Dec 23, 2008, 7:49 AM
Post #1 of 3
On December 18, 2008 during Leg 3 of the Volvo Ocean Race, Delta Lloyd experienced a massive failure to the port side bulkhead that attaches the keel canting hydraulic ram to the boat. They were able to center the keel and complete the leg, and while it was the first significant keel failure of the 2008-2009 VOR, it did bring back memories of the many failures from the 2005-2006 event.
Canting keel failures
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From Andrew Troup, New Zealand:
Once again keel canting rams - or the structures mounting them - are failing offshore. The most recent example is the wonderfully cosmopolitan Dutch/Irish VOR entry with the highly respected Argentinean designer. This is actually a 'much-modified' ABN Amro One, so some of what I have to say may not be applicable in this specific instance, depending on the extent of the modifications.
If a pattern of failures emerges in a new generation of VOR competitors, this may be a cause for deeper concern. Ahead of this possibility, while there's still potentially time to do something about it, I care to provide a few observations, hopefully in sufficient depth to at least be meaningful, possibly useful. It goes without saying that this is the last area one would *want* to see failures. It is also the last area I would *expect* to see failures, for reasons I hope to explain:
1) Hydraulic rams, and their support structures, hardly ever fail in other well-resourced disciplines
2) For relatively new installations, overloads which could cause catastrophic failures are likely to be up around the nominal breaking load for the ram.
I say this because there shouldn't have been enough load cycles already to trigger fatigue failure, which can happen at much lower, and much less predictable, load levels.
Furthermore, the structure around the ram should be at least as strong as the ram.
3) Externally applied loads which exceed internally generated forces can be difficult to handle in engineering mechanisms. Such loads are notoriously high and unpredictable for canting keels.
Hydraulic rams are almost uniquely well suited to routinely shedding the energy content of external overloads, protecting themselves and their surroundings, hence point 1 above.
In this mode, the keel would "relax" away from the set angle, in much the same way a suspension strut retracts when a vehicle hits a severe bump.
Having shed suddenly applied, high-energy loads, adding the capability to return automatically to the desired angle is straightforward.
4) The devices and circuit design to achieve all this - in spite of equipment failure - are relatively well understood by industrial engineers with experience in fields where such load situations are common.
Individual hydraulic components and peripherals are not immune to failure, but the natures of these individual failures tend to fall within a fairly small set of possibilities, and hence 'fail-safe' design, IN RESPECT OF SETTING AN UPPER LIMIT TO EXTERNAL OVERLOADS, can be implemented with such rigour as to be (almost) true to label.
Such circuits and devices need not entail a serious weight penalty.
The fact that ram and structural failures have been so common in this application suggests to me that substantial pockets of people have been engaged in this field despite unperceived deficiencies in knowledge and/or experience relating to design of these mechanisms and circuits. This might arise because the latter are relatively newly applied to sailing, although not at all new elsewhere.
No individual should take offence at this suggestion: taken on its own, any one failure could easily arise from hidden inherent material defects, rather than faulty design.
However there have been so many instances that this simply cannot be true in all cases.
Where the failure is structural, arising at loads within the nominal working load for the rams, the problem could hypothetically be with the engineering of the structure itself.
However I am confident that this is unlikely to be the case, because the people concerned are very experienced and skilled at this sort of structural challenge, and are well accustomed to working with the materials concerned.
To me, as a mechanical engineer, the other structural challenges routinely solved in design of these yachts are very difficult, because the loads are so hard to quantify.
In stark contrast, the challenges around the canting loads of the keel should be relatively straightforward, because a strict ceiling can be set for the loads which can be developed or applied.
Hence my frustration and perplexity that the problems have not been laid comprehensively to rest, even at these elite levels of practice.