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Old 10-12-2003
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Keel Bolt Concerns


Keeping the keel on a classic boat is a matter of good boatkeeping.

How trustworthy is the keel attachment? If your boat has a bolted-on ballast keel, this is a question that has increasing importance as the boat ages.

Because a definitive answer to this question is so hard to come by, let’s see if we can at least come up with some means of assessing the risk.

Galvanized Keel Bolts    First you need to know whether your keel is iron or lead. This is important because it gives you a clue about what the keel bolts are made of. Many European boats are fitted with iron keels. To the surprise of more than a few American owners, their keel bolts often are galvanized mild steel.

Selecting galvanized bolts is not entirely a matter of "cheaping out." Mild steel is galvanically compatible with cast iron. Stainless steel is less so.

How long will a galvanized keel bolt last? That’s the wrong question. The right one is how soon might a galvanized keel bolt fail? You should not count on galvanized keel bolts lasting more than 15 years. Galvanized keel bolts must be inspected beginning no later than when the boat reaches this ageand preferably five years soonerif you are to have confidence in the keel attachment.

"Visual inspection is foolproof, but withdrawing keel bolts can turn into a very big job."
Steel keel bolts can be tested in situ by X-ray, but the cost of this might exceed the cost of withdrawing a bolt or two for a visual inspection. Another inspection technique is ultrasonic testing, but since the reliability of the results depends on the operatorwith no check unless you subsequently pull a "good" boltbe very careful not to buy a pig in a poke.

Visual inspection is foolproof, but withdrawing keel bolts can turn into a very big job. Mild steel bolts, in particular, are likely to seize with rust, requiring persuasion with a sledgehammer to free them. All but the most financially strapped should leave this job to the boatyard.

The forward-most keel bolt might be the one exception. It is typically shorter, making it easier to drive out. And because groundingeven sailingtends to open the seal at the forward end of the keel, there is a good chance this keel bolt has had the longest exposure to seawater. A sound first bolt is no guarantee that the others are in equally good shape, but it is a good sign.


A properly faired keel shows little sign of its joint with the hull.

Galvanized keel bolts should be checked at least every five years, with a different bolt or pair of bolts extracted at each inspection. As most of the cost is in the extraction, it makes good sense to replace old with new unless the old bolt is completely corrosion-free. As a general rule, if any extracted bolt has lost 10 percent of its diameter to corrosion, all the keel bolts should be replaced.

Stainless Steel Keel Bolts    To the dismay of some authorities, iron ballast keels are also attached with stainless steel bolts. Stainless has two advantages over galvanized: the life expectancy is about twice as long, and stainless bolts are less likely to seize in their holes, making them easier to extract.

This second advantage points out a disadvantage of using stainless bolts in an iron keel. Because the iron is less noble than the steel alloy, galvanic action between the two tends to enlarge the hole in the keel. To mitigate this, stainless keel bolts are typically coated with some insulating material before they are installed.

Stainless keel bolts have one more drawbackthe most significant one. Stainless steel loses its resistance to corrosion when immersed in stagnant seawater. Because of this, it is an inappropriate material for underwater fittings. It is used for keel bolts based on the optimism that the bolts will remain dry. If they do, the bolts are good for half a century. If they don’t, life expectancy can drop to half a decade.

In seawater, stainless steel is subject to pitting, a particularly virulent form of corrosion that literally destroys the metal from within. If you have ever snapped a stainless steel chainplate in your handI haveyou will have a healthy concern about stainless steel keel bolts.

"The first step in inspecting stainless keel bolts is to chip away this resin cap."
Here again, X-ray or ultrasound may be employed to determine the condition of the bolts without extracting them. "Ringing" the bolts with a rap of a hammer can reveal a badly corroded bolt to the discerning ear, but I have little doubt that many bolts suffering from corrosion would "pass" this test.

If the tops of your keel bolts are encapsulated in resin, they are almost certainly stainless steel. The encapsulation is there to seal the bolts from bilge water. The first step in inspecting stainless keel bolts is to chip away this resin cap. Everything inside should be as shiny as a new dime. If not, water has found its way to the bolt, presumably from the keel joint since the top was sealed.

It is a common (and questionable) practice to use mild steel washers on the stainless steel bolts. These can lose thickness to rust and loosen the keel, but they do dramatically confirm water intrusion. If the washer is rust-covered, the bolt ought to be inspected. If the washer looks new, the bolt is less likely to reveal damageunless the washer is sitting on a bed of sealant. In that case, a clean washer may only confirm the effectiveness of the sealant.

Be sure you re-encapsulate the bolts after inspection.

Stainless steel keel bolts are used extensively in lead keels. Except for being more compatible, galvanically, they face all the same problems, plus one more. In lead keels, the keel bolts are more often actually keel studs, cast in place when the lead is cast. They typically have an L or J shape inside the lead keel, so they cannot be extracted. And the part of the bolt in the keel cannot be x-rayed because it is shielded with lead.

Bronze Keel Bolts    Bronze cannot be used in a cast-iron keel (unless the bolts are electrically insulated from the iron) but it is a superior choice for attaching a lead keel. Unlike stainless, bronze likes stagnant seawater. A good marine bronze can lose less than one thousandth of an inch to corrosion in 40 years of immersion. Bronze keel bolts generally succumb to overtightening rather than to corrosion. Under normal circumstances they are good for the life of the boat.


Cracks between hull and keel are obvious signs of movement and indicate trouble.

Warning Signs    The classic sign of keel-bolt trouble is a leak into the bilge. If the keel moves, it elongates the bolt holes and begins to leak. Leaking is almost certain to occur before the keel is in immediate danger of separating from the boat, but this may be of little consolation when the leaking starts 1,000 miles offshore. And when the tops of the bolts are encapsulated, you may not be aware of the leak until you chip off a cap or two.

Inspect the hull-to-keel joint. A hairline crack in the paint bridging this joint is normal, but anything wider suggests that the keel is moving. You can (and probably should) retorque the nuts, but if the bolts are stainless steel you are closing the barn door after the horses have already escaped. Water has almost certainly penetrated the seal and reached some or all of the keel bolts.

Watching the keel joint at haulout can confirm this diagnosis. It should dry out immediately. If it continues to weep, seawater has penetrated the seal.

Look also for rust streaks. Rust bleeding from a lead keel can come only from the bolts. If the keel is iron, a pattern to the streaks can suggest that the source is the bolts rather than just the keel.

Look for swelling or discoloration under the resin encapsulation in the bilge. If your boat is 20 years old or more, chip off a couple of these caps and examine the ends of the exposed bolts. Try tightening them. Breaking one this way would be a fortunate misfortune.

Some Reassurance    Except on radical race boats, keels rarely fall off. The reason is that a huge safety factor is nearly always designed into the attachment system. Nevin’s Rule calls for one square inch of bolt section for every 1,500 pounds of ballast. Both galvanized steel and bronze have tensile strengths of around 60,000 pounds per square inch, so if a builder follows this rule, the bolts attaching a 4,500 pound keel are capable of supporting 180,000 poundsa margin of safety of 40 to 1. Of course, the static load increases with heel, and we can only guess at the dynamic loads, but in a recreational boat neither is likely to test or even approach such bolt strength.

You can calculate the safety margin for your own boat by totaling the cross section (p r2) of all your keel bolts, and multiplying that times 60,000 for bronze or galvanized and 80,000 for stainless steel.

While this margin gives some comfort, it should not make you complacent. Check galvanized bolts after 15 years, stainless after 30 years, and either of them sooner if you see any warning signs.

Next  I will try to cover the spectrum of options that are available to restore full strength to the keel attachment. Stay tuned.

10M2Bill, wolfhouse and flyingjib like this.
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