Force required to pull a cruise liner or cargo ship

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    Force Pull Ship
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Discussion Overview

The discussion revolves around calculating the force required to pull a cargo ship using a kite as the sole means of propulsion. Participants explore various factors influencing this force, including speed, wind conditions, and resistance values, while considering the theoretical implications of using a kite for such a large vessel.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants note that the force required will depend on the speed through the water, wind direction, and strength, as well as current conditions.
  • There is a suggestion that using a kite could reduce fuel consumption when combined with engine power, but concerns are raised about the practicality of using a kite alone.
  • One participant proposes calculating the force required for a cargo ship moving at 25 knots while ignoring air resistance, which is considered negligible compared to water resistance.
  • Another participant questions the feasibility of achieving 25 knots solely through wind speed, suggesting that the effective speed may be lower due to wind conditions at different heights.
  • Participants discuss the efficiency of pulling versus pushing the ship, with some arguing that pulling may be more efficient due to reduced friction losses.
  • Mathematical estimates are provided, with one participant calculating the force required based on engine power and speed, arriving at an estimate of approximately 3 million Newtons.
  • Concerns are raised about the assumptions made regarding drag forces and the need for a more precise calculation of the pulling force required to move the ship.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the feasibility and calculations of using a kite for propulsion. There is no consensus on the exact force required or the assumptions about drag forces and wind conditions.

Contextual Notes

Participants acknowledge limitations in their calculations, including uncertainties about resistance values, the impact of wind speed at different heights, and the assumptions made regarding engine power and efficiency.

  • #31
Baluncore said:
The vessel displacement is reduced by the lift.

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... Furthermore, it is not at all said that lifting the bow will reduce the drag since wave resistance can become worse and frictional resistance is hardly affected.
 
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  • #32
Baluncore said:
The vessel is driven horizontally by the drag.

If you use the drag of the kite for propulsion you're doing it wrong ;). The lift is much higher than the drag and the lift vector of the kite should have a significant component in the horizontal direction (and of course as much as possible in the forward direction). So you should optimize the lift over drag ratio and get a high as possible/practical amount of lift.
 
  • #33
So something like this:
kite.png
 
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  • #34
Arjan82 said:
If you use the drag of the kite for propulsion you're doing it wrong ;). The lift is much higher than the drag and the lift vector of the kite should have a significant component in the horizontal direction (and of course as much as possible in the forward direction). So you should optimize the lift over drag ratio and get a high as possible/practical amount of lift.
The advertising information for the kite in question indicates that they do just this. The kite is flown back and forth in a figure eight pattern at high speed (100 kph) so that it is primarily lift that tensions the cable rather than drag.

The claim is that this increases the "effective area" of the kite by a significant factor. Much like three bladed wind turbines effectively harvest wind energy from a region larger than just the area of the blades.
 
  • #35
snorkack said:
The underwater part of the ship must be efficient in producing lift!
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.
 
  • #36
Arjan82 said:
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.
 
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  • #37
Arjan82 said:
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.
 
  • #38
jbriggs444 said:
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).
 
  • #39
Arjan82 said:
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.
 
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  • #40
Baluncore said:
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.
 
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  • #41
snorkack said:
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.
 
  • #42
jbriggs444 said:
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.
 
  • #43
snorkack said:
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.
 
  • #44
Arjan82 said:
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.
 
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  • #46
jbriggs444 said:
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...

jbriggs444 said:
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...

jbriggs444 said:
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...:

sailingtrips-in-holland.jpg
 
  • #47
Baluncore said:
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 :) )
 
  • #48
Arjan82 said:
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.

Arjan82 said:
... these micky mouse ears on the side ...
Those are called leeboards. I sailed one of those back in the 1970s.
 
  • #49
Baluncore said:
No, I am not.

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

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

true

Baluncore said:
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.

Baluncore said:
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...

Baluncore said:
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.

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

Ah, didn't know the English term :)