Scholar Gypsy

Here's an approximate sum: 15 metres x 2 metres x 13 cm = 4 cubic metres displacement = 4 tonnes. So you might need to remove some of the existing ballast. Here is what can happen if your air vent into the engine compartment (if you have one) was previously 12cm above the waterline.

This is what I remember from my childhood, on the beach at Southport in the 1960s & 1970s. I think by then mainly used for a bit of shrimping, and rescuing cars that got stuck in the sand on a rising tide. http://www.southportgb.co.uk/showthread.php?t=50357366

MAIB safety bulletin published recently: http://www.maib.gov.uk/cms_resources.cfm?file=/Safety%20Bulletin%203_2013.pdf

I think it might be helpful to distinguish between i) the hydraulic effect as a boat travels along a narrow channel. This is determined by the displacement of the boat. For each boat length a narrowboat advances you have to move (say) 15 tonnes of water from in front of you to behing you, and this creates a current in the opposite direction equal to (on average) speed of boat x (ratio of cross section of boat to the cross section of the canal not filled by boat). So if your boat occupies 1/4 of the total cross section, then if you travel at 1.5 kts then there will be a current of 0.5 knots in the other direction. If you occupy half if the canal (eg going through a narrow bridge, or into a lock) then the current will be the same as your foward speed. ii ) the wash which is more of a surface wave phenomenon, and depends on the hull shape and your speed, and the depth of the water (I think - I need to revise my fluid mechanics...) Waves travel at an angle to the direction of travel. Now a speedboat or a RIB on open water may well create lots of wash (which will rock you around a bit, and may snap ropes if they are too tight) but no significant hydraulic effect (cross section is essentially infinite) .. A Thames Clipper on the plane at 25 kts creates very little of either. On the other hand a narrowboat in a narrow channel can create quite a hydraulic effect (and this is what pulls mooring pins out) but no wash. A cruiser on a canal will not create much hydraulic effect but might create a bit of wash. The latter is not very likely to pull your pins out.

I am finding these last two sentences a bit hard to follow. Are you referring to the hatch in the stern deck (if you have one) that you lift up to gain access to the weed hatch itself? The latter is usually held in place with a clamp system of some sort, and thick rubber seals. Or do you have the setup (quite rare I think) where the weed hatch is surrounded by a vertical steel tunnel so that it is impossible for it to leak into the engine space?

I was alarmed the first time I saw a (new) boat where the rudder tube leaked into the engine compartment (this was because it was a wheel steered boat, with hydraulic mechanism under the stern deck. We were running the engine in gear for 15 minutes, while stationary, to check all was well after a diesel pipe blockage on the Thames tideway (near Lambeth Bridge). A lot of water was coming in. Would a weed hatch leak be another possibility?

Suggest you try measuring that voltage when turning the key. If it still stays the same then it looks like a stuck solenoid, or as Chop suggested the negative return from the engine / chassis to the battery

I agree Eynsham - the second photo is very helpful, and that does indeed look like Wytham Woods. Next challenge: what does it say on the sign leaning against some sort of box on the left hand side?

I think the view is definitely downstream - the lockkeeper is leaning on the far end of the balance beam. Google maps shows me I was wrong about Cleve - there is no weir on the right, and the curve of the river doesnt look right either.

I think the weir is to the right in the photo? There's a crane as well. If so it might be Cleeve, which is quite shallow compared with other locks in the area? ; but not Hambleden where the weir is on the left. Of course there is a risk in assuming that locks today are in the same place they were 100 years ago. Some are but some (eg Abingdon, probably the most extreme example) aren't ...

This type of stuff is all over the London Underground system (well OK, the parts that are not underground) to deal with the flying rats: http://www.birdstop.co.uk/bird_spikes.asp Like nails but you would not injure yourself if you trod on them.

On the second point, I confess to choosing numbers to make the sums easier. On the first point, the issue is more the momentum of the boat, and the time it takes to come to rest when it hits something, Mass x velocity = Force x time = impulse (or momentum). So if a 20 tonne boat hits a gate at 0.5 metres a second (a bit over 1 mph), and comes to rest in say 0.1 seconds, then the force exerted is (on average) 100 kN, or the weight of a 10 tonne boat. That should certainly be enough to move (or break!) the gate. So the next question is whether the thrust that a typical narrow boat prop can exert would be enough to hold the gate open for long enough for the levels to equalise. So I now need some data on how long it takes a boat to accelerate from rest to 1 mph. If this is 10 seconds then the thrust is of the order of 0.1 tonne (1 kN), likely to be a rather marginal? All of this reminds me of my A level physics teacher who, when introducing us to electromagnetism and the theory of electric motors, talked at length about couples having their impulsive moments in fields. I didn't quite understand what he meant....

For a lock gate 2m deep and 3m wide, the total force from water pressure (P in my picture ...) is equal to a weight of 3 tonnes (Force = g.density.Area.height/2) where g = gravity and density is 1 tonne per cubic metre. That gives you an idea of the scale of the forces involved, both those holding the gate in place against the pressure, and also the lateral forces in the plane of the gate. So to answer the question, to get a rough idea of scale a 1 tonne force distributed though a sealing strip of oak 3m x 33mm is 10,000 Newtons through an area of 0.1 m2, which is 100,000 Pascals or about 15 psi. That's nearly 800 mm of mercury, so not good for the circulation in any trapped extremities ...

Sorry, but I don't think that is correct. There is a difference between the net resultant force exerted by the water in the uphill pound on the lock and surrounds, which is constant, and the detailed forces within and between the elements (whuich is what would make a structure fail). In my simplified model, if the angle theta increases, then the size of the gate increases (proportional to 1 / cos(theta) ) and so the total pressure force increases. The end result is that the forces perpendicular to the gate are proportional to (1 / cos(theta)), and the forces along the gate are proportional to 1 / sin (theta) - which will get very large as theta gets smaller.

Yes, I think I agree with that. It is worth noting that there are some lock gate designs - eg the radial gates at Limehouse - where the gates are held in place via a vertcal pillar, and the forces in play at the top bottom and centre seals are essentially minimal. So you could remove all the rubber seals on that lock and the gates would not collapse, though they would leak rather a lot eg through the inch or two gap between the gates. With mitre gates the gates do need to be in contact, otherwise they will collapse. Sorry, I am indeed a mathmo rather than an engineer (and my foreign languages are essentuially useless). And I am a fairly useless carpenter. The equation with tan theta is there because the force between the gates is actually across the lock - ie at 90 degrees to the axis of the lock. One way of justifying this statement is to recall (Newton's laws) that the contact force gate A exerts n gate B is equal and opposite to the force gate B exerts on gate A, and then to apply symmetry. I need to draw a plan view of the lock to explain why tan(theta) = M/X. In the diagram. X Y and Z act in the plane of the gate, MH F and P are perpendicular to the gate. As the angle theta gets smaller, the forces X Y and Z get bigger, which creates challenges for the carpenter and for the mason. On the other hand if you increase the angle you need more wood fo the gate, and more space to fit longer balance beams...

Thinking some more while cycling home. We know where the water pressure acts on the gate - midway between mitre and heelpost, two thirds of the way down. To consider an over-simplified situation, if we imagine the lock structure (cills, quoin, mitre, hinge etc) is in contact with the gate at precisely 3 points, then we can work out what the reaction forces are that hold the gate in place. This is essentially the same problem as supporting any object with a flat bottom on three (horizontally) coplanar points (assuming the centre of gravity is within the triangle). An example is worked out here - with the gate held in place just in three places - top and bottom of the hinge, and midway up the mitre. One can then work out all the forces, as a multiple of the pressure force. This includes the transverse forces along the gate - the source of the original sleeplessness I think? This is of course not reality, though you could simulate this situation on the Nene by wedging a large solid object between the upper gates, half way down, on one of the few remaining locks that are not electrified, and then opening the guillotine bottom gate). In reality, if there are more than three contact points (or even worse contact surfaces along the edges of the gate) then one cannot use this sort of technique - there is not enough information to get a solvable set of equations (cf a flat object supported on four or more coplanar points). The precise solution will depend on how the gate flexes under pressure, and would only be solvable using some quite complex computer modelling, I fear, to calculate the stress and strain within the gate.... It would be quicker to measure the forces in play on an actual gate ... I hope this helps, but rather doubt that it does.

Thanks for that - a good one of me (well I think so anyway) standing on the roof of Indigo Dream ...

Just a couple of points to add. First the charge for using the lock is pretty horrendous (£hundreds), SPCC manage to negotiate a bulk discount (this year the planned trip is 3rd-6th January 2014). Second, arranging moorings in the Royals is not straightforward - a pity as there are literally miles of dock wall to tie up to. I have tried in the past to arrange a temporary mooring with the marina at Gallions point, but when I spoke to them they said it was not safe for narrowboats to go downstream of the barrier. I emailed them some photos, but to no avail....

I am not sure I would agree with "at the moment" - with apologies to Pelican, I suspect I am always like this. I am also not sure if I can solve Pen n Ink's insomnia, This problem will require some more thought and also a three dimensional approach. Even if one assumes a weightless, perfectly rigid and fitting gate (so the forces at the mitre are zero) I have not yet got a solvable set of equations. I have remembered how to show that the pressure from the water on the gate acts (in effect) at a point 2/3rds of the way down from the water level to the bottom of the gate. I had better save this for a weekend task....

I think we need to distinguish two forces The hinge mechanism needs to exert a lateral force along the gate, in reaction to the contact forces at the mitre. This will be provided by a combination of the hinge pin at the bottom of the gate, the hinge collar, and possibly the stonework. In each case the contact forces are nothing to do with providing a seal. Indeed the pin is upstream of the sealing surfaces, and the hinge collar is out of the water. This lateral force can also be provided in part by friction forces at the bottom of the gate, once it seals. The forces providing the seal are (as others have noted) at right angles to the gate surface, along the bottom edge and at the hinge end of the gate. The sealing forces at the mitre will be at 90 degrees to the axis of the lock I should draw a diagram. It's not obvious there is a unique solution to the problem (I think at least four variables to solve for, given the total water pressure acting on the gate, and only three equations - 2 force resolutions, and one taking moments). Next problem: how does a radial gate like that at Limehouse (which can operate in either direction) seal. One can see the big squashy rubber seals at the visible edges, but what happens at the bottom ??

Thanks - there are lots of websites that purport to sell 12v neon bulbs, but I suspect they are indeed wrong, fpr the reasons noted! So a normal panel light is indeed what is needed. I used LED panel lights for my recent fridge fan project and they are almost too bright (even the blue one). Would make a good stern light though ...

I like Keeping Up's circuit - and may pinch that idea (PS it needs a neon, not a LED bulb, as the current needs to be able to flow in boh directions in the various modes). I agree about bright stern lights, but I think 10W ones are a good idea for reasons others have noted ... I suppose blue might be a better colour, but that's not what it says in ColRegs....

That's one reason why I have my "headlight" at the back. (Also means I can point it to the right when passing oncoming boats, look into side shafts etc). I have now found a more elegant way of mounting it than is shown in this picture...

I have filled in the survey. What follows is perhaps slightly off topic, but I thought it would be worth noting I got an email from CRT a couple of weeks ago to say my boat had been spotted on the visitor moorings at Thrupp recently, and would I like to fill in a brief CRT survey saying what I thought of the new arrangements? I rather approved of this approach to collecting feedback.

I would say: 1) Turn on most/all of the interior lights. They will be more use to you - in terms of knowing where you are - than a headlight pointing forward anyway. After all you know that the tunnel is pretty much dead straight. A torch is a good idea as well. 2) Check the weed hatch, then (unless you know how to bleed your engine which is the next most likely reason for a stall) start legging (or poling), or wait for someone to give you a tow. 3) I would estimate about 10% of narrowboats have a white (10 watt) light at the back. This is a good idea as it is surprisingly easy to run into a stationary boat in front of you - estimating distances can be tricky, and smoke/fumes/fog don't help. (On our first boat (17" cruiser) we navigated through tunnels using a camping gaz light in the cabin, suspended from a bit of wood fixed across the hatch opening.... Very effective and did not dazzle the steerer)