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Hi all, 

Only my second-ish post here and I wondered whether anyone could offer any advice re. rust in the bilges. I bought my boat a couple of months ago and have made a bit of a start on insulating it with celotex. After stripping away some panelling and polystyrene I found a load of flaking rust where the hull meets the bilge. I took one of the flooring panels off and found a lot of flaking rust in there. (Have included pic). The boat has been overplated and a kind soul on the canal told me that that's encouraging...that possibly this is old rust. Does anyone have any opinion re the pic attached? 

IMG_20180225_142604414.jpg

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One of the main things to remember about rust is that it only happens in the presence of oxygen and water. So keep the under floor areas dry and it won’t get any worse.

Also, for each millimetre of steel, when it rusts you get about 6 mm of rust, so it often looks worse than it is.

Welcome to the forum :) 

Edited by Stilllearning

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Ok, thanks for the reply. It's not currently wet at that spot. And other bilge areas further down are also dry. I have a fair bit of condensation and rust in places on the hull and gunwale but I'm thinking there's no actual ingress of water from outside. So I'm hoping it's old rust. By the sounds of it, you don't think that surface needs treating, right? Just keep it dry?

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In an ideal world you could take the lot back to clean metal and paint it but that's not going to happen. If its been overplated then effectively that is no longer the bottom. As Still learning says, you need oxygen and water/damp to get rust. The picture shows some quite severe old corrosion but if its kept bone dry then it is ok. If I were you I would concentrate on keeping everything as dry as possible in the future. Your photo though is a worry for people who have no idea what is happening under the floor, If you docked a boat and found that on the outside you would burst into tears. I really think that boatbuilding needs to re think some things here. It's lovely to have fitted carpets and nice flooring and it sells boats but it is much more important to be able to lift the floor up. Houses have a hell of a lot of work under the ground but boatbuilders just bung big sheets of ply down and build everything on top. Its not good practice.

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Presumably the picture shows rust from the baseplate.  Has your boat's baseplate been overplated?  I'd be very careful before poking around on rust like that.  I've posted before my tale of woe when I was removing rust from the inside of the of the boat where it had been overplated.  The overplating had been quietly rusting away from the inside.  As has been pointed out many times, the quality of the overplating is paramount.  What did your surveyor say?

 

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@Bee Many thanks for the reply. Yeah, interesting point. Someone here in the canal has advised putting floor hatches at intervals. I currently just have two hatches, both at the stern. I had to saw around rusted screws to get that floor board up. 

For my own understanding (apologies for revealing ignorance), once I have covered the steel of the hull with flag paint, celotex, silicone for joints, and ply lining, where will condensation condense? Is the hope that I will block the steel off from air such that there is no water vapour with access to it, and that the ply will not be cold enough for water vapour to condense? 

(I am referring to the walls of the hull now - but I'm guessing a lot of the moisture that caused that rust trickled down from condensation on the walls)

@koukouvagia Again, thanks for reply. The surveyor did a full ultrasound and seemed confident. I don't have the survey with me but the readings were good. In general the baseplate was 10mm but I do remember that there were areas that were a bit thinner and i raised this with him but he said it was ok. 

Since it's not actually wet down there do you think I can just let that rust lie? Is it possible my poking will damage the overplating?

I'm planning on laying insulation, new ply (on top of old ply) and vinyl down. 

 

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If the new baseplate is around 10mm, then you have plenty of steel underneath you. As has been said by Bee, keep access as easy as possible to the under floor area: damp or condensation can and will appear all by itself in spite of what you do. Keeping the boat warm dry and lived in is the best way to maintain it in a dry state. A warm boat with adequate high level ventilation, ideally from roof vents, should give a reasonable level of circulation and so drive out any moist air.

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@stilllearning Ok, so it's just a case of monitoring things via hatches. Ok, thanks a lot for advice. (I'm finding it hard to keep this boat warm! 10mm polystyrene insulation above the gunwale right now. Also, brazier smokeless coal damn awkward to light).

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3 minutes ago, klgilfillan said:

10mm polystyrene insulation above the gunwale right now.

I guess that’s marginally superior to having nothing at all ;)

Are you aiming to replace/add to that?

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Marginally superior to just sleeping under the stars, yeah. Marginally. Yeah, gonna do 25mm celotex above gunwale, 50mm below gunwale and I think maybe 12mm on the floor. Currently 25mm poly on the ceiling. Was maybe going to leave that for later - do you think ceiling is critical? 

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The original insulation is probably responsible for the corrosion that you have discovered, when you redo the insulation it should stay drier down there. If you are insulating under the floor it would be wise to clear the loose rust whilst you're there, as this will hold onto any moisture that finds its way down.

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I have never noticed condensation on the bottom plating of a boat, maybe the air is just 'dead' down there? Maybe I've always been too scared to look.

7 minutes ago, klgilfillan said:

Marginally superior to just sleeping under the stars, yeah. Marginally. Yeah, gonna do 25mm celotex above gunwale, 50mm below gunwale and I think maybe 12mm on the floor. Currently 25mm poly on the ceiling. Was maybe going to leave that for later - do you think ceiling is critical? 

Ceiling? 25mm poly? I would say that's a bit thin especially if its laid between the roof 'frames', problem is that they will be cold spots and you will get lines of condensation on the ceiling. I would add another 25mm over the lot. Poly is a bit of a fire risk so be careful, Maybe use the 12mm on the floor for the ceiling if on a budget.

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9 minutes ago, klgilfillan said:

do you think ceiling is critical? 

If everything else is well insulated the roof will become the prime candidate for condensation in any gaps between the poly so at the minimum I’d suggest you seal all gaps and fit a vapour barrier. 

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3 minutes ago, Bee said:

I have never noticed condensation on the bottom plating of a boat, maybe the air is just 'dead' down there? Maybe I've always been too scared to look.

No, neither have I, but it's a rare boat that doesn't have a plumbing leak at some time.  Then moisture can hang around for ages.

 

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Ok, I'd better just do the ceiling then. I'll be getting a loan to cover it all anyway. I can actually see rust under some of the light fittings. So 50mm celotex on the ceiling, I'm guessing? Or is 25mm good enough (since it's more effective than polystyrene)?

@BWM  Ok thanks for advice about removing loose rust. Will make sure to do that.

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Have you considered using glass mineral wool insulation slab instead of Celotex?  This is designed to be friction fitted - i.e. it can be stuffed into all the many nooks and crannies and stays put when wedged between battens. Any uninsulated area, however small, is a condensation  magnet. It is much is much more difficult achieve complete coverage using a rigid product.  Also slab insulation is much cheaper, if you're on a budget.

The disadvantages are that the insulating properties are not quite so good (although I've never had any problems)  and you need to have a vapour barrier (building plastic sheets stapled to battens in front of the insulation slabs).

Link to slab insulation from Wickes  https://goo.gl/8S1saB

 

 

 

 

Edited by koukouvagia
  • Greenie 1

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Ok, thanks, I'll look into glass mineral wool. But does it come in slab form and also loose, stuffing, form? If one stuffs it in, without using adhesive, will it be airtight enough against the steel? Or do you use adhesive?

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I assume that having lifted the floor and found all that mess you've swept it all up and removed it. Stuff like that on the steel bottom encourages damp which then lurks there converting more steel to rust.

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20 minutes ago, klgilfillan said:

Ok, thanks, I'll look into glass mineral wool. But does it come in slab form and also loose, stuffing, form? If one stuffs it in, without using adhesive, will it be airtight enough against the steel? Or do you use adhesive?

It doesn’t particularly need to be airtight against the steel (although it’s good if it is) as long as it’s covering the whole area, leaving no bare steel cold spots. The vapour barrier achieves the airtight bit. 

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19 minutes ago, klgilfillan said:

Ok, thanks, I'll look into glass mineral wool. But does it come in slab form and also loose, stuffing, form? If one stuffs it in, without using adhesive, will it be airtight enough against the steel? Or do you use adhesive?

Picture shows how the slab insulation is friction fixed between the battens.  You can also see the odd corners where small pieces are stuffed in.  There is no need to use adhesive and it is airtight against the steel.  

Everything is bone dry after ten years.

 

 

Mar17 010.jpg

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1 hour ago, Bee said:

I have never noticed condensation on the bottom plating of a boat, maybe the air is just 'dead' down there? Maybe I've always been too scared to look.

Ceiling? 25mm poly? I would say that's a bit thin especially if its laid between the roof 'frames', problem is that they will be cold spots and you will get lines of condensation on the ceiling. I would add another 25mm over the lot. Poly is a bit of a fire risk so be careful, Maybe use the 12mm on the floor for the ceiling if on a budget.

 

1 hour ago, WotEver said:

If everything else is well insulated the roof will become the prime candidate for condensation in any gaps between the poly so at the minimum I’d suggest you seal all gaps and fit a vapour barrier. 

A lot of wisdom in both the above posts. 

The reason for the constant dampness that must have been there to cause the corrosion is very likely to be down to the scanty insulation, with condensation forming on both the interior of the cabin top and the sides, it is only going to run downward into the bilge area. Before finishing the fit out of our current craft I could find a surprising volume of water during cold spells.

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3 hours ago, klgilfillan said:

... (I am referring to the walls of the hull now - but I'm guessing a lot of the moisture that caused that rust trickled down from condensation on the walls)... 

When I had Innisfree spray foamed I finished it at floor level and blacked the bottom few inches down to the baseplate, my reasoning being the steel would never get cold enough there for condensation to form. I was wrong, condensation did form on those few inches in very cold weather and before it could evaporate it would run down to the baseplate only for condensation to reform and run down again. Baseplate wasn't a prob as it was horizontal so allowed condensate to evaporate. In hindsight I should have sprayed down to baseplate. Even better would have been a sheet of Celotex/Kingspan (thickness dictated by ballast)  on the baseplate but finished an inch or so from the side foam so as to form a drainage channel to direct any water spillage back to the stern bilge area for mopping up. This channel could be blacked to protect the steel at this point. 

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@ BWM Ok, right, I understand. Thanks very much for advice. 

@nb Innisfree Hm interesting point about the drainage channel. I didn't know about that. Will figure it into plans. Thanks.

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  • Yellow = professionally applied sprayfoam
  • Pink = fireresistant sprayfoam applied to bits they missed
  • Bricks = ballast, Engineering grade A, since replace in part by iron
  • White = underfloor heating pipe, abandoned
  • Drainage channel = none

 

IMGP5025.JPG

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