Sailing without wind, but with strong currents

Question

Context

On my world, there is a lot of water: sea, oceans, ... There are also firm land and islands, so it's interesting to travel on the water. The problem is that there is NO wind at all, and lots of currents. These currents are mostly tidal: basically, they go in one direction for some time, then in the other for nearly the same amount of time, at more or less the same speed. It is hence impossible to practice normal sailing, the boats would just be moved around by currents and getting nowhere but reefs.

The technological level is equivalent to what was available on earth during the 17^th century.

Question

The boats must be able to choose where they are going, and not merely be moved around like cork would be.

I'm thinking about two possibilities, but I am unsure if these are viable:

• Use an anchor when the current is not going where you want to go. (Problem: it's difficult to move sideways from te current).
• Reverse the concept of a boat: use small "sails" underwater for propulsion, and enormous sails in the air to steer and prevent drifting.

Is there a possibility to use the currents in order to move a boat toward its goal?

Answer 1

Relativity

According to relativity, a strong wind with negligible current is locally identical to a negligible wind with a strong current. All of the sailboats that exist today would work just fine in this new environment without any modifications. The issue is they would sail relative to the current.

This means if the current is 30 knots, on the sailboat it would feel like you have 30 knot winds and you could sail using the exact same techniques. The difference would just be that in addition to whatever normal sailing speed you were managing, there's a 30 knot offset in the direction of the current.

With the current

This one is pretty easy. When you're trying to sail with the current, you're effectively trying to sail upwind. There are a lot of boats that can sail upwind at about 60% of the wind speed.

(source)

In this image we can see how fast you can travel on still water with a given wind speed and direction. Basically, you can travel fastest to the sides, at nearly 100% of the wind speed (high performance boats can exceed 100% to the side), around 55% upwind, and 65% downwind.

Then you get to add the current speed back in so you should be able to sail a normal sailboat at about 155% of the speed of the current when you're sailing with the current.

Against the current

If we repeat the math from above now you get that we'll sail at 65% speed downwind (upcurrent) but then when we add the current in now we're going at -35% speed. Uhh Oh.. now we'd be making negative progress.

Solutions

Just deal

If the currents change direction often enough we could just deal with it. If half the time we're making 155% progress and half the time we're making -35% progress we'd still average 120% progress. So we'd still be able to make it to our destination faster than the average current speed.

Propellers

So we'd like to be able to sail downwind faster than the wind. Sounds impossible, but we've actually already achieved something similar: A wind power car than can go 2.86 times the speed of the wind. In the case of the car, the wind pushing on the propeller gives the car thrust to move forward. The wheels then take a portion of this forward energy and direct it back to the propeller to blow backwards. This additional energy allows the wind to provide additional thrust, which then goes into an amplifying cycle only limited by friction and wind resistance (as the vehicle is going faster than the wind so it net blowing against it)

So we can adapt this same technique to a boat, but instead of wheels we'd need a water propeller. The idea would be that the wind would push on the air propeller driving the boat forward. Some of this forward energy would be absorbed by the water propeller which would drive the air propeller into action giving us more thrust to move forward more.

I doubt we'd be able to achieve the amazing 2.86 times the wind speed, but we may still be able to achieve something meaningful using oldmill technology. If this technology worked well, it could actually be used to sail upwind faster as well, by reversing the gear ratios to let the wind propeller power the water propeller.

High Performance Sailing

Turns out it rather than using a typical earth sailboat, you use something like an 18 foot racing skiff, you can sail faster than the wind in pretty much any direction:

With a boat like this you'd have no problem sailing upcurrent. I took the liberty of making a modified graph of how these boats would behave relative to solid ground:

Of course this probably requires modern materials to achieve.

Answer 2

You can't control your direction in a sailboat when there is just one current. Sailboats are only able to move into a different direction than the wind is blowing because they have a "second sail" under water, called a keel. The actual physics on a sailboat are a complex topic. But the bottom line is:

When the aerodynamic force on the sail and the hydrodynamic force on the keel act in different directions, then the sailboat can move into a direction which is neither.

However, when the wind speed over land is 0 but you are drifting in a very fast current, then you actually feel an airstream just from your movement. That airstream can be used to maneuver by sail. You won't get a lot of speed and you certainly won't be able to sail up-current, but it might be enough to jump from one current into a different one which goes in a different direction.

It would be like sailing, but in reverse. Normal sailing is exploiting air currents in stationary water. In your universe, it would be exploiting water currents in stationary air.

Answer 3

Rowing

If you don't have motors, it's back to good ol' muscles, like Egyptians and Romans used to do. The only alternative is that if somehow there is another, intelligent enough marine species that could be trained, you could use those animals to pull the ship along the seas.

EDIT: TECHNOLOGY LEVEL

On Earth, the first functioning steam-propelled ships appeared in the 18th century, but they were prototypes prone to break down within a short time (often less than one hour).

Only in 19th century there would appear the first true steamboats. And they were affairs made more to cross rivers than oceans. Of course, given the pressure of obtaining a steamboat as fast as possible with no other options available, it's reasonable that the Watt engine is conceived even two centuries before than on Earth.

NOTE: Chimneys cannot be vertical tubes, or when you turn them on, first thing you cover the ship with soot. Ew. I think they should should be shaped like a car's exhaust tube, horizontal with the holes in the stern and additional filters to collect the soot and emptying it in the sea. (yes, very little ecological, but any civilization at its first invention will do ecological damage before improving their transportations)

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Further considerations for this world

No winds at all implies that any kind of gas pocket will disperse by its own pressure along the surface, or rise in accordance to its weight, but limited by the pressure it finds.

In other words, a volcanic gas column will keep falling vertically on itself (after the main phase of the eruption has subsided) and keep spreading all around the volcano, permanently poisoning a growing area until much more land is lost for a very, very long time.

Any fume caused by the people activities (sewers, chemicals, even the animal wastes in the streets and in the farm) will stay where it is, progressively polluting the area to the point of suffocation.

All hot air will stay where it is, in pillars that would make flying for birds a near impossibility. In order to fly, an animal should be like a gigantic manta with low dense tissues and humongous floppy membranes to catch all the thermals they can in one sweep.

Plants that need impollinators would need land mammals to coat with their pollen. These mammals could look like tiny kangaroos so that they can hop faster in a field and spread the good stuff.

On a good note, viruses and bacteria that need currents to move would stay where they are along with their victims. Once their preys are consumed, they will revert to a sporal state so to be carried by passing animals or unsuspecting people.

Tall plants would also be very different: to transmit DNA and warn each other of biological attacks, they'd be connected to each other in a thick web of creepers, that would also serve to reinforce each other with nourishment. And people could use the creepers as roadways in this new upper-level ecosystem.

Swamps would be pots of pure undiluted poisonous CO2 and other nasty gases.

Deserts in daytime would be even more suffocating without any breeze to disperse the heat. presumably, they'd even be hotter.

Lakes would, well...stink! Not that their water wouldn't be drinkable, but you wouldn't want to stay in a place that stink of dead fishes and dead algae

Of course, you don't want to venture in a canyon without a gas mask, unless you like your deposits of CO2

Also, humanoids would much look like Voldemort! In a currentless words, no species needs sense of wind-carried smells anymore. They could breathe through gills that would also serve as air purifiers.

Every species' tastebuds would be extremely more refined to distinguish toxic from edible. Possibly, the standard tongue would look more like a snake's.

Predators should rely on augmented vision to better hunt. Instead of scent tracks, they's release a substance that leaves a visible mark at night.

I can think of only one reason why this world is not reduced in a hellish nightmare coated from pole to pole by a layer of heavy toxic gases, and that reason is: Bacteria. A form of microlife evolved to eat up all the nasty stuff and spew oxygen 24/7. On land, they'd appear as plants, on water as a thick plancton presence culled by fish so that it doesn't turn the oceans in a soup.

Answer 4

Sailing in any direction other than straight downwind is an exercise in balancing wind resistance against water resistance. Since the wind doesn't apply as great a force on your vessel you need larger sails relative to the size of the submerged hull.

To move solely with the current, you do not have to make any changes to the design of your hull, however you will not be moving in a controlled manner, to have control of a vessel you must have movement relative to the water or otherwise exert a force on your vessel relative to the water.

Currents are not simple laminar flows. There are weaker and stronger areas of flow, there are eddies where the flow effectively runs backwards.

In a steadily flowing straight river, the flow will be strongest in the centre and weaker towards the edges. On a bend in a river the flow will be strongest on the outside of the bend and weaker, even to the point of forming an eddy on the inside. Wide shallow rivers have slower flow than narrow rivers. Obstructions in the river will cause faster flow around (and over) the obstruction and an eddy with backward flow behind it.

What this all means in the long run is that with only minimal paddling or rowing you can have considerable control of a vessel without having to constantly power your movement.

Consider the competitive canoe slalom event. Visually you see powerful paddlers with a large blade in their hands fighting the current to pass through gates in a given order as fast as they can. They're not fighting the current though, they're riding the current, dodging in and out, finding stronger and weaker flows and eddies as they need. The "upstream" gates are not against the flow, but in eddies where the flow is running backwards, then out into the strongest flow to get back up to speed. Winning is not in fighting the water but in best reading the flow and using the water to your advantage.

The same is true of ocean currents, they're not simple things, they shift and swirl around islands and continents. There's a correct and a wrong way to sail round the world as a result of all this. Getting the currents right makes a massive difference to the speed of a crossing.

All your boatmen will be expert readers of current, where to be and where not to be and how to place their boats to travel their routes.

Answer 5

I'm afraid your suggestion of huge sails won't work for a couple of reasons:

1. Engineering. 17th century ships had wooden masts, and you simply wouldn't be able to make a wooden mast large enough for your purposes
2. Physics is also against you here. For this to work, the force exerted by the air has to be similar to that exerted by the water (you don't need to stop your movement downwind - so to speak - just reduce it enough that you can make headway towards your destination). The issue is our old friend $F=MA$ - water is around 800 times denser than air, so (assuming roughly equal speeds through water and air) you'd need several hundred times greater sail area than the area exposed under the water. Unfortunately for you, boats displace a fair amount of water and so tend to have a large underwater area. Check out this image of a galleon:

While the design would of course change, the hull volume is probably going to remain fairly constant.

So, what can we do instead?

Anchoring

This is a good idea, and one that sailors already make use of. It's normal to try and travel 'with the tide' so as to increase your speed over the ground. Where this might fall down is the depth of your oceans. There are practical limits to the limit of anchor chain you can carry, and if your ship finds itself somewhere deeper than it can anchor it might be in trouble.

Rowing

This is a nice, low tech solution and on your world it might work even better than on Earth: one of the big drawbacks of galleys is that they need to be long and light to work well, which means that they can't handle rough seas. Fortunately for you, the absence of winds means that your seas should be exceptionally calm. Humans do get tired rowing for long distances, but people have rowed across the Atlantic, so it's definitely feasible.

Treadmills, Propellors and Paddle wheels

Paddle wheel powered ships (think paddle steamers and pedalos), like the galleys, struggle with rough seas - this is why you tend to see paddle steamers only on rivers or lakes in the real world. Here, we have no such problem.

The treadmill has been around since Roman times - we can simply connect a couple of them up to paddle wheels on the side of our ship and get people (or animals) to walk in them (this has actually been done). This has the benefit of being a reasonably simple mechanism, and allows people to power the ship with their legs instead of their arms. Humans are incredible endurance walkers, so this should be much easier on your crew than rowing.

Somewhat more complex would be to use a treadmill to power a propeller - these are more efficient, but would need more engineering to make work.

Development of steam

The development of steam engines covers about the same period as you're looking for - Jerónimo de Ayanz y Beaumont had a steam engine working in a silver mine in 1611 and Newcomen's Atmospheric Engine dates from 1712.

Given this, and the massively increased benefits of developing steam locomotion for ships, it's not unreasonable to think that these steam engines would have been used in ships much earlier than they were in history - the first steam-powered ship was built in 1704 by Denis Papin. The widespread use of mechanical (albeit human-driven) drives would also speed this development.

Answer 6

As an amateur sailor:

Sailing using the Tide

As Philipp stated, sailing exploits the kinetic energy of air with respect to the water. If the air is fixed with respect to the land while the sea moves with respect to it, there will be relative wind when drifting with the current. If the moving air has non-zero density then it will have momentum and thus exploitable energy.

As an example, the practical sailing technique of 'lee bowing' uses the effect of tidally-induced relative wind to increase ground speed to windward when sailing across a tideway. For an explanation see, http://www.pbo.co.uk/seamanship/nav-nutshell-lee-bowing-defined-41887.

Anchoring in a tideway

Don't go backwards!

As you surmised, anchoring (even in relatively deep water) is practical and useful when becalmed in an adverse tideway. It is much better to stop than to drift backwards.

'Sheering'

When anchored in a tideway it is possible to 'sheer' off to one side by using the rudder to angle the keel to the current. This is a bit like flying a kite, but sideweays. You could then drop another anchor, raise the first and repeat the process.

In practise it may be easiest to have a tender help raise the anchor from which you are sheering, using a 'tripping' line.

Sheering (once) is useful when rafting up to another anchored vessel.

Controlled, continuous sheering?

Now for the clever bit: if we could controllably drag the anchor from which we are sheering then we would not need the step-and-repeat process. What we want is a sort of underwater tether that resists sideways motion (with the current) but allows motion across the current. In its simplest form a mushroom anchor with its shank bent and its disc free to rotate might do the trick when you have a smooth, sandy bottom.

Given that this should work, I would be surprised if this has not been tried, somewhere.

Conclusion

Relative motion implies exploitable energy. Air versus water is sailing. Water versus land is practical but rarely employed.

I have used lee bowing, anchoring to stem the tide, and sheering (through a relatively small angle) while sailing small vessels in the English Channel / la Manche.

Answer 7

Yes, you could turn a sailboat 'upside down' and generate lift from the currents.

Sailing doesn't work because the wind pushes on the sails. It works by creating pressure differentials which suck the boat forwards. You control the direction of the boat with the 4 foils (or more) - the main, jib, keel and rudder.

Most boats only have one adjustable foil below the water: the rudder, but it is entirely possible to design a boat where the keel also rotates.

In your boat, the pressure differential of the sails would come from apparent wind, the force you feel on your face when you drive with the window down. This force would balance against the pressure differential created by the keel and rudder, moving your boat forward.

It's hard to know how fast you could go - in most places in the ocean, it is rare to see a current that passes 2kts, but there are places where the currents go up to 17kts.

Answer 8

As mentionned in some answers, the key lays in the difference in speed between the water and the air.

But in your world you can't, so you need to exploit the difference in speed between water and water. For this you need multiple contacts with the water, and there is two solutions for this :

• One boat with multiple keels, something like a trimaran but probably with more keels and extremely large. Some keels will stay in the center of the current where the flow is the strongest, some will be stay far from the center, or even outside the current. This way, by changing the underwater area of some keels, for example by using fins, you can change the balance of forces and make the boat rotate.

• The other solution will also exploit the difference of speeds between inside and outside the current, not by using one boat, but by using a fleet of small boats connected together.

In both case, the keels or boats at the center of the current will act as the engine, dragging the whole boat or fleet, and the keels or boat in slower water will act as the rudder, by increasing and lowering the underwater area or by changing their directions, thus pulling the boat or fleet at one side or the other of the current.

I hope it is clear enough to give you a lead. By the way, hello Stack, this is my first message here :)

Edit :

An other idea for using huge sails : if your currents are really fast, instead of building sails supported by a heavy mast, you can transform them into kites so that they support themselves in the air once the boat is fast enough. Thus you can have larger area for the same weight, but you need to go fast enough to keep them in the air.

You can also use hot-air balloons that can also support themselves, but it implies the need to generate hot air.

Answer 9

It is a question of technology.

A modern Americas cup sailboat can easily move against the current with no wind. Here's the link to to designers discussing it:

https://www.youtube.com/watch?v=WBG1g8s3BT0#t=8m40

The example he sites is on the Amazon river with a 12 knot current. Specifically he states the boat could move against the current at twice that speed.

However your requirement that the level of tech on your world is 17th century probably rules out such designs, due to the lack of material (carbon fiber) needed to construct the foils, so that the boat can lift itself clear of the water.

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That said, it may be possible to construct a boat with 17th century tech that can sail faster than the true wind speed due to apparent wind:

http://www.boatsafe.com/kids/bramp1099.htm

However in your scenario this would still be relative to the motion of the water, so probably the best you could do would be to move quicker (1.5x) than the current at an actual ground track of about 45 degrees from the direction of flow **.

If the currents are predictable, this amount of control may be sufficient to plan/navigate between islands.

**- I am using a boat angle of about 75 degrees to the flow and a speed through the water of about 130% of the current flow, which is consistent with the second link.