Offshore Optimising
February/March 1997
The spate of dramatic failures in the southern ocean
in January has brought yacht design and construction into the limelight in a
way that only disasters can. Whereas we all accept a certain rough and tumble
in round the world races this last lot of failures really made me pause. And
it wasn't just the Vendee Globe either since a number of Hobart entries either
retired with broken rudders or failed to make it home for the same reason with
one yacht even losing its keel. As a result of these occurrences and the alarming
photos they produced many things were discussed in a more public way than usual.
The issue always seems to boil down to responsibility at the end of the day.
The general belief is that designers themselves are constantly pushing the envelope
to produce lighter (i.e., weaker) boats and that this is promoted by the expanded
use of modern materials which in themselves are assumed inferior to good old
(steel, timber, aluminium, whatever). First I believe it is safe to say that
no one sets out to design an unsafe yacht. Second let us be clear that it is
not the material that fails but rather its application. Application has two
parts, design engineering and manufacture or process engineering. I will deal
with these separately.
Design engineering does not start with a material but rather a load case. Modern
rules governing the design of marine vessels basically apply a load to every
panel and structural member which the designer must then meet with a material
application. The second step them is not material selection . It is deciding
whether the intended use of the vessel makes these load cases adequate. After
all, going to Hobart fully crewed is a tough job at times but is it quite as
severe (at its worst) as what might be encountered in a solo round the world
race? This is an area where the designer is unguided and experience and common
sense may need to be equally applied. A little more here and there may be just
the thing to allow the race to be finished in glory as opposed to an ignominious
rescue somewhere off the coast of Australia. Also keep in mind that the only
thing that did govern race yacht construction was the ABS Guide. It is now defunct
due to legal pressure applied to it and the system is in limbo. I believe that
it was a good guide in the end particularly in the area of keel attachment and
its requirement as a condition of entry into Cat 0,1,and 2 events was generally
supported by the design community. However, in the case of single handed round
the world events there is no design / construction standard applied as requirement
for entry. This clearly should be addressed.
Anyway, having thus added value judgement to calculated load case one can then
determine if the final load on a panel may be met by using a 3mm steel plate
or a carbon sandwich panel with a low density foam core or anything in between.
Both can be made equal in the eyes of the rule and thus equal to the job so
the decision boils down to intended use, budget, and familiarity . What difference
would material choice have made to the three rescued sailors? In Thierry Dubois'
case probably nothing as his self righting problem was independent of construction
and in violation of the race rules themselves. In Dinelli's case the mast ,
which was deck stepped , broke free of the step and punctured the hull while
it was inverted causing it to sink. OK, a stronger deck may have withstood the
pounding but then the butt of the mast should have been fixed in place. The
fact is decks are not made to take that sort of pounding regardless of the material
used so it has to be viewed as a freak situation. Finally, there is Tony Bullimore
whose carbon keel strut broke. I would love to say a steel strut would not have
broken but the boat that de-keeled on the way back from Hobart this year had
a steel strut so it goes back to the original question : what was the design
load assumption ? I can say that in the case of David Adams' True Blue we chose
steel for the fin but had the budget allowed we would have built it in carbon.
The point is that the load case would have been identical in both cases.
Now if we assume that all yacht designers are qualified to engineer the vast
array of materials that are available and they have the experience and good
judgement to back up the numbers then where is the next place that things can
go wrong? Obviously on the shop floor where process engineering comes to play.
It is all well for the designer to have a perfect set of figures and a flawless
design but if it is build incorrectly or cannot be built as is sometimes the
case then things can come unstuck once again. In the case of steel and aluminium
construction the issues generally revolve around welding ability as well as
quality and compatibility of disposables (gas, rods, etc.). These rarely produce
problems though they do provide a sufficient range of variables that when a
metal structure fails it is usually weld related.
Composites have a far greater array of variables which belies the basic simplicity
of the concept which is something akin to making a peanut butter sandwich. First
off the actual properties of the finished structure are the result of the actual
way the materials are used and things like resin mix, surface preparation, fibre
to resin ratios, laminate compaction, accuracy of orientation of the fibres,
and quality of the initial materials all come into play. In aerospace, the object
is made then destruction tested . A new set of figures called design allowables
are derived and the whole engineering package is re-examined. In the yacht world
the design allowables are calculated independently and usually well before the
builder is selected. The actual product itself is rarely tested. This is usually
not a problem as most design allowables use conservative test figures and make
pretty primitive assumptions about the laminating process. However, when the
object is highly critical and advanced process engineering is assumed (pressure,
heat, high temp resins and so on) then the room for error is smaller. Without
belabouring the point this is where failures are most likely to occur . This
again is not a material problem in itself but is related to the complexities
inherent in advanced composite design and fabrication.
Beyond the designer and builder there is, of course, the client who I will define
in most cases as the person paying the bill. In most cases the client really
only wants a boat and has some budget he or she believes is adequate for the
purpose. Design and its more abstract elements such as materials testing are
really not easy things to sell into a limited budget. Time also comes into play
- how long do we have to stuff around when we have a deadline to meet? These
are very real practical concerns that effect all aspects of private yachting.
In commercial design this element has been legislated out and tests are conducted
on everything from composite panels to engine shafting. Obviously when the lives
of innocents are at stake someone else must take control and all at a cost of
course.
The client comes into play a bit more in a single handed race as their experience
and desire to win will often dictate a mutual understanding with the designer
as to what is expected. I can say that I would not design the same boat for
an experienced skipper and a first timer but I would see it very much as the
designer's job to make sure all design assumptions and options were well spelled
out in advance so that all parties were well aware of the capabilities of the
yacht.
The next situation owner driven situation is then when a boat changes hands
. A good example of this were the course racing IOR 50s bought second hand to
do the Hobart during which they duly fell apart. While these boats met ABS and
should have survived (assuming ABS to be "correct") they were not only built
on the critical edge (i.e., opportunity for design assumption and actual construction
to not mesh) they were being used well outside the range of the original client's
intention which, in the case of the 50s, was to stay tied up in over 25 knots.
Were these then examples of composite failure or a simple misapplication mixed
with misunderstanding?
Finally , a closing comment about masts. Masts are the great unregulated area
and it is only the odd offshore class yachts such as the various Whitbread/Mt.
Gay classes that exercise any control in this area through simple application
of minimal weights and CG locations. I have always felt that this is the way
to go taking pressure off owner's insurance policies as well as levelling the
playing field in this crucial area. It is all good and well to point the finger
at maintenance but the best maintenance in the world will not keep an under
designed mast in the air. Even material choice is a mute point as we saw world
class examples of both fail on maxis off Sydney last December. Where masts generally
differentiate themselves when it comes to ultimate survival is that a lost mast
usually does not sink you. You can jury rig and get somewhere eventually. Keel
failures leave you with substantially fewer options.