Question on seawater hydraulics

[Lahara]

In a rash moment a few weeks ago while sailing my 30 foot keel yacht on the Brisbane River, I cut across a point bar at low tide to make an overtaking catamaran work hard to pass me. He ended up doing this with ease because I went aground in soft mud.

The tide was coming in and I would have floated off on my own in 30 minutes but expected to be bounced off almost immediately in the wake of a large container ship I could see coming down river. However, as it approached, I could see it was moving so slowly (being pushed by tugs) that it had no discernable bow wave or wake wake. Given this, I didn't expect anything to happen but as it drew abreast of me, with about 80 metres between us, I started to heel over as a hell of a lot of water was sucked towards the ship. About 30 seconds later it all came back and lifted me off the mud. There was no evident wave involved; the sea level simply fell then rose.

My question is: What exactly caused this?

I don't know whether it is pertinent but the ship had one of those large underwater snouts poking out in front of the bow.

Stuart Hawthorne

 

[snowleopard]

the action of a large prop in a narrow channel sucks the water from the sides and in front of the ship and pushes it out behind.

when going along narrow canals you sometimes find that if the prop gets too close to one of the sides it sucks the stern in towards the bank with embarassing consequences.

 

[david_bagshaw]

It is a case of Bernoulli's Principle,

see web page showing the theory in practice


where the ship is in the chanel and moving it creates a low pressure area under and at the sides, which sucks the water away from your mud flat. If you had been totally observant you would have seen a very small rise before the water left.(from water being pushed down the channel by the ship)

As SL mentions the same happens in canals, again the faster moving water down the side closest to the bank gets sucked towards the side..

Another demonstration of this is to hold 2 sheets of paper anout 2 " apart and blow between them, they get pushed together.

 

[Lahara]

David and SnowLeopard

Thanks for your replies. I had thought it was something like this though I thought the width of the river (about a kilometre) would have dissipated the Bernoulli effect. Mind you, it was a very large and very deeply laden ship so it was probably causing all kinds of underwater turbulance, as I found out.

Thanks again

Stuart

 

[Bodach na mara]

It is not as simple as water being drawn under the ship by the prop. The effect also happens with non-powered craft and with waves of any type. The peak of a wave always seems to be preceded by a trough, even for a single wave event.

This has always seemed strange to me and my physics education does not enable me to explain it, but I have observed it when sailing in narrow channels and tidal waves are always preceded by a drop in water level.

 

[catmandoo]

Would suggest that in addition to the static head due to the depth of water energy and the friction energy inherant in the bow wave, energy is also being added to the water by the ships motion which translates as velocity head thus depressing the overall level below the static level . Further down stream it turns back into to static head and turbulence losses .

 

[Lahara]

Thanks Ken and Catmandoo. I understand (I think) your comments. But despite Ken's observation that a fall in water level precedes a rise, I'm still unclear on the sequence of events in this case. They seem to be in reverse order.

One would expect a large deeply laden ship to first cause a rise in the local water level equal to the volume of its displacement, and then a lowering of the water level when the ship had passed and the water ran back into the 'hole' in the water the hull occupied.

Is it the case that the initial lowering of the water level was caused by the water running back into the hole in the water behind the ship further upstream, before the ship drew abreast of me?

If not, where exactly did all the water go before it was returned to the original level?