The ability for an enclosed cockpit to drain rapidly has long been a concern among yacht designers, and safety guidelines have been in place for decades to prevent cockpit flooding.
World Sailing, the governing body for sailboat racing, has two principal standards pertaining to the cockpit’s ability to shed water quickly. The first restricts cockpit volume (Offshore Rule 3.09.4); the second stipulates the minimum dimensions for cockpit drains (Offshore Rule 3.09.5).
With regards to cockpit volume, the maximum allowed combined volume below the lowest coaming of all contained cockpits for offshore boats is 6 percent of the waterline length x maximum beam x freeboard abreast the cockpit. For in shore racing, the limit for cockpit size is increased to 9 percent of these same dimension.
For boats launched after March 1992, the lowest coming doesn’t included any area aft of the FA station and no extension aft of the working deck will be included in the cockpit size. (The FA station is the transverse station at which the upper corner of the transom meets the sheerline. In other words, extensions beyond the design transom ( boarding platforms, swim platforms, etc.) are not included in the cockpit volume.
A boat with an open transom is effectively exempt from these volume restrictions, since the cockpit is not enclosed. However, a transom washboard is considered a coaming, so if you are going to install a washboard, you need to be aware of how this will impact its ability to shed water.
In the case of the F-24, the maximum cockpit size would 32 be cubic feet under the rule for inshore boats (ignoring the fact that it is a trimaran with two more hulls). The cockpit easily exceeds the World Sailing limits, thus the need for an open transom. To stay in compliance, our DIY washboard could only enclose the small footwell, which is all we wanted to do anyway.
World Sailing takes a simple approach to regulating cockpit drainage requirements by designating the minimum size of the cockpit drains. For boats that are smaller than 28 feet long, there must be at least two drains measuring 1 inch in diameter (25 mm). For boats greater than 28 feet long, four drains of at least 3/4-inch in diameter are required.
These drain diameters are hardly adequate in our view. For comparison, transom bailers on a 420 Class Dinghy are 20 times that size. Of course, the bailers on a 420 are intended to help clear the half-full cockpit of a freshly righted dinghy, but how is this so different from a boat that broached or took a big wave?
The minimum size drains designated by World Sailing can take as long as an hour to drain a fully flooded cockpit. This is hardly quick enough to put the boat back on her feet and allow one to safely access the cabin without downflooding. We’ve seen cockpits on cruising sailboats flooded with no more than 6 inches of water take 15 minutes to empty.
If we had designed our do-it-yourself transom washboard based on World Sailing’s requirement for drain size, a 1/32-inch sliver under a transom washboard would have sufficed. That would have drained far too slowly. Instead, we cut holes totalling about three square inches. This allowed it to meet the recommended drain time.
Another solution is a small gap under the washboard—common on many cruising boats. A 1/4-inch gap will quickly drain most footwells in the required time. If the cockpit is long or the board extends above the footwell, increase this gap to 1/2- to 1-inch. In the case of our F-24 footwell (measuring about 20 cubic feet, or 150 gallons), a 1-inch gap would drain it in about 5 minutes.
You could also consider using a dinghy-style, flapper-valve bailer, but remember that the valve will be subject to fatigue in rough weather and that it must be able to open at about 1 ouce per square inch of pressure. Multiple valve bailers might work. However, because the cockpit is far above the water, unlike dinghies, a gap is more reliable and will keep the big waves out.
SAFETY GUIDELINES: ESTIMATED RATE TO DRAIN CORSAIR F-24 COCKPIT AREA
|WATER VOLUME (WITH DIY SPLASHBOARD)