70 New Hazards to Navigation

Will Gilmore · Mar 4, 2018

Thanks Dan,
That's a busy area. I hope there is no more damage past the loss of those containers.

"WSC reported that for the years 2011, 2012, and 2013 there were around 2,683 containers lost each year [3]" from an article in Yachting World http://www.yachtingworld.com/news/c...the-danger-to-sailors-real-or-imagined-107508

- Will (Dragonfly)

Al Schober · Mar 24, 2018

The thought of hitting one of those things gives me the willies. Helped a friend build a 39' custom, which included a collision bulkhead just forward of the mast with a sturdy door that closes swinging aft (gasketed). As time goes by, it seems like a better and better idea!

Will Gilmore · Mar 24, 2018

Al Schober said:
a 39' custom, which included a collision bulkhead just forward of the mast with a sturdy door that closes swinging aft (gasketed).

How about a sealed cabin floor just over the keel in case the obstacle is below the waterline?

It won't save you if you lose your keel, but if there is a breach around the keel bolts or the garboards.

- Will (Dragonfly)

SG · Mar 25, 2018

Bulkhead helps if really sealed. However, the anchor locker, or even the aft of a normal v-berth position is going to be challenged by a partially submerged object.

Look at the underwater profile of your boat. If you hit a container floating a foot, or so, below the surface, I think you will tend to slam up on it, scraping away until buoyancy, ripping, or the keel intervenes. You're also looking at striking a metal, sharp box at what likely is an angle.

The boat may be heeling, coming off of a wave ot a trough.

Regardless, not good.

Al Schober · Mar 25, 2018

Like I said - gives me the willies..

Parsons · Mar 26, 2018

SG said:
If you hit a container floating a foot, or so, below the surface, ...

I'm not a physicist, submariner, or nautical engineer, but we have some of them on this board who hopefully will chime in. Things don't 'float' below the surface of the water, they either float or sink. If the average density is less than water, it goes to the surface, perhaps just at it, but not below it. If it's fully below the surface, then the average density is greater than water and it's headed down. https://en.wikipedia.org/wiki/Buoyancy
I can understand the comment in a dynamic situation as you described, with moving boat, waves, etc. that a barely floating container would have portions below a moving boat. Just did not want to confuse people with the concept of floating below the surface. Submarines 'hang' at a specific depth by careful adjustment of their average density, water movement over dive planes, and more slowly rising or sinking, but they do not stop.

Will Gilmore · Mar 26, 2018

There is, in diving, the concept of neural buoyancy. An object can be static in water. As it sinks, the increasing water pressure will actually cause it to stop its descent because to go lower requires more density, to go higher requires less density. So, yes you can have objects that float just below the surface. Shipping containers are designed to protect their contents from the weather, thus water doesn't flood them quickly. They are vented in limited ways, so they do eventually fill. The force or inertia generated by waves may also push them down where they don't appear on the surface for long periods of time. The time window for this danger would be relatively limited, but out still exists.

- Will (Dragonfly)

SG · Mar 26, 2018

I'm not a physicist, submariner, or nautical engineer, but we have some of them on this board who hopefully will chime in. Things don't 'float' below the surface of the water, they either float or sink.

Inanimate objects have buoyancy too. Sometimes they have have enough positive (i.e., upward from the center of the earth in a fluid) buoyancy to maintain their position just below the surface.

The terms "float" and "sink" may have binary characteristics. (It is, or it's not, etc.) I know not if the term float is misapplied. :^)))

Equality, is of course, solely a mathematical abstraction. -- nothing (animal, mineral or vegetable) is EXACTLY equal to anything else in an absolute sense in reality. Equilibrium and equality are similar concepts, are relative, and (like everything and everyone) transient.

When learning to swim as a child, my Red Cross instructor had us try the: "Dead Man's Float". Like many, I wondered about that, but complied with the general instructions and simulated the "float". I found floating on my back much more useful; and never understood why the Red Cross sought to show a swimmer-to-be such a demonstration. Sadly, many years later, I can attest to having found one who was actually in that situation. He was in a stable position just below the surface (and had been stable for the least an hour, or so) in substantially the same position that my childhood swimming instructor taught for some reason. I'd still say he was "floating" when we came upon him. Lifeless, just below the surface, with his back up, arms and legs hanging down.

There are objects in the water which maintain equilibrium relative to the surface only nominally below that surface. Once, awash, now just un-awashed. Floating, maybe not in a pure sense?

jssailem · Mar 26, 2018

Will Gilmore said:
the concept of neural buoyancy

That is when your mind is just there resting in the cosmos. Maybe like a Dr Who experience. Or near death moment.

Will Gilmore · Mar 26, 2018

or underwater on a tropical reef

- Will (Dragonfly)

Hayden Watson · Mar 26, 2018

Will Gilmore said:
There is, in diving, the concept of neural buoyancy. An object can be static in water. As it sinks, the increasing water pressure will actually cause it to stop its descent because to go lower requires more density, to go higher requires less density. So, yes you can have objects that float just below the surface. Shipping containers are designed to protect their contents from the weather, thus water doesn't flood them quickly. They are vented in limited ways, so they do eventually fill. The force or inertia generated by waves may also push them down where they don't appear on the surface for long periods of time. The time window for this danger would be relatively limited, but out still exists.

- Will (Dragonfly)

Will, when a diver attains neutral buoyancy they do so by adding air to the buoyancy compensation device (BC) from the air tank. As soon as you drop below the surface in flat water, the rate of decent increases unless you add buoyancy due to the compression of the air contained within. A container cannot add air so once it goes negative it will sink. That is not to say that a container that is still floating will not be below the surface. As they loose buoyance, they tend to not rise with the swell so if you meet a container with very little buoyancy at the top of a big swell it will be below the surface.

Will Gilmore · Mar 26, 2018

Hayden Watson said:
As soon as you drop below the surface in flat water, the rate of decent increases unless you add buoyancy due to the compression of the air contained within.

You are right, I was thinking about non compressible substances, but there would still be trapped air that compresses and becomes more dense as it descends.

- Will (Dragonfly)

Hayden Watson · Mar 26, 2018

It is the barely floating objects that cause the most problems. They stay at the mean swell elevation so sometimes they are above and sometimes below as the swells roll by. That can be hard to see.

TomY · Mar 27, 2018

Hayden Watson said:
It is the barely floating objects that cause the most problems. They stay at the mean swell elevation so sometimes they are above and sometimes below as the swells roll by. That can be hard to see.

The biggest floating item I've hit, was a large cable reel, at least 8' in diameter. A friend and I were powering through NYC harbor and hit the reel, right under the TappenZee bridge. It hit just below the bow. Due to the round reel shape and the slack shape of the boats underbody, we just careened off (the reel didn't move much). We could not see the reel floating about a foot below the chop, until we were just above it. Then it was startling.

MitchM · Mar 27, 2018

i beg to differ with the author of "Things don't 'float' below the surface of the water, they either float or sink..." Yacht Forums just had a very informative post from a captain of a commercial transport ship who stated that when containers fall off the transport ship, often they slowly absorb water and float 1 foot or so below the surface for some time , where they are impossible to see even with sonar. once they completely absorb enough water, they sink. some float for days just below the surface...

MitchM · Mar 27, 2018

check out the whole thread at yacht forums.com search for: Maersk
Maersk containers in the waters off of Oregon Inlet N.C.
...its fasteners and make them fly overboard. My special friends from Maersk have pretty good records of loosing boxes during voyage. Recently, a...
Post by: HTMO9, Mar 13, 2018 in forum: General Yachting Discussion

Hayden Watson · Mar 28, 2018

MitchM said:
i beg to differ with the author of "Things don't 'float' below the surface of the water, they either float or sink..." Yacht Forums just had a very informative post from a captain of a commercial transport ship who stated that when containers fall off the transport ship, often they slowly absorb water and float 1 foot or so below the surface for some time , where they are impossible to see even with sonar. once they completely absorb enough water, they sink. some float for days just below the surface...

Sorry to disagree with a ships captain but the laws of Physics do not support that statement. An object will float when it has positive buoyancy but as soon as it goes negative it will start to sink and become more negative with each additional foot below the surface due to compression of the air contained within. What he was observing was a container that was just barely positive so that it just floated. The tops of the swell will make it "appear" to be below the surface but it will partially exposed in the troughs.
Try and interesting experiment some time. Take an empty milk jug and tie a weight to it and carefully fill it with water until it just floats. If you push it down a little bit it will slowly come back up. If you force it down 5'-10' the air will compress and it will stay down.

Will Gilmore · Mar 28, 2018

Hayden Watson said:
...Try and interesting experiment some time. Take an empty milk jug and tie a weight to it and carefully fill it with water until it just floats. If you push it down a little bit it will slowly come back up. If you force it down 5'-10' the air will compress and it will stay down.

then try that same experiment with a non compressible object. It will find an equilibrium because to go deeper is to sink into higher atmospheric pressure and to rise higher is to enter lower atmospheric pressure. Just as you point out, increasing pressure on descent compresses the gas to greater and greater density so it will sink faster and faster, but if it doesn't behave as a compressible gas, it will find stasis.
Very interesting to think about.

- Will (Dragonfly)

Hayden Watson · Mar 28, 2018

Will, you hypothesis is valid only if every buoyant object inside the vessel is a sealed and non-compressible but that is not usually the case. In diving we talk about neutral buoyancy but that really is not correct. We get to "near neutral" and then use thrust hand & flippers to provide the power to control depth. The same is true for subs. If you remove all propulsion from a sub and do not carefully monitor and adjust the buoyancy tanks, it will either rise or sink. Being neutral below the surface is like balancing a inverted broom in the tip of your finger. It can be done but only with great control and adjustment of the balance point.