Catenary Effect on Horizontal Pull, Yaw and Damping

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I believe that catenary has little effect in strong winds but now I am not so sure. I have been paragliding behind a sports boat and I noticed that there was always a noticeable catenary in the thin line. This was a big parachute, powerful boat at speed and about a F4 tops, line about 15mm. Even when the winch kicked in to bring down the chute, the line stayed curved, viewed from the boat. When it was my turn to be winched down, there was still a noticeable snatch at the harness as the windlass started, but the catenary did not change.

Idly musing, I wonder if I have underestimated the impact of catenary by dismissing it. Anyway, it was surprising to me how much gravity influenced catenary on a light line, subjected to big forces.
 
I think you are seeing the effects of drag. The drag on round objects like wire and rope is surprisingly high which is why plane designers were so keen to eliminate the wire bracing that was seen in early aircraft.
 
In deep sea towing, which is not quite the same thing, it's always been "the rule" that you have chain to take the chafe at the fairleads of the tow, secured at a Smit bracket or an AKD stopper, then wire, then a double nylon warp to provide shock absorption, then a lot ( some hundreds of metres) of wire paid out from the tug's winch. So much catenary, in fact, that you shorten up in passing through shallower water.
 
looking at it logically:rolleyes: To have a catenary and be able to tow something or for that matter retain a catenary against an opposing force be it tide wind or both then the force applied to achieve that state must be less than the force required to lift and straighten the weight of the cable.
Neeves will be along in a minute.
 
noelex said:
I think you are seeing the effects of drag. The drag on round objects like wire and rope is surprisingly high which is why plane designers were so keen to eliminate the wire bracing that was seen in early aircraft.
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I bought that for a few seconds then realised that drag would have the opposite effect - in other words an upside down catenary with nearer horizontal at the top and nearer vertical at the bottom.
 
Fr J Hackett said:
looking at it logically:rolleyes: To have a catenary and be able to tow something or for that matter retain a catenary against an opposing force be it tide wind or both then the force applied to achieve that state must be less than the force required to lift and straighten the weight of the cable.
Neeves will be along in a minute.
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If you are towing something at sea, I think you will find that any tide will affect both tow and tug equally. :rolleyes:
 
NormanS said:
If you are towing something at sea, I think you will find that any tide will affect both tow and tug equally. :rolleyes:
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The thread initially started of about anchoring Minn introduced the concept of towing I used the force v weight of catenary to explain both situations, sorry you couldn't understand that.
 
BlowingOldBoots said:
I believe that catenary has little effect in strong winds but now I am not so sure. I have been paragliding behind a sports boat and I noticed that there was always a noticeable catenary in the thin line. This was a big parachute, powerful boat at speed and about a F4 tops, line about 15mm. Even when the winch kicked in to bring down the chute, the line stayed curved, viewed from the boat.
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I have spent many happy hours driving glider winches typically a 200hp+ engine, half a mile of 5mm galvanised wire rope and then somewhere between 250kg and a tonne of aircraft. The catenary sag is huge, even when you're launching a heavy glider on a windy day, but the wire still transmits the force and there is very little resilience in the system. Every swaged ferrule splice going over the winch's guide rollers gives a noticeable bump at the glider.

vyv_cox said:
When I have dived on my anchor and chain in a variety of stronger winds the chain has never been completely straight. My view is that the amount of curve was insufficient to provide any noticeable damping.
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I agree. There is always a catenary; it just doesn't provide much damping, as you say.
 
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In the case of deepsea towing, the catenary and the snubbers are both there to provide damping; the catenary is the primary damper and the elasticity of the nylon only comes into play once the "spare catenary" has been used up or if the tow is shortened up for shoal water.

I think we need to distinguish between the catenary that is always there and what I want to call "spare catenary" - such as a length of chain lying on the bottom, or a weight sitting on the bottom.
 
RupertW said:
I bought that for a few seconds then realised that drag would have the opposite effect - in other words an upside down catenary with nearer horizontal at the top and nearer vertical at the bottom.
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Drag was also the very first thing thought of when reading the post. Not certain how you get an upside down catenary with something aloft being pulled forwards by something on the ground (or in the sea). I must be missing something, might need a more detailed explanation of the forces generating an upside down catenary with a towed object on a long light line in F3-4.
 
Fr J Hackett said:
The thread initially started of about anchoring Minn introduced the concept of towing I used the force v weight of catenary to explain both situations, sorry you couldn't understand that.
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Oh dear, I have no wish to argue with you, but,,,,,,in what respect did the thread start about anchoring? Sorry you couldn't understand that. :rolleyes:
 
NormanS said:
Oh dear, I have no wish to argue with you, but,,,,,,in what respect did the thread start about anchoring? Sorry you couldn't understand that. :rolleyes:
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Yes it started with Boots aerobatics but then passed onto the effects of the catenary on a boat at post #4 and on a sailing forum that is the logical point of interest. However the forces and their resolution are essentially the same in all cases where a catenary is involved. I think:rolleyes: Now do you have any contrary thoughts on the matter.:p
 
The reason for my wittering on about deepsea towing is that the catenary is most definitely there to act as a shock absorber. It's not particularly unusual to find the tow taking up a considerable sheer, and somewhere I have a photo of a tanker broad on the port beam of a big tug, which puts a lot of drag on the wire of course.

There's a whole article in Wikipedia:

https://en.wikipedia.org/wiki/Catenary
 
Mistroma said:
Drag was also the very first thing thought of when reading the post. Not certain how you get an upside down catenary with something aloft being pulled forwards by something on the ground (or in the sea). I must be missing something, might need a more detailed explanation of the forces generating an upside down catenary with a towed object on a long light line in F3-4.
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It was me that was upside down - the upside down catenary would only happen if it was the airborne object towing the one at sea level, and even then only if the line was light but with a lot of air resistance
 
Minn said:
The reason for my wittering on about deepsea towing is that the catenary is most definitely there to act as a shock absorber. It's not particularly unusual to find the tow taking up a considerable sheer, and somewhere I have a photo of a tanker broad on the port beam of a big tug, which puts a lot of drag on the wire of course.

There's a whole article in Wikipedia:

https://en.wikipedia.org/wiki/Catenary
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But as you said the catenary exists whilst forward motion of the towed vessel is maintained therefore the force exerted to move or tow the vessel must be less than that required to straighten the catenary. Now the interesting question is that force equal to the weight of the towing equipment? I am beginning to wonder.
 
RupertW said:
It was me that was upside down - the upside down catenary would only happen if it was the airborne object towing the one at sea level, and even then only if the line was light but with a lot of air resistance
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Maybe in an aero tow situation where the towed plane is at a lower altitude. It does introduce thoughts on long anchor rodes and water current effect.
 
Fr J Hackett said:
Yes it started with Boots aerobatics but then passed onto the effects of the catenary on a boat at post #4 and on a sailing forum that is the logical point of interest. However the forces and their resolution are essentially the same in all cases where a catenary is involved. I think:rolleyes: Now do you have any contrary thoughts on the matter.:p
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Logic on a sailing forum! Ha.

Contrary thoughts, yes, if only to point out the absurdity of your suggestion about the effect on catenary of towing against the force of the tide.
 
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