On 31/03/2014 16:39, rowin...@hotmail.com
> Hmmm, so certification wouldn't actually ensure that 'Saturday's fiasco couldn't happen again' because it relies on all 420 crews not being under boated and having their crew weights distributed as certified. Sadly most clubs don't have the luxury of being able to provide boats that necessarily fit these criteria for every race/ crew/ squad season.
First of all, it is normal engineering to build a safety margin into any
provision, just as your shell should be stronger than the biggest
service load it's likely to encounter.
And then: If you do choose to overload your boat, I've already
explained that the slight increase in depth of immersion results in a
bit more of the crew's bodies going below the waterline, thus enhancing
the computed flotation capacity.
If a vessel descends to the point where it has no further above-water
reserve volume, then it sinks. That's never going to happen with a
crewed & adequately buoyant rowing shell since part of the bodies (below
the 5cm above the seat level specified in the standard will be already
immersed & any increase in depth of immersion continues to increase the
displacement by immersing more of each crew member, more of the boat, etc.
One reasons why many shells filled rapidly on Saturday was that most
wing-rigged boats have cut-down saxboards. The pictures taken that day
from Hammersmith Bridge show waves running along the sides of the hulls
with crests at, or over, the sax tops. It is these waves brimming over
the edge, not the showy but really rather insignificant spray flying off
oars & riggers, which cause rapid swamping. And lowered saxboards
greatly increase this problem.
Any wave field has a wave height spectrum (there are no "rogue waves",
just fewer, bigger ones at that edge of the spectrum). Water flow over a
lip is easily calculated & depends strongly on the height of the wave
above the lip. Until the first few big waves reach the sax top, almost
nothing comes in. Then a quite small increase in average wave height
starts a sudden influx - as the waves rise the flow per unit length of
wave escalates & the length of sax over-topped by waves increases, so
with just a small increase in wave height the water just gushes in.
That's how an eight can go, in just a few strokes, from seeming secure
to taking on a tonne of water within 10 strokes.
One bonus from full shell buoyancy is it greatly reduces surging of
water inside the boat. This helps in 2 ways: first, it greatly reduces
energy dissipation by friction & impact of sloshing water; second, it
stops masses of water from flowing to one end. Thus it makes a more
effective (& stiffer) racing shell while reducing the tendency to get
bow or stern heavy. Remember how small differences in wave heights
above the sax hugely increase influx? Every lump of water you take on
lowers the boat in the water & rapidly increases how fast you fill up -
it's a 1-way trip, especially if 1 end goes down first.
This isn't a simple concept, but any club wanting to do well in racing,
be safe in training & finish high in the HoRR & other stormy heads is at
best ill-advised to use under-buoyant shells. And - wing-rigged shells
will always take on water faster than boats with higher saxes.