"Doing the dance" when passing

[Rob-M]

I find the opposite, passing very close the water movement between the boats stops them from touching.

[alan_fincher]

2 hours ago, Rob-M said:

I find the opposite, passing very close the water movement between the boats stops them from touching.

 

Absolutely!

 

To me it seems fairly obvious that the water which you are attempting to "squash" between the boats will attempt to push them apart.

I can't rally visualise how passing close together could ever suck the boat towards each other.

 

 

On 16/08/2023 at 21:16, MtB said:

 

Indeed, and this leads to occasional collisions.

 

Doesn't it, Alan!! 

 

 

 

Only if you are very unlucky about who it is that is coming the other way!

Edited August 18, 2023 by alan_fincher

[blackrose]

4 hours ago, alan_fincher said:

I can't rally visualise how passing close together could ever suck the boat towards each other.

 

That's because it's not something easy to visualise. It is however a known hydrodynamic effect. Look it up.

6 hours ago, Rob-M said:

I find the opposite, passing very close the water movement between the boats stops them from touching.

 

As I said, it depends on a number of variables, but you can also get sucked towards the oncoming boat.

4 hours ago, alan_fincher said:

To me it seems fairly obvious that the water which you are attempting to "squash" between the boats will attempt to push them apart.

 

In that case you are obviously unaware of the low pressure effects which act at the same time. The water isn't only getting squashed between the boats but it's flowing between them creating low pressure.

 

The field of physics is called hydrodynamics. There isn't just one effect happening but a number which occur at the same time. If one force is stronger than the other then that force will dominate.

[howardang]

As Blackrose has said, it is extremely an well know effect and happens between two vessels when underway and passing close to one another. It is caller Interaction. Google “Interaction between ships”. 
 

Howard

Edited August 18, 2023 by howardang

[David Mack]

Bernoulli's principle.

[GUMPY]

It's never happened to me when passing in opposite directions as I push a wall of water Infront 😱. It always happens when I try overtaking another boat.

[blackrose]

5 minutes ago, GUMPY said:

It's never happened to me when passing in opposite directions as I push a wall of water Infront 😱. It always happens when I try overtaking another boat.

 

In that case the water flowing between the boats is all travelling in the same direction so the effect is increased. It still happens between boats passing in opposite directions but there are other effects too. It can also occur between a boat and the bank. They think it's part of the reason the Ever Given container ship ended up stuck across the Suez canal a couple of years ago.

[IanD]

5 minutes ago, howardang said:

As Blackrose has said, it is extremely an well know effect and happens between two vessels when underway and passing close to one another. It is caller Interaction. Google “Interaction between ships”. 
 

Howard

 

That's true, but also some of what is published about this for boats passing *in opposite directions* is not entirely correct -- see here for example...

 

https://owaysonline.com/ship-to-ship-interaction/

 

The pressure changes are caused by changes in water speed/direction, hence the pressure rise at the bow and drop at the stern, and also a drop in level if water is flowing backwards past the hull in a narrow channel, as often seen in a canal. So when overtaking there is a tendency for the two hulls to be pulled together when level with each other as water flows backwards between them, which is the case usually being referred to (and when collisions occur as a result).

 

It's also correct that as the bows pass (opposing directions) they're pushed apart by the positive pressure, and as sterns passes they're pulled together by the negative pressure.

 

However some thought shows that with two similar ships passing in opposite directions at the same speed the water in between the two doesn't move (no pressure rise or fall) because they're trying to push it in opposite directions, and on the outer side of the two hulls water flows backwards past one ship and the opposite way (but also backwards) past the other, with low pressure zones on these outer sides *but not between the two hulls* since there's no water flow there -- to be technical, the system is mirror-image antisymmetric.

 

It's *not* the same as taking the pressure distribution from two isolated hulls and adding them (like the drawing in the above reference), because the water flow is very different with two hulls close together going opposite ways.

 

This stacks up with my experience "doing the dance" -- the bows are pushed apart as they meet, you can then travel parallel and close together while passing with no pulling-together problem,then the sterns tend to pull together as they pass and may collide if you don't counter-steer. And that conversely when overtaking there is a strong tendency for the two hulls to be pulled together while passing.

 

It's all down to fluid dynamics and what actually causes water flow and pressure/level changes around hulls, which applies equally to narrowboats and ships -- but differently in a narrow channel to the open sea... 😉

[howardang]

Interaction is a very common and well known  phenomenon. If you read the MCA Notice below (link below)  you will see more information which you may find useful. The link is the MGN issued by the MCA on this subject, and I'm afraid it is fact, not something that can be disputed.

https://www.gov.uk/government/publications/mgn-199-dangers-of-interaction

 

Howard

 

Edited August 18, 2023 by howardang

[Hudds Lad]

There was a boat dumped just before Bridge 104 at Lower Shuckburgh earlier in the year with a very slack bow line, as you approached heading for Braunston all looked well. As you began to pass under the bridge the bow then came out to meet you, the rope being so slack that it hit you before reaching the limit of the line. So either hydrodynamics is real or I imagined the whole thing  

[IanD]

Just now, howardang said:

It is a very common and well known  phenomenon. If you read the MCA Notice below (link below)  you will see more information which you may find useful. The link is the MGN issued by the MCA on this subject, and I'm afraid it is fact, not something that can be disputed.

https://www.gov.uk/government/publications/mgn-199-dangers-of-interaction

 

Howard

 

If you read that in detail -- and all the other papers on the subject, some far more difficult to understand -- you'll find they agree with what I wrote. Most of them are dealing with two vessels overtaking (same direction) when this is indeed a big problem. But when going opposite ways it isn't, the problems only happen when the bows and sterns pass each other.

[howardang]

21 minutes ago, IanD said:

If you read that in detail -- and all the other papers on the subject, some far more difficult to understand -- you'll find they agree with what I wrote. Most of them are dealing with two vessels overtaking (same direction) when this is indeed a big problem. But when going opposite ways it isn't, the problems only happen when the bows and sterns pass each other.

I have read it in detail  and many other similar warnings. I am more than aware of the interaction between two vessels, and  I have been familiar with the phenomenon professionally for more years than I care to remember, both on large and smaller vessels,  on rivers, canals, both in this country and abroad, and also as a dockmaster controlling shipping in large port, where vigilance is very necessary to avoid accidents.   MGN 199  clearly warns mariners   that this effect can occur both when overtaking and also when passing, especially in narrow waterways or at excessive speed, and I have occasionally seen the result when the warning was disregarded. As far as I am concerned it is not really a subject for argument or interpretation; it is too well know in professional marine circles. 

 

Howard

[GUMPY]

48 minutes ago, blackrose said:

They think it's part of the reason the Ever Given container ship ended up stuck across the Suez canal a couple of years ago.

 

Watched an interesting program on the EG the other night and the conclusion by the authorities about the cause of the accident was what they referred to as  bank effect. Travelling too fast in a shallow narrow channel and the stern gets pulled to one bank and stuffs the bow in the other bank

 

Much the same as taking a 60x11'6 barge too fast down the GU  DAMHIK

 

Edited August 18, 2023 by GUMPY

[IanD]

28 minutes ago, howardang said:

I have read it in detail  and many other similar warnings. I am more than aware of the interaction between two vessels, and  I have been familiar with the phenomenon professionally for more years than I care to remember, both on large and smaller vessels,  on rivers, canals, both in this country and abroad, and also as a dockmaster controlling shipping in large port, where vigilance is very necessary to avoid accidents.   MGN 199  clearly warns mariners   that this effect can occur both when overtaking and also when passing, especially in narrow waterways or at excessive speed, and I have occasionally seen the result when the warning was disregarded. As far as I am concerned it is not really a subject for argument or interpretation; it is too well know in professional marine circles. 

 

Howard

I'm not arguing that it's very significant when overtaking and in general when large vessels are near each other or passing at speed -- and when bows and sterns pass when travelling in opposite directions, narrow channel or not.

 

I'm saying there are very good reasons -- both in theory and in practice -- why two narrowboats on a canal passing in opposite directions do *not* have this particular problem (being pulled together while parallel).

 

I've passed plenty of boats this way (coming the other way, not overtaking), and from my experience (and GUMPY's) the interaction happens when bows and stern pass (as I described) not when they're parallel -- they just slide past each other without being pushed apart or pulled together.

 

What MGN 199 says agrees with this:

 

3. PASSING VESSELS
When vessels are passing there are two
situations: (i) overtaking and (ii) the head-on
encounter.

(i) Overtaking: Interaction is most likely to
prove dangerous when two vessels are
involved in an overtaking manoeuvre.
One possible outcome is that the vessel
being overtaken may take a sheer into the
path of the other. Another possibility is

that when the vessels are abeam of one
another the bow of each vessel may turn
away from the bow of the other causing
the respective sterns to swing towards
each other. This may also be accompanied
by an overall strong attractive force
between the two vessels due to the
reduced pressure between the underwater
portion of the hulls. There are other
possibilities, but the effect of interaction
on each vessel during the overtaking
manoeuvre will depend on a number of
factors including the size of one vessel
relative to the other, the smaller of the two
vessels feeling the greater effect.

(ii) The head-on encounter: In this situation
interaction is less likely to have a dangerous
effect as generally the bows of the two
vessels will tend to repel each other as they
approach. However, this can lead indirectly
to a critical situation. It may increase any
existing swing and also be complicated by
secondary interaction such as bank-
rejection from the edge of a channel.

Edited August 18, 2023 by IanD

[Paul C]

My boat obeys the laws of physics. I can't believe the last page or so is still being 'debated'!!

[howardang]

4 minutes ago, Paul C said:

My boat obeys the laws of physics. I can't believe the last page or so is still being 'debated'!!

It is not being debated by me. For  some reason  I  notice from time to time  that there are members on this forum who seem to think that canal boats behave differently to other marine craft, which of course is patently not the case, but they will argue their case until they are blue in the face. 
 

Howard

[IanD]

19 minutes ago, howardang said:

It is not being debated by me. For  some reason  I  notice from time to time  that there are members on this forum who seem to think that canal boats behave differently to other marine craft, which of course is patently not the case, but they will argue their case until they are blue in the face. 
 

Howard

No, I think they behave exactly like other marine craft, just like MGN 199 says, as well as both theory and experience of narrowboats on canals... 😉

 

If you can find evidence to prove what you seem to be claiming about boats being "sucked together" while passing in opposite directions -- not other different cases like overtaking! -- then I suggest you do so, because so far you haven't, and the laws of physics says they don't.... 🙂

Edited August 18, 2023 by IanD

[howardang]

36 minutes ago, IanD said:

No, I think they behave exactly like other marine craft, just like MGN 199 says, as well as both theory and experience of narrowboats on canals... 😉

 

If you can find evidence to prove what you seem to be claiming about boats being "sucked together" while passing in opposite directions -- not other different cases like overtaking! -- then I suggest you do so, because so far you haven't, and the laws of physics says they don't.... 🙂

As the bows approach each other they are repelled and swing away from each other, but as the boats continue to pass the stern swings inwards because of this swing with two possible issues. One, the bow of one or both boats can strike the bank and two, the sterns can swing towards each other because of the reduced pressure caused by Bernoulli's effect caused by the boats passing each other in close proximity and If not corrected this swing can (and does) cause collisions. Its indirectly referred to in your extract from the MGN above towards the end of (iii) and I think it is spoken about in 4, although I haven’t  got it in front of me at the moment.

 

There are many tutorial videos available on YouTube which also show this.

Howard

 

[IanD]

40 minutes ago, howardang said:

As the bows approach each other they are repelled and swing away from each other, but as the boats continue to pass the stern swings inwards because of this swing with two possible issues. One, the bow of one or both boats can strike the bank and two, the sterns can swing towards each other because of the reduced pressure caused by Bernoulli's effect caused by the boats passing each other in close proximity and If not corrected this swing can (and does) cause collisions. Its indirectly referred to in your extract from the MGN above towards the end of (iii) and I think it is spoken about in 4, although I haven’t  got it in front of me at the moment.

 

There are many tutorial videos available on YouTube which also show this.

Howard

 

 

Not arguing with any of that, both theory and experience say that's what happens at the bow and stern.

 

What both also say is that with two long boats with parallel sides -- like narrowboats -- passing in opposite directions, while they're level with each other there's no water flow either way along the gap between them, and therefore no pressure drop, and therefore no force pulling them together, and therefore no interaction due to this. The opposite is true when overtaking, *then* they get strongly pulled together....<crunch>

 

Which means that ignoring what happens as the bows pass each other (high pressure, pushed apart, oh no here comes the bank!) and the sterns pass each other (low pressure, pulled together, crunch!) you can be as close as you want to the other boat *while you're passing it* with no problem... 😉

 

You can use the rudder to counteract both the bow and stern swings if you (actually, both boats...) anticipate them and react quickly enough -- but try and pass too closely and a collision is the most likely outcome, unless both steerers know *exactly* what they're doing... 😞

Edited August 18, 2023 by IanD

[howardang]

42 minutes ago, IanD said:

 

Not arguing with any of that, both theory and experience say that's what happens at the bow and stern.

 

What both also say is that with two long boats with parallel sides -- like narrowboats -- passing in opposite directions, while they're level with each other there's no water flow either way along the gap between them, and therefore no pressure drop, and therefore no force pulling them together, and therefore no interaction due to this. The opposite is true when overtaking, *then* they get strongly pulled together....<crunch>

 

Which means that ignoring what happens as the bows pass each other (high pressure, pushed apart, oh no here comes the bank!) and the sterns pass each other (low pressure, pulled together, crunch!) you can be as close as you want to the other boat *while you're passing it* with no problem... 😉

 

You can use the rudder to counteract both the bow and stern swings if you (actually, both boats...) anticipate them and react quickly enough -- but try and pass too closely and a collision is the most likely outcome, unless both steerers know *exactly* what they're doing... 😞

I would gently remind you that this part of the topic was about the interaction between two vessels when passing and overtaking,  which as you know is not just about about suction. That does play its part in causing collisions of course, and hitting the bank etc. but so does excessive speed, slow reaction on the tiller etc. , lack of steerer’s

experience and shallow water. 

 

Howard

 

 

 

[IanD]

29 minutes ago, howardang said:

I would gently remind you that this part of the topic was about the interaction between two vessels when passing and overtaking,  which as you know is not just about about suction. That does play its part in causing collisions of course, and hitting the bank etc. but so does excessive speed, slow reaction on the tiller etc. , lack of steerer’s

experience and shallow water. 

 

Howard

 

 

I'm aware of all that --  the original subject ("the dance") was about passing (how closely?) *not* overtaking, which is why I kept objecting to comments about interaction which were not relevant to this case... 😉

 

If *both* steerers are skilled and know the channel and how their boats respond, it's perfectly possible for boats going opposite ways to pass with very little clearance -- a foot or so? -- with no problem or slowing down, and no pulling together *while* they're passing once the bow and stern interaction is dealt with.

 

Which is presumably what happened in the good old days -- but the odds of this working in reality nowadays are small, so even if you know what you're doing it's safer to leave a bigger gap in case the other steerer doesn't... 😉

Edited August 18, 2023 by IanD

[beerbeerbeerbeerbeer]

Passing closely is a common enough event with no disasters or scrapes,

particularly when boats are passing each other by moored boats,

Rugeley is a good example,

or passing in narrow stretches like what you get on the Shroppie.

 

perhaps in theory boats ought to clang together, 

but in practice it don’t seem to happen 🤷‍♀️

[nicknorman]

I find myself in agreement with IanD. Which is worrying!

[MtB]

1 minute ago, nicknorman said:

I find myself in agreement with IanD. Which is worrying!

 

Reminds me of what my dear old departed Dad used to say about quantum physics. "If you think you understand then I can't have explained it properly"

 

I think he nicked that from someone famous.

[IanD]

12 minutes ago, Goliath said:

Passing closely is a common enough event with no disasters or scrapes,

particularly when boats are passing each other by moored boats,

Rugeley is a good example,

or passing in narrow stretches like what you get on the Shroppie.

 

perhaps in theory boats ought to clang together, 

but in practice it don’t seem to happen 🤷‍♀️

The point is that passing boats didn't ought to clang together even in theory -- but that doesn't mean they won't with steerers who don't know what they're doing, or panic... 😉