Wash powered ships?

[Thistle]

Hmmm. The post about the Lymington ferries got me thinking. One of the problems - and it is common to many vessels - is the wash pushed out sideways. The wash is potentially uncomfortable / damaging / dangerous. Because it is going out sideways it is also surely a waste of energy.

Is there any way the wash can be caught and shoved backwards? Catching it will reduce its effect on others; shoving it backwards should use its energy to push the vessel forwards.

What about plates that can be lowered into the water parallel to the hull? Thin so that they don't cause much wash themselves and some sort of pattern on the inner surface to push the wash astern? Any other ideas?

 

[sighmoon]

All you could achieve with plates would be to deflect the bow wave back towards the ship. deflecting it backwards would require the plates to be at an angle and thus producing wake.

A bulbous bow is the real answer. A wave made at one part of the ship cancels out a wave made at an other.

 

[Thistle]

Are we trying to solve different problems? Isn't the bulbous bow designed to be effective at cruising speeds? I'm trying to think of ways of reducing wash for use at lower speeds (and thus probably in restricted waters where the effect of wash on other water users and the environment is likely to be greater).

I agree that something needs to be done to prevent waves being reflected straight back at the ship and that angling the plates isn't the answer. Hence the suggestion of thin plates parallel to the hull (or, more accurately, parallel to the ship's centreline) and with some sort of patterning on the inner face to deflect the wash astern. The "pattern" might be something like sections of an archimedian screw or ...

 

[Twister_Ken]

Might make mooring alongside a problem!

 

[Danny]

It's a question of relativity /forums/images/graemlins/smile.gif . The wash is moving at the same speed as the ferry! I know there is a sideways component relative to a (fairly) static observer in a small nearby boat but there is practically no sideways component relative to the ferry, not in close proximity to the ferry anyway. Anything fastened to the ferry would be unable to deflect the wash as there would be no relative motion between the wash and the gadget. All it would do is create a wash of its own.... ....I think.

 

[Thistle]

Surely wash is [wave | energy] travelling away from the ship at roughly 90°? This [energy | wave] is most intense near the ship and dissipates as it moves outwards. Without a sideways component there would be no wash ... ... I think /forums/images/graemlins/wink.gif

 

[chiefeng]

Wash or waves if deflected back towards the hull would only be deflected back again outwards as metal cannot absorb the energy in the wave. Anything that is moving in water creates drag and therefore more power is required to maintain speed. That of course brings up the whole global warming, carbon footprint, NOx emmisions, cost effectiveness, increase use of fossil fuels argument! but I won't go there!

 

[Thistle]

[ QUOTE ]
Wash or waves if deflected back towards the hull would only be deflected back again outwards as metal cannot absorb the energy in the wave.

[/ QUOTE ]

... which is why I would want some way of deflecting the wash towards the stern.

 

[dt4134]

There might be some advantage to a second ship riding the bow wave of the first, albeit maybe it is quite small.

Just think, you could have flocks of container ships heading out to China in formation, just like migrating birds. /forums/images/graemlins/smile.gif

 

[rhinorhino]

Joking apart, there is, extensive testing on aircraft revealed savings of about 20% in power, but much higher crew laundry bills as required proximity was quite close. Why do migrating birds fly in formations?

 

[Twister_Ken]

It works - we once 'towed' a kayak up the river. He just sat on our stern wave, with an occasional dip of a paddle to keep him in the right place.

 

[DownWest]

Volvo did some studies on trucks. In train, the second in line saves 20% the third and subsequant ones save 30%in mpg.
Migrating birds in V ride the wakes of the leading bird, and swop places to ease the load. Enables them to extend their range to the surprising ranges we see.
Andrew

 

[Cantata]

[ QUOTE ]
[ QUOTE ]
Wash or waves if deflected back towards the hull would only be deflected back again outwards as metal cannot absorb the energy in the wave.

[/ QUOTE ]
... which is why I would want some way of deflecting the wash towards the stern.

[/ QUOTE ]
If you deflect the wash, that will require energy, which can only be provided by the ship, so it will either slow down or require more power to maintain speed.

 

[LittleSister]

Just to clarify (is this thread drift?) that the 'wash' problem (or one of them) with the new Lymington ferries is not the waves spreading from the ferries as they proceed through the water, but the substantial turbulence caused by their propulsion system (one Voith-Schneider 'propellor' on each corner) as they accelerate, turn, move sideways, or try to hold station against the wind in the confines of the river and harbour.

No doubt someone will come along in a minute to correct me, but I guess that in order to move a ship of X thousand tonnes displacement Y metres in one direction, you've got to shove an equivalent amount of water an equivalent distance the other (or something along those lines).

If only you could trail deflector plates behind your ship to catch all that power!

 

[JerryHawkins]

Interesting... logically (to my mind anyway!) "towing" the kayaker on your wake should have slowed you down, or you would have had to open the throttle a little to maintain your speed. Just one light weight canoe would probably make the difference almost imperceptible, but it must be there - you don't get something for nothing; the energy to keep him going must have come from somewhere, that somewhere being your engine and the fuel you paid for to run it!

I can't prove this; maybe someone else can?!!

 

[dt4134]

Resistance is made up of frictional resistance and wave-making resistance. Flippantly, the work done to overcome wave-making resistance is the work done to produce wash.

Frictional resistance has a pretty linear relationship to speed so predominates at low speed whereas wave-making resistance increases dramatically as speed increases. That's why you move crew weight forward in light airs when racing to reduce frictional resistance at the expense of a smaller increase in wave-making resistance and move it aft in stronger winds to reduce wave-making resistance at the expense of a small increase in frictional resistance.

For a given speed wave-making resistance is proportional to the square root of the length. Hull speed is the speed that corresponds to the speed that produces a wave (wash) of wavelength equal to the boats length; it requires a lot of power to exceed it. Mobos get around that by planning.

Anyway, that's my amateur's understanding. I'm sure there are naval architects on this forum that can give you a better technical description if you want the full details.

 

[Thistle]

[ QUOTE ]

If you deflect the wash, that will require energy, which can only be provided by the ship, so it will either slow down or require more power to maintain speed.

[/ QUOTE ]

The ship has used energy to produce the wash. That energy is then in the wash. Putting a passive plate of some description in the wash will use energy from the wash to deflect the wash. If the wash is deflected backwards, won't there be a corresponding tendency for the plate (and the ship it's attached to) to move forward?

 

[dt4134]

One way to improve efficiency is to reduce the wash. I think there was a boat a couple of years ago that produced near zero wash by having enough buoyancy in the keel to keep the bottom of the hull flush with the sea. I understood that as a trial it worked, but it must have been very sensitive to small changes in trim.

 

[chiefeng]

The size of the wash i.e. how far away from the ships side it is at any given point, measured from the bow at any given speed varies with the speed of the ship. To deflect any wash in a forward direction relative to the ship and use it to give the ship a push, the wash would have to be moving faster than the ship and the plates used would need to be at an angle other than parallel to the hull. This on its own would mean that, a) the plates would need to be adjustable, to account for the variation in ships speed and b) the angle of any plates would cause drag and turbulance in it's wake.
The QE2 did use a freewheeling propeller attached to the hub of the driven propeller and the flow of water through it did increase efficiency. Trouble was the blades fell off on the first Atlantic crossing it was used.

 

[Cantata]

[ QUOTE ]
The ship has used energy to produce the wash. That energy is then in the wash. Putting a passive plate of some description in the wash will use energy from the wash to deflect the wash. If the wash is deflected backwards, won't there be a corresponding tendency for the plate (and the ship it's attached to) to move forward?

[/ QUOTE ]
I'm no engineer but I'm sure that what you are suggesting is contrary to the laws of physics, in that you can't get owt for nowt. As others said, even if your deflector plates diverted the wash so it became favourable (which I don't think they could), they themselves would create drag.