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Old 30-06-2003, 03:37   #1
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Question Ferro Cement Hulls ?

My partner and I are currently researching live-aboard yachts, and despite doing a search of this site I have not found any mention (or discussion) of ferro-cement hulls.
We have spoken to a couple of people who own them and who say they can be very capable (although warning that there have been some very bad 'backyard' ones built) however others speak of them like the Black Death!

I would appreciate the opinions of those in this forum,
Thanks,
Marlene
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Old 30-06-2003, 12:15   #2
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My take on ferro-cement is that it is, in fact, pound for pound the weakest of all of the commonly used boat building materials. Ferro-cement operates by the same principle as fiberglass, in other words, a high tensile strength reinforcing held by a high compressive strength, low tensile strength cement. The cement in ferro-cement ideally is a high strength Portland cement. The cement in fiberglass is polyester, vinylester or epoxy resin. The tensile reinforcing material in ferro-cement is steel (sometimes with glass fiber), and in fiberglass its glass in a variety of forms, kevlar, carbon and all kinds of new variations on these materials.

Ferro-cement's weight comes from a number of sources. First of all, no matter how small the boat, there is a practical limit to how thin ferro-cement can be. ferro-cement needs to have a minimum thickness in order to have sufficient depth of material to protect the reinforcement from moisture. Because of this boats below 40 to 45 feet are generally considered too small to use ferro-cement efficiently. (i.e. they weigh way more than they would or should in some other material.)

The implication of the weight issue is not readily obvious. At the risk of sounding like a broken record, Weight in and of itself does nothing good for a boat. It does not make it stronger, or more comfortable or more stabile. Weight does increase the stress on the various parts of a boat. It increases the size of a sail plan required to achieve a particular speed. It increases drag and typically means that for a given draft a boat will have a less efficient keel (i.e trading off greater drag for the same amount of leeway.)

In order to carry more sail area the boat needs greater form stability, which comes at the price of a choppier motion and greater drag, or greater ballast or deeper ballast which adds more weight and drag and perhaps depth.

To keep the weight down, many ferro-cement cement boats have reduced ballast ratios when compared to other construction techniques. This means that they need more sail area because of their weight but they can't carry more sail area because of reduced ballast ratios without using lower aspect rigs which are by their very nature less efficient and further compromises performance.

This is further complicated by the fact a higher proportion of the weight in a ferro-cement boat is carried in the in the topsides (and sometimes decks). This means a high center of gravity which has a variety of implications; reduced stability, wider roll angles, smaller angles of ultimate stability, and more prone to excitation rolling (which may be slightly offset by the greater inertial moments due to weight).

Then there is maintenance costs. In a study performed some years back looking at the life costs of various materials, ferro-cement-cement came out as the highest maintenance cost material (if I remember worst to best was ferro-cement, steel, conventional wood, aluminum, fiberglass, cold molded wood) Of course, as with any generalized study there will be case by case exceptions and given the comparatively small sampling of non-FRP boats it can be easily skewed by a few bad apples.

Other problems with ferro-cement are the difficulty of connecting things to it, and prevention of rot in wood in contact with ferro-cement. The difficulty in bolting to ferro-cement is that ferro-cement hates localized loadings. It's hard to glue things to ferro-cement since secondary bonds are greatly weaker than primary bonds.

Ferro cement requires a high level of skill and a large labor force to build properly. The best materials, either galvanized or epoxy coated steel reinforcing rod, are very expensive making a well built ferrocement hull more expensive to build than a glass boat.

Then there is the market value thing. Ferro-cement has a poor reputation in the States that does not match the comparatively high regard that it is held in other countries. Some of this is just plain unfair prejudice, but most of this distrust comes from real shortcomings in the materials as noted above. A well-built ferro-cement boat can be a good cruising boat. But the image of the crudely finished 'hippie' built cement and rust buckets still clouds the perception of ferro-cement for many North Americans.

The other problem is telling whether the boat that you are looking at is a good boat. It is very hard to do non- destructive survey techniques to tell whether the original work was done well and is in good condition. While sounding will reveal any major separations in the cement to reinforcing bond, it does little to determine the affects of fatigue, poor curing practices or cold joints. With Ferro-cement it is particularly important to maintain the ferro-cement and non-ferro-cement parts in good condtion. That can be very significant. People who buy boats because they are priced well below the market, often are overly frugal or just plain do not have the money that it takes to properly maintain a boat. An otherwise good Ferro-cement boat left to poor maintenance and miss-handling can quickly become a poster child for why North American's don't trust Ferro-cement.

To me the real cost of owning a boat is the difference between what you paid for the boat, the cost of upgrades and maintenance and the price that you can get when you sell the boat. The problem with a lot of low value boats is that the sales price is always limited no matter how much you put into the boat. This too works against ferro-cement boats as thier prices will generally be limited by perceptions of Ferro boats and their poorer sailing performance.

I guess my conclusion is if you are strictly looking for an low initial, upfront cost boat and don't mind putting some sweat equity in, and you can look past the sailing shortcomings, and you actually find one that was well built and well maintained, a ferro-cement boat might work out fine for you. For most of us, they do not.

Respectfully

Jeff
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Old 01-07-2003, 19:36   #3
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Ferro Cement

Jeff's exhaustive sermon on ferro cement leaves little, if any at all, room for questions.
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Old 02-07-2003, 04:50   #4
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Thank you Jeff for giving us considerable food for thought! I appreciate the time and detail of your reply.

Some of what you say though is what I have already read in various books and web resources... which leads me to ask whether or not your comments are based on general opinions, or whether they come from actual experiences or contact with ferro-cement owners? My partner and I are finding it very difficult to make any decisions because in our own (limited) experience we have found that those who actually own these yachts usually speak highly of them....and those who don't, run them down! Perhaps its just a 'pride of ownership' thing?

Is there anybody out there who actually has one, has had one in the past...or who at least knows somebody who does? I'd really appreciate your views...but of course anybody's view will help us with our decision-making.

Thanks so much,
Marlene
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Old 02-07-2003, 11:18   #5
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My information comes from a wide collection of sources. In the 1970's I thought I wanted to build a boat and live on it. Ferrocement sounded very appealing. I read everything I could get on the material and talked to maybe a dozen ferrocement owners. My plan was to design and build a ferro boat.

I eventually participated in 'plastering' one. I, along with probably over a dozen other people spent a very long weekend applying the portland cement to the hull of a roughly 40 foot 'Ingrid' variant. In theory you need to work non-stop until done. In a very long weekend we were only able to finish roughly 2/3 to 3/4 of the boat so that there was a 'cold joint' in the hull. What we ended up with, despite a high level of care was a terribly unfair and uneven hull, thick in places and thin in others. The guy nearly a year fairing the hull and putting a finish on it. 7 or 8 years later I saw the nearly completed boat and there were sections where we must have been thin because there was a telltale pattern of rust.

Also in the 1970's, I also worked in a boat yard in Florida where we hauled a Ferro cement boat for an insurance survey and when we did, the layer of cement that was over the reinforcing peeled away in the area of the boat where the travel lift straps were bearing. The surveyor who was supposed to be an expert in Ferrocement tapped the boat out and said it was other fine but when I was working with the owner to make the repairs, we were able to peel away large areas of the skin to reveal a lack of bonding at the reinforcing and rusting reinforcing material.

Some of my comments come from discussions with yacht designers. When I worked as a yacht designer this was a popular topic in the office. Some of my comments come from discussing ferro boats with marine surveyors. Some of my comments on the properties of ferrocement are based on my knowledge of concrete that I obtained during my masters degree in architectural structures and during my career as an architect. Some of my comments are based on conversations with past and present ferrocement boat owners over the past 30 years. Some of my opinion comes are simply based on basic yacht design theory. Some comments are based on conversations with brokers, some of whom will not even take a Ferro listing. And, yes, some of my opinion comes from books and from 'generally held opinion', which, right or wrong, none the less affects the resale price of a ferrocement boat.

I know that there are ferrocement boat owners who love thier boats but to paraphrase Lincoln,"I have never met a man who had an ugly wife, a dumb kid, a bad boat." In my life, talking to thousands of boat owners over the years, I have consistently been amazed at the glowing reports that I have heard from owners and crew who own boats that by any fair and objective standard are really poorly designed and built.

In the end I stand by my conclusion. I know that there are well designed and constructed ferrocement boats out there. I know that there are owners for whom the compromises in sailing ability, greater maintenance, reduced stability, limited resale value, low strength to weight, and carrying capacity for a given size, etc, really are not significant, but for most of us, at least here in North America, the risks and realities of ferrocement, especially when there are so many better choices out there, really are not worth it.

Respectfully,
Jeff
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Old 02-07-2003, 14:33   #6
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ferro cement

Marlene:

You say you are "researching". From this I cannot tell whether you are planning to buy an existing boat or to build one.

If you are planning to 'buy', don't bother to read any further.

If you are planning to build, consider this. Ferro cement has been used as a building material for boats for about a hundred years. Some of the early hulls, especially in Europe, are still in use. In areas where wood was becoming scarce, it was a cheap and readily available alternative. That being said, I believe that all of Jeff's comments are valid and should be given careful consideration -- with special emphasis on those dealing with aesthetics, modification, and re-sale.

In recent years, advances in marine adhesives have resulted in hull building methods that enable home builders to turn out hulls that can be indistinguishable from those professionally built. As an example, the W.E.S.T. System, or as referred to by some designers, a wood/epoxy system, has been used by many ameteurs with great success. Many naval architects, Bruce Roberts, for example, have a number of designs specifically drawn for construction by this method.

Both W.E.S.T. System and Bruce Roberts websites contain a wealth of information and can easily be found.

Good luck with your quest,
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Old 08-07-2003, 04:22   #7
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I agree with Jeff, there are a lot of old "Samsons" out there. Have seen some that looked great, but most are for sale in the US for the cost of the rigging. Yes you can get a 40-50 ft boat for under 30k, and can live aboard, and cruse. If you are 60 yr old and take it on you will most likely out live the boat. Insurance will be tough to get, but you might be better to self insure, except liability.

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Old 03-09-2003, 02:03   #8
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FERRO CEMENT

Marlene I own a hartley southseas whitch is a 38ft ferro. The hull was professionally built and plastered 1974 it has never shown any rust or seperation on the hull. The first time the hull was moved a crane lifted it without using spreader bars much to my horror but no damage was substained. Impact strength posted by hartley boats is for 38-40ft hartley construction method hull .75 inch thickness 30lb steel content per sq ftis as follows approx 8000 lbs per sq inch after 14days curing,11000 lbs sq inch after 28 days,19000 lbs per sq inch after 90days. As far as the wood problem epoxy resin sticks to ferro cement like sh!! to a blanket then you seal the timber the same and mastic them together.Fittings such as chain plates, bowrollers, anchor winches should have had plates welded in the frame before plastering. All that i need to do on haulout to the hull high pressure water wash down ,anti fouling 2coats,clorinated rubber paint 2coats So i strongly recommend that you only purchase a pro plastered hull. A good web site is www.ferroboats.com Cheers Greg ..
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Old 03-09-2003, 03:39   #9
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My neibor has a ferro cement boat:
It is ugly as the sin with rough sides and rust streaks all over it.

Seems that the cement is absorbing water which get to the chicken wire used for re-inforcement, the wire corrodes then swells out and cracks the cement, more water gets in, etc....

(Granted, the boat is probably amateur built and in the cheapest possible way: The masts are wood and they are rotting away, the guy keeps scraping the rot away every year and stuffs bondo in the voids. The anchors are home made and the bilge pump float switch is from the innards of a toilet tank...Not making this up folks, the whole thing is a disaster.)

From what I have seen and heard over the years, would not own a cement boat even if given one for free....
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Old 03-09-2003, 04:18   #10
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CSYMan,

When the bilge pump "dumps" its contents,does it sound like a toilet flushing?
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Old 03-09-2003, 04:27   #11
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Stede:

Don't know what it sounds like, trying to stay away from that boat.
Looks like it could sink anytime and many a' times I have hoped it would....Preferbly in deep water..

Not to poop too much on ferro cement, that particluar boat would have been a wreck regardless of building material, the builder or owner is the problem in this case, not the construction material, he would have been able to screw up a steeel or plastic boat just as well, all it takes is lack of common sense...
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Old 08-09-2003, 10:16   #12
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Ferro is an excellent material

Hello folks! I've been lurking for a bit, and I thought I would jump right in with a hull materials post. What am I thinking?!?!

There have been several long, authoritative sounding posts by Jeff H. While I am sure his post seems correct from his side of the dock, the reality is very different.

"My take on ferro-cement is that it is, in fact, pound for pound the weakest of all of the commonly used boat building materials. "

Jeff! You couldn't be more incorrect! A properly made FC hull is stronger than any other material or method except steel! With a compresive strength in the high teens of thousands, demonstrated and not remarked, this cannot be debated.

The issue with FC hulls, and the decline of FC as a building material is twofold:

1) Badly designed and/or constructed FC hulls

2) Basic perception

"Rocks don't float!" people think.. when all the while they are taking cruises on steel cruiseships. So thats the basic perception problem.

The other problem is more of an issue. You see, with Ferro you just CANNOT do it wrong and come out with anything usable. You CANNOT cut corners. You CANNOT use too much water. You CANNOT take 2 days to plaster. You CANOT cure it wrong. Any of these things, done incorrectly, could easily compromise the hull.

Case in point, a professionally designed ferro hull was lovingly crafted by an amateur builder. He did a fantastic job on the armature but when he plastered he didn't insure the sand he used was washed sand, and so salt got into the mix. Now in concrete this is not too much of an issue, but with FC.. OOF. His hull was spalling and refused to cure properly. He had to jackhammer the material out and redo it properly.

Anyhoo, as a result of these amateur designed and/or built boats where quality was not observed, FC has gotten a very bad name. I have never owned an FC boat, but have seen a stress test of a 25 year old Samson built hull. That is, a fella was swinging a sledge hammer at it trying to redo a section that had gotten puncture damage. I could not believe how strong that stuff was. In most cases, in order to get the plaster to fall off the whole armature, he had to swing the sledge five or six times, and that was just the OUTER plaster.

Another case, a 100+ year old FC barge was broken up in the UK recently, and the steel was not rusted. Not a bit. And it took hydraulic wrecking machines to do it.

Let me address a couple of Jay's points:

"Ferro-cement's weight comes from a number of sources. First of all, no matter how small the boat, there is a practical limit to how thin ferro-cement can be. ferro-cement needs to have a minimum thickness in order to have sufficient depth of material to protect the reinforcement from moisture. Because of this boats below 40 to 45 feet are generally considered too small to use ferro-cement efficiently. (i.e. they weigh way more than they would or should in some other material.) "

Ok Jay lots of errors here. First of all, a 32 foot ferro boat broke the world circumnav racord in the early 1980's. And its not 40-45 feet, its 30 feet thats considered the lowest limit for oceangoing boats. There are FC dinks at around 10 foot. As far as the weight issue, that's simply not so. The plaster is supposed to barely cover the reinforcing, like an 1/8" of material that is grouted on after the basic fill.

This being said, smaller ferro boats are indeed a bit heavier than their wood or lightly built FG counterparts, but not by loads. And if you get a professionally designed FC boat that was done by Hartley or Samson, or a real NA you should be fine.

"Ferro cement requires a high level of skill and a large labor force to build properly. The best materials, either galvanized or epoxy coated steel reinforcing rod, are very expensive making a well built ferrocement hull more expensive to build than a glass boat."

Building an FC boat requires so little skill its not funny. We're talking about using wire ties and mesh.. how does that translate to high skill? And if you follow the boiler plate process provided by the designers to begin with, you wont run into trouble plastering. Plus, I never knew that galvanized steel rebar was expensive... Also, the only 'labor force' you need is when you finally need to plaster. Then you need about 12 people, including 4+ professional plasterers. For 2 days. Also, you never use epoxy coated anything rods! The rust on the rods and mesh is what causes the proper bonding for the elastomeric properties to manifest..umm.. I mean the steel needs to be rusty for the thing to work Sorry! Engineering showing!

I won't get into the rest.. this is getting very long!! But sufice to say, yes FC has low resale value, but this is a market thing, not a real value thing. And yes, FC boats are not trivial to survey, but then wooden boats are hard too. It is easy enough to require the seller to expose the armature in some location specified by a surveyer. This is easily patched back to new condition.

My point is, FC is not a bad material to build a boat from. It 's just not popular, and cannot be mass produced because it is a labor heavy process.

If anyone would like to know the real story of FC, I suggest you cough up a few pennies for Colin Brookes' "Ferro Cement Boatbuilding" which is offered for sale at http://www.ferroboats.com/

Jeff, I am sure you are very experienced but I'm sorry.. the tens of thousands of successful FC boats over the years, boats that have hauled cargo and people over oceans reliably and safely even when having encountered rocks and reefs, are a testament to FC as a good material to build boats from. It only takes one counterexample to disprove a hypothesis Jeff, and I think I can provide thousands! The general feeling about FC has been manufactured by culture as well as ticked off builders and designers who found they couldn't cut corners on Ferro and survive.

FC, once the Darling of materials is now the red headed stepchild, not because it is a bad material, but because folks realized that they couldn't make any money on it. So it is in everyone's very best interests to badmouth the material. And they have succeeded well, so much so that I dont believe many insurers are left in the US whom will even consider liability on FC, no matter WHAT the survey.

All bad news for FC owners, but then you can just sail to New Zealand and get insured

Good luck hunting!!

--T
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Old 08-09-2003, 20:57   #13
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Knome, You make some pretty strong statements but do not provide any numbers to back the engineering aspects or explain the items in my are incorrect. Then again neither did I provide the basis of my comments either. To provide a basis for an understandable discussion I will explain my assumptions and we can go from there.

Lets start with the basics, most ferrocement manuals recommend a minimum ratio of 10% steel to concrete(by cross sectional area) and a maximum of 40% steel to concrete in areas were tensile stresses are expected to be large such as near the keel or in the region of rigging loads. Well into the mid- 1980's most ferrocement boats were constructed using steel with a tensile strength of 36,000 psi and concrete with a compressive strength between 3,000 psi and 6,000 psi with roughly 4,500 psi generally cited as being a very typical cured strength for hand laid up ferro-cement.

In engineering ferro cement, (because it is considered critical to avoid hairline cracking and cement develops small cracks when exposed to comparatively small amounts of tension), the cement was calculated as taking no tension with the steel taking all of the tension. Similarly, because the steel is used in such small columnar sections and therefore prone to buckling and damaging the matrix, for the purposes of engineering the the sectional properties of a ferro-cement section, the steel is seen as taking no compression. This discounting the concrete in tension and the steel in compression does result in a very small additional margin of safety.

If we look at the combined properties of a matrix with as close to a balanced steel to concrete ratio (roughly 12% 36,000 psi steel) you end up with a section with a tensile strength in the range of 4300 psi and a compressive stength closer to 4000 psi. Increasing the strength of the steel or the proportion of the steel in the matrix will make small gains in the strength of section but as the section becomes unbalanced in tension the compressive strength of the concrete will limit the strength and stiffness of the overall section when exposed to bending.

In terms of modulus of elasticity (E) the area of the steel is seen as governing, so in our balanced section, E for steel is 29,000,000 which for 12% of the matrix would result in an E= roughly 348,000 psi.

So lets see how ferro-cement fairs against other materials, (and I really wish that I could insert an excel spread sheet at this point). It is often assumed that steel is the strongest boat building because it has such wonderful properties when compared in a strength per square inch basis. But steel is very dense. When its relative density is factored in does pretty poorly on a pound for pound basis. Still it serves as a useful benchmark.

If we start with steel plating and assume that for the basis of comparison we use an 1/8" section (which is quite thin but might be used for a 25 to 30 or so footer) a one foot square section would weigh roughly 5.1 lbs. For A60 steel you would have an Fb of 36,000 psi and a section modulus of .0026 inches cubed resulting in a bending strength of 93.75 psi. From a stiffness stand point steel has an E around 29,000,000 times and the section in question results in a moment of Inertia of 0.000163 or a stiffness of roughly 472.

Looking at fiberglass, a 5 lb one foot square section would be just over 0.5 inches thick. With a Fb of roughly 10,000 psi and an E of 280,000. The section would have an S of 0.043 and an I of 0.0111 producing a bending strength of 435.317 and a stiffness of 3114.67. In other words on a pound for pound basis, fiberglass is roughly 4.6 times stronger than steel. Coring the section brings the section thickness to .67 inches and the strength to roughly 8 times the strength of steel.

If we look at a cold molded wood veneer construction with fir outer plies and cedar inner plies and glass skin, you end up with a section that is roughly 2 inches in thickness for the same 5 lbs per square foot weight. While the combined Fb is low around 1300 psi and the E is also low on a unit area basis around 1,100,000 psi when combined with the sectional properties of the much lighter wood section, the section would have an S of 0.79247and an I of 0.864013. This produces a bending strength of 1030 and a stiffness of 95041. In other words, cold molded wood comes out to roughly 10.98 times stronger than the original steel panel.

Now to ferrocement. a 5 lb one foot square section would be just over 354 inches thick. With a Fb of roughly 4300 psi and an E of 3,480,000 psi. The section would have an S of 0.020843 and an I of 0.003685 producing a bending strength of 89.62 and a stiffness of 1282.5. In other words on a pound for pound basis, ferrocement is only .64% of the strength of steel making it by far the weakest material on a pound for pound basis.

On the second point that you questioned from my original post, most ferrocement design manuals agree that the minimum cement cover on the steel needs to be 3/8" to develop full strength in bonding. If you use 1/8" pencil rods, you end up with a total hull thickness of 7/8". To achieve that kind of thinness you need to very precisely place the steel so that it is properly within the thickness of the matrix. Precision placement is next to impossible with rod this thin so typical Ferro cement hull thicknesses end up being over 1 inch in thickness. That is a 14 lb per square foot hull section which is substantially heavier than the 7 1/2 to 9 pound per square foot fiberglass hull section that would be typical on a 40 to 45 foot boat.

On the last point, I stand by my statement that Ferro cement requires a high level of skill and a large labor force to build properly. You and I seem to agree that properly built the boat should be laid up in one continuous operation. We are talking about hand mixing, and placing literally tons of low slump portland cement to a uniform thickness and with a fair surface. Anyone who has ever tried to trowel even a sidewalk knows that getting a smooth level surface is harder than it looks, only on a boat we are talking about a huge curving surface where as little as an 1/8 inch deflection in 4 feet will be very obvious. That takes skill. Consider plastering on around the clock for a period of days, that requires a large workforce a large number of which need to be skilled.

Even building the male mold capable of rigidly supporting the wet cement, which is how most one off ferro-boats are built, requires a lot of wood working skills, and female molds require a lot more skill yet.

Again the current thinking on building a durable ferro-cement boat says that the best reinforcing materials are either galvanized or epoxy coated steel reinforcing rod. Galvanized or epoxy coated steel reinforcing is recommended because cement is a fatigue prone material and over time will develop small hairline cracks. These cracks allow moisture and air to reach the steel and cause rust. When steel rusts it expands with tremendous force prying damaging the matrix. Galvanized rod or epoxy coated rod greatly increase the longevity, especially in lighter weight ferro-cement where flexure will occur. When you consider the tons of reinforcing involved, these are very expensive material which makea well built ferrocement hull more expensive to build than a glass boat when you factor in the cost of the forms, high strength cement and reinforcing involved.

Small amounts of tight rust can improve the bond between the steel and the concrete but loose or powdered rust damages the bond. Clean steel develops nearly the bond of lightly oxydized steel. Zinc or epoxy coated steel rods actually develops higher bonds. If you are not familiar with them epoxy coated bars are the green reinforcing that you see in highway construction these days.

So Knome, if one of us has a whole raft of misinformation in our post, I don't think it was me, but I am certainly open to seeing how you will refute the above.

Respectfully,
Jeff
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Old 09-09-2003, 06:28   #14
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FC: Authority

Jeff,

I don't intend on taking up this boards bandwidth with numbers! Its a forum, not a symposium, and since those kinds of numbers are already available from published sources, it is easy enough for you to check those sources.

Second, I am basing my statements on conversations I have had with the pre-eminent FC engineer alive today, Colin Brookes, mSNAME. He was awarded Master because of his work with FerroCement.

You have some very nice numbers there, and I am sure that fiberglass has some lovely 'strength' to it, that is.. when built out of solid glass like they used to thirty and forty years ago. Nowadays you are lucky to get 1/2 that strength. Instead you get fancy equations and quotes about how strong Kevlar is, and then the boat takes a hit from a reef and goes under.
As far as 'Cored' FG panels, those numbers you give are based on FLAT panels. I wonder how many flat panels are in a boat? And once the panel aint flat, it starts losing strength. You also WAY overstate how thick FC is supposed to be.

You say that manuals say the coverage should be 3/8".. well, to Quote Mr. Brookes, a guy who has put literally thousands of FC boats in the water:

"..the thickness applied in the first instance must allow for levelling and finishing at an ideal covering of around 1/16" to 1/8" over the netting. "

--Colin Brooks/Ferro Cement Boats, p67

Heck, there was a guy who put 3/8" of covering over his boat and Colin had him jackhammer the whole damn thing and recoat! See, you're right. 3/8" cover makes it too heavy.

The stringers are best made from 'Bright hard drawn wire mild steel' They are recommended to be 1/4". Diagonals are recommended to be 1/8"

So, what you end up with is 1/8 + 1/4 + 1/8 = 1/2". Of course, we will allow for a bit more than that , and in fact it does turn out that this sort of hull ends up being 5/8" or so.Cecil Norris, NA for Samson industries, agrees with this layup as well.

You mention mold building.. and that being the way most FC boats are built. That also, is not the case. The very best method is the Web Frame method, and I suggest you check out the website I talked about earlier. Samson used this method almost exclusively once they were in production and produced some long lasting, easy to maintain boats. Hartley-Brookes use this method almost exclusively.

At the end of your statments you talk about FC as if it were just plain cement. Jeff, FC has so much steel in it, it has its own rules of conduct. Well made FC doesn't crack enough to 'let in' moisture EVER. In fact, osmosis works to prevent that. We are talking about something with at least 8 layers of netting and a layer of reinforcing bar going in two directions. That is 5/8" thick. The grout coat covers the base cement anyway, and then you have that miracle sealant that you put over the FC.. paint.

Edit: I really need to speak to this one...

Jeff you say:
Quote:
Precision placement is next to impossible with rod this thin so typical Ferro cement hull thicknesses end up being over 1 inch in thickness.
As mentioned above, there are THOUSANDS of boats sailing out there that have 5/8" thick hulls and 1/4" rod for the matrix. How could it be next to impossible? And if there was an FC boat with a 1" hull thickness that was less than 100foot long, I would bulldoze it myself!

One more edit:

Quote:
On the last point, I stand by my statement that Ferro cement requires a high level of skill and a large labor force to build properly. You and I seem to agree that properly built the boat should be laid up in one continuous operation. We are talking about hand mixing, and placing literally tons of low slump portland cement to a uniform thickness and with a fair surface. Anyone who has ever tried to trowel even a sidewalk knows that getting a smooth level surface is harder than it looks, only on a boat we are talking about a huge curving surface where as little as an 1/8 inch deflection in 4 feet will be very obvious. That takes skill. Consider plastering on around the clock for a period of days, that requires a large workforce a large number of which need to be skilled.
According to many, many accounts I have read about, and according to several owner/builders I have spoken to, and according to Mr. Brookes himself, as well as Cecil Norris and John Samson and Jay benford, it takes a crew of 4 - 6 professional plasterers plus that amount of assistants to plaster up a 50' hull in one day. The cement is NOT hand mixed. According to all accounts this takes between 12 and 16 hours of continuous work to accomplish to a finish.

I have a great deal of respect for Mr. Brookes. I have had many conversations with him about this topic and as far as I am concerned, he is a primary source for information on FC. There were many people who tried to get into the field during its heyday, but they didn't understand the material nor did they have any experience with it. Hartley-Brookes have designed many FC boats over a long period of time and have had such success that I don't think it is even reasonable to question them on these matters. It would be like saying Carl Sagan didn't know anything about astronomy.

So, as I said before, please pick up his book. Its cheap and contains all the information you need to see my points. If you really want hard numbers, try the man himself. Or perhaps, go to www.ferrocement.net and get in touch with some of those FC engineers. Or perhaps www.ferrocement-consultant.com where there is a fella who seems to also know quite a bit about the topic. I could talk to you in math, but I don't have the background these guys do so you would be better off talking to them. I would imagine they have boilerplate answers containing lots of numbers. You could also talk to Larry Mahan of www.larinda.com. She is FC and he is a recognized expert on FC.

In the final analysis, there are and have been so many FC boats of so many types and sizes.. boats that have endured rough seas and even wartime conditions, and have stood up to the tests of time with little to no hull maintenence.. That ALONE should convince anyone that FC is a good material to build from. And like any material it is good for a certain set of circumstances that may or may not fit a particular persons needs.

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Old 10-09-2003, 06:00   #15
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While you are correct that this is not a symposium, I think that when statements about relative strength of materials are being made it is important to look at the relative strength of the materials involved and I don't see how that can be done without "the numbers". The properties of these materials can be verified on Material Property Search in English Units http://www.matweb.com/searchenglish.htm and are consistent with other sources of information on weight and strength properties that are widely published. The calculations are based on the combined sectional properties (strength of material and cross sectional behavior) and not on either a flat or curved panel. All of these panels would gain strength proportionately with 2 dimensional curvature.

Here are couple quick responses to some of the specifics within your response.:

" I am sure that fiberglass has some lovely 'strength' to it, that is when built out of solid glass like they used to thirty and forty years ago. Nowadays you are lucky to get 1/2 that strength."

That is totally fallacious. The resins, additives and mixing procedures of thirty and forty years ago resulted in very brittle resin. Similarly the fiberglass reinforcing and the handling of the reinforcing resulted in a more brittle and fatigue prone matrix as well. Laminates of thirty and forty years ago tended to be resin rich and to use a larger percentage on non-directional reinforcing materials (Mat or chopped glass). Today's laminate tends to use more ductile resins and improved fibers and handling techniques have actually resulted in much stronger fiberglass than was in use in earlier boats. In terms of strength (including from small area impact) cored composite hulls have been found to offer enormous strength advantages and durability over a very long test period. Based on test data, current thinking on increasing the strength of a FRP hull calls for a less resin and non-directional reinforcing. This not only reduces weight but also results in an extremely sturdier hull. While the use of vinylester resins (as used in motorcycle helmets) and higher strength reinforcing materials would greatly increase the numbers that I used for my calculations, the numbers shown are at the conservative end of conventional fiberglass construction as it would have been done 10 years ago.

"..the thickness applied in the first instance must allow for leveling and finishing at an ideal covering of around 1/16" to 1/8" over the netting. "
--Colin Brooks/Ferro Cement Boats, p67

In principle I do not disagree with that statement, but the netting is installed over the reinforcing cage. You still end up with a minimum of 3/8" cover over the primary reinforcing because the netting goes over the reinforcing. A typical lay-up schedule for a ferrocement boat would have the covering over the netting, a layer(s) of netting, then the longitudinal bars and then the diagonal bars and in areas of high tensile loads, transverse bars, then the outer layers of netting, and then the covering over the netting.

And there is not even agreement amongst experts in the field on the proper thickness of the covering over the net. Quoting from the ferrocement Institute site "There was quite a discussion between Tony Naaman and Dr. Paramasivam about the minimum covering of matrix on the outside of the elements. That is the amount of matrix over the outside layer of mesh. Param says a 5mm (3/16") minimum covering is needed whereas Tony says 1mm (1/20") is fine. This is another main difference between RC and FC. In RC the code says you must have a 20mm minimum thickness of covering over any reinforcement. This is to prevent cracking deep enough to expose the rebar to corrosion."

Using your 1/8" number for the covering of the mesh layer, you end up with a minimum thickness of"
covering over the netting, = 1/8"
layer(s) of netting, = 3/16 to 1/4"
longitudinal bars,1/8"
diagonal bars = 1/16"
outer layers of netting = 3/16 to 1/4"
covering over the netting, = 1/8"

Adding those up you come out with 13/16" thickness which is quite close to the 7/8" that I used in my post that we are discussing. As noted that panel is roughly 14 lb. per square foot hull section which is substantially heavier than the 7 1/2 to 9 pound per square foot fiberglass hull section that would be typical on a 40 to 45 foot boat. If you use the heavier 1/4" stringer bars (longitudinal) and 1/8" diagonal bars that you mention that would add 3/16" taking the thickness to the 1" that I mentioned in my post. I don't know where you come up with 5/8" hull thickness for a ferrocement hull.

"You mention mold building.. and that being the way most FC boats are built. That also, is not the case. The very best method is the Web Frame method."

I believe that you are actually describing what is generally referred to as a "truss frame" in ferro cement literature. It was first advocated by Hartley. In truss frame construction a male mold of welded steel trusses are fabricated and that mold then becomes a part of the actual hull. The only shortcoming with that method is that it requires stiffer, larger diameter longitudinal reinforcing for building purposes than would be required in the finished hull. According to the website you reference, this results in a heavier hull. And frankly, at least from my sense of your average amateur boat builder (or professional boat builder), producing accurately shaped welded steel trusses requires highly skilled labor. At least in the US most of the ferro boats I have seen were built over wooden male molds which permits a more precise placement of the cement.

"At the end of your statements you talk about FC as if it were just plain cement. Jeff, FC has so much steel in it, it has its own rules of conduct. Well-made FC doesn't crack enough to 'let in' moisture EVER. In fact, osmosis works to prevent that."

I disagree with the idea that I talk about Ferro cement as if it was regular reinforced concrete. Quoting from the Ferrocement institute again "According to Paramasivam the volume of mesh in a properly designed FC element is 3%-8%" that is actually less steel than would be used in an engineered reinforced structural concrete and is less than had previously been recommended for Ferrocement. While it is true that osmosis continues to 'harden' the concrete over time, that hardening results in the formation of microscopic passages for water and air to move through, the presence of either will promote rust on the steel reinforcing. Again quoting from the Ferrocement Institute site, "the addition of 1% fiber greatly increases the resistance to cracking by spreading the load..... Fibers range from relatively low cost polypropylene to relatively high cost fibers such as kevlar and aramid."

I need to get back to work here but hopefully the above will clarify where we agree and disagree. Again, if you would like to provide substantive input I am certainly open to seeing it.
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