Wash powered ships?

[bbg]

I think you are looking at things backwards. Rather than deflecting the wash towards the stern, you have to ask whether the plates would be able to impart any thrust to the ship. Once the wash leaves the side of the ship, it is gone, as far as the ship is concerned. Whether it is deflected (as it would be when it approached a shore, or a headland, for example) is irrelevant.

So to get back to your example. I imagine the plates you are talking about would be attached to the ship. Let's say, for the sake of simplicity, that each plate is attached with two bars to hold it the proper distance away from the ship. BUT the bars are hinged at the inboard end (where they attach to the hull) and at the outboard end (where they attach to the plate). They need to be able to swing in so they can lie next to the hull for berthing.

Now - in the deployed position and running at 20 knots - do you think they need a bracing member / cable to prevent them from swinging (A) forward (towards the bow) or (B) backward (toward the stern). If you (or anyone) can design a system for (A), then you will have captured some energy in the plates that will be imparted to the ship as forward thrust. If not, you've just added drag.

 

[bbg]

No, towing the kayaker won't slow the bigger boat down. The bigger boat has used energy to create the wash, which then goes out and transmits that energy elsewhere - to the shore, a boat sitting peacefully at anchor, or a kayak a few feet away. I am no physicist or marine engineer, but I can't think of a mechanism that would transmit increase the drag on the big boat just by having a kayak sit in its quarter wave.

It is, in fact, a well known phenomonon in racing. On certain points of sail (reaches) a large displacement racing boat can tow a smaller boat. If the smaller boat can sit in the quarter wave so it is always sailing downhill, it can keep up with the larger boat. Doesn't slow the big boat down, but it pisses him off because he should be pulling away on handicap. It is very difficult to maintain for more than a minute or so, requires very close distances and the conditions need to be just right. I have seen it done but only for very short times. One or to short sharp luffs by the big boat (or other rapid course changes) is usually enough to disrupt the situation and spit the freeloader out the back.

 

[Thistle]

I think that's probably the most persuasive argument I've seen so far, though I'm not yet fully convinced. As said before, I'm really thinking about slow speeds - rivers, harbour entrances, etc - where the wake is more likely to inconvenience others or harm the environment. At those speeds, where frictional resistance is much lower, would the same apply? There's still a directed flow of energy outward from the hull: can't some of that be captured or re-used in some way?

It's also interesting that almost all posters have concentrated on the issue of using the wake to push the ship forward (perhaps my fault for the original post title) but the question of wash reduction has been ignored. Does this mean that everyone has accepted the suggestion as a possibility or are they ignoring it as not worthy of comment?

 

[bbg]

Wash reduction would be a different issue. Again, I am no naval architect, but I think, at a given speed, wash will be a function of the cross-sectional area of the object being propelled through the water, and the 3D shape. Cross-sectional area is related to the boat's displacement (and of course design), and 3D shape is almost certainly optimised (given the displacement and other parameters) for fuel efficiency at cruising speed, not for wash reduction at minimal speeds. I don't mean to be rude (just a bit flippant - please forgive me) but good luck with your campaign to get shipping companies to optimise their designs for the latter rather than the former.

 

[Thistle]

Again I find myself in considerable agreement with you: no-one in their right mind is going to design or use a ship just to reduce wash unless there is some benefit to them in doing so. One possible benefit struck me as being the possibility of recovering and reusing some of the energy in the wake and it continues to be interesting to hear the view of forumites on this issue. Another, perhaps more significant, benefit might be for them to be able to show that they can care for the environment and for users of smaller vessels nearby

There is, of course, no need for you to apologise, not least as I don't see this in any way as a campaign! It's simply that an idea struck me as I was reading about the new Lymington - Yarmouth ferries (and, as is often the case with such ideas, it wasn't, as has been pointed out by Carvel, actually relevant to the Lymington problem.) I wanted to bounce the idea off other forumites who, as ever, have been generous with their helpful comments and have been slow, in public at least, to rubbish the idea: my thanks to them all!

 

[Danny]

A similar principle can be used when towing a dinghy. IE adjust the tow so that the dinghy is surfing down the forward face of the following wake-wave.

Maybe, if you had a large enough dinghy and a small enough yacht and a rod instead of a tow rope, the dinghy would push the yacht along nicely with no other power source required /forums/images/graemlins/cool.gif

 

[Danny]

[ QUOTE ]
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

[/ QUOTE ]At any point near the ship the wash is actually travelling at the same speed as the ship and in the same direction. OK the waves appear in a V shape spreading out from the sides of the ship but this V shape (and the waves that it is composed of) moves along with the ship.

If you consider standing on the bank watching the wash breaking you'll see the wash isn't coming towards the bank at 90 degrees from the boat, it's running along the bank at a speed approaching the speed of the boat.... ...I think /forums/images/graemlins/confused.gif

 

[Thistle]

The wash must be moving away from the ship ... how else would it reach the bank? It is in a V-shape because it isn't all produced at the same time. At any given instant the wave that was produced one second ago has moved a certain distance from the ship while the wave produced two seconds ago has moved twice that distance away from the ship, the wave produced three seconds ago has moved three times that distance ... and so on. It's a continuous process (ie it doesn't just happen every second) so you get a continuous V-shaped wave moving away from the ship.

 

[Danny]

[ QUOTE ]
The wash must be moving away from the ship ... how else would it reach the bank?

[/ QUOTE ]First consider the bow wave. I think you'll agree it's always there when the ship is moving and it's always the same height and it moves forward at the same rate as the ship. The second wave, which angles out and back from the side of the ship is also always there and always the same distance from the bow wave. The third wave following the second wave...etc. etc. etc.

If you suspend anything in the water (from the ship) it will be moving at the same speed as the ship. None of the wake waves will move anywhere in relation to it as, once the ship is in constant motion, the whole wake system moves along with the ship, the waves appearing as standing waves in relation to the ship.

The waves "reach the bank" because the wake system has a V shape, extends to the shore, and is moving along at the same speed as the ship. If you're standing on the shore then the waves will break at your feet when the leading edge of the V reaches your position.

 

[Danny]

BTW I've no argument with your original idea about reduction in wake and reclaiming the energy that went into producing it. It's just that I don't believe it's possible to do this in the way you suggest.

One could design a wave power device which could be used on a moored boat to reclaim energy from the wakes of passing ships (and it would therefore reduce the wake in its vicinity), but this wouldn't work on the ships themselves.