Keep Your Keel On
Originally published in the September 2008 issue of SAIL
When the keel of the 38-foot sloop Cynthia Woods fell off in the Gulf of Mexico during an early-June 2008 race from Galveston, Texas, to Veracruz, Mexico, five of the six crew, all members of the Texas A&M University at Galveston Offshore Sailing Team, were rescued by a Coast Guard helicopter two days later. But the boat’s safety officer, Roger Stone, drowned after he helped two students escape from the flooded cabin.
This tragic accident, along with a number of other keel-related incidents here and in Europe (most recently the loss of the 40-foot Cheeki Rafiki) has drawn increased attention to what can happen when a keel fails. It is therefore worth taking a look at how keels have evolved into the foils now found on many modern designs and what is required to keep them in a safe condition.
In fact, keel failures are not a new phenomenon. Years ago, when external ballast was attached to integral full keels with forged iron bolts and copper plating was used for antifouling, there were any number of keel failures. The reason was that the copper caused the iron bolts to corrode, and over time that put the ballast in great danger of falling off.
TRADITIONAL KEEL-BOLT DESIGNS
On some older fin-keel boats, keel bolts descended into pockets inside the keel and were threaded onto nuts inside those pockets. The arrangement made it possible for a surveyor to pull a keel bolt to check for possible corrosion.
This type of keel attachment is time-consuming and costly to construct, and eventually the methods used on many production boats were changed. One approach was to bend the lower end of the keel bolt so it would form an angle, with a shape resembling a “J” or an “L,” so it would be (hopefully) permanently anchored within a poured-lead ballast keel. Another method was to either weld or bolt together a larger keel-bolt assembly around which lead could be poured.
Some builders concluded that threaded rods encased in lead would be sufficient to hold a keel in place. This anchoring method was, however, inherently weak, and over time the keel would work against the hull and the lead could start to slip along the bolt threads. Eventually the keel could drop off, and—if they were lucky—the crew would have to sit on the bottom of the overturned hull and wait for someone to come to their rescue.
MODERN KEEL-BOLT DESIGNS
Modern high-aspect bulb keels with their very thin blades create much larger engineering problems than do the more moderate fin-keel assemblies found on typical production sailboats. Some modern keels can’t even accommodate keel bolts within their blades, because the foil’s footprint is too small and narrow. Instead, the lead bulb is bolted to the bottom of the keel blade (which may be constructed of metal or a composite material), and the top of the blade is fastened to the hull in one of several ways.
A popular method is to construct a vertical keel box inside the hull: the top of the blade slots into the box and is fastened to it with horizontal through-bolts. Another method is to weld the top of the foil to a horizontal plate and then bolt that to the bottom of the hull.
What can cause these keel installations to fail? Welding flaws and loss of strength, which can occur when metal is heated and then cooled, have caused some thin-blade bulb keels to fail. Another important problem is the huge loads created by a modern keel’s long lever arm when a boat is under way. Combined with the constant movement of the bulb, these factors can weaken a weld, which can eventually lead to a keel failure.
Keel failures often start with a grounding. Many modern keels, particularly those on raceboats, are not designed to absorb the shock of hitting the bottom at any speed. When a keel strikes the bottom, there is often damage to the hull area just above the after end of the keel foil. (Photos of the overturned hull of Cheeki Rafiki show evidence of corrosion in the area of some of the keel bolts.) There may also be some separation between the leading edge of the keel and the hull due to the compression of the hull structure directly above the front of the keel as the keel tries to rotate aft.
The loads on a highly stressed thin-bladed bulb keel produced by a grounding may be too much, and the force will crack a weld or shift bolts. On a composite keel, the bulb may be dislodged from the composite fin or, if the keel has a long lever arm, the force may damage the hull or blade structure.
And there are other problems not related to groundings. If the boat comes off a large wave, the hull-keel joint may be overloaded, potentially leading to hull delamination and keel failure. Traveling at high speeds through waves may overload composite structures, particularly those with poor laminate schedules, and this too could cause a structural failure.
In short, there are a number of ways modern keels can suddenly become detached from their boats. Unfortunately, unless the keel is recovered, the cause of the failure may never be completely clear.
|A grounding pushes a fin keel aft and up, which can lead to serious structural damage||Poor laminate schedules could cause a structural failure|
KEEL PROBLEMS: WARNING SIGNS
While these are by no means the only indicators of potential keel problems, any of following should prompt an in-depth inspection by a professional.
• Cracked fiberglass or staining due to corrosion around the keel and fillet
• Any gap, no matter how small (but not just cracked paint), between keel and hull
• Keel bolts that appear to be compressing the fiberglass structure in the bottom of the hull
• Keel bolts that do not pass through structural floors, or frames, in the bottom of the hull
• Any noise made by the keel when the boat heels during a tack or moves in a seaway
Whenever a boat’s keel strikes something at sea or during a grounding, its condition should be carefully checked either by hauling the boat or by having a diver make a thorough inspection. Groundings can often lead to serious structural problems with the keel and its attachment points.
Veteran ocean cruiser and race Roger Marshall has designed many boats over the years, both power and sail.