Wash On Inland Waterways

[Vaughan]

A discussion has developed on a thread about Friday Girl's travels, all about going too fast. (Not the Admiral, I hasten to add). I didn't want to steal his most interesting thread, so I am opening this one. John advised me as to the title, as I was going to call it "squat effect" but then he reminded me of what all you lot would make of that title!

So what is squat effect?

Let us first think of displacement, and here is a question -

Imagine a stone arched aqueduct bridge, which is taking a canal over a river valley. What will be the extra weight on the span of the arches, when a 400 ton barge comes across, on the canal? The answer is zero, as the barge is displacing its own weight in water. But that water has to go somewhere and so the barge is actually pushing it in front of it. As water finds its own level, there will be a rush of water going back, alongside the barge, which lowers the level alongside it, and once the barge has passed, the level will rise again. This is called "squat effect".

Two examples of this : When the Potter Bridge pilot takes a boat through at high speed, it will drop down by about 6 inches (or more) and thus clear the bridge. On Thorpe Island, when sitting on board Morning Flight having lunch, the boat would suddenly move against the moorings, until the lines came tight. Sure enough, and 20 minutes later, a coaster would come past on the back reach, on its way up river. The surge on the mooring lines had been caused by all the water that the coaster was pushing in front of it, on its way up the Yare. This is squat effect.

The Broads navigations are narrow and shallow, and so this has a big effect on wash, as well as boat handling. If you are going down a narrow dyke such as Rockland, or even the Ant, you can see that water is being "sucked" away from the bank alongside your boat. This is because the boat is displacing water in the dyke as it passes, and the rush of water past the boat causes much worse bank erosion than "waves" from a wake. This also causes a strong resistance against your boat and so increases your fuel consumption. If you slow down until the squat effect is minimal, you will actually be going faster over the ground.

So a GPS will only tell you your speed over the ground, which is fine for radar traps but cannot control any damage that you might be doing to the river bank. The only way to do that is look around you, and see how much wash you are making.

That'll do for now - if you want me to talk about the effect of coarse pitch propellors, we can do that as well!

[ExUserGone]

Any chance of a talk about the effect of course Pitch propellers Vaughn?

[BroadScot]

I distinctly remember reading many times in the Hoseasons and Blakes booklet that came with your holiday booking, about checking your wash and speed by looking at the river bank. 5mph was a brisk walk! Know wonder my legs are knackered today ! The same was in, and I think still is in, the Skippers manuals. Two words come to mind.....common sense. 

Iain

 

[LondonRascal]

You have some of us who are considerate and will - even when doing the speed limit -still slow when passing moored boats, and others who don’t give much of a damn.

What bothers me more about wash these days are the new bank works that have been undertaken in ever more places – I remember the ones that were completed on the bend (after you have passed Ranworth Dam and are approaching the mouth of the River Ant) that has clearly been eroded far from being smooth mud.  

This erosion means only one thing – more silt in the water stream and it has to go somewhere.  When there was quay heading the wash would hit this, sure it meant every 20 or 30 years it would need to be all ripped out and replaced but the wood would absorb the impact of the wash (and tidal current).  No quay heading, day by day inch by inch the muddy bare bank is being eroded and shaped by the river.  I guess from an environmental point of view it could be seen as being more natural and allowing the river to carve its own course more freely but I am not so convinced in the science and think it is more a case of cheaper to do now and review the results in 20 years when others will be left with the job of dealing with the issues or reaping the benefits however it may swing.

End of the day – less speed = less wash so maybe it is better to not think so much o speed limits but have in areas like Wroxham. Horning etc ‘wake free zones’ so if you produce wash you are liable to a fine which would mean smaller craft which may cause more wash than a longer slender craft might indeed have to actually cruise slower thus not produce wash.

[JennyMorgan]

To save any other coarse type responding thus:

There is a story that early photographs of the Pyramids often included local natives squatting in artistically convenient positions within the composition. Apparently it wasn't until many thousands of such cards had been sold that the publishers were told exactly what was going on under the natives flowing robes! Needless to say the cards were withdrawn from sale, nevertheless they still appear either as post cards or in aged encyclopedia. How's that for deviation then?.  

[Vaughan]

JM, the Admiral warned me of such as you, before I started this! I knew I could count on you!

[Hockham Admiral]

I rest my case, Vaughan........................   

[Vaughan]

10 minutes ago, LondonRascal said:

This erosion means only one thing – more silt in the water stream and it has to go somewhere.  When there was quay heading the wash would hit this, sure it meant every 20 or 30 years it would need to be all ripped out and replaced but the wood would absorb the impact of the wash (and tidal current).  No quay heading, day by day inch by inch the muddy bare bank is being eroded and shaped by the river.  I guess from an environmental point of view it could be seen as being more natural and allowing the river to carve its own course more freely but I am not so convinced in the science and think it is more a case of cheaper to do now and review the results in 20 years when others will be left with the job of dealing with the issues or reaping the benefits however it may swing.

Robin you are quite right right but you are talking of an "alluvial" river which is flowing out to sea the whole time and so the bends will "meander" with the erosion caused by the current. The Thames is an example and especially the Rhone, hence its delta when it reaches the sea.

The Broads rivers are tidal and so any erosion, in principle, is caused by boat wash.

[Poppy]

2 hours ago, BroadScot said:

I distinctly remember reading many times in the Hoseasons and Blakes booklet that came with your holiday booking, about checking your wash and speed by looking at the river bank. 5mph was a brisk walk! Know wonder my legs are knackered today ! The same was in, and I think still is in, the Skippers manuals. Two words come to mind.....common sense. 

Iain

 

And just how many bother to read that these days?  They should be sat in front of a screen with a video presentation before being allowed to their boat IMHO !

[JennyMorgan]

7 hours ago, Vaughan said:

JM, the Admiral warned me of such as you, before I started this! I knew I could count on you!

Only too pleased to oblige