Force required to pull a cruise liner or cargo ship

[jbriggs444]

The ship in my post has a displacement of 245,000,000 kg. I don't think the lift of a kite is going to have any significant influence on that...

Kite thrust is some significant fraction of the approximately 3.5 million N of engine thrust versus about 2.5 billion N of gravitational down-force. Yes, we are down about three orders of magnitude from the kite's upward force on the ship being significantly helpful.

 

[jbriggs444]

So something like this:
View attachment 318232

As in all pictures I've seen, the ship has a tailwind, the kite is flying dead ahead. There is no tacking going on. The hull is not being called upon to act as a water foil. There is no yaw torque to interfere with steering.

 

[Arjan82]

As in all pictures I've seen, the ship has a tailwind, the kite is flying dead ahead. There is no tacking going on. The hull is not being called upon to act as a water foil. Nor is there any yaw torque to interfere with steering.

Actually, the kite in the picture does not have tailwind but the wind is somewhat from the side. This means the angle of attack of the wind on the kite is from the side such that it can generate a forward lift. Otherwise it would have to rely solely on its drag resistance, which generates a much lower force (and you can never sail faster than the wind for it to generate a forward force)

That however doesn't dismiss your other conclusions. There is definitely no tacking going on, much too difficult for such a ship (and prohibitively increases time and costs due to the longer path taken).

 

[jbriggs444]

Actually, the kite in the picture does not have tailwind but the wind is somewhat from the side.

Based on the waves, the absolute wind appears to be about twenty, maybe thirty degrees port of directly astern. Call it a quartering tailwind if you like.

If the absolute wind was from directly to port, a kite flying directly ahead of the ship would have zero lift and would be falling into the sea.

But yes, certainly a kite can fly directly ahead in a relative wind that is coming from a direction somewhat aft of directly abeam.

 

[snorkack]

If you generate lift from the hull, you generate bigger waves.
We are discussing heavily loaded displacement hulls here, not surface skimming hydroplanes or hydrofoils.

For the ship underwater part, remember that the lift is horizontal! It is the force that prevents the ship from making leeway - travelling sideways through the water.

 

[jbriggs444]

For the ship underwater part, remember that the lift is horizontal! It is the force that prevents the ship from making leeway - travelling sideways through the water.

Leeway... An etymology lesson learned. Obvious in retrospect.

 

[Arjan82]

Call it a quartering tailwind if you like.

Ok, that's what I meant by 'somewhat from the side'... I didn't mean exactly 90 degrees. Sorry for being so wooly :).

But this picture is also fake (intended for commerce), I don't think the kite is effective for tailwind.

 

[Baluncore]

For the ship underwater part, remember that the lift is horizontal! It is the force that prevents the ship from making leeway - travelling sideways through the water.

You are kidding yourself. The length-way section is symmetrical, designed to minimise drag and wave generation, while the sideways section is very close to rectangular, all drag.

 

[jbriggs444]

I don't think the kite is effective for tailwind.

If the relative wind is coming from abeam or anywhere forward of that then one must change strategy. Instead of flying the kite directly ahead, one must fly it somewhat leeward of that.

If the kite has a lift to drag ratio of 10 to 1 then one should be able to fly it anywhere in a 170 degree arc. 85 degrees right or left. So with a relative wind 10 degrees away from directly ahead, one should be able to fly the kite at five degrees forward of directly abeam.

If the ship's hull had a lift to drag ratio of 10 to 1 (unlikely) then one could then start getting forward propulsion from a kite flying as little as five degrees forward of directly abeam.

That is called "sailing close hauled" (you haul in on the sheets so that the sails are about as close to parallel to the ship as you can arrange). However, I doubt that a cargo ship could tack as close-hauled as 1 point from the wind. We generally did 4 points (45 degrees) when I was learning to sail.

 

[bob012345]

I wonder how this compares to a Magnus effect powered Flettner ship?

https://en.wikipedia.org/wiki/Rotor_ship#:~:text=A Magnus rotor used to,sail is the Flettner rotor.

 

[Arjan82]

If the relative wind is coming from abeam or anywhere forward of that then one must change strategy. Instead of flying the kite directly ahead, one must fly it somewhat leeward of that.

In normal sailing you have a change in strategy between tailwind and about quarterly tailwind. Because if the wind is coming (almost) directly from aft, a sail cannot get an angle of attack that is low enough to prevent full separation of the flow. So instead it must work like a 'bag' which is just 'catching' and slowing down the wind as much as possible, but it has nothing to do anymore with a wing. This is what spinnakers are especially good at. But between quarterly tailwind and close hauled I don't see a clear change in strategy, only a kite that is pulling gradually more sideways.

But the kite is really designed as a wing and thus should be used as a wing. I do now remember that they are proposing to do 'figure eights' which would still use the kite as a wing at tailwinds. But I would wonder how that would work in a practical sense with a kite like that, the thing is huge! Try to control that properly and safely...

If the kite has a lift to drag ratio of 10 to 1 then one should be able to fly it anywhere in a 170 degree arc. 85 degrees right or left. So with a relative wind 10 degrees away from directly ahead, one should be able to fly the kite at five degrees forward of directly abeam.

If the ship's hull had a lift to drag ratio of 10 to 1 (unlikely) then one could then start getting forward propulsion from a kite flying as little as five degrees forward of directly abeam.

Unlikely indeed :) [EDIT: sorry I thought you meant the ship with some leeway angle, not the kite... for a kite 1/10 seems much more likely]. I don't think such a configuration can even come close to 170 degrees... [EDIT: ok, I need to eat some dinner before I answer stuff... :( Here I meant the total angle around the into-the-wind direction (i.e. 85deg to either side) for which 170deg is already pretty bad :)] I would expect not any further than abeam winds. Also, in the picture I've shown the kite is attached near to the bow. If you put a significant side force at that point on the ship you introduce a significant steering force (i.e. moment, aside from the side force I mean) which you need to compensate with the rudder or your angle in the water (or both..). This introduces a serous amount of extra drag...

That is called "sailing close hauled" (you haul in on the sheets so that the sails are about as close to parallel to the ship as you can arrange). However, I doubt that a cargo ship could tack as close-hauled as 1 point from the wind. We generally did 4 points (45 degrees) when I was learning to sail.

45 degrees is actually really close to the wind and is only achievable for well designed yachts. With the sailing ships that I'm used to sail on it is more likely to be 50 to 60 degrees... And those are still somewhat meant to be able to do this, unlike the above cargo ship...:

 

[Arjan82]

You are kidding yourself. The length-way section is symmetrical, designed to minimise drag and wave generation, while the sideways section is very close to rectangular, all drag.

You are misunderstanding @snorkack. You are thinking vertical lift (more lift = bigger waves, which only applies to hydroplanes not these displacement vessels) But @snorkack is thinking horizontal lift, the lift generated by your hull in horizontal and sideways direction that prevents a large velocity component in sideway direction. The idea here is indeed that the 'sideways' section as you call it has a much higher drag than in forward direction, this is exactly what prevents sideway velocity and this is why sailing yachts have keels (or daggerboards, or these micky mouse ears on the side of the ship on the picture of my previous post :) )

 

[Baluncore]

You are misunderstanding snorkack.

No, I am not.
With a symmetrical hull, to develop a sideways lift, requires a side-slip to generate a non-zero angle of attack. That involves a non-zero angle on the rudder, turning the boat slightly into the wind to compensate for drift by changing the heading. What a waste of energy that would be, when a change of heading could compensate for the drift.
The next thing you will be telling me is that an aeroplane, flying in a cross wind, gains lift from its fuselage by flying slightly sideways, to compensate for the cross wind.

... these micky mouse ears on the side ...

Those are called leeboards. I sailed one of those back in the 1970s.

 

[Arjan82]

No, I am not.

Ok... then I really don't know what you meant...

With a symmetrical hull, to develop a sideways lift, requires a side-slip to generate a non-zero angle of attack.

true

That involves a non-zero angle on the rudder,

Not necessarily true, at least not continuously (of course you need corrections for a varying force). It all depends on where the forces apply and if they generate a moment and in which direction that moment is. Some badly trimmed sailing ships need a constant rudder angle turning the ship away from the wind (I'm really hampered by the fact that I don't know the English sailing terms... only the Dutch ones...) This then adds to the already leeward direction due to drift due to the side force.

turning the boat slightly into the wind to compensate for drift by changing the heading. What a waste of energy that would be, when a change of heading could compensate for the drift.

I don't understand this part. "turning the boat slightly into the wind to compensate the drift" to me is equal to "when a change of heading could compensate the drift", yet one seems to be good and the other bad...

The next thing you will be telling me is that an aeroplane, flying in a cross wind, gains lift from its fuselage by flying slightly sideways, to compensate for the cross wind.

No... That is some ill founded logic I hope I would never say... A ship works on the interface of wind and water, unlike an airplane, that makes a big difference.

Those are called leeboards. I sailed one of those back in the 1970s.

Ah, didn't know the English term :)