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Mad Harold

Turning Around

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Can someone settle an argument for me please?

I know that an aircraft manoeuvres in flight around it's centre of gravity regardless of where it's centre of lift is.

I am guessing that a boat does the same,but some people I know reckon that a boat manoeuvres around it's centre of buoyancy not it's centre of gravity.

Does anyone know for certain weather a boat moves around it's centre of gravity,or it's centre of buoyancy?

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Aren't they both the same thing? All I need to know is that our boat turns from the middle, any more information would be overload... 🧐

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11 minutes ago, Mad Harold said:

Can someone settle an argument for me please?

I know that an aircraft manoeuvres in flight around it's centre of gravity regardless of where it's centre of lift is.

I am guessing that a boat does the same,but some people I know reckon that a boat manoeuvres around it's centre of buoyancy not it's centre of gravity.

Does anyone know for certain weather a boat moves around it's centre of gravity,or it's centre of buoyancy?

There are two factors in play, one being angular momentum and for that, the boat will turn about its centre of mass. But also, with water being quite viscous (compared to air) a boat will turn according to the resistance to the lateral movement. I’m not sure that centre of buoyancy is the relevant term for this, for example a keel won’t have much effect on centre of buoyancy but it’s position will affect how it turns.

 

Anyway the answer is that it is a combination of centre of mass effects, and centre of “resistance to sideways movement” effects. With a narrowboat, I expect the latter is the predominant effect.

5 minutes ago, Jennifer McM said:

Aren't they both the same thing? All I need to know is that our boat turns from the middle, any more information would be overload... 🧐

Most narrowboats, being heavier and deeper at the back, tend to turn about a point forward of the middle. How much depends on how they are ballasted.

  • Greenie 1

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17 minutes ago, Jennifer McM said:

Aren't they both the same thing? All I need to know is that our boat turns from the middle, any more information would be overload... 🧐

Yes,if your boat sits absolutely level in the water.  However,lots of boats sit a little "tail down" which means the centre of gravity is aft of the centre of buoyancy.

You're right in that it is a matter of no importance,I am looking for an answer because of a  friendly discussion.

I think nicknorman has answered the question quite well.

Edited by Mad Harold
  • Greenie 1

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If you read "Boater's Guide to Boating" by "Christopher Norton Deuchar" you will learn that the turning point of a narrow boat moves and it explains why it moves. Well worth a read for even the most experienced skipper

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12 minutes ago, mrsmelly said:

I dont think either are relevant. It seems to me the more gongoozlers that are watching the more the boat does what the hell it likes!!

Especially when trying to reverse with a bit of wind blowing!

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29 minutes ago, Tonka said:

If you read "Boater's Guide to Boating" by "Christopher Norton Deuchar" you will learn that the turning point of a narrow boat moves and it explains why it moves. Well worth a read for even the most experienced skipper

I would drink to that, a chap with years of boating experience unlike some of the Youtube experts who bought their first boat last summer.

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22 minutes ago, Mad Harold said:

Yes,if your boat sits absolutely level in the water.  However,lots of boats sit a little "tail down" which means the centre of gravity is aft of the centre of buoyancy.

You're right in that it is a matter of no importance,I am looking for an answer because of a  friendly discussion.

I think nicknorman has answered the question quite well.

That doesn't sound right to me, putting aside that I have no experience of naval architecture. Buoyancy is a force acting vertically upwards and gravity is a force acting vertically downward. Furthermore, the buoyant force is a reaction to gravity acting on the mass of the boat so the centre of buoyancy surely moves with the centre of gravity in plan view? If it didn't you would produce a couple and have some sort of aquatic fairground ride. The pitching and rolling of a boat is the process of them finding that balance as conditions change and momentarily throw them out of alignment.

 

Centre of buoyancy is a product of the shape of the submerged area of the hull; centre of gravity is a product of the distribution of mass of the whole boat. Therefore they are not necessarily at the same height and that's a key issue in stability of vessels as you don't want the centre of gravity above the centre of buoyancy otherwise once a boat starts to roll it might not come back.

 

i did notice Nick didn't answer the question directly but appears to have voted for 'none of the above'.

 

I think it's far more important to recognise where the steering force is applied rather than the precise position of the centre of rotation.

 

JP

 

 

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I go with Nick from my dingy sailing days when they turned just about around the centreboard  and with a pivoting board the point of turn changed as you lifted the board. I was always told it was the centre of resistance. and the sails were adjusted to get the centre effort just behind the centre of resistance and the boat would try to come round to the wind. Weather helm.

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45 minutes ago, mrsmelly said:

I dont think either are relevant. It seems to me the more gongoozlers that are watching the more the boat does what the hell it likes!!

Yep, and the perfect manoeuvre will only happen if there's no one watching 🙂

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The steering mechanism of a boat uses similar principles as an aeroplane which are very different from a road vehicle. It relies on a destabilisation of forces acting on the body by means of inducing roll and yaw.

 

The rudder induces yaw around the centre of gravity and also roll in the opposite direction to which the rudder turns. These in combination result in hydrodynamic forces acting on the hull that turn the boat. The boat pivots around the centre of gravity - generally aft of the midpoint - but it doesn't turn around a specific point. There has to be an element of forward momentum to produce a turning force on the bows resultant to the yaw.

 

Whilst a car is generally going in the direction it is pointing and turns in direct proportion to the steering input a boat or aeroplane does not. They often don't go where they are pointing even when going in a straight line, particularly aeroplanes.

 

JP

Edited by Captain Pegg

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A boats pivot point is aproximately just forward of amidships when at rest. When the boat starts moving forward the pivot point moves progressively but only slightly towards the bow as the speed increases. However, when moving astern, the pivot point moves progressively towards the stern as the speed increases.

 

For a more detailed explanation you may find this paper from Warsash Maritime Academy gives much more information.

 

https://www.warsashacademy.co.uk/news-events/resources/use-of-pivot-point-in-ship-handling.pdf

 

Howard

 

Edited by howardang

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33 minutes ago, howardang said:

A boats pivot point is aproximately just forward of amidships when at rest.

 

Surely that depends upon the boat? It may be true in general for ocean going ships but does it hold for narrowboats? I think Mad Harold was correct in his assertion that a boat will rotate about the centre of gravity in terms of yaw (rotation in the horizontal plane) and that won't change with forward and back motion (surge). The term pivot point seems to be have a different definition in the attached paper to that you use above.

 

Anyway aren't we both saying that it's all a bit academic because the boat doesn't prescribe a circle around either of them when turning?

 

JP

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It would rotate around the centre of gravity/buoyancy if it was just freely rotating, however since the thing that makes it rotate is the rudder pushing the stern sideways this moves the centre of rotation forwards -- the amount depends on speed, for a typical narrowboat I'd say it's usually around a third of the way from the bow to the stern. Especially when entering a lock from an angle, this estimate works for me...

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21 minutes ago, Captain Pegg said:

Surely that depends upon the boat? It may be true in general for ocean going ships but does it hold for narrowboats? I think Mad Harold was correct in his assertion that a boat will rotate about the centre of gravity in terms of yaw (rotation in the horizontal plane) and that won't change with forward and back motion (surge). The term pivot point seems to be have a different definition in the attached paper to that you use above.

 

Anyway aren't we both saying that it's all a bit academic because the boat doesn't prescribe a circle around either of them when turning?

 

JP

The principle applies to both boats and ships under power, big and small. Each vessel will vary depending on its particular characteristics, but there is nothing different regarding the pivot point between a narrow boat and any other powered craft.it's just a matter of degree.

 

It might seem a bit academic to some but that sometimes seems to be a theme on this forum. However, the fact that the pivot point moves may help to explain to some people why their own boat steers the way it does, if they take the trouble to learn something about it, and may also go some way to partly explain why a vessel - ship or boat - handles differently when going astern - a subject which is sometimes spoken about as the devils work!j:giggles:

 

Howard

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With the motive force of the prop being only at one end, the pivot point is also hugely dependant on the amount of power applied and the reaction from the rudder.

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Knowing where the pivot point didn't really help yesterday.

 

After rising in the lock, the top gate opens, and the boat gently leaves. Towpath is to the right,  and on the left there's a permanent mooring for a very nice vintage boat which is rather close to the lock's exit (about 30 ft away). Also on the left to the lock, and behind the vintage boat, is a weir where the water is obviously running down, having the potential to push the boat to the left.  Just to make the situation interesting, there's a strong wind coming from the right, pushing the boat towards the moored one. 

 

It's impossible to start to steer the boat until our boat is out of the lock, and is halfway along the vintage boat. In this hairy situation, I wasn't really thinking of the pivot point 🙏 😂

 

On top of that, a boat appeared in front on the way to enter the lock, my OH hadn't seen it, and didn't hear my screeching to leave the gate open, then of course, with the wind it wasn't easy to get out of the way of the oncoming boat! Boating can be very interesting.... 

 

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So many variables. Our boat sort of turns around about 1/3  of the way back going forward. In reverse it turns further back but only in one direction, in the other it kind of goes sideways. Anyway whatever you do it has considerable side slip depending on its speed and the depth of water. It turns best at slow speed with few revs to reduce side slip (not with a load of throttle and water flying everywhere) and just a bit more throttle when nearly all the way round. Sometimes the b****** doesn't turn at all!.

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7 minutes ago, Jennifer McM said:

Knowing where the pivot point didn't really help yesterday.

 

After rising in the lock, the top gate opens, and the boat gently leaves. Towpath is to the right,  and on the left there's a permanent mooring for a very nice vintage boat which is rather close to the lock's exit (about 30 ft away). Also on the left to the lock, and behind the vintage boat, is a weir where the water is obviously running down, having the potential to push the boat to the left.  Just to make the situation interesting, there's a strong wind coming from the right, pushing the boat towards the moored one. 

 

It's impossible to start to steer the boat until our boat is out of the lock, and is halfway along the vintage boat. In this hairy situation, I wasn't really thinking of the pivot point 🙏 😂

 

On top of that, a boat appeared in front on the way to enter the lock, my OH hadn't seen it, and didn't hear my screeching to leave the gate open, then of course, with the wind it wasn't easy to get out of the way of the oncoming boat! Boating can be very interesting.... 

 

I think the clue to the problem with that event is in “the boat gently leaves”. So many people come out of locks on tickover doing 1 mph or less, then wonder why their boat gets affected excessively by modest wind and slight currents. If in doubt, come out with plenty of power on and then you are out of the lock and can steer properly, before the wind and current have time to do anything much.

 

It is the same when entering a lock. There seems to be a choice between creeping in sufficiently slowly that when you miss the aperture by a foot, the inevitable collision (due to lack of steering authority) isn’t too severe. You even see people hitting reverse when the 1mph they are doing feels too fast for them. Inevitably, they then bash the lock entrance.

 

Or coming in with plenty of power on and hence plenty of steering authority, and just slide straight in. We always do the latter but it is amazing how many people think that doing the former and hitting the lock wall is normal.

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10 minutes ago, nicknorman said:

It is the same when entering a lock. There seems to be a choice between creeping in sufficiently slowly that when you miss the aperture by a foot, the inevitable collision (due to lack of steering authority) isn’t too severe. You even see people hitting reverse when the 1mph they are doing feels too fast for them. Inevitably, they then bash the lock entrance.

 

 

We call those "brick counters" 🙂 

 

haggis

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14 minutes ago, nicknorman said:

I think the clue to the problem with that event is in “the boat gently leaves”. So many people come out of locks on tickover doing 1 mph or less, then wonder why their boat gets affected excessively by modest wind and slight currents. If in doubt, come out with plenty of power on and then you are out of the lock and can steer properly, before the wind and current have time to do anything much.

2

Yes, you're most probably right. Thinking about it, it's a bit like learning to ride a bike, the trick is not to look down at the road because for sure you'll hit the road. Coming out of the lock with confidence (more speed) and looking ahead, instead of 'concentrating' how near the other boat is getting - probably would have been better. Except if my 'calculation' was wrong, it would have caused a bigger impact.🤔

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9 minutes ago, Jennifer McM said:

Yes, you're most probably right. Thinking about it, it's a bit like learning to ride a bike, the trick is not to look down at the road because for sure you'll hit the road. Coming out of the lock with confidence (more speed) and looking ahead, instead of 'concentrating' how near the other boat is getting - probably would have been better. Except if my 'calculation' was wrong, it would have caused a bigger impact.🤔

Well that is where experience comes in. Knowing whether adding plenty of power is going to fix the problem or make it worse!

  • Greenie 1

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Just now, nicknorman said:

Well that is where experience comes in. Knowing whether adding plenty of power is going to fix the problem or make it worse!

And the only way to gain said experience is keep using the boat in slightly different ways to learn what you need to do.

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