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Old 28-07-2011, 08:57   #1
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Testing Cement Hulls for Defects . . .

... Firstly, how long does it take for the steel mesh inside a well maintained cement hull to rust significantly? I have heard that rebar inside concrete buildings can rust, even if treated against rust, in about 30 years. The rebar oxidizes and expands, cracking the concrete and causing it to fall off.

I have seen one concrete sailboat hull where the same thing seemed to be happening on the deck, so I was wondering how long this kind of "rot" takes to happen. Also, how and where do I check for it?

What would the tell tale signs of such rusting look like if it is occurring, but there were no visible signs of cracked concrete?

What are the other signs of a bad hull, and what are the problems typical of a concrete boat? What should I be looking for?

Thanks for all the help you guys have been on the various threads posted. Don't know what I'd do without ya!

G2L
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Old 28-07-2011, 09:06   #2
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Re: Testing Cement Hulls for Defects ...

This may be the major problem with ferrocement boats: it's too hard to tell what's going on inside the hull. This is why some surveyors won't even look at them, and many insurance companies won't write policies for them. It might be easy to tell if the construction has gone bad, but there's no non-destructive way to insure that it's still good.
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Old 28-07-2011, 17:17   #3
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Visual only...

First, don't ever hit ferro with a hammer. It will explode a chunk of concrete from the inside and the owner will be as mad as a bunch of cut snakes when they find out.

A screwdriver is similarly, useless.

A good visual inspection may tell you enough to start looking for a surveyor who knows ferro cement.

As iron oxide (rust) takes up more space than the original steel evidence of rusted reinforcing should be visible on careful inspection.

The big issues on ferro cement that I am aware of are 1) Voids caused during the plastering process, 2) insufficient reinforcement 3) contamination by diesel and salt water and 4) electrolytic erosion of reinforcement.

If they exist these problems may be evident with a careful inspection, a surveyor may find them or ultimately, if you really like the boat, they need to be factored into the price.

When using a new to you ferro boat it would be wise to do initial voyaging in sheltered waters.
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Old 29-07-2011, 05:45   #4
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Re: Testing Cement Hulls for Defects . . .

Just curious, how many here have have built, or been intimately involved in building a ferro boat.
Without googling first, who knows the difference between type 1 and type 5 portland, and knows the curing procedure and times for the types.
Would you use uncoated, galvanized, or epoxied metal work and mesh.
What are your thoughts on the "heating coil" effect of a lightning hit charge passing from mast thru to mesh, and the resulting vaporisation of the matrix moisture content.
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Old 29-07-2011, 09:34   #5
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Re: Testing Cement Hulls for Defects . . .

Have not built a ferrocement boat have built structures using ferrocement technique. Wonderfully strong stuff, with proper care will last longer then any of us. Rust of the destructive nature (expansion great then the cements natural expansion) only happens if the armature has been allowed to be exposed to oxygen/water (which means it was an improper build to begin with as you want all metal covered by the cement). The building I did was with a modified CEM I cement that pretty much equals Portland type I.

Cement and Iron expand at the same rate so as long as there is no reason for the iron to continue expanding past the original bond formed, it won't.

As far as building a ferro boat I would use CEM II or III type cement with 20-30 replacement of the sand with either fly ash or blast furnace waste. Both of which increase the strenght as well as the bonding of the cement to the mesh. Another nice bonus is increased resistance to Chlorides and Sulfates. As far as mesh, wire and rod are concerned, uncoated is best from every source I have seen.

Interesting on the lightening point. Taking to a few folks who have ferrocement for greater then 10 years and have brought this issue up with their insurer. There seems to be no issue; as there has never been a claim of that type according to their insurance agent (getting insurance is all in the documentation, same as with any boat).

From a surveyor here in Scotland, I was advised that a good solid rubber mallet will do the job as you will hear the dead spots very easily. Also ask for a build log (as you should with any boat built). As far as I am concerned you can keep slagging on the ferrocement boats all you want. Then again it deepens on what you want from a yacht. Different folks have different views. I would suggest that folks do a good bit more study or real world work with ferrocement before you slag the material on its physical properties, as that just makes you look like fool.

As with any boat building, ferrocement is time intensive and requires you either know very well what you are doing with cement or that you hire folks who do for the plastering. Before you even do that you need to be darn sure that the armature is fair in and of it self as that will directly effect the finish of the final hull.

Only other types of inspection you can do are core sample or if you can afford it ultrasound. If the surface is flaky or showing lifting in sections (hard for me to describe, hopefully some one else will be able to) walk away from it. If there are minor cracks that have NOT been sand blasted, they NORMALLY are of no import and can be painted over with a two part epxoy without any worry. Now remember MY experience is in structures and foundation work as well as embankment re-enforcement. You want more information go talk to some one who has more direct information.

Hope some of this helps and good luck on finding one that is sound and that you like.

Michael
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Old 29-07-2011, 20:22   #6
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For Boracay, et all ... More questions on hulls ...

... Thanks to all again for the tips.

Boracay,

I was thinking along the same lines in that, if there is a problem with rust, the hull would bulge where the rusting is taking place, no?

If the boat has nice, smoth lines, along the hull, and especially at the waterline or below, probably there is no serious problem with it.

The same seems to be true with wood, although, with wood you can actually feel the soft spots as well.

I have been told that the same is true, to a lesser extent with glass boats. In other words, if the hull looks smooth, its probably OK, and that spots will actually be soft, if there has been poorly repaired damage or rot in key places like the glassed wood which forms the keel well.

Does all this make sense to you?

Lastly, folks have advised tapping (gently) the hulls of both fibreglass and wood boats for "soft spots" or hollow sounding spots. As a novice to construction materials and techniques, I have been trying this tapping system on various boats I have looked at over the last few weeks, and the problem for me is that even good hulls give off different pitched sounds when you tap them, depending on what part of the hull you are hiting.

For instance, on a glass boat with a heavy keel as you tap down toward the keel, the pitch of the sound gets higher and sounds more like a "crack" than a "thump" (low pitched sound). Obviously that is because that part of the boat is very "solid" with some kind of heavy reinforcement behind the glass, so that the keel can be attached.

However, as you move up the hull toward the water line and deck, the sound will change and even sound "hollow" at times. As you move forward or aft, the sound will also change from more like a "crack" to a hollow sounding "thump", apparantly because where one hears the "thump", one is tapping at the center point between reinforced elements of the hull's structure. When one hears the crack, one is tapping on a reinforced section of the hull.

All this being said, how can one tell much from the sound of tapping the hull?

Any advice would be appreciated.

Regards to all,

G2L
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Old 30-07-2011, 02:19   #7
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Metal detector?

I have never tried it but if I had to find a non destructive way to test a ferro hull I'd beg, buy or borrow a metal detector.

One with an audio output or headphones that gives a different tone for different metals.

Once one got used to waving it over the hull it should be possible to get some idea of whats in there.
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Old 30-07-2011, 05:16   #8
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Re: Metal detector?

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Originally Posted by Boracay View Post
I have never tried it but if I had to find a non destructive way to test a ferro hull I'd beg, buy or borrow a metal detector.

One with an audio output or headphones that gives a different tone for different metals.

Once one got used to waving it over the hull it should be possible to get some idea of whats in there.

Thanks again,

Such may not be available where I am at the moment, but your advice is sincerely appreciated. Any comments on my other questions, or would you suggest I post them on another thread?

Best regards,

G2L
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Old 31-07-2011, 23:27   #9
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Re: Testing Cement Hulls for Defects . . .

Hi Gone2long,

Concrete doesn't `rot' in a certain timeframe, in fact it gets harder the longer it exists. The problem comes when there are fractures caused by impact damage, stress fracture etc which open up the steel inside to corrosion. Only then does rust `rot' set in on the steel mesh and/or rods.

The keel, gunwhales and bulkheads, and probably a few other places too, are the most susceptible to cracks due to the stresses that are put on them.

What looks like localised damage on the plaster can result in a slightly wider area of hidden damage because the mesh can rust away inside BUT any well built ferro boat has closely spaced rods in the structure and when one of these rusts you WILL know, believe me, the rusted steel expands a lot and splits open the concrete. I have bulkheads near windows that have experienced this when left for a long time without being fixed. This kind of damage is easy to fix too, just knock out the concrete (usually its flaking off), reweld and remesh and replaster.

I will echo what I have heard ferro savvy people saying a million times. You need to build a ferro properly to get a good boat, and then you will get one that will probably last a few lifetimes. Some people put crap mesh with big holes, some people put only one layer of mesh instead of three, some people put the steel rods too far apart, some people plaster the hull in 20 shots, some people do this, some people do that...there is a method to ferro and it must be stuck to, I mean you can build a perfectly good space shuttle if you have the recipe and follow it but otherwise its probably going to be a heap of junk.

I would probably only buy a Hartley boat second hand but then if the thing is still floating with minimal problems after 20 years its as good a guarantee as any that it was well built, since even professionally built boats of all kinds can have mistakes. It will NOT just start rotting at some appointed time.

That's all there is to know really. The idea that you can never tell what goes on inside doesn't work out when you see how much a steel rod blows up when rusty. A bad hull will start breaking apart with weeping cracks within a few years so if it is still smooth and its been around a while there's a pretty sure bet that its OK.

On my Dad's home built ketch, bulkheads and gunwhales have some cracking but Im knocking it out and replastering with Bondcrete mixed into the concrete.

Rust spots in the bilge etc can come about when you have wires from the mesh sticking out from the plaster, or the mesh is at the surface. I have a few of them. I'm planning to dig it out a teeny bit and put a bit of epoxy filler over. I can't vouch for the mesh in the immediate area of the rusty spot though.

The concrete quality (mix and curing etc) is something thats hard to gauge but again time is the only true tester.

Voids are not my area of expertise but the book I got from Hartley says they're not hard to fix.

Another thing I've recently learned is that one reason why ferro got a bit of a bad name is because home builders think they can just redesign parts of the boat at will. So it might be perfectly well built materials wise but sail like the brick sh**houses I keep hearing people talking about.

Anyways, that's what I've learned, I'm sure there are plenty of knowledgeable ferro owners who've been in the business longer than me who can give you the best advice. But my take is that unless the bloody thing is cracking from stem to stern all you have is a little bit of chiselling and plastering to do. Ferro boats are not maintenance free, although they nearly are.

Boracay, I don't suppose the owner would like if if the hammer went thru his hull but its a dud boat that would have chunks of concrete falling out, even if you did give it a good whack with a steel hammer. You should've seen me sweating over the chisel trying to put a dint in my window frames just to get a good key!

Cheers
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Old 01-08-2011, 08:16   #10
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Re: Testing Cement Hulls for Defects . . .

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Originally Posted by billy4184 View Post
Hi Gone2long,

Concrete doesn't `rot' in a certain timeframe, in fact it gets harder the longer it exists. The problem comes when there are fractures caused by impact damage, stress fracture etc which open up the steel inside to corrosion. Only then does rust `rot' set in on the steel mesh and/or rods.

The keel, gunwhales and bulkheads, and probably a few other places too, are the most susceptible to cracks due to the stresses that are put on them.

What looks like localised damage on the plaster can result in a slightly wider area of hidden damage because the mesh can rust away inside BUT any well built ferro boat has closely spaced rods in the structure and when one of these rusts you WILL know, believe me, the rusted steel expands a lot and splits open the concrete. I have bulkheads near windows that have experienced this when left for a long time without being fixed. This kind of damage is easy to fix too, just knock out the concrete (usually its flaking off), reweld and remesh and replaster.

I will echo what I have heard ferro savvy people saying a million times. You need to build a ferro properly to get a good boat, and then you will get one that will probably last a few lifetimes. Some people put crap mesh with big holes, some people put only one layer of mesh instead of three, some people put the steel rods too far apart, some people plaster the hull in 20 shots, some people do this, some people do that...there is a method to ferro and it must be stuck to, I mean you can build a perfectly good space shuttle if you have the recipe and follow it but otherwise its probably going to be a heap of junk.

I would probably only buy a Hartley boat second hand but then if the thing is still floating with minimal problems after 20 years its as good a guarantee as any that it was well built, since even professionally built boats of all kinds can have mistakes. It will NOT just start rotting at some appointed time.

That's all there is to know really. The idea that you can never tell what goes on inside doesn't work out when you see how much a steel rod blows up when rusty. A bad hull will start breaking apart with weeping cracks within a few years so if it is still smooth and its been around a while there's a pretty sure bet that its OK.

On my Dad's home built ketch, bulkheads and gunwhales have some cracking but Im knocking it out and replastering with Bondcrete mixed into the concrete.

Rust spots in the bilge etc can come about when you have wires from the mesh sticking out from the plaster, or the mesh is at the surface. I have a few of them. I'm planning to dig it out a teeny bit and put a bit of epoxy filler over. I can't vouch for the mesh in the immediate area of the rusty spot though.

The concrete quality (mix and curing etc) is something thats hard to gauge but again time is the only true tester.

Voids are not my area of expertise but the book I got from Hartley says they're not hard to fix.

Another thing I've recently learned is that one reason why ferro got a bit of a bad name is because home builders think they can just redesign parts of the boat at will. So it might be perfectly well built materials wise but sail like the brick sh**houses I keep hearing people talking about.

Anyways, that's what I've learned, I'm sure there are plenty of knowledgeable ferro owners who've been in the business longer than me who can give you the best advice. But my take is that unless the bloody thing is cracking from stem to stern all you have is a little bit of chiselling and plastering to do. Ferro boats are not maintenance free, although they nearly are.

Boracay, I don't suppose the owner would like if if the hammer went thru his hull but its a dud boat that would have chunks of concrete falling out, even if you did give it a good whack with a steel hammer. You should've seen me sweating over the chisel trying to put a dint in my window frames just to get a good key!

Cheers
Thanks Billy - Appreciate the insights - G2L
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