Ferro-Cement Multihull Scantlings

I have just popped an 18 foot cement Proa hull from its mold today. This is a trial run for a larger proa, 40 to 60 feet. I have been working on a different technique using chopped kevlar strand mixed directly into the mortar mix to augment the strength from the multiple layers of steel mesh and eliminating all the heavy gauge steel. None of the steel is over 19 gauge in thickness.

Here's my question: What type of testing should I do to calculate what thickness is needed for a boat of this type and size using my composite method?

Any help would be greatly appreciated.

Cheers,
DrCraze

 

I (and everyone else?) is wondering why the material choice?

 

GOSH....I wonder myself about the choice of the material, where plywood would be far lighter, simpler to make and probably far cheaper...

 

Strength Ilan! I guess strength is the reason....















..no not the strength of the material choosen, of course not.
But nothing is stronger than a well established preconception.
Regards
Richard

 

I know the material from my own experience to be very durable. The proa under construction is mainly a test bed before going bigger and for testing repairs. Cost is 1/3 the price of the epoxy it would take to build in ply, using scrap wood for the mold. I have nothing against butt-blocking molds that I am just going to destroy when removed.
I build ferro cement using the inverted technique. I find it to be a more accurate process and having a solid backing to pack the mix into reduces the amount of labor on the plaster day. I have to admit getting the mix perfect and using it properly isn't something easy to learn because of timing, humidity, curing times, knowing when to stop and when to start again.

To clarify one more thing, this will be fitted out very traditionally. Absolutely no fasteners will be used for rigging or structural support. Everything will be lashed together.

Now back to the scantlings. Should I test my material against 3/4 inch ply and match its failure load?

 

You should calculate against the ply thickness to which the original design was calculated. Was that 3/4 in.?

You would need some hard data about your composite first. Is that available?

 

Cement is unlike any other to some degree. Any hard data has to be derived from from the actual material to be used on the project, sand cement, etc.....
Once compression tensile strength test have been performed on the site specific materials plastering can then begin.

I suppose my question should be "How are other composite materials tested, sample size, fatigue cycles so on. Or maybe a link to some good info?

 

Try "Lamicens" for GRP layup as a comparison.

And get off my back.............

 

Let's take that rationally:

1-Material itself: density 2700 kg/m3, if a lot of steel inside until 3500kg/m3. It's heavier than aluminum and I doubt you can make a ferrocement able to have the same weight/modulus ratios than aluminum. Plywood has far better weight/modulus ratios than aluminum. It's a basic principle of strength of materials, thus it seems that ferrocement is poorly suited for boat building.

2- Being obliged to make a mold even in scrap is a waste of time and money for just one boat. More if you have to dismantle that mold after. A mold has to have a minimal quality of surface or the time spent for fairing will be prohibitive.

3- A true ferrocement is not so dirt cheap; fresh puzzolane cement, 100% silica sand, water low in salts, microfibers, chemical additives (plastifier, impermeabilizer etc...) plus galvanized mesh and good steels bars painted with epoxy add to the price.

4- 3 or 4 weeks of curing (unless using steam) that's long...

5- After curing, the time of drying the excess of humidity of the ferrocement adds another long delay.

6- installing all the mesh, wires, making the steel bar structures is rather tedious and long.

7- With the thin thickness of the ferro you'll have to use in the trial to keep a decent weight, you won't get enough cement to insulate the steel from the sea water, so you'll have to use the same amount of epoxy resin as on plywood to waterproof the ferrocement. Or the steel will begin to rust with all the known consequencies.

8- The worst: after all that job, you'll have a too heavy boat in an inferior material.

The FAO has published a book ( that I read carefully) about ferrocement boats (fishing mainly) and you can get from the New Zealand authorities a lot of information about ferrocement; rules, scantlings, etc... A certain number of fishing boats in ferro have been made in this country; they do not seem to be highly successfull.

If ferrocement was a superior material with a low price, we would have a lot of boats made in this material on the seas...it's not the case and for good reasons. I have no prejudice against any material, each have pros and cons and its range of use.
If I have the scientific proof that cow dung glued with Chechen tree sap added with a few drops of bat ***** is a very good material for boatbuilding I'll use it. I'll simply put in the cons the smell and the flies.

 

All of what you have said is true....40 years ago. I happen to have proof of ferros capabilities in my front yard. You see I am not building some cut throat racing machine. I am building a traditional micronesian proa.

My method of mold making uses a wood lath form made mostly from scrap I then fair the form with plaster of paris and sheetrock compound. completely different from what you're thinking.

To erode steel you need more than moisture.... you need air as well. A very thin layer is all that is needed. I can show you with a simple test.

Repairs are actually a breeze and can be performed one day and get you back sailing the next. I use a grace product called Polar Set that accelerates the curing time without all the bad effects of the old chloride type accelerants. No need to steam just keep it moist.

I use cement everyday with small scale castings and precast panels and countertops so I am very very familiar with it. I have been for 15 years.

All I am saying is cement has come a long way since the ferro cement witch hunt began.

By the way my vaka on my proa cost me under 100 bucks. At 18 feet that is saying something. I bet I could build a 35 footer for under 1000.

I am doing this for the fun, experience and to demonstrate ferros capabilities using modern methods.

Any bets on how fast it will be with 110sqft crabclaw?

 

If you say it...my 18 feet cats (compounded plywood) 18m2 rule were all under 100 kgs, and all able to go to the 23-25 knots. My monodromic 18 feet proa made in 1986 was 72 kg.

100 bucks seems a slightly low budget even for the hull alone.

I'm impatient to see the finished boat with all the pics, weights (list and price of materials) and perfs (not impressions, but good runs of 0.25 NM with chrono and GPS). You're a believer so you won't be accesible to any engineering discussion.

Sorry, it has never been a witch-hunt of ferrocement, just the visible results spoked alone. There was a fashion in the seventies that died by itself and the so called NA who promoted this material vanished in the anonimity.

Most of the ferro yachts were pure crap, some profesionnally made not bad but not withstanding. There are excellent barges in ferro, but I have to see a good fishing boat in ferrocement because all those I surveyed were close to crap.

Boat engineering is stubborn: ferrocement simply has too bad ratios weight/strength, in simpler words not strong enough for its weight. Too much work for the mediocre results. Economically not viable. Even in Third World countries, where price is the most important factor and the need of cheap boats is great, ferrocement did not last.

PS The data I have is 12 years old, and chlorides are not more used since 30 years at least in ferrocement if I believe my NZ data.

Good luck and post some pics. But weight, gives us the ciphers.

 

Traditional proa's were the original light weight craft - and have only become viable with the use of modern - ie light materials....
Not my material of choice but will follow progress with interest.
At those prices - go for it!
Interested in rig - more info on your cc? Am thinking cc myself....

 

Most ferro yachts are still in use today. You cant tell the difference by simply looking at them.
The latest fashion In modern boat design is to create the lightest and fastest. I'm not into that, I prefer leisurely cruising and minimal maintenance.

Ilan I don't see why you have to try to and mock the material. It has everything to do with engineering. If it didn't then there wouldn't be concrete canoe competitions for civil engineers every year. I would say more engineering goes into a ferro boat than ply or steel. And before you start lobbying against ferro because of the resale value let me just say I plan on scuttling this off the coast of Florida for reef propagation if/when the time ever comes to do so. Don,t worry its legal and welcomed and the best material for that.

To clarify, I am not advocating cement for amateur construction. The skills needed to properly mix, finish, and cure take a very long time to master.

Proas were viable with stone age materials they have become marginally faster with the use of modern material. In sailing that margin is huge I know Howevever my proa is lighter than a traditionaly built proa.

Here's a photo just before I pulled the hull off the mold. I have since polished the hull and scalloped tubericals on the bows. Yes tubricals. they work on wind turbines, why not in the water where they have been developed through eons of evolution? Never know unless we try:idea:

 

I am modeling my crab claw from a maple leaf, If you compare a maple seed to that of a modern windsurfer sail you will see the glaring similarities. Once you have made that connection you will see how much in common a crabclaw has with such a modern rig.

 

I have also been dabbling with cored ferro-cement. The additives I use to enhance the strength of the cement cause it to bond chemically with Styrofoam.

If I have time I will make a foam cored test panel and get this idea out of my head. Im not all that familiar with styrofoams degradation so this probably wont be usable for below the waterline but Im sure bulkheads and floors would benefit from this immensely.

Does anyone here have "any" experience testing materials?