Bullseyes

[Chris Pink]

I have just removed two from a boat, one was positioned upwards, one was positioned downwards.

 

What are the arguments for each way?

 

My understanding is that up distributes light out for locking etc and down brings more light in.

 

I have one that faces up and there is a hot spot 6" below it but no danger further down.

[Chertsey]

Mine's up, and that makes sense for me, to bring more light into the cabin. It's been scratched to bits by the (tin-covered) slide though. When we took it out to reseal round it it was all broken round the edges, so we stuck it back together with clear silicone - all under the brass, it doesn't show. It's pretty old - maybe even original?

 

The heat focusing argument for installing them upside down doesn't hold water as far as I can tell - as you note, the natural focal point is too close to the glass for anything to be accidentally caught in it, certainly in a traditional installation.

 

Just thinking about it logically, without drawing on knowledge of any actual practice, the argument for distributing more light outwards seems thin - it would depend on having a good strong light inside the cabin, which, if it were available might be better deployed outside - and how common was locking at night?

[Derek R.]

We all know the effect of a magnifying glass on combustibles, and I once saw a handlebar end mirror on a motorcycle & sidecar actually begin to smoulder the fabric of the sidecar seat once. A swift movement of the handlebars stopped the effect, and the owners attention was brought to bear.

 

However, with bulls-eyes of a modern pattern there does at times seem to be a similiar problem. I'm sure this has been aired before at some time, and various solutions provided for the elimination of a burning focal point. Some say the larger older Bulls-eyes do not suffer from this effect, but as to dome up or down and light efficiency, I wouldn't have thought there was much in it.

 

Any roof light will increase the light distribution within a cabin greatly, but the only way of testing the efficiency and distribution of light within an enclosed space would be to use a Lux meter with a Bulls-eye mounted loosely in the top of a box with a fixed strength light source from above. Measure the light strength and distibution at several points with the box, Bulls-eye first one way up, then the other. Most 'deck lights' are flat on top and domed or 'peaked' beneath, this in the main is to prevent a trip hazard, but I fancy might actually be better for light distribution below deck. Only testing would tell, and the domed or peaked element of the glass would need to show lower than the surface of the deck-head beneath.

 

The other issue is water. A flat glass topside in a raised rim will hold water until mopped or evaporated away. A domed topside will shed water. A domed bottomside might allow any condensation to be drawn away from the edges of the glass and the surrounding woodwork to drip from the centre, but I've seldom experienced any condensation from a back cabing Bulls-eye - the range and open slide have seen to that. Most Bulls-eyes I have seen on working and ex-working boats have been dome side up.

 

Edited to add (does he never stop?) I'm minded of loco headlamps that are also domed - outward from the light source, but this to act as a magnifyer of the light. This may well show that dome outwards will give more light outwards, though the suggestion that it might improve night locking has to be with the consideration that curtains and low powered cabin lights situated away from the 'lens' will almost negate any beneficial effect. Non starter.

 

The external scratching on a 'dome up'from boots etc. may well aid preventing such a focal hot spot, though of course such is accidental.

 

That's enough prattle from me, hopefully the experience of others will further enlighten!

Edited May 12, 2011 by Derek R.

[Tam & Di]

Just thinking about it logically, without drawing on knowledge of any actual practice, the argument for distributing more light outwards seems thin - it would depend on having a good strong light inside the cabin, which, if it were available might be better deployed outside - and how common was locking at night?

 

I agree with the argument that any light amplification from an upside-down installation would be meaningless. Working in the dark was hardly uncommon though. As an example, the limejuice job meant loading at say 7.00 and getting away at perhaps 10.00. It was a 12 hour run Brentford-Boxmoor for a loaded pair, and we were expected at Boxmoor for 7.00 next morning. In the winter it would be dark about 4.30 in the afternoon so almost half the boating was done in the dark - we'd keep going till we got there, but it would not make any difference if you stopped early as you be up and a-going in the dark next morning. Our own experience was certainly nothing unusual, and even on a starless and moonless night you can still see well enough when you are out in the open.

Edited May 12, 2011 by Tam & Di

[Timleech]

We all know the effect of a magnifying glass on combustibles, and I once saw a handlebar end mirror on a motorcycle & sidecar actually begin to smoulder the fabric of the sidecar seat once. A swift movement of the handlebars stopped the effect, and the owners attention was brought to bear.

 

However, with bulls-eyes of a modern pattern there does at times seem to be a similiar problem. I'm sure this has been aired before at some time, and various solutions provided for the elimination of a burning focal point. Some say the larger older Bulls-eyes do not suffer from this effect, but as to dome up or down and light efficiency, I wouldn't have thought there was much in it.

 

Any roof light will increase the light distribution within a cabin greatly, but the only way of testing the efficiency and distribution of light within an enclosed space would be to use a Lux meter with a Bulls-eye mounted loosely in the top of a box with a fixed strength light source from above. Measure the light strength and distibution at several points with the box, Bulls-eye first one way up, then the other. Most 'deck lights' are flat on top and domed or 'peaked' beneath, this in the main is to prevent a trip hazard, but I fancy might actually be better for light distribution below deck. Only testing would tell, and the domed or peaked element of the glass would need to show lower than the surface of the deck-head beneath.

 

The other issue is water. A flat glass topside in a raised rim will hold water until mopped or evaporated away. A domed topside will shed water. A domed bottomside might allow any condensation to be drawn away from the edges of the glass and the surrounding woodwork to drip from the centre, but I've seldom experienced any condensation from a back cabing Bulls-eye - the range and open slide have seen to that. Most Bulls-eyes I have seen on working and ex-working boats have been dome side up.

 

Edited to add (does he never stop?) I'm minded of loco headlamps that are also domed - outward from the light source, but this to act as a magnifyer of the light. This may well show that dome outwards will give more light outwards, though the suggestion that it might improve night locking has to be with the consideration that curtains and low powered cabin lights situated away from the 'lens' will almost negate any beneficial effect. Non starter.

 

The external scratching on a 'dome up'from boots etc. may well aid preventing such a focal hot spot, though of course such is accidental.

 

That's enough prattle from me, hopefully the experience of others will further enlighten!

 

As I understand it, the idea of a bullseye or deck prism is that the projection should be into the cabin so helping to distribute light around the cabin.

However, as you rightly say, 'flat side up' they are likely to collect water. For this reason I think it was normal to fit bullseyes 'dome up' into wooden cabin tops because it's easier to keep them watertight, they were often fitted into a circular rebate in the woodwork and sealed with some simple zinc or similar flashing. That way they may collect a bit more light, but distribution below will be poor. With a steel cabin, a bullseye or prism (or dioptric deck light as was sometimes used) can be securely fitted with a brass frame and much less risk of leakage, so fitting them the 'right' way up is more viable.

 

Tim

[ditchcrawler]

How about these new ones from Davey item number 2418

[furnessvale]

The heat focusing argument for installing them upside down doesn't hold water as far as I can tell - as you note, the natural focal point is too close to the glass for anything to be accidentally caught in it, certainly in a traditional installation.

 

 

I can confirm two instances of scorching of woodwork inside the cabin with a bullseye fitted upwards. After the first one you would think I would have learned my lesson, but I inadverently left an internal door open which placed the top nicely in the "hot spot". Fortunately, I was on board and smelled something burning! A friend of mine had a more serious fire involving soft furnishings.

 

I have no knowledge of whether bullseye down changes the situation and I have no desire to find out, I always cover the bullseyes on my unattended boat.

 

George Sandbach ex nb Alton retired

[madcat]

mines flat side up n has a stainless fruitbowl over it if I'm not home ,wanting a lie in or nursing a hangover. Had a hole scorched in the curtain before I changed it which I did cos it also leaked.Thats what caused the fruitbowl roof decoration!

[dave moore]

Hi all

I've posted this before, but it may be of interest again. Resolute's bulls eye is fitted curve up and yes, we had a nasty scorch mark below it. I was discussing the issue with another boater at Norton Canes, a scientist by trade, who explained that the bulls eye would focus sunrays regardless of which way it was fitted. He suggested that a solution would be to fix a circular piece of frosted perspex to the flat side, thus dispersing the rays. I cut a piece and stuck it in place using silicon. No further problems 10 years later!

Hope this helps.

Dave

[Laurence Hogg]

All these items if used as intended should point down leaving the flat surface on the outer.

Bullseyes are in reality "Deck lights", not only do they come as round "eyes" but you have square ones, one like a orange squeezer and oblong toblerone style. Mosy are still manufactured by Davey who have introduced a equal sided one. All the old ones are fitted with the prism or lense DOWN, this is because they are deck lights inset into a deck and you need to be able walk on them. If no one believes this then go and look at some older chandlery catalogues, they are not made for narrowboat use but were adapted. For some insane reason Yarwoods of all people drew the up cabin plans with the deck light shown fitted incorrectly. This was a satandard 8H-014 lense encapsulated in a frame and sealing ring (exactly what Davey still sells) but wasnt fitted as it would have been in normal use.

 

links: http://marinestore.co.uk/davey-deck-hardware-deck-lights.html

 

http://www.davey.co.uk/pdf/deck_hardware.pdf

 

This famous works makes the glass: http://www.nazeing-glass.com/manufacturing/index.html

Edited May 15, 2011 by Laurence Hogg

[Admiral]

Brinklow boats use a novel solution. They give the flat side of the bullseye a light "shot blast" then install them dome up, thus giving the same light into the cabin but there is no magnifying effect at all. From the outside you would never know.

[Laurence Hogg]

Brinklow boats use a novel solution. They give the flat side of the bullseye a light "shot blast" then install them dome up, thus giving the same light into the cabin but there is no magnifying effect at all. From the outside you would never know.

 

We tested this method years ago at Boatmans Cabin, it doesnt work you still get a hotspot although a tad dispersed but enough on a bright day to be risky. Many of the so called "bullseyes" sold years ago by Midland Chandlers wer signal lamp lenses they aquired in bulk from the old Reading signal works, needless to say their focal length was short, these were the first on the market to cause concern and if my memory serves correct a boat at Calcutt was damaged by one.

[station tug]

my tug has small port holes and the engine room and back cabin are going to be dark...do bulls eyes make a huge difference??

[Derek R.]

my tug has small port holes and the engine room and back cabin are going to be dark...do bulls eyes make a huge difference??

 

Yes - massively.

 

Side windows and portholes will allow light in from the sides. In places where there is open daylight, this will be sufficient. But when against hedges, trees, and buildings, the amount of available light will be much reduced. The clear example of where the most light comes from is in the simple experiment of looking vertically upwards. Who does nor blink or shield their eyes from open sky?

 

Even a comparatively small Bulls Eye or roof light/deck light, will bring in as much if not more than a window three times the size in the side. How many house lofts are as black as the Ace of Spades, until a roof light or 'Velux' is installed.

 

Focal points causing scorching can be eliminated, but should always be considered when fitting any 'lensed' light.

[Chris Pink]

The focal point with the one I have facing upwards is about 4" down so not in a particularly dangerous spot.

 

Would it change if I turned it the other way up?

[Laurence Hogg]

One thing left unmentioned here is "what" is your bullseye? If it wasnt intended to be one thats where issues may lie. If you have a signal lamp lense or a similar lense which is made to act as a optical lense then the quality of the glass will be higher, more pure, polished and clear and thus more able to concentrate light. As Chris points out his focal point is 4", that is about right for the ex railway signal lamp lenses, reversing them may help.

The Davey bullseyes I would imagine are still made at Nazeing glassworks for that purpose, these tend to be non optical grade and the glass surfaces are not polished, very often on the 8" you fnd a little square pattern indent on the flat face, this I believe is from the cooling surface they are placed on when fresh out of the die. We never experienced any problems with Nazeing products but I have seen some badly charred surrounds (usually in a arc) from signal lenses type bullseyes.

With the powerful sun we are experiencing in recent times it would be a good idea to inspect yours, on Barnet we have a signal lamp lense (not fitted by me!!) this has a brass liner fitted underneath which dissipates the heat from the hot spot which burned the original liner. its a MC one BW fitted and its replacement will be carried out as time and funds permit!

[c c]

When light from anywhere in the sky hits the curved side of the glass it emerges from the flat side in straight lines which do not converge so can not focus and concentrate heat. If light was to hit the flat side it would emerge from the curved side to converge at its focal point where its heat would be concentrated, assuming it was a polished lense and not just a piece of low grade moulded glass.

Next time you see a bull, look at its eye and you will see where the name comes from - curved bit outwards. Or even easier and certainly safer, next time you see an old photo of a working boat you will see the Bulls Eye fitted with the curved side upwards to the sky.

Edited May 15, 2011 by canalchef

[Timleech]

When light from anywhere in the sky hits the curved side of the glass it emerges from the flat side in straight lines which do not converge so can not focus and concentrate heat. If light was to hit the flat side it would emerge from the curved side to converge at its focal point where its heat would be concentrated, assuming it was a polished lense and not just a piece of low grade moulded glass.

Next time you see a bull, look at its eye and you will see where the name comes from - curved bit outwards. Or even easier and certainly safer, next time you see an old photo of a working boat you will see the Bulls Eye fitted with the curved side upwards to the sky.

 

http://www.glassperfection.com/images/plano-covexImage1.jpg

[c c]

http://www.glassperfection.com/images/plano-covexImage1.jpg

 

Is your light source from the left or the right?

Put a bulb at the focal point in your diagram and you have a searchlight, but not many back cabins have a light source placed just under the Bulls Eye to light up passing aircraft.

Decrease the radius of the curve to copy a Bulls Eye and draw the lines of light coming at it from all angles from the left in your diagram and the result should be difussion of light in the cabin interior.

Next time the sun comes out I shall try some experiments in Badseys back cabin.

[Timleech]

Is your light source from the left or the right?

Put a bulb at the focal point in your diagram and you have a searchlight, but not many back cabins have a light source placed just under the Bulls Eye to light up passing aircraft.

Decrease the radius of the curve to copy a Bulls Eye and draw the lines of light coming at it from all angles from the left in your diagram and the result should be difussion of light in the cabin interior.

Next time the sun comes out I shall try some experiments in Badseys back cabin.

 

From the left. The sun's rays will be parallel, and focus to a point as per the diagram. I have to say I'm rather surprised that ordinary cabin bullseyes can be a fire risk, I'd have expected the focal length to be rather short so nothing at risk would normally be anywhere near. There are enough scorching stories around to contradict that idea, though.

 

Tim

[furnessvale]

From the left. The sun's rays will be parallel, and focus to a point as per the diagram. I have to say I'm rather surprised that ordinary cabin bullseyes can be a fire risk, I'd have expected the focal length to be rather short so nothing at risk would normally be anywhere near. There are enough scorching stories around to contradict that idea, though.

 

Tim

 

On both occasions I had trouble, the scorched area was about 6" from the bullseye. The rather more serious damage suffered by a friend of mine was also about 6" away.

 

I note from the thread there are some doubters (not yourself Tim). Perhaps they could wait for a bright sunny day then offer up a piece of paper to the bullseye and find the focus point.

 

George ex nb Alton retired

[RLWP]

On both occasions I had trouble, the scorched area was about 6" from the bullseye. The rather more serious damage suffered by a friend of mine was also about 6" away.

 

I note from the thread there are some doubters (not yourself Tim). Perhaps they could wait for a bright sunny day then offer up a piece of paper to the bullseye and find the focus point.

 

George ex nb Alton retired

 

You could probably work it out with an assistant holding a torch

 

Richard

[furnessvale]

You could probably work it out with an assistant holding a torch

 

Richard

 

True but I was hoping the accompanying pyrotechnics would ram the message home:-)

 

George ex nb Alton retired

[Derek R.]

I'm sorry to have to disappoint Canalchef, but Tims image of light being bent through a plano convex lens is correct, and that light from either direction will produce the same focal point on the opposite side of the lens at the same distance given the same lens dimensions.

 

I have amongst my collection of tat from childhood, an almost complete and 'as given' Christmas present from my parents some time in the late fifties, an Optical experiment kit known as an 'Optikit No.1'. I dug it out (finding a little emotion in discovering Christmas tree Pine needles still within), and following the instructions using a slice of cylindrical lens of plano convex format, a torch, a mask with two vertical slots, and black paper, and in a suitably darkened place, found that the bending of two nearly parallel light beams from the torch through the slotted mask onto; first the convex side of the lens, and secondly the flat (plano) side of the lens, do in fact produce a focal point at the same distance from the light source.

 

This therefore is to say, that either way round, a Bulls Eye as mounted in a roof, will produce a focal point - given the glass is of first class clarity - that may indeed scorch any combustible materials within its range. That range will vary of course, with the angle of the Sun in the sky. As many Bulls Eyes fitted to boats are indeed convex side uppermost, they will be subject to wear and tear - abrasions, that will ultimately affect the ability to transmit light in a manner likely to cause a burning beam. There is also the factor that such glasses as supplied for deck lights were not of optical perfection for this very reason - hence, little has been heard of back cabin disasters in ex-working boats, though some folk did paint a disc on the underside.

 

The fact that railway lamp lenses and others of their kind are fitted convex side outward, will possibly be down to the fact that the flat side of the lens within the lamp housing gives moe room for the source of light keeping the light source (often a flame) further from the glass, and possibly some small nod to aerodynamics. One other factor, a Melon shaped decklight placed into a back-cabin of a narrow boat may cause some headaches of a physical kind, though most as fitted barely extend below that of the head-lining - so no problem.

 

By all means carry out your own experiments, but either way up will make no difference to light beams. Only a trip hazard factor need be considered, and for those who wish to reverse their current configuration with regard to reducing or eliminating 'scorch factor' - there will be found no benefit.

[carlt]

True but I was hoping the accompanying pyrotechnics would ram the message home:-)