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How much hull thickness do you actually lose??


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Hi

 

I was wondering how much hull thickness you actually lose from what looks like a lot of internal rust/flaking inside the bilge?

 

I have read some where that 1mm of flaking equates to 0.1mm of actual loss? Does this sound accurate? Or is there a better guide?

 

Cheers

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I had just been trying to find an reference source but I can't find one quickly. I agree with Alan it's the figure you often see quoted and nothing in my experience gives me reason to doubt it.

 

 

edit to add apostrophe before the punctuation police attack

Edited by John V
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Many years ago electric motors on trawlers was dipped into a white paint? solution this protected them from corrosion in a very harsh environment would seem to be ideal for bilges alternatively can you still get white lead paint ?

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I'd say it depends on how you measure the 'thickness' of the rust flakes.

 

If you firmly crushed the rust down flat then yes I'd image about 10:1 is about the right volume ratio but surface rust flaking usually contains a high proportion of air. When you actually scrape off a significant volume of rust/paint flakage, the metal below has often lost virtually nothing.

 

MtB

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We have our boat out now on your marina just inside the gate, after rescuing her from sinking last saturday..It was surveyed yesterday and only fails on the swim and the plate above after spending many years with a wet through bilge and alot of rust...best advice we were given was to remove rust dry and recoat and keep it dry and well ventilated.We did the majority of the boat floor when we had her 5 years ago and its lost barely anything and most measurements were 4.5 and above.

Were just doing the last 8 feet now since its being redone inside and it looked like this throughout with wet soggy rust holding the water all the time.(sorry for pic quality)

 

257osp3.jpg

 

After scraping,

 

2rhu3bb.jpg

 

Then painting,

 

zjdq4o.jpg

 

After all this the base plate has suffered very little if any, most important thing being get the rust out as ours rotted from the inside out.

 

 

 

 

.

Edited by Orakal
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We have our boat out now on your marina just inside the gate, after rescuing her from sinking last saturday..It was surveyed yesterday and only fails on the swim and the plate above after spending many years with a wet through bilge and alot of rust...best advice we were given was to remove rust dry and recoat and keep it dry and well ventilated.We did the majority of the boat floor when we had her 5 years ago and its lost barely anything and most measurements were 4.5 and above.

Were just doing the last 8 feet now since its being redone inside and it looked like this throughout with wet soggy rust holding the water all the time.(sorry for pic quality)

 

257osp3.jpg

 

After scraping,

 

2rhu3bb.jpg

 

Then painting,

 

zjdq4o.jpg

 

After all this the base plate has suffered very little if any, most important thing being get the rust out as ours rotted from the inside out.

 

 

 

 

.

 

That looks like a very strong framing structure for a baseplate to me. Most modern NBs just have cross-members at 18" centres, nothing running lengthways. My widebeam has cross-members at 18" centres and 3 running along the length of the boat but it's not going to be as strong as yours. Then again I have a 10mm baseplate. Someone will be along shortly to correct my framing/scantling terminology I'm sure.

Edited by blackrose
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That looks like a very strong framing structure for a baseplate to me. Most modern NBs just have cross-members at 18" centres, nothing running lengthways. My widebeam has cross-members at 18" centres and 3 running along the length of the boat but it's not going to be as strong as yours. Then again I have a 10mm baseplate. Someone will be along shortly to correct my framing/scantling terminology I'm sure.

The reason its so strong might be because it was built in 1976 in the middle ages plus it was built in imperial so its about 6mm only.

The other reason might be because its a through bilge something todays boats dont incorporate thank god as its a horrid idea.

Added to this the length way strips support the ballast not the floor which i prefer personally, i wont even stand on our floor although it should be sound enough.

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The reason its so strong might be because it was built in 1976 in the middle ages plus it was built in imperial so its about 6mm only.

 

Ermmm, 6mm is metric!

 

Anyway UK engineering went metric in 1971/1972 IIRC.

 

 

MtB

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Ermmm, 6mm is metric!

 

Anyway UK engineering went metric in 1971/1972 IIRC.

 

 

MtB

Yeah im aware 6mm is metric hence saying "about".

 

It must have been made from 1/4 inch which i think is 6.35mm but im not pedantic enough to put that so 6mm sufficed as conversions carry too many decimal places for general chat.

Given the fact every original bolt and fitting ive come across is imperial im happy to believe the boat uses an imperial steel thickness, something the surveyor also mentioned to me friday, who knows....she floats ,just....it'll do me.

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One thing that I've found to be really useful Is to find a piece of steel the same thickness as your hull that is seriously pitted so you have a point of reference. Of course steel rusts in different ways but its interesting how dramatic it looks when the steel has pitted to half its original thickness.

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