Update
foredeck at the time when longitudinal compression is maximum. For a boat travelling at 20kt the water crashing on the deck will have a dynamic pressure of the order of 0.5 bar (ie 5m water head) plus whatever static pressure due to the wave height. The resulting combination of longitudinal compression plus pressure on the deck can entail a local buckling of the deck and subsequent catastrophic situation with the forward part of the boat folding up. Figure 2: illustrates the local behaviour of a sandwich deck panel subjected to in-plane compression in the case of delamination of one skin from the core. Normally the compression is shared between the two skins of the sandwich making for a stable and stiff structure. The delamination of one skin entails a loss of longitudinal
stiffness of that skin no longer stabilised by the core and therefore free to move up (bubble appearing on the deck). That loss of stiff- ness results in the transfer of the total compression load to the other skin which is still attached to the core. However, the core is a very flexible material which, being detached from the upper skin, no longer sufficiently stabilises the lower skin which now also becomes free to move off plane and lose stiffness. The failure mode is as follow: the local loss of stiffness results
in an increase of the compression in the adjacent area of the deck which can also see an excessive compression load, resulting in propagation of the buckling. As long as the elastic limit of the skin material is not exceeded, once the compression load is removed the deck will regain its normal shape. However, the forward part of the boat may now feel ‘more flexible’ compared to that of the new boat. That is an early warning of the next failure mode. Figure 3: illustrates the next failure mode and the impending catastrophe. The application of a transverse load on the sandwich deck under compression. The two skins are being forced to move off-plane by the transverse load, they lose stiffness (buckling) and compression rupture of the deck will occur even under apparently limited in-plane compression load. This situation occurs when the boat bumps into a wave and water crashes on the foredeck. It is to be noted that, even if there is no delamination to start
with, catastrophic buckling can occur at relatively low in-plane compression load – it all depends on the ratio of the deck sandwich thickness and elasticity modulus of the skin material, versus the span between the beams and stringers supporting the deck. The larger that span the lower the load needed to buckle the deck. As this failure mode can be catastrophic, safety should demand
that under the large and flat sandwich deck there is a structure able to take simultaneously the compression and pressure load of wave crashing even if the deck sandwich is destabilised.
q SNAPSHOTS Brought to you in association with
Those were the days when the tricks used to optimise your IOR rating really were limitless. When Bruce Farr drew this 1970s Half Tonner he wanted the engine in the front regardless of the large centreboard trunk in the way of the prop shaft. With an extra length of shaft and a couple of additional universal joints the problem soon went away. But mind your feet when motoring
20 SEAHORSE
l Class… immediately after winning their thrilling final round robin race against Luna Rossa Ben Ainslie went ashore to phone Bob Fisher to whom he dedicated the Ineos Team UK victory l Nice tonic… the weekend after being relieved of America’s Cup duties Dean Barker and his daughter Mia won the first Cherub regatta to be run in New Zealand in over 20 years l Second… 10-year-old Merrick Sanderson with dad Mike (the same) l Nice work, Moose… on helping drive a timely revival of a great class l The… editor’s Cherub sank on contact with water many years ago l Small consolation… Boris Herrmann lost a chance to win the Vendée Globe by crashing into a Spanish fishing boat l Which by the way… did have its AIS switched on (eh, Boris) l But he has… been declared Imoca World Champion for his accumulated results over the past three years l In passing… Boris also told us that he never found a way to use his big foils safely or effectively in waves of any reasonable size l 53.31kt… American Magic went home still holding the record for the highest speed recorded by an AC75 in anger l Sorry, Rod… our resident OK champion and columnist had his new mainsheet system ruled illegal l Plenty more… in that tank, though l Why ever not… time for another new Seahorse campaign perhaps, make the wearing of a working kill-cord a legal requirement? l Surely… we’ve all had enough of the family tragedies? l Standing by… a small US group is trying to organise a new regatta in Key West in January 2022 for boats over 30ft l Would be great… but… l Brinkmanship… Grant Dalton inviting bids to host the next Cup before defending this one? l Brinkmanship… Ineos cutting the deal as Challenger of Record before the Prada Cup final l Tempting fate… doing it in public, yes sir l Of course… anything is/should be better than having a repeat of Luna Rossa’s so often churlish efforts as CoR this time around l Of course…. you cannot really hold that against them… l This is… the America’s Cup (popular fount of unsporting behaviour) l Although… the rescue of Patriot was an uplifting moment that went some way to taking our eye off Prada’s (rather self-defeating) yellow card attempt to take our home boys out in ‘the room’ l Progress… Maybe? Watching the AC75s, Cam Lewis reminded us that the mighty Stars&Stripes wing-rig cat of 1988 required only one grinder to win the America’s Cup (Cam Lewis!) l Really… as if American Magic hadn’t suffered enough… l While… packing up the compound the Auckland Harbour Authority took the opportunity to hand deliver a $200 fine for buzzing the harbour ferry last September... to loud cheers from the passengers l Yawn… when the AC news’ well ran dry we heard that Alinghi and Ernesto Bertarelli are going to challenge next time around l Of course… even a stopped clock is right twice a day l Builder woes… Laser Performance have been told by the Supreme Court to cough up another US$2.5 million to designer Bruce Kirby l On top of… previously awarded damages of $4.5 million l Ouch… Indeed! l Builder woes… Australian-built Etchells are faster, the unanimous opinion of every Etchells skipper who does not yet own one l We shall… see l Poisoned chalice... US Sailing is searching for a CEO (again…) l Win, lose… or draw. The first (stunning) new MC63p powercat from McConaghy is going to Grant Dalton l iTunes access… good question. At last look the eBay bidding for a working original 1976 Apple 1 had reached US$1,500,000 l Sorry, can’t stop… we are off to the shed to start making a bunch more Apple 1s l But for everything else…
RaceboatsOnly.com l Including… a very decent Ultim which can still be rented for some summer fun (remember that?) l Or for a piece… of history… l Visit… Sandeman Yachts for a look at Sir Francis Chichester’s globe-girdler Gypsy Moth IV l And finally (of course)… head to
EurosailNews.com
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