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Canal boat on a tidal estuary.


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Hi, we are totally new to canal boats and are planning to buy our first very soon. We have had two smaller sea going boats so are not complete water novices. We plan to travel the inland waterways for 2 or 3 years and then have it lifted and transported back to Cornwall where we are from.....my question is.....due to the fact that a canal boat has a flat bottom, would the suction effect of it sitting on a tidal estuary mudflat at low tide cause it to "stick" to the mudflat on a rising tide, meaning there would be a potential for it to get swamped by the rising tide. Strange question I know but physics was never my strong point.

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Yes is a quick answer.

It's by no means certain it would happen but it is possible.

Narrow boats that used to moor on the tidal bit behind Three Mills used to use ropes connected from the offside of the boat under the boat to the floating pontoon to "roll" the boat out of the mud if it did stick.

 

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4 minutes ago, Loddon said:

Yes is a quick answer.

It's by no means certain it would happen but it is possible.

Narrow boats that used to moor on the tidal bit behind Three Mills used to use ropes connected from the offside of the boat under the boat to the floating pontoon to "roll" the boat out of the mud if it did stick.

 

Thank you for your fast answer. I did suspect that it could happen, it was a bar of soap that stuck to the flat part of the the sink the other day that made me think this of all things. If it did happen where I was planning to eventually moor it on Penryn river, then the range of the tidal rise there would certainly be high enough to swamped a canal boat. Better to realise this could happen now in theory than to put it to the test!! ..... thanks again.

10 minutes ago, robtheplod said:

What an interesting question!  I've not idea, but i suppose its also possible for it to release quickly also?

 

It is looking that it is possible that it could happen, bit late though to get it released if it did get swamped, best not take the risk I think is the best answer haha.

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1 hour ago, Shaky Start said:

Thank you for your fast answer. I did suspect that it could happen, it was a bar of soap that stuck to the flat part of the the sink the other day that made me think this of all things. If it did happen where I was planning to eventually moor it on Penryn river, then the range of the tidal rise there would certainly be high enough to swamped a canal boat. Better to realise this could happen now in theory than to put it to the test!! ..... thanks again.

 

It is looking that it is possible that it could happen, bit late though to get it released if it did get swamped, best not take the risk I think is the best answer haha.

 

It would very much depend on the bottom - if it was sand /gravel / shell you may be OK, but mud / ooze would be a problem.

 

You would need to ensure that 2 people were on board at each tide.

 

Run a rope under the bow and with a person on each side, as the tide comes in you 'saw' the rope side to side pulling slightly towards the stern as you do so.

This will generate a small gap allow the water to fill it and will break the vacuum.

 

Best advice - do not put a flat bottomed boat on a drying mooring, but you could always have some short bilge-keels weded onto the base plate to keep it out of the mud.

 

We have our boat at Plymouth and we have seen some fun and games on the drying moorings.

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I would also be wary of mud. People beach narrowboats all the time crossing the wash but that's sand rather than mud. I've never seen it done on a narrowboat but lots of people use the mudflats outside Portishead marina and that is most definitely mud. I'm sure it must have been done by narrowboats there too?

 

The only time I've done it was on the tidal Thames and that was gravel.

 

 

DSC00344.JPG

Edited by blackrose
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48 minutes ago, Alan de Enfield said:

 

It would very much depend on the bottom - if it was sand /gravel / shell you may be OK, but mud / ooze would be a problem.

 

You would need to ensure that 2 people were on board at each tide.

 

Run a rope under the bow and with a person on each side, as the tide comes in you 'saw' the rope side to side pulling slightly towards the stern as you do so.

This will generate a small gap allow the water to fill it and will break the vacuum.

 

Best advice - do not put a flat bottomed boat on a drying mooring, but you could always have some short bilge-keels weded onto the base plate to keep it out of the mud.

 

We have our boat at Plymouth and we have seen some fun and games on the drying moorings.

 

I am not quite sure if I am replying to people in the correct way so please forgive me if I am making a mess of this thread. Thank you Alan, your reply is insightful. It is definitely mud on the Penryn river at low tide, the thick oozy stuff, yuk. Your idea about having bilge keels added is definitely a thought but may be too costly. The reason we were thinking about doing this is because over the 3 years on the inland waterways we intend to put our own stamp as it were on the interior of the boat. Also a 40 ft canal boat is a lot more spacious than say a 40 ft motor cruiser or yacht to live on. Food for thought all of this..... I know Plymouth sound and the Tamar, it used to be a regular place we visited on our boat. I can well imagine the fun and games you have there lol.

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18 minutes ago, blackrose said:

I would also be wary of mud. People beach narrowboats all the time crossing the wash but that's sand rather than mud. I've never seen it done on a narrowboat but lots of people use the mudflats outside Portishead marina and that is most definitely mud. I'm sure it must have been done by narrowboats there too?

 

The only time I've done it was on the tidal Thames and that was gravel.

 

 

DSC00344.JPG

 

Hi Blackrose, great photo by the way. Yes, if it was shingle/gravel as it is in your photo then it wouldn't be a problem but the whole of Penryn river is thick oozy mud at low tide. I am seriously thinking now that I would need to have some sort of addition to the bottom adding, whether it is bilge keels or something similar to stop the flat bottom settling on the mud....question is, how deep is the mud , I have just had a vision of 10ft high bilge keels on it lol.

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Does anyone actually know of a narrowboat that has come down on a tidal mudbank and hasn't risen with the incoming tide?

 

It might be possible to do a calculation if one knows the buoyancy of the boat and has some idea of the tensile strength of the vacuum formed between steel plate and mud/area2. I guess it's the properties of the mud that are the unknown factor.   

Edited by blackrose
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14 minutes ago, Loddon said:

How are you going to get to/from the boat at low tide if it's surrounded by mud ?

 

Good question but this won't be a problem because where we have in mind is a riverbank mooring so we would just use a gangplank o0n and off the boat.

Just now, blackrose said:

Does anyone actually know of a narrowboat that has come down on a tidal mudbank and hasn't risen with the incoming tide?

 

It might be possible to do a calculation if one knows the buoyancy of the boat and has some idea of the tensile strength of the vacuum formed between steel plate and mud. I guess it's the properties of the mud that are the unknown factor.   

 

This is a very good question. I am using my bar of soap stuck to the sink equation on this lol but yes, a little more science may be needed I feel.......that bar of soap though don't have stick to the sink haha.

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6 minutes ago, blackrose said:

Does anyone actually know of a narrowboat that has come down on a tidal mudbank and hasn't risen with the incoming tide?

 

It might be possible to do a calculation if one knows the buoyancy of the boat and has some idea of the tensile strength of the vacuum formed between steel plate and mud/area2. I guess it's the properties of the mud that are the unknown factor.   

The late Nigel Moore recounted a tale where a narrow boat was aground in mud and the tide came up almost over the sides then the boat suddenly lurched upwards. 

 

 

One approach I have seen used down near Brentford on the Thames was to put a load of old tyres around where the boat lands when the tide goes out. That would in theory break up the mud effect.

 

 

Whether this is allowed I don't know and I think the tyres would all need to be chained together and anchored so it could end up being quite a complicated process. 

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22 minutes ago, Alan de Enfield said:

 

You have a 'tender behind' and arrive / depart whilst there is still water available,

Of course why didn't I think of that.🤭

Doesn't help much if you want to get on/off and the tide is out.

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12 minutes ago, magnetman said:

The late Nigel Moore recounted a tale where a narrow boat was aground in mud and the tide came up almost over the sides then the boat suddenly lurched upwards. 

 

 

One approach I have seen used down near Brentford on the Thames was to put a load of old tyres around where the boat lands when the tide goes out. That would in theory break up the mud effect.

 

 

Whether this is allowed I don't know and I think the tyres would all need to be chained together and anchored so it could end up being quite a complicated process. 

 

1 minute ago, magnetman said:

That's when one uses the helicopter!

LOL Haha.......the expense just keeps rising lol

 

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14 minutes ago, magnetman said:

The late Nigel Moore recounted a tale where a narrow boat was aground in mud and the tide came up almost over the sides then the boat suddenly lurched upwards. 

 

 

The mud suction has to be greater than the buoyancy of the boat and that seems unlikely to me. However if the mud suction keeps the boat down for long enough as in the case you cite above, and the rising water floods engine vents or gas locker/bow well deck scuppers, then that likelihood increases dramatically.

Edited by blackrose
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16 minutes ago, magnetman said:

The late Nigel Moore recounted a tale where a narrow boat was aground in mud and the tide came up almost over the sides then the boat suddenly lurched upwards. 

 

 

One approach I have seen used down near Brentford on the Thames was to put a load of old tyres around where the boat lands when the tide goes out. That would in theory break up the mud effect.

 

 

Whether this is allowed I don't know and I think the tyres would all need to be chained together and anchored so it could end up being quite a complicated process. 

 

How would that work as the boat floated and swung around with the tide / currents.

Mooring at sea is very different to tying up on a canal tow-path.

 

This plot shows my boat movement at anchor over (I think) a 14 hour period each red dot is a movement. We travelled 2.36 miles over the period, whilst attached to 25 metres of chain.

 

Anchor-Movements.jpg

Edited by Alan de Enfield
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8 minutes ago, Loddon said:

Doesn't help much if you want to get on/off and the tide is out.

 

We were moored on a swinging mooring in Holyhead, and I decided to go back ashore and stay overnight in our static caravan a few miles away, as the forecast was 'not good'.

I took the water taxi back into shore and left SWMBO alone on the boat as she thought it would be good fun to rock & roll with the storm.

 

Middle of the night the phone goes "come and get me, its 'orrible" I kindly explained that she had the tender in the lazarette and the water taxi did not start again until 9 am.

 

She was not happy when I arrived at a few minutes after 9.

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1 hour ago, Shaky Start said:

 

I am not quite sure if I am replying to people in the correct way so please forgive me if I am making a mess of this thread. Thank you Alan, your reply is insightful. It is definitely mud on the Penryn river at low tide, the thick oozy stuff, yuk. Your idea about having bilge keels added is definitely a thought but may be too costly. The reason we were thinking about doing this is because over the 3 years on the inland waterways we intend to put our own stamp as it were on the interior of the boat. Also a 40 ft canal boat is a lot more spacious than say a 40 ft motor cruiser or yacht to live on. Food for thought all of this..... I know Plymouth sound and the Tamar, it used to be a regular place we visited on our boat. I can well imagine the fun and games you have there lol.

 

I know that a narrowboat was moored and fitted out in a floating "dock" dug out by a farmer upriver from Plymouth, and they published a bit about it somewhere. I don't recall them having suction problems, but maybe they did something like the chained tyre trick or the bottom was fairly hard clay etc.

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13 minutes ago, Alan de Enfield said:

 

How would that work as the boat floated and swung around with the tide / currents.

Mooring at sea is very different to tying up on a canal tow-path.

 

This plot shows my boat movement at anchor over (I think) a 14 hour period each red dot is a movement. We travelled 2.36 miles over the period, whilst attached to 25 metres of chain.

 

Anchor-Movements.jpg

 

That is an interesting plot indeed. Quite a distance travelled too over that time period. I can only answer how the tyre option for me would work. On the Penryn river where I would intend to permanently moor, it is literally right at the top of a tidal creek. The boats bow is tethered to the path along the riverbank and the stern is moored to a permanent concrete mooring block a few feet behind the boat. This only allows for a very small drift factor and as the out going tide would pull the boat towards the same direction every tide, the place of rest on the estuary mud would be almost the same every time.. well this is the theory but as we all know, theory and practice are never quite the same in the boating world haha.

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I think I would try to get in a low tide to investigate just how deep the mud is. It might be possible  to lay some concrete posts or similar transversely along the mooring so they stand a little proud to allow the boat to settle on them. It's even possible tidal action might scour the mud between the posts. A sort of gridiron.

Edited by Tony Brooks
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6 minutes ago, Tony Brooks said:

I think I would try to get in a low tide to investigate just how deep the mud is. It might be possible  to lay some concrete posts or similar transversely along the mooring so they stand a little proud to allow the boat to settle on them. It's even possible tidal action might scour the mud between the posts. A sort of gridiron.

 

That is a good idea, better idea though to send the wife in to investigate haha. The concrete idea has legs too. This is 3 years down the line at the moment. We are looking forward to travelling the inland waterways first, that'll give us time to investigate all of the estuary options.

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30 minutes ago, Tony Brooks said:

 

I know that a narrowboat was moored and fitted out in a floating "dock" dug out by a farmer upriver from Plymouth, and they published a bit about it somewhere. I don't recall them having suction problems, but maybe they did something like the chained tyre trick or the bottom was fairly hard clay etc.

That was me and no, there was no problem in the wee 'dock' or, prior to that, in the drying harbour which had a muddy bottom. Was there for a couple of years.

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