Canting Keel failures

by Don Jones on 19 Jan 2006 Given the spate of recent problems experienced by yachts taking part in the Volvo Ocean Race which has sparked off a very public debate on the causes of the problems, and what should be done about them (particularly as it relates to the safety of crews and the good of yachting). Unfortunately thus far, much of the comment is ill informed and as a result there seems to be more heat than light being produced. I felt it would be helpful to put pen to paper.

I divide the main problems being experienced as, firstly, structural and, secondly, hydraulic. They are separate but to a considerable extent related.

While the structural detail of the boats may vary considerably and I am not privy to the detailed design of boats other than ING Real Estate Brunel, they were all designed and built to the requirements of the one Rule. While the Rule is short on in specific requirements, it clearly did not anticipate the dynamic loads which can and are applied to the hulls at the speeds they have been found to be capable of. That is, they are too fast for their strength. In particular high dynamic loading while running is a fairly new experience. This suggests the question; why were they not designed stronger?

The answer to this is in two parts, firstly I doubt that the designers realised the full potential speeds of the boats and even if they did the maximum weight imposed by the Rule prevented added structural weight if a bulb of adequate weight to meet the Rule stability requirements was fitted. We in our team realised the danger of the maximum weight rule which we branded 'the suicide rule.' I do not doubt that other teams shared similar concerns.

It may be that some local strengthening of the boats is possible at stops between legs however if design standards for sending crews to sea in these boats require a safety factor of at least 2:1 (say) then clearly some or probably all of the present hulls are hopelessly inadequate.

Turning to the failure of hydraulic components of the keel systems I draw on experience with the three different keels and hydraulic systems which have been fitted to Skandia Wild Thing and the keel and its associated system on ING Real Estate Brunel.

The failure of the cylinder rods on Skandia in the 2004 Rolex Sydney to Hobart race has been well documented previously and I will not repeat this here. Since then the systems on the two boats have been well instrumented and load measurements have been taken in widely varying weather conditions and many hours of sailing.

These measurements have indicated that loading in bad weather and high speeds has produced no surprises on the high side and we cannot subscribe to the theory that it is the higher than expected speeds of the Volvo 70’s which are overloading components and causing the spate of cylinder rods failures.

In designing Skandia we contemplated using trunnion mounted cylinders with the trunnion axes parallel to the keel pivot and the accuracy of mounting determined by accurate machining, an arrangement most commonly used in canting keel boats. However a strain analysis quickly showed that if the mounting accuracy was even slightly impaired or if the accuracy varied with hull distortion due to dynamic or rig loads, then distortion of the cylinder rods would occur associated with high bending stresses which may be cyclical in a seaway.

While I believe it is possible to design such an arrangement to produce a safe set up it would be difficult and probably heavy. On both boats we adopted a system where the cylinder mounting and the rod clevises were free to rotate in two planes at right angles and therefore self aligning, making the flexibility of the hull of no consequence in determining cylinder loading. On the down side there is a significant weight penalty. We have not experienced any surprises to date.

Therefore, assuming I am correct in my analysis, my opening comment holds true, and the structural issues being experienced out there on the high seas impact on the hydraulic problems to the extent that they cause distortion of the hull and hence cylinder misalignment.