Re: Close call in DC

James. wrote:
To carl:
apologies

None needed!

I didnt really make my point at all there- we as the rowers
wouln't be able to do it (those who have the skills don't tend to have
the time, those who have the time don't have the skills)- certainly I
could hack up some material and stick it in place, I wouldnt feel
confident or comfortable that I had made something airtight (or good-
looking) that would save a crew. Therefore we would want someone who
knew what they were doing to do it. Given it is usually £250 for a
shoulder replacement, I can't believe that getting a professional to
do it would be cheaper than the bags...? (Carl, if you can do it for
less we may have some work for you..!!). Not being cheap in that we
don't want to spend the money (not at all), its very simply that we,
as most unis it seems, don't have the money to spend.

I think, & I'm sure Robin will agree, that you underestimate yourself & your colleagues. Remember the old adage - busy people find time. Are you really saying that no one in your club has manual skills & is willing to rise to a simple challenge? This is a job you can play at doing, with bits of cardboard instead of the stuff you will eventually use. And when you are good at it with cardboard, you'll be good at it - period.

As for making it leak-proof, there are some pretty tolerant engineering considerations in your favour & we need to understand these from the outset:
1. It is not well-understood that any enclosed compartment must be able to breath. If you leave your boat in direct UK summer sun for a while, its temperature may easily exceed 50C (depending on colour, wind, etc.), but the water in which you then place it may be at 20C or less. Air in an airtight compartment will undergo a ~10% change of pressure for that 30C/54F temperature change - i.e. 1.5psi or 100mbar. It is a good idea for this pressure difference to be able to vent, since much higher differentials are possible at lower latitudes.
2. Small seepages do not materially compromise your boat's buoyancy. It is much easier for air to pass through a small gap than for water which, due to capillary forces, may be unable to penetrate finer holes at all. And if you do get a cupful of water into a 50 litre underseat volume, how much does that matter? Not at all. Provided your hatch is reasonably airtight, you could (but I certainly do not advise it!) have a pretty big hole near the bottom of your bulkhead & still get almost nothing into the compartment.
3. Averagely competent application of a sealant bead (as around sanitary ware) will completely seal & retain any but a really lousily fitted bulkhead.
4. It is very easy to pressure test your work. Slosh a detergent solution around the sealed edges and blow through the hatch with the _outlet_ from a vacuum cleaner (many cheap workshop vacuums have both suck & blow facilities). Leaks will reveal themselves with a bloom of bubbling foam & can be dried off an rectified.
5. The pressures of the water trying to enter the sealed compartments are quite low. At the most you'll have about 300mm/1ft of water pressing at the bottom of the boat, & half that at slide bed level. So the water pressure on the bulkheads & hatches will, at the bottom of the boat, be ~1.5psi/100mbar - the same figures I mentioned earlier - & half that at slide bed level. That pressure is also pushing everything you've installed shut tight against the original structure.

So you really think this is a difficult task? And remember that you can do it in stages, 1 bit at a time.


To TidewayUmpire,

yes- please go and have a lie down! Im sure you were there- did you
consider I was too? Not only was I there but my crew and I rowed down
after the cancellation aquiring no more than a few inches in the
bottom of our (fully buoyant) boat. How? We taped the (carl douglas
finest) riggers to block the waves coming over (strips of tape, not a
whole canvas that could hold water and feasible pull the boat over).
We also did (as Cambridge appear to do) ignore the racing line go
where the water was calmest, strangely that was actually near the
middle of the river. If you look at AK and leander lwts going down
they are going down stern first, i.e. they have plenty of forward
momentum. During that stretch of water round hammersmith there is no
way that a bailing mechanism would work, I accept that- after that
point conditions improved *those boats would have been emptied by the
time they got to barn elms*. If the Italians were not in a buoyant
boat then thats their look out!

Actually, those boats by being sufficiently buoyant but very low at the stern were unintentionally self-bailing. Water was coming in over the saxboards & running out around cox. But it was neither a competitive nor desirable state of affairs, & was a tad marginal for effective flotation. Eights do generally sink stern-first because their enclosed volumes (esp. bow decks) are distributed more towards the bow.


Under ordinary circumstances I would agree with Carl, the bailer
should be a performance aid, but in circumstances such as Horr07 crews
could have gone from rough patch to rough patch relatively unscathed
because the bailer would have got rid of the water in the mean time,
lifting the saxboards again and regaining lost buoyancy.

It is a dicey equation. As the boat goes that little bit lower, the water influx rises at a shocking rate. A pump or self-bailer will work well with small influxes, but once swamping starts for serious then only full buoyancy will save the day. An eight can be sunk by just 1 or 2 waves hitting it in the right way - the first puts it lower in the water & the second, even if no bigger, then has a so much easier way in as a result & can completely inundate it.


To suggest that the wintech is unbuoyant is frankly stupid. A simple
tube under the buoyancy chambers would allow water to move to the
stern and out of the bailer without losing much of the inherent
buoyancy. Doubtless those in the stern would get wet but those in the
bows could essentially row the crew from trouble.

A method well understood & preferred by some coastal clubs & many sailors is to install through-hull self balers - like little stern-facing trap doors. Opening these by a modest amount allows the forward motion of the boat to entrain & suck water out at prodigious rates for very little drag penalty. When closed, they incur no drag. But no bailing system can substitute for full buoyancy. Self-bailing is complementary to full buoyancy but is in no way an alternative.


For those who weren't there or can't remember what it was like, here
is some footage for you. I would stress that those laughing on the
bridge at the beginning hadn't realised there was a problem and were
assuming racing was going ahead as normal.
http://video.google.co.uk/videoplay?docid=1659429580355039897

If I can dig it out Ill put the footage of our novice crew rowing down
without any issue whatsoever.

Thanks for that video!

Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Email: carl@xxxxxxxxxxxxxxxxx Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
.

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