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| Keel Bolts and General Topics on Keel - Report on the Capella |
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Join Date: 01 April 2007
Posts: 106 |
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Report on the Capella Dear Friends, I have recently read the report on the loss of the keel of the Capella. Capella was a 30 feet yacht designed and owned by the famous designer Frans Maas who was drowned in the incident. After investigation it was discovered that the the keel bolts were broken after they had haircracks caused by (a) grounding(s). The bolts corroded at the haircracks which eventuelly caused the loss of the keel. I am not a specialist and perhaps I do not fully understand the report, but it got me worried. Many Swans are/were pushed to the limit on regatta's or just on sailing trips. I myself have been grounded several times trying to win or just by being stupid. And always thought: no problem it is a Swan. But this incident got me worried. Should we examine the keels ? Must all keels be dropped to check the bolts ? If we have not been grounded then perhaps the previous owner ? How can we be sure on the keelbolts ? Perhaps Lars/the Professor can give us some guidelines ? Fair winds ! jan kooistra yulunga/swan 38-110 |
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Join Date: 27 January 2011
Posts: 140 |
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One thing special in this incident was that the keel bolts were welded to an inox plate that was inbetween keel and boat, which likely weakened the steel of the bolts (as per the article in YACHT this week). Christian 411/028 IF |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Jan Thank you for bringing up this subject.
I have read the Capella report, and the fundamental problem was that the main keel bolts could neither be pre-tensioned nor tensioned afterwards, as they were welded to a steel plate on the outside of the hull.
The keel was quite extreme with a short and narrow upper chord, and the bolts nearly on the centerline.
The main bolts below the plate slacked slowly through a combination of repeated grounding loads and lead creep. The keel and bolts would then bend slightly on each tack, and this finally, after many years of bending back and forth, caused the bolts to break.
Cracks must have been visible where the keel met the hull, probably almost from the beginning. It is likely that the owner noted this, and attempted to tension the bolts, but this did not work, as their lower part could not be tensioned. A very tragic case.
To answer your question - as long as there is no cracking along the keel to hull joint when the yacht is hanging in the air, there is pre-tension in the bolts. Then the keel can not fatigue the bolts in bending, and the keel to hull joint remains watertight. There may be some cracks in the joint forward and aft caused by rig loads, and the recommendation is to remedy these, as their pre-tension has been lost, and water may enter the cracks.
Quiescent water standing in narrow cracks for a long time will develop oxygen starvation, and in contact with a stainless keel bolt this will cause pitting on the material, the lack of oxygen damages the protective oxide layer. Therefore keel to hull joint cracks should better be avoided. The keel bolt upper ends in the bilge see oxygen-rich water, and their oxide layer is then fully protective.
Christian referred to a statement that the welding of the bolts weakens them by 30 to 50%. It should be noted that all steels are not alike, this statement refers for hi-tensile bolts, but for Capella and the S&S Swans low-tensile AISI 316 was used, and the strength properties are affected very little by welding. The corrosion properties may suffer if improperly welded, and then indirectly turn into a strength problem, but professional welders are generally aware of this.
Further it could be mentioned that the keel bolts are sized with large safety margins. For example Swan 38 has 17 bolts, and each of them can alone carry the weight of the keel with a good margin.
AISI 316 is a very ductile material, it will elongate about 50% before it breaks, and elongation will provide an early warning of overload.
Questions?
Kind regards
Lars
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Join Date: 01 April 2007
Posts: 106 |
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Dear Lars, Thank you very much for studying the capella report and your explanation on this matter. If I understand correctly, there is no need to worry as long as the keel is firmly to the hull and the bolts are tensioned. If that is the case, then even severe groundings will not cause haircracks or other problems. And your advice is to always check if there is play in keel-hull joint after groundings. And 17 keel bolts is indeed very reassuring. Thanks again professor Lars !!! kind regards, jan yulunga 38/110 |
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