Passing moored boats - water depth

[doratheexplorer]

Just wanted to share my experience this morning.

 

I've been moored in the same spot since last thursday (Shroppie - not far from Chester).  Yesterday morning there was a lot of passing traffic and I noticed that my boat was bumping a little as they passed.  So I went out and tightened my lines a little and that solved the problem.  Then yesterday evening and overnight we had rainfall here which I can only describe as biblical.  Possibly the heaviest rain I've ever seen.  When there was a slight lull at about 9pm I could hear water pouring into the canal from surrounding fields and I became concerned that it would affect the canal level.  I was right, my lines had become extra tight!  Even though it's a long pound I'm on, the level was up a few inches.  So I loosened my lines a fair bit, to allow for further rises.  Come this morning and the level is almost to the top of the bank, somewhere between 6-12" rise which is crazy for a canal IMO.  Now my lines are about as loose as they were yesterday morning, but here's the interesting thing - when boats go past, there's virtually no movement in my boat.  It seems clear that the extra water depth has made all the difference.  This is something I'd assumed before from my experience of mooring in shallow spots - the boat moves a lot more when boats go past.  But my experience of the last 24 hours has confirmed it for me.

[Rob-M]

The draw of the passing boat will have less impact due to the increased depth which is why on rivers and deep canals like the G&S people don't slow down.

 

There is probably a bit of science to explain it all.

[doratheexplorer]

26 minutes ago, Rob-M said:

The draw of the passing boat will have less impact due to the increased depth which is why on rivers and deep canals like the G&S people don't slow down.

 

There is probably a bit of science to explain it all.

It's certainly what I've seen on the G&S and bigger rivers.  I've been surprised how much difference just a few inches of water has made to a fairly narrow canal.

[Ex Brummie]

The draw of a passing boat will displace the a fixed  amount of water as it passes. If you double the width and depth of a waterway, you will probably only have 1/4 of the effect on a moored craft. The same principle also explains why you travel faster for the same revs on deeper water, as you don't pull the boat down to the bottom.

[Mike Todd]

14 hours ago, Ex Brummie said:

The draw of a passing boat will displace the a fixed  amount of water as it passes. If you double the width and depth of a waterway, you will probably only have 1/4 of the effect on a moored craft. The same principle also explains why you travel faster for the same revs on deeper water, as you don't pull the boat down to the bottom.

Not quite the same effect: the interaction between the boat and the bottom is a proximity effect rather than one of volume. 

[doratheexplorer]

1 minute ago, Mike Todd said:

Not quite the same effect: the interaction between the boat and the bottom is a proximity effect rather than one of volume. 

Yes.  This is what I was thinking.  Something about there now being a decent space between the hull and the canal bed, giving the water a gap to slosh about in, rather than just forcing my boat around.

 

Certainly the width of this waterway hasn't changed and it's got a few inches deeper.  Maybe a 10% increase in water volume for a massive decrease in boat movement.

[Mike Todd]

5 minutes ago, doratheexplorer said:

Yes.  This is what I was thinking.  Something about there now being a decent space between the hull and the canal bed, giving the water a gap to slosh about in, rather than just forcing my boat around.

 

Certainly the width of this waterway hasn't changed and it's got a few inches deeper.  Maybe a 10% increase in water volume for a massive decrease in boat movement.

Indeed - once you are into the range of a proximity effect with the bottom then the impact of a percentage change will be disproportional.

[doratheexplorer]

4 minutes ago, Mike Todd said:

Indeed - once you are into the range of a proximity effect with the bottom then the impact of a percentage change will be disproportional.

I find it all quite interesting but it's really not my area.  It occurs to me that when the level was lower, I might have just be lined up with some kind of bumpy bit which made it worse too.  A sample size of one, in an uncontrolled environment, isn't the ideal situation from which to draw conclusions.

 

I do remember times when moored on very shallow moorings with a v shaped canal profile, and everytime a boat passed, my boat would list over as the water was taken from beneath and the boat would just tilt as if it were lying on the bottom.  Nothing I did to the mooring lines, including springs, made any difference.

[twbm]

... and in extremis, the breaking wave on the shallows tells you you're going too fast, or not in the channel, or both.  

[Detling]

Your 15 ton boat has to move 15 cubic meters of water from in front of it to behind it, to move a boat length. If you have deep wide water there will be very little resistance. On a narrow ditch the water is pushed into a hump in front of the boat, this hump empties down the sides and under the boat into the dip the propeller has created by pushing water backwards. As you pass a moored boat some of the water under that boat also flows into the dip your prop has created. This is why your boat goes slower in bridge holes, and why if the bottom is sloped your stern gets sucked towards the shallow side.

[Sea Dog]

If you're missing the rock and roll Dora, simply moor with the centre line!

 

[Dave123]

Yep this is why even deep boats can charge past on rivers and cause hardly any movement to ropes. Unusual to see it on a canal!

[Wanderer Vagabond]

5 hours ago, Dave123 said:

Yep this is why even deep boats can charge past on rivers and cause hardly any movement to ropes. Unusual to see it on a canal!

Not sure that is entirely the case.

 

Had an experience today whilst moored at Hundred Acre Moorings on the Great Ouse. I was sitting working inside the boat when suddenly I became aware of the mooring lines tightening and the boat starting to lean. I looked out of the window and saw the sort of flow that you'd get moored just above a lock when someone lets the paddles go, except that the nearest lock is at Hermitage, about 11 miles away. Now having some concern, my next thought was that there had been a breach somewhere, except we are on a river and the land around here is a flat as a pudding. I'm just getting ready to go outside to slacken the mooring lines (just in case water is going down) when a large, sea-going cruiser comes into view out of the window. By now he had slowed to the 4mph limit but given how much water had been shifted, I would guess that on the approach to the moorings he had wound it up to 7mph or above. This is speculation since I didn't actually see him travelling at that speed, but it is the only explanation I can think of for such a sudden rush of water, that stopped after he had passed (although the river continued to 'slop about' for 10 - 15 minutes after he had passed).

 

In fairness to him, I don't suppose that he was remotely aware of just how much water he was shifting when/if he had the burst of speed. I've been moored at a lift bridge whilst the Exon Humber Princess has gone past so I am aware that boats can shift a bit of water, but today's cruiser was drawing water from under us whilst he was still over 100 metres away. Being moored properly, no harm was done and no-one was showered with boiling water but it is really surprising just how much a passing cruiser can disturb the water if it really tries.

[doratheexplorer]

On 14/08/2020 at 14:59, Sea Dog said:

If you're missing the rock and roll Dora, simply moor with the centre line!

 

Or put another dime in the juke-box?