Ask SAIL: Add to Keel

With regard to keel-bolt strength, both 304 and 316 stainless steel have a tensile strength in the neighborhood of 80,000 pounds per square inch. The cross-section area of a 3/8in bolt...
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DAVID CACCIA, Honokaa, Hawaii

Q: I have a 23ft Hunter built in 1986. This is a very soft boat that does not do well in winds above 15 knots, even with a double reef. The keel is 2ft 3in inches and weighs 800 pounds. I would like to add about 100 to 150 pounds of ballast. Will this compromise the keel bolts? Any related thoughts?

Don Casey Replies

A: With regard to keel-bolt strength, both 304 and 316 stainless steel have a tensile strength in the neighborhood of 80,000 pounds per square inch. The cross-section area of a 3/8in bolt (minor diameter of 0.30in) is about 0.07 in2 (πr2), so each bolt can carry around 5,650 pounds. Six such bolts can carry almost 34,000 pounds. If the keel bolts on your Hunter are 1/2in, that number goes up to around 60,000 pounds. The point here is not the absolute strength, but the relative increase of just 150 pounds. The bolts will not notice.

That said, you should keep in mind that adding weight and wetted area (assuming you are adding area to the keel) will likely have a negative effect on the boat’s light-air performance. That could be an acceptable price to pay if you often sail in windy conditions, but it is something to consider before making any modifications.

Don Casey has written many books and articles on marine maintenance and repairs

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