Falling into water from moving ship question

[tater]

I never said anything about a slower, non-cavitating ship. I observed that the (relatively few, I live in a desert) ships I have been on in the recent past have left frothy wakes, and that might explain a lower density in the wake.

They were more like boats in most cases, a few ferry trips in NY, San Francisco, and Italy last year. I've never been on anything as big as a liner under way, however. None the less, the props were entirely underwater (unlike a cargo ship in ballast), and the bubbles have to come from someplace.

 

Edited April 25, 2017 by tater

[Green Baron]

3 minutes ago, p1t1o said:

*snip* ... it doesnt explain how a slower, non-cavitating ship would produce a frothy wake. *snip*

I don't think it does. The dinghi propeller did (a 2.5hp suzuki). The 3 blade folding prop of a volvo penta 75hp of a 43ft Hanse did not. Your ego is safe :-))

Edited April 25, 2017 by Green Baron

[tater]

I also don't know anything about the flow along the hull at the air/water interface. Certainly that provides a route for entrained air as well I would think. It's certainly an interesting problem, though it never occurred to me to think about it until this discussion, lol.

[Green Baron]

11 minutes ago, tater said:

I also don't know anything about the flow along the hull at the air/water interface. Certainly that provides a route for entrained air as well I would think. It's certainly an interesting problem, though it never occurred to me to think about it until this discussion, lol.

This is of concern for sporty sailing boats and anything that shall move without too much friction. They know quite well what happens at the limiting layer between laminar and turbulent flow. Sailing boat construction, manufacturers of antifouling paint, profiles of sails and aircrafts, there is where i would start to search .... but i am an ignorant in these things.

Edit: a desert doesn't keep Australians in Alice Springs from performing a boat race. Though cavitation only plays a role later the day when satisfying thirst :-)

Edited April 25, 2017 by Green Baron

[tater]

Yeah, I suppose the control case would be a sailing vessel wake, and whatever the bubble density is at various scale sizes in the wake (and their location within the wake, I guess).

For large, slow moving stuff, again, I don't even have 2 clues to rub together, the largest vessel under way I have been on in decades has been the Staten Island Ferry, lol. Regardless, I've also never tried to swim in such a wake... there are guys who surf in ship wakes, right? Wonder what happens if they cross from the outside of the wake into the center vs regular ocean? Of course they are also not close, I'd assume any bubbles would rapidly head to the surface vs being... emulsified (?).

I'm playing the fun google game of reading tangential papers I am finding... I never knew about "squat/sinkage" in ships before, pretty fascinating... Have about 1 more cup of coffee before I have to take my son to the pediatrician. 

Edited April 25, 2017 by tater

[peadar1987]

So outboard motors tend to vent their exhaust below the water, close to the prop (apparently to reduce noise). This seems to be the cause of most of the bubbles I've seen in the wake when driving powerboats.

From seeing large prop-driven ships moving at low speed, I don't seem to remember the wake being too frothy, mainly just turbulent water. Most of the froth, if there is any, seems to be cause by air being entrained by the bow slamming down after going over a wave.

[Green Baron]

Can't check it any more. Ran out of money and had to sell the boat (including dinghi) :-/

[1101]

On 24/04/2017 at 11:21 AM, p1t1o said:

So the common wisdom is that if you fall off a moving ship, you are in dire risk of being "sucked" under the water by the turbulence of its passing, or possibly drawn into the propellers.

Does anyone know if this is true?

Well, I don't know for certain, and I don't really want to go too far into the fluid dynamics right now, but it certainly used to be possible:

Quote

 

At 08:30, two lifeboats from the boat station assigned to Third Officer David Laws were lowered, without his knowledge, through the use of the automatic release gear. Those two lifeboats dropped some 2 metres (6 ft) and hit the water violently. The two lifeboats soon drifted back into the still-turning propellers, which were beginning to rise out of the water due to the water flooding into the front of the ship. As they reached the turning blades, both lifeboats, together with their occupants, were torn to pieces.

From:

https://en.wikipedia.org/wiki/HMHS_Britannic#Last_voyage

I can't remember where exactly I read it, but I believe there were accounts from RMS Titanic's sinking that they did not immediately lower the lifeboats for the same reason.  Bear in mind of course, that this was a 110 year old ship design, there may be mitigating factors these days.

That being said, this concerns rowed lifeboats - I would presume it would be much worse for someone swimming.

[p1t1o]

15 hours ago, 1101 said:

<snip>

 

O_0  ...dang...

 

[Green Baron]

16 hours ago, 1101 said:

Bear in mind of course, that this was a 110 year old ship design, there may be mitigating factors these days.

 

The Cap series of ships from the 50s had an underwater hull like a yacht. Beautiful ships ... :-)

Modern ships like container freighters or tankers seem to me just like humongous boxes with a bulb at the bow and a flat keel as wide as the beam of the ship. Maybe economic considerations override hydrodramatic concerns ...

 

[1101]

11 hours ago, Green Baron said:

The Cap series of ships from the 50s had an underwater hull like a yacht. Beautiful ships ... :-)

Modern ships like container freighters or tankers seem to me just like humongous boxes with a bulb at the bow and a flat keel as wide as the beam of the ship. Maybe economic considerations override hydrodramatic concerns ...

 

Yes, that is a nice looking ship....

If I were to guess, hull design would probably be a mixture of streamlining, maximised cargo volume, redundant hulls (and watertight sections - presumably horizontal as well as vertically oriented), and (even for an ocean going vessel) mass ratio (what good is a ship with all the strength but no remaining buoyancy).  Presumably tied into streamlining and useful mass would be the issue of engine power and propulsion.  Given that steam and older diesel would be less efficient per unit weight (and obviously, a complete lack of CFD), I would presume that older designs like the Olympic class would be designed more toward a knifing bow to reduce hydrodynamic drag.  The propellers being mounted quite far into the flow (and huge) along with a fairly streamlined hull to get the water to flow smoothly (-ish) toward them would, I assume, increase the risk of being pulled into them if you are in the water.

I may be wrong on this.

https://www.encyclopedia-titanica.org/titanic-deckplans/tank-top.html

Useful link - it should give you the lowest deck plan, with an idea of the shape of the hull.