Three feet underwater.

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BurnitBlue

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Just read that a 50 foot Benny hit a submerged object and foundered within 30 minutes. Clost to the N cyprus coast. The skipper noted that the submerged object was three feet below the surface. Container or steel mooring bouy.

I thought it was impossible for a submerged object to stay stationary just below the surface.

OTOH how does a submarine do this very same thing? I recall a schoolteacher at my Tech saying that sinking ship will find balance at a certain depth. Sound BS to me.
 
Very difficult to achieve precisely neutral bouyancy. In practice something will either float or sink but maybe one end of the container was 3ft below the surface and the other end was just breaking the surface. Waves complicate the issue too!

Submarines actively control their bouyancy by flooding and blowing tanks and they fly higher or lower in the water using diving planes.
 
Measuring the depth of the submerged object in these circumstances would be the last thing on my mind.
 
If it was 3 feet down wouldn't it have missed the hull and just hit the keel or rudder? Perhaps it was pushed down by the bow wave of the boat and the impact was lower than the object was before meeting the wash

Hope everybody was OK
 
Would be interesting to know the nature of the damage. Having watched how hard it was to smash a hole in a grp hull in the YM crash boat videos, I was left with the impression that boats are tougher than they look.

Maybe YM/PBO could get an old eBay boat and tow it into a container at 5kt to see what happens?
 
Mavanier said:
Would be interesting to know the nature of the damage. Having watched how hard it was to smash a hole in a grp hull in the YM crash boat videos, I was left with the impression that boats are tougher than they look.

Maybe YM/PBO could get an old eBay boat and tow it into a container at 5kt to see what happens?
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I was sent a link to another forum (cruiser forum I think). The main damage was in the bow forecabin. The skipper used bedding and pillows to control the damage. However the skipper says that the hollow stringers were a conduit for the flood water to get passed the forecabin bulkheads into the saloon. Liferaft and handheld VHF saved him but not the boat. He was single handed and moving almost 7 knots when brought to a sudden halt. Hospital for broken ribs.
 
I think you have to take the 3ft down with a pinch of salt rather than literaly.
The sailor was sinlge handed at night and the first he knew of the object was after he hit it.
Nuetral or near nuetral bouyancy, and awash seems likely.
Would be hard to see in daylight let alone darkness.
He is guessing what the object was and its depth.
Makes no difference he hit somthing large and partitialy submerged at night, which holed and sank his boat, He is lucky to have survived.
Although much of the luck may have been due to his quick actions in assesing the situation and preparing to abandon and having the nessessary gear on board.
 
Uricanejack said:
I think you have to take the 3ft down with a pinch of salt rather than literaly.
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Perhaps not, Jack. The unlucky sailor could well have been basing his estimate of depth on the depth where the damage and thus (probably) the impact occurred. On the other hand , from the cruisersforum account I've read (link above) he never actually found a hole in the hull, and never mentioned three feet or any other particular depth. Did he do so elsewhere?

As to something 'floating' below the surface: surely there's no need for such a scenario. He simply hit a part of it that was three feet under. Or is there a law of buoyancy which dictates that containers, say, must float with two axes parallel to the surface?
 
As far as I understand it water gets more dense the deeper you go, due to pressure and drop in temperature. So, while something might not float on the surface it will a metre or two below.
 
I once came across a big heavy open boat like a ships' whaler, floating awash but effectively just under the surface, it would spoil the day of most yachts hitting it.

I did report it and position to the nearby Alderney HM, don't know the outcome.
 
Seadog17 said:
As far as I understand it water gets more dense the deeper you go, due to pressure and drop in temperature. So, while something might not float on the surface it will a metre or two below.
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Not really, water is practically incompressible. But many bouyant objects (like polystyrene packaging or submarine hulls) are compressible and so become more dense with depth. So such objects will sink faster the deeper they go.
 
DJE said:
Submarines actively control their bouyancy by flooding and blowing tanks and they fly higher or lower in the water using diving planes.
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I've often wondered why the air they use to 'blow' the tanks doesn't make the sub have buoyancy? Why only when it's in the tanks of expelled water!??
 
DJE said:
Not really, water is practically incompressible. But many bouyant objects (like polystyrene packaging or submarine hulls) are compressible and so become more dense with depth. So such objects will sink faster the deeper they go.
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+1. Unless you should sail on a distant and very dense star, in which case hitting containers would be the least of your problems, water (like all solids and liquids) is incompressible.
 
Seadog17 said:
As far as I understand it water gets more dense the deeper you go, due to pressure and drop in temperature. So, while something might not float on the surface it will a metre or two below.
Click to expand...

Any trapped air will compress as it goes deeper thereby reducing the buoyancy. It's a positive feedback situation. Once it has sunk below the surface it will go to the bottom. Subs avoid that by 'flying' on their planes.
 
NPMR said:
I've often wondered why the air they use to 'blow' the tanks doesn't make the sub have buoyancy? Why only when it's in the tanks of expelled water!??
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Hiya, here's the quick version!

The goal of a dived submarine is to achieve neutral buoyancy such that whetever depth you are at, you will neither rise or sink. Internal tanks inside the pressure hull are pumped out/ flooded into from the sea as necessary to maintain this balance.

A poor trim can be overcone with speed and therefore the effect of the control surfaces (hydroplanes) but you will be stuffed if you slow down.... trimming is carried out at any change of depth plus taking into account useable fluids, salinity and other operational issues.

The other tanks, fitted to the submarine are external ballast tanks. These are filled with air on the surface to enable the submarine to float. Venting these tanks allows them to fill with water to give the sm some negative buoyancy which can be corrected underwater by trimming.

Blowing high pressure air into those tanks will expel sea water to surface the boat with positive buoyancy.

So, ballast/air to dive/surface. bodily weight (trim) and speed for underwater control.

Hope this helps!
 
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