Skin Fitting - d'oh

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efitz

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I did something rather silly the other day. As part of winter maintenance in the yard, I was trying to clean some mussels and crud out of the scoop for the engine water intake.
Then I had a brainwave - oh dear. I saw that the aft end of the outer scoop was screwed into the hull, so I assumed that the scoop was a cover and could be removed (new boat - first experience of this type of skin fitting). So I took out the screw and began tapping the fitting with a hammer expecting it to pop off. After a couple of minutes of this and of nothing happening, the penny drops - it's all one fitting. Then I think oh sh**, what have I done?
The fitting didn't turn a fraction at all so I am assuming that I didn't break the seal. It seems to be bedded with sikaflex, and the fitting is rock steady on the inside. I really don't want to remove, clean and rebed the fitting, but my own stupidity is nagging me and making me feel that I should.


Any advice would be welcome.
 
Something similar to this

watermark.php


Its all held and sealed by the big retaining nut.

Almost certainly still water tight if it did not move.


You did say tap not whack?
 
point of information, please VicS.

I have come across thermal embrittlement a bit, but are any of the metals used in boat fittings likely to become fragile in the temps we have had recently (Say down to -12 or so ) ?

thanks

PS I looked in K&L, and ended up with longitudinal in-plane shear modulus, longitudinal through-thickness shear modulus, - at which point, I thought,....No. :)

Limits of understanding.
 
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Vic,

The fitting is similar to that but a bit more rounded in shape. I did tap it firmly but didn't whack it. The prospect of removing all the old sikaflex before refitting puts me off if it isn't really necessary.
 
efitz said:
Vic,

The fitting is similar to that but a bit more rounded in shape. I did tap it firmly but didn't whack it. The prospect of removing all the old sikaflex before refitting puts me off if it isn't really necessary.
Click to expand...


If you don’t re seal it, it will be a worry, on of your mind all season!
 
sarabande said:
point of information, please VicS.

I have come across thermal embrittlement a bit, but are any of the metals used in boat fittings likely to become fragile in the temps we have had recently (Say down to -12 or so ) ?

thanks

PS I looked in K7L, and ended up with longitudinal in-plane shear modulus, longitudinal through-thickness shear modulus, - at which point, I thought,....No. :)

Limits of understanding.
Click to expand...

The phrase you are looking for is ductile-brittle transition. Carbon steels have a pronounced transtion at about room temperature, dependent upon carbon content. At arctic temperatures there is considerable risk that some steel components will break in a brittle manner.
Neither brass nor austenitic stainless steels display a transition temperature, which is why the latter is used throughout LNG plants. For other metals I suggest you Google the phrase.
 
Titanic

vyv_cox said:
The phrase you are looking for is ductile-brittle transition. Carbon steels have a pronounced transtion at about room temperature, dependent upon carbon content. At arctic temperatures there is considerable risk that some steel components will break in a brittle manner.
Neither brass nor austenitic stainless steels display a transition temperature, which is why the latter is used throughout LNG plants. For other metals I suggest you Google the phrase.
Click to expand...

It has been suggested that ductile-brittle transition was the cause of the loss of the Titanic. This is based upon analysis of some rivets that were recovered from the wreck. The heads of the rivets might have survived impact with the iceberg but being brittle they broke at low stress levels, opening a plate. The Oceanic, a sister ship, also lost a plate after impact with a rock, although not at low temperature.
 
vyv_cox said:
Neither brass nor austenitic stainless steels display a transition temperature, which is why the latter is used throughout LNG plants.
Click to expand...

I spent a fair bit of my early career (sic) developing high strength, low temperature materials. Stainless undergoes a shear transition to martensitic BCT at some horribly high (to us cryogenics people) or low (to everyone else) temperature, giving it, as I recall, a 30% UTS boost by 77K. I don't suppose that's terribly relevant to boats, is it?

Mumble, mumble, mumble.
 
what happens, for instance, to CO2 bottles in liferafts and PB Aids when they discharge ? I know they become quite cold, but is it cold enough to affect plastic or metal piping taking the gas to the tubes or discharge horn ?

I heard from our local Crime Prevention Unit (bless them !) that some villains use CO2 extinguishers to freeze locks, and they just tap them with a hammer and the lock shatters. :eek:
 
sarabande said:
I heard from our local Crime Prevention Unit (bless them !) that some villains use CO2 extinguishers to freeze locks, and they just tap them with a hammer and the lock shatters. :eek:
Click to expand...

Some of our local crims are into that mis-use of technology, but being Wiltshire, they didn't go on to do the advanced course.

This is what 'going equipped' looks like in Swindon....

Sledge-Hammers.jpg


:)




Back to the thread... Should you choose to keep the fitting undisturbed to 'see what happens', do put your mind ( and mine ) a bit at rest by securing the regulation right-sized 'softwood plug' on a string close by the inner side of the fitting. Then you'll have something positive to tell your insurance company....

:)
 
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sarabande said:
what happens, for instance, to CO2 bottles in liferafts and PB Aids when they discharge ? I know they become quite cold, but is it cold enough to affect plastic or metal piping taking the gas to the tubes or discharge horn ?

I heard from our local Crime Prevention Unit (bless them !) that some villains use CO2 extinguishers to freeze locks, and they just tap them with a hammer and the lock shatters. :eek:
Click to expand...

After some of the spectacular failures that occurred when large welded structures began to be built ( see Schenectady), it was found that adding manganese to a ratio of 4:1 against carbon lowered the ductile-brittle transition to a value where it was no longer an issue. Ships' hulls, bridges, pipelines, etc are now built using this type of steel and I suppose that CO2 extinguishers are similar.

As for plastic, many types become brittle not far below 0C so they are probably at risk.
 
Just a bit of useless information, the T2 tankers were turbo-electric propulsion!

They were kept in service for some time after the war unril replacement tonnage could be built. They had long gone before I went to sea, but I did sail with a few old engineers who had sailed on them. I was told, although I don't know if it was true or not, that some T2's were taken up to the head of remote Norwegian fiords when their sailing days were over, parked alongside and the turbines used to generate power for the local town.
 
vyv_cox said:
It has been suggested that ductile-brittle transition was the cause of the loss of the Titanic. This is based upon analysis of some rivets that were recovered from the wreck. The heads of the rivets might have survived impact with the iceberg but being brittle they broke at low stress levels, opening a plate. The Oceanic, a sister ship, also lost a plate after impact with a rock, although not at low temperature.
Click to expand...

Sorry for Fred Drift, but the Titanic's sister ships, were Olympic and Brittanic (I'm not being pedantic: my point, such as it is, comes at the end).

The Oceanic was an earlier ship with the same White Star line, designed by Thomas Ismay, father of Titanic designer Joseph. Ironically she was involved in the Titanic rescue. She was wrecked off Shetland in Sept 1914.

The Brittanic sank after striking a mine off Greece in 1916.

The Olympic had an eventful career, serving as a troopship, ramming and sinking a U-boat (the only merchant vessel to do so in WW1), hitting various other vessels including a lightship. She was finally scrapped in 1937.

Anyway, my reason for mentioned all this is that I've read that the same woman served as a nurse on all three sister vessels, went down with the first two, and can't have been the most popular crew member when she joined the third. (I imagine post-traumatic stress wasn't uppermost in her employer's mind.) Does anyone know more about this doughty person?
 
If i was you id change it, it will take you 3 hours at most to do it, its not hard at all. That way you have your own piece of mind that its all sealed up. Not only that if its old, say 10 years plus then id change it anyway. My ten year assumption may open a can of worms here, some say if its fine just leave it alone. The truth is, you dont know what type of metal it is, (unless you bought and fitted it) and you dont know what condition its in untill you remove it.
 
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