The area close to domed decklights (bullseyes)

[canals are us?]

It would seem that my 4x pigeon box glass does not have bulls eyes as my glass is flat with an engraved star in it and toughened. Safe hopefully safe as I also have the same type in 2 larger portholes.

 

Like this

 

James

Edited May 18, 2014 by canals are us?

[dave moore]

Hi all

 

I'll say it again.....

 

 

The standard bull eye fitting will act as a focusing lens regardless of which way it's fitted. They originated on working boats and are often found on traditionally styled canal craft. We had the problem of a scorched liner, below the lens years ago. The solution was to glue a piece of frosted Perspex , using Silicone, to the flat bottom of the bull eye. The theory is that the frosted Perspex disperses the sun's rays preventing a fire risk. Job done!

 

Dave

[David Mack]

From what I remember from school I don't think which way up it is makes much difference to the focusing of the sun.

 

No. This diagram shows that the beam is more tightly focussed when the curved side faces the light source, and more diffuse when the flat side is towards the light. So the traditional narrowboat dome up arrangement is less safe.

 

 

 

 

Those bullseyes are just idiotic things to fit on boats. Prisms or grapefruit juicers may be ok and they will do the same job, so why anyone would put a magnifying glass in the roof of a boat I have no idea?

 

Might be better from a fire point of view, but a grapefruit juicer in a back cabin could do serious damage to your scalp!

 

[Bobbybass]

THere WAS a publicised fire..probably 4 years back...and I 'think'...Wigrams...?...

 

It was caused by a bullseye focusing the sun onto a can of (I 'think again')...expanding foam that had been carelessly left nearby.

 

Sketchy memory.....but sure I'm right..?

[Dorlan]

Our bullseye has been in place for 29 years and there is no sign of scorching anywhere near it.

 

 

The top surface of the glass is textured.

 

[MtB]

 

No. This diagram shows that the beam is more tightly focussed when the curved side faces the light source, and more diffuse when the flat side is towards the light. So the traditional narrowboat dome up arrangement is less safe.

 

 

 

 

Might be better from a fire point of view, but a grapefruit juicer in a back cabin could do serious damage to your scalp!

 

 

This type of focusing never happens in real life though, as in the UK the sun never reaches a directly overhead position.

 

For a bullseye to cause burning of the surrounding woodwork firstly the sun would be to be quite low in the sky to shine at an acute enough angle to focus on the roof surround. Secondly the focal length of the bullseye would need to be really short.

 

I wonder if the focal length shortens, the further the angle of incidence moves down from 90 degrees.

 

 

MtB

Edited May 19, 2014 by Mike the Boilerman

[furnessvale]

Our bullseye has been in place for 29 years and there is no sign of scorching anywhere near it.

There won't be. Looking at the top photo there is nothing beneath the bullseye within the area of focus.

 

Scorching will mainly occur where there are curtains or lace nearby. Sometimes the bullseye itself can be very near a cross member.

 

I once had scorching on the top of a door which was normally closed. On this occasion it was swinging loose and had come to rest directly under the bullseye. Fortunately I was onboard and got the whiff of smoke.

 

My bullseyes have old pan lids on them when unattended.

 

George ex nb Alton retired

[mark99]

I did a practical experiment today.

 

Sun was shining at approx. midday - my bullseye has top scratched/ground up and the dome is facing up.

 

By using hand/finger to find a hot zone (where sunrays were concentrated/converged) I replaced hand with a thermal probe to measure temp - it hit nearly 180c before the temp probe outer sheath started smoking. That was in about 90 seconds.

 

Conclusion - my bullseye is a potential source of ignition.

Edited May 25, 2014 by mark99

[ditchcrawler]

I hung a piece of loo paper on the edge of the bulls eye frame after putting a finger there and finding it was hot. I couldn't get t he paper to smolder and ti looks as if the focal point is fight at he edge of the rim.

 

[Clifford]

THANKS! Very grateful for this heads up.

 

At first I thought it didn't apply to me. We have no fabric anywhere near the bullseye in the kitchen ceiling, and the picture on here shows the sunlight being focussed several inches below the ceiling which would be harmless on our boat.

 

But in a sunny moment one morning a few days ago, I idly thought I'd have a look at our bullseye.

 

OK, the dome is outside; the flat side is well recessed inside a hardwood liner like a mini porthole liner. I tried to find whether the bullseye was focussing the morning sun. Hand few inches below bullseye, no heat felt. Hand higher up, nothing. Hand inside liner, almost touching glass, nothing. That's all right, then.

 

But as I removed my hand, I noticed that one side of the inside of the liner was much brighter than the other. Touched it.....

 

HOT! The morning sun was hitting the dome not from above, but on one side, and being focussed in a VERY short distance ( 2 cm below the bottom surface) onto the opposite side of the liner. I put my finger in the beam and it rapidly got too hot to bear.

 

Looking at the liner closely, there is a fine very dark crack running horizontally all round. It was not obvious before because the wood is dark anyway, and who looks up there normally?

 

It hasn't set fire to the boat in its seventeen years, but I can't now ignore an obvious sun scorch mark. I'll be emery papering the glass next time I'm on the boat.

 

Score another reason for being on here. Thanks again.

Edited May 30, 2014 by Clifford