Voyaging Tips, June 2021

Flooding Rate Table

A hole or break in your boat’s hull is among the worst situations you can face offshore. If your boat is sinking, sort out what to do immediately and what can wait. The first priority is to determine how much water is coming in the hole. Flooding means water is coming in faster than the bilge pump(s) can pump it out. Any lesser amount is leakage, not flooding. Though the difference may seem obvious, it is an important distinction. Leaks can be controlled by pumping alone — no other immediate action is required. Flooding, on the other hand, must be stopped or slowed. Pumping alone will not be enough.

Flooding Rate Table
Flooding rate table

How much water is coming in the hole, and how fast can you pump it out? In estimating the capacity of your bilge pumps, you must realize that it will be considerably less than the figure marked on the side of the pump. The rated capacity is for free flow from the outlet (i.e., no hose connected, no three-foot lift to the discharge point). The rated capacity is determined with a battery at full charge. Supply less voltage, add a hose with a lift of several feet, and the real flow rate drops quickly — usually about half the rated capacity. Compare that to the 47 gallons per minute (GPM) flooding rate from a two-inch hole located just one foot below the waterline. Even with a small hole, pumping is not enough.

The first action for flooding, then, must be to stop the inflow. If you can pump at the same time (either turn on an electric pump or have another crewmember man a manual pump), fine; otherwise, stop the flooding, then pump. Stopping the flooding may be as simple as shutting a seacock or stuffing your shirt in a small hole. Try the quick, simple approach first; put something in the hole, or put something over it. Even if the flooding slows but doesn’t stop, you have gained ground. The Navy uses wooden plugs (called damage control or DC plugs) to stop leaks. The approach is simple and effective. Even putting a single square plug in a round hole could reduce the flooding to a leak. For example, a two-inch hole one foot below the waterline passes about 47 GPM. Drive a tapered square plug into the hole, though, and the rate drops to about 17 GPM. Now your 30 GPM electric bilge pump can gain ground on the flooding while you further reduce the inflow rate by stuffing rags around the plug.

Plugs work best if you wrap a rag around each plug before inserting it. The cloth tends to keep the plug in place and fills some of the gaps between the plug and the edges of the hole.

Wooden plugs usually don’t make a watertight fit. You can reduce the leakage around the plugs by driving in smaller plugs or wedges, then driving in rags. Cedar shingles work well as a finishing touch. They are easy to split to the right size and swell when wet.

Larger holes require slightly more elaborate techniques. If the hole is so large you can’t put something in it, put something over it. Even the flooding from a large hole could be almost stopped by a crewmember pressing a throw-able cushion over the hole. For long hull cracks (e.g., from a collision with another boat), use a settee cushion stuffed into the crack, with another cushion over it backed by the top of the dining table. You could probably limp back to port with the first mate holding such a patch in place, but to go any distance, the patch should be self-supporting.

A makeshift support can be made using materials at hand, but truly sturdy patches can be constructed by adding just a few items to your toolbox. Judge what repair material you want to carry by how far you venture from help. The important thing is to think through the process now.

You might also use some sort of box over the hole. Depending on the size and shape of the hole, a skillet or baking pan wedged in place and caulked could be effective.

Actual flooding rates can be sobering — 47 GPM is a lot of water. How much water would it take to sink your boat? Multiplying waterline area times freeboard gives you a rough approximation of the interior volume of your boat, which is a measure of the reserve buoyancy. A relatively narrow 45-foot voyaging sailboat might have a waterline area of about 250 square feet and freeboard of about 2.5 feet. This yields a reserve buoyancy of about 625 cubic feet, or a little more than 4,500 gallons (there are about 7.3 gallons per cubic foot). With a flooding rate of 47 GPM, the boat would sink in 4,500 divided by 47 = 96 minutes. If the hole is big, say five inches in diameter one foot below the waterline, the time is cut to 4,500 divided by 294 = 15 minutes. Actually, the boat would sink faster because as the boat settles in the water the hole gets deeper and the flooding rate increases. The time is further shortened since sinking boats never go down on an even keel (an optimistic assumption we made in approximating the reserve buoyancy).

Also, the flooding will affect your boat’s stability. As the water comes over the sole, any motions of the boat will cause the water to slosh around, and that will exaggerate the movement of the boat.

With a hole of any size, therefore, it is imperative to stop the flooding. Five minutes go by awfully quickly in the panic that follows a collision or holing. Think about your actions now so you won’t spend the first two minutes deciding what to do.

All holes should be partially plugged if they cannot be completely plugged. Remember, reducing the flooding rate is not as good as stopping the flooding, but it’s a start.

Don’t forget holes above the waterline. They may not appear immediately dangerous, but as the boat rolls or settles, they could become submerged and provide a new source of water.

In coastal waters, consider the option of intentionally grounding your boat if you can’t control a flooding casualty. Your boat would be easier to salvage in three or four feet of water than it would in 40 feet.

Your repairs don’t have to be pretty. They just have to stop the water, or at least reduce the flow rate. Don’t worry (yet) about whether the patch will last.

Once you have the flooding under control, then think about the big picture. Can you make way? Will your temporary patch hold until you reach port? How can you rig a more permanent repair?

Most of the “temporary” fixes will hold at sailboat speeds. With the extra drag from your patch, you won’t be going very fast anyway

By Ocean Navigator