Bullseyes

[c c]

Just to prove I was indeed talking spherically..........

http://www.suite101.com/content/bullseye-decklights-may-pose-fire-risk-a258954

(I am sure your clever ones can make this into a clicky thing)

Don't place anything a few inches under the dome.

After that, the light does indeed distribute itself around the cabin.

[RLWP]

[David Mack]

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.

 

Indeed, the focal length is the same, whichever way you use the lens, but that is not the whole story.

 

If the dome side faces the light source the all light rays pass through or very close to the focal point, whereas if the flat side faces the source, the focal length is still the same, but less of the light passes through it, as illustrated below (from here). So with the dome up, more heat is concentrated at the focal point, and thus the risk of fire is higher.

 

 

David

[RLWP]

So, that's two reasons to mount dome down - diffused focus and avoiding a trip hazard

 

Richard

[Speedwheel]

So, that's two reasons to mount dome down - diffused focus and avoiding a trip hazard

 

Richard

 

But then the water collects on/in it and it sticks down into the cabin and you keep wacking your head on it.

Edited May 17, 2011 by Speedwheel

[RLWP]

But then the water collects on/in it and it sticks down into the cabin and you keep wacking your head on it.

 

Is it really deeper than the roof lining? The one in the photo isn't

 

Richard

[Speedwheel]

Is it really deeper than the roof lining? The one in the photo isn't

 

Richard

 

They can be, in the back cabin of old boats.

[Chertsey]

They can be, in the back cabin of old boats.

And if it didn't protrude below the roof lining, wouldn't you lose the benefits of the diffusion and be just as well off with a porthole?

[RLWP]

And if it didn't protrude below the roof lining, wouldn't you lose the benefits of the diffusion and be just as well off with a porthole?

 

No. A plain porthole would let in a round "column" of light. The bullseye will spread out a cone of light as shown in David Mack's diagrams

 

Richard

[Davidss]

And if it didn't protrude below the roof lining, wouldn't you lose the benefits of the diffusion and be just as well off with a porthole?

Not according to the line drawing in post 28. This shows that the light beams don't come through the glass and then spread out (when some could hit the roof lining). Rather the light beams comes through the glass and then diagonally across the centre of the glass, so away from any roof lining that is around the edge of the glass.

 

I only have one experience I can recall of a roof mounted Bullseye. IIRC it was curved side up, but I recall with more certainty how very impressed I was with the amount and spread of light within the cabin. I made up my mind that 'my' boat would have these scattered about the boat as they gave light with privacy.

 

The only disadvantage is that my determination has never been tested, because I've never had 'my' boat, but that's another matter.

 

Regards.

[Derek R.]

This is beginning to delve into the realms of fantasy now.

 

Whether dome up, dome down, or flat both sides, the amount of light allowed into a space through a roof light will be sufficient to illuminate that space with precious little difference. A flat skylight will not allow just a 'column' of light in one place, the area at large will be lit. Much the same as with a domed light.

 

One that does catch my eye (and probably head too) would be the big 'orange squeezer' upside down. Extending beneath the deck-head that would give much diffused light. But no way would that be a practical proposition in a motor cabin with little more than five foot headroom as it is.

 

Trip hazard: It's set so close to the slide you're going to be watching where you put your plates anyway - pick 'em up! It's only a slight dome, and also mounted in a raised rim, itself not flush with the deck. Too pedantic by far.

Edited May 17, 2011 by Derek R.

[RLWP]

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.<snip>

 

 

This is beginning to delve into the realms of fantasy now.

 

<snip>

 

Trip hazard: It's set so close to the slide you're going to be watching where you put your plates anyway - pick 'em up! It's only a slight dome, and also mounted in a raised rim, itself not flush with the deck. Too pedantic by far.

 

 

Well Derek, I'll bow to your superior knowledge. Tawny Owl doesn't have any bullseyes fitted, so I can't really comment on the practicalities of them

 

Richard

[Laurence Hogg]

Indeed, the focal length is the same, whichever way you use the lens, but that is not the whole story.

 

If the dome side faces the light source the all light rays pass through or very close to the focal point, whereas if the flat side faces the source, the focal length is still the same, but less of the light passes through it, as illustrated below (from here). So with the dome up, more heat is concentrated at the focal point, and thus the risk of fire is higher.

 

 

David

 

When we looked at damage caused by Bullseyes whilst manufacturing them for the industry it was when the sun was low in the sky that problems to liners occured, the sun is rarely directly overhead as the drawings presume. I can assure that what we saw was off a low sun.

[Derek R.]

Knickers; twist; in.

 

Richard, you sound patronising.

 

The roof of a narrow boat is no ballroom. There are sufficient 'trip hazards' in pigeon boxes, exhaust pipes, mushroom vent, slide, can, mop, chimney, and perhaps a coil of line (not to mention some boaters equivalent of Kew Gardens) that a domed Bulls eye is the least of anyones problem. Most are placed dome up. Why? doesn't matter, it lets in light, and anyway - water sheds off it that way.

 

I have trouble seeing how the diagram shown in David's post can have different angles of light 'arrows' for the same lens. The upshot is: light from either direction will focus at a common point to make little or no difference. My experiment with optical quality lenses in my superior laboratory to anything that has been done by inferior learned professors in the past (tch!) has demonstrated sufficient for this impoverished dolt quite enough.

 

Laurence's point about the angle of Sun is valid. The Sun in this part of the Globe never shines from directly overhead, and the lower angles will move the focal point around the cabin, hence scorching. It may be that smaller contemporary Bulls Eyes have a greater propensity for this due to better quality glass, I don't know. I do not have sufficient in my superior shed laboratory tat equipment to conduct further tests.

 

Excuse me - kettles boiling.

[alan_fincher]

Interestingly when the BSS office issued their warning on this, the picture used for the illustration showed damage at very close proximity to the bull's eye.

 

In fact what had caught fire in that case was the actual roof lining.

 

Perhaps not one might expect, (although it is neither explained which way up it was, nor how far above the roof lining which ignited).

 

BSS Warning Flyer - Link (PDF document)

[Davidss]

Interestingly when the BSS office issued their warning on this, the picture used for the illustration showed damage at very close proximity to the bull's eye.

 

In fact what had caught fire in that case was the actual roof lining.

 

Perhaps not one might expect, (although it is neither explained which way up it was, nor how far above the roof lining which ignited).

BSS Warning Flyer - Link (PDF document)

Gosh, I echo your comments, how thick is that roof lining? Certainly a lot thicker than I was thinking about when I posted earlier. I'm also embarrassed to admit I'd neglected to think about the 'low angle' situation that Laurence Hogg has advised us about.

[station tug]

the place were im building my boat are ex mod serplus dealers...iv found a quantity of what look to be 5" lamp lense's the glass is domed and seems to be 1/2 thick in a brass surround...i'll do my best to put a pic up...could these be used as bulls eyes???

[Laurence Hogg]

I will state this once again in case anyone has missed it. I manufactured bullseyes for years, MC copied mine and Davey still make some which go back an awful long time. Whether its a bullseye, a prism, the orange squeezer shape they are ALL intended to be used with the FLAT surface upwards as these are all "DECK LIGHTS" which are meant to be walked on.

Below is part of the Grand Union Canal Co drawing for a "Royalty" class motor showing the item as a 8" decklight.

This is from Yarwoods drawing for the "Town" class, it is shown as 8" Bullseye.

The 8" Decklight consists of a frame with a recess into which the glass is sealed, then a ring which covers the edge of the glass and effectivly encapulates the glass into a double sided brass frame, the outer ring is polished as this is the item seen on the deck floor.

Why this was reversed on the GU fleet boats is a mystery as shipbuilders when then using a component in a way it was not designed for and normally the raised surface of the rebated casting would not have been finished to a polished standard.

A look in any old (1940/50's) Simpson Lawrence catalogue will allow you to see all the sizes and types of decklight which were available, none were used proud of the deck surface.

[Chertsey]

The fact that the cabin slide so often scrapes over and/or wbbbles on the dome lends weight to Laurence's view that the designers of the boats didn't intend them to be fitted that way up, I guess.

[c c]

That all sounds very sensible and leaves little room for doubt. Question is:

Should those who try to keep a GU boat back cabin as original as possible, have the bulls eye up or down?

[Chertsey]

That all sounds very sensible and leaves little room for doubt. Question is:

Should those who try to keep a GU boat back cabin as original as possible, have the bulls eye up or down?

I'm not taking mine out again to turn it over, it would definitely fall to pieces this time!

[Mike C]

Indeed, the focal length is the same, whichever way you use the lens, but that is not the whole story.

 

If the dome side faces the light source the all light rays pass through or very close to the focal point, whereas if the flat side faces the source, the focal length is still the same, but less of the light passes through it, as illustrated below (from here). So with the dome up, more heat is concentrated at the focal point, and thus the risk of fire is higher.

 

 

David

I have seen scorching on a number of craft over quite a number of years. My experiences are that:

 

1. It makes no readily decipherable difference whether the lens is up or down. On one of the incidents that I examined a few years ago for insurers the lens was removed after the boat fire and used to create a focal point either way up in order to prove that the boat builder had not been blatantly negligent as was being suggested.

 

2. It is easier to create a focal point with sufficient intensity to burn combustibles with a polished lens used as a deadlight, but it is also possible, though a little harder to do so, with an unpolished or non-dressed cast glass deadlight.

 

I have seen both a thin layer of white wash, (or white undercoat) applied on the inner cabin side of the dead light and also adhesive privacy film, as sold in hardware stores for bathroom windows. On the examples that were examined no effective focal point could be created in midsummer sunlight.

 

My own, in a big Northwich motor with an unpolished original deadlight, fitted flat side in the cabin and has a thin layer of white undercoat on it, no decipherable light loss and no scorching since but there were signs of scorching from the summer of 2006 before the paint was applied.

[Laurence Hogg]

I have seen scorching on a number of craft over quite a number of years. My experiences are that:

 

1. It makes no readily decipherable difference whether the lens is up or down. On one of the incidents that I examined a few years ago for insurers the lens was removed after the boat fire and used to create a focal point either way up in order to prove that the boat builder had not been blatantly negligent as was being suggested.

 

2. It is easier to create a focal point with sufficient intensity to burn combustibles with a polished lens used as a deadlight, but it is also possible, though a little harder to do so, with an unpolished or non-dressed cast glass deadlight.

 

I have seen both a thin layer of white wash, (or white undercoat) applied on the inner cabin side of the dead light and also adhesive privacy film, as sold in hardware stores for bathroom windows. On the examples that were examined no effective focal point could be created in midsummer sunlight.

 

My own, in a big Northwich motor with an unpolished original deadlight, fitted flat side in the cabin and has a thin layer of white undercoat on it, no decipherable light loss and no scorching since but there were signs of scorching from the summer of 2006 before the paint was applied.

 

Yours will be a 8H - 014 from Nazeing glassworks as supplied original, I have never seen a problem with these and we used to sell a lot, we never sold the smaller signal lamp lense (but did make surrounds)as we considered it dangerous. These are "Decklights" btw "Deadlights" are what people mis call "portholes" fixed or opening!

[marcus]

Yours will be a 8H - 014 from Nazeing glassworks as supplied original, I have never seen a problem with these and we used to sell a lot, we never sold the smaller signal lamp lense (but did make surrounds)as we considered it dangerous. These are "Decklights" btw "Deadlights" are what people mis call "portholes" fixed or opening!

 

like so many nautical terms I think it depends on where you are or what your circumstances are but my understanding is that an 'Dead light' is any heavy galzed item (not window) that is not opening - thus it is 'dead'.

 

A quick seach reveals both of the below:

 

deadlight [ˈdɛdˌlaɪt]

n

1. (Transport / Nautical Terms) Nautical

a. a bull's-eye let into the deck or hull of a vessel to admit light to a cabin

b. a shutter of wood or metal for sealing off a porthole or cabin window

2. (Fine Arts & Visual Arts / Architecture) a skylight designed not to be opened

 

dead·light (ddlt)

n.

1. Nautical

a. A strong shutter or plate fastened over a ship's porthole or cabin window in stormy weather.

b. A thick window set in a ship's side or deck.

2. A skylight constructed so that it cannot be opened.

 

with a deadlight becoming a porthole when it can be opened - thus not 'Dead' and only a porthole when it can be opened which would be when not at sea and so when in port.

[Mike C]

like so many nautical terms I think it depends on where you are or what your circumstances are but my understanding is that an 'Dead light' is any heavy galzed item (not window) that is not opening - thus it is 'dead'.

 

A quick seach reveals both of the below:

 

deadlight [ˈdɛdˌlaɪt]

n

1. (Transport / Nautical Terms) Nautical

a. a bull's-eye let into the deck or hull of a vessel to admit light to a cabin

b. a shutter of wood or metal for sealing off a porthole or cabin window

2. (Fine Arts & Visual Arts / Architecture) a skylight designed not to be opened

 

dead·light (ddlt)

n.

1. Nautical

a. A strong shutter or plate fastened over a ship's porthole or cabin window in stormy weather.

b. A thick window set in a ship's side or deck.

2. A skylight constructed so that it cannot be opened.

 

with a deadlight becoming a porthole when it can be opened - thus not 'Dead' and only a porthole when it can be opened which would be when not at sea and so when in port.

 

Yes that is my understanding

Edited May 19, 2011 by Mike C