gunwales. One big wave breaching the gunwale can easily add six inches of water to the interior of the boat. The more water you take on, the less stable the boat becomes. One cubic foot of saltwater weighs 64 pounds. If a wave breaches the gunwales of a boat that has an interior say 8 foot wide by 10 foot long (the dropped portion of a fishing boat, for instance), and 3 inches of water are added to the interior of the boat, the result is an additional 1,280 pounds (.25 x 8 x 10 x 64) to the boat. If the boat rocks side to side, consider the effect of 1,280 additional pounds on that rocking motion. Heavy rocking motion from just three inches of water can easily capsize a small boat. If you add even more water, then the boat tends to roll over and capsize.
If only the Captain had headed for port when the weather started getting rough and the tiny ship was tossed, the Minnow would not have been lost. When the wind causes wave height to increase, consider calling it a day and heading in. If you do take on water, do your best to bail it out as the extra weight will make your boat increasingly unstable. While most boats have bilge pumps, it is easy to overcome their ability to remove water and the Coast Guard recommends that a secondary dewatering device be kept aboard. These are in most cases hand pumps that will remove about a gallon at a time.
Overloading
Overloading is the number one cause of vessel instability leading to the vessel capsizing. The root cause is simple: the more weight you put into the boat, the lower it sits in the water. This makes a boat that is normally quite stable in one foot seas become unstable in the same seas, making it easier for a wave to breach the lowered gunwales thus flooding the deck. Most recreational boats have a capacity plate that tells you the maximum carrying capacity of the boat both in actual weight and in hypothetical persons. If you divide the weight capacity displayed on the capacity plate by the number of persons listed on the plate, you will note that the hypothetical boat passenger weighs from 140-150 pounds. Too many people just poo-poo that capacity plate and wing it when figuring how much weight their boat can safely handle.
Initial Stability vs. Overall Stability
Initial stability is the stability felt by the crew or passengers during normal operations in calm seas. Initial stability does not indicate whether the vessel’s overall stability is good, bad, or borderline. Initial stability can give a false sense of safety because calm seas rarely last for the entire trip. Overall stability is a measure of the vessel’s ability to handle increasingly rougher seas. For instance, my boat is a center console bay boat with low gunwales. I know from experience that my boat can handle one-foot seas and maybe a little more with me and a neighbor. I know that when I encounter two-foot seas I need to head for port because it would be only a matter of time before my boat would become swamped and capsize. Overall stability should never be overestimated. If your freeboard (amount of hull above the waterline) is only one foot, then there is no way you can handle waves any higher than those that hit you from the side.
Weight Creep
Weight creep is the weight added due to the accumulation of extra spare parts, fishing gear, and just plain junk over a long period of time and in small amounts. Every extra pound of gear you carry aboard causes your boat to rest lower in the water. When you first got your boat, you loaded it with gear, your neighbor, and a tankful of gas, and you measured your freeboard. It was a good 16 inches, just fine for a calm day out on the bay reefs. But after a couple of years if you were to measure your freeboard again you might find that you only have 14 inches of freeboard. That may still be good for a calm day out on a bay reef in six feet of water and no wind. But don’t pay attention to that darkening sky and that cooling wind and you may find yourself fighting to get back to port while your neighbor bails the water out. So that’s what that squawking noise was on your weather radio! If you are getting your boat ready for the upcoming recreational boating season, while you are checking your fire extinguishers and visual distress signals for expiration, clean out the junk! Note: “weight creep” is a nautical term and is not safe to use when talking about your significant other.
Free Surface: Water On Deck
Well, now we have water in the boat. It covers the entire deck to an even depth as long as the boat isn’t rocking. It may have been that mythical rogue wave that put about 20 cubic feet of water on the deck (1,280 pounds of water). That’s the equivalent of adding 8.5 hypothetical boat passengers! But the boat doesn’t stay still. It heels (rocks) from side to side. If that water was frozen, we would be mostly okay as half of the ice would stay on the port side and half on the starboard side. But it’s not ice. It’s liquid water, and as you heel from port to starboard, the entire mass of water also heels to starboard, exaggerating the effect and causing the boat to heel over much farther than it would if there was no water on the deck, and the next thing you know you are in the water next to your capsized boat.
Just this past week two recreational boaters noticed water on their deck. They decided the best way to get rid of the water was to get the boat on plane and the water would rush to the rear and out the scuffers. Well, the water did rush to the rear, but the transom sank lower in the water and the bow pointed to the sky and the boat capsized. Water rushing in any direction is never a good thing. Sit where you are and pump it out with the bilge pump and backup dewatering device. If you can’t overcome the amount of water entering the boat, it is time to get off that Mayday alarm and prepare to abandon your vessel. If it capsizes, the best place to be is on top of the capsized boat. The next best place to be is alongside and holding onto the boat.
Summary
The key to having a stable vessel is to make sure there will always be sufficient stability to overcome the tendency to capsize due to weather, waves, loading, or other conditions during the entire trip. Watch for the warning signs that your boat is or is becoming unstable and take action to remedy the situation. If your boat suddenly feels sluggish, check for water in the bilge or on the deck. Take immediate action to remove the water. Let the Coast Guard know you are in trouble, and you are taking on water. Give your location, number of persons on board, and answer all questions as fully as possible.
[BC: May-7-2024]
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