In the early 70's, I worked for a company that built ferrocement hulls using a patented process called Fibersteel.  We built the hull, bulkhead and deck for on 55" "Valeo" and six Tom Colvin designed "Saugeen Witch" hulls. (I considered buying one of these and rigging it like Colvin's Gazelle.)

The Fibersteel method is the same as that for fiberglass in a female mould. The "gelcoat" consisted of a mixture of cement, Pozzolith and acrylic latex (paint base). After it set, acrylic latex was sprayed on the surface and a proprietary mix of cement, sand and other additives sprayed on. Then sheets of expanded metal (steel) were laid up, overlapping like shingles. Rolled to ensure mesh was well nested. Repeat as necessary. We laid up a 55' hull in two days. Deck done the same way. Bullheads laid out on a loft surface.  Very efficient but the devil was in the details. Assembly was slow and messy.

To make a long story short, the company went under for lack of cash flow. Sales too slow. 

Ferrocement is a wonderful material when properly used. Some boats from that era are still around and doing well. Google "CHI LIN junk" But as I said, the devil is in the details.

Problems with air pockets or lack of perfect lamination seems to be an issue with quite a few construction methods.

Back in the seventies, quite a few large boats were constructed from ferrocement in my area. On one of them I spent a lot of time assisting in “knitting” together the rebar steel and chicken wire, and I also participated on the big event for two of these vessels  -  the plastering day.

Luckily, some experience had already been gained by then, so it was decided that two and two people worked together, one inside the boat and one on the outside. Only one pushed cement onto and through the mesh, while the other acted as an inspector on the opposite side. The inspector just smoothed out penetrating cement on his side. This method (compared to pushing in cement from both sides) ensured complete penetration, but was hard work, so the cement pusher and the inspector frequently swapped roles. Later, when holes for seacocks and for chainplate bolts were drilled, they revealed absolutely perfect penetration.

Samson, well known to those who joined the rallies in Stavanger a few years back, was the last and biggest ferrocement project I can remember. These hulls appear to last extremely well  -  that is  -  when properly built.

Arne

 

Oh, gosh, Malcolm, that would be a nasty shock!  Thus far, everything I have cut through seems to be well glued - that was why the little cracks between the two layers somewhat shouted at me.  Truly, I'm very, very glad that I found them and the time spent, cutting out and replacing the plywood and filling voids was time well spent.  When you are pounding to windward, clawing off the proverbial lee shore, you don't want to have to worry about the bow delaminating!  I am filling, sanding, cursing the bits I've missed, filling and sanding.  The 6mm, not surprisingly, is full of little dents and hollows, so I have effectively crawled all over it several times now.  It all sounds reassuringly solid.  Moreover there are two bulkheads close together and a fair bit of framing around the bow.  I think I can sleep in peace.  But as I am the world's worst worrier - the tiniest unfamiliar sound makes my heart race, when I'm sailing - I had to do the job as well as I could.

I found it very reassuring that the Gougeon Bros talked about filling voids by drilling holes and pumping in epoxy, as though it were an everyday occurrence.

I had a bit of a setback.  While getting the bottom of the hull ready for the end-grain to be sanded and sealed and dowels to be fitted, I came across a 30mm crack between the two layers of 6mm ply at the chine.  I cut a sliver of paper and slipped it down.  It all disappeared.  So then I cut a rather longer sliver and got the same result.  Time for a cup of coffee.  I remembered having real difficulties getting the second layer to pull down here, in spite of heaps of screws.  The trouble is that 6mm plywood doesn't give you a lot to screw into.  I had also been aware of a definite bump in the plywood and had put it down to a high spot on the bulkhead.  Well, perhaps that wasn't the explanation.  The coffee told me to get out my router and remove the top layer of plywood near the void.  I could see that it simply hadn't stuck, away from the chine where I could use longer screws to pull it in.  So I routed a larger and larger area until I came to well-adhered wood.  Then I cleaned it all up, made a pattern and stuck another piece in place.  That done I went back and looked at the other side.  At just about the same place there was about a 10mm void between the planks of ply, which I could just see after sanding it perfectly smooth and using the vacuum cleaner.  Rude word.  Out with the router.  Forewarned is forearmed so I was more methodical this time, and stapled strips of ply onto the hull to act as guides for the router, ensuring that I ended up with straight lines.  This area was much bigger and indeed went from chine to chine at its worst. This was the first of the two panels I'd put on - obviously I had learnt enough to do a better job of the second one.  My only consolation was that at least I'd spotted it when I did.  Once again, I got back to glued laminate and made a pattern.  But when I came to fit the wood, I then realised what the problem had been in the first place.  It simply did not want to lie flat following the fore and aft curve as well as the up and down one.  So I split it in two and glued it on in two pieces, which sat down very happily. 

Somewhat disconcerted by this lack of harmony between the two layers, i went back over the whole sheet very carefully, and found several more voids.  It took rather more than a cup of coffee to help me decide on this one.  Should I remove the whole lot and start again?  But mightn't that leave me with the same problem?  And what if, actually, most of it was well adhered?  I spoke to SibLim club member (and boatbuilder) Marcus: drill holes, inject resin and make sure there are air holes as well, was his advice.  I spoke to JRA member, Mike Hayes, also a boatbuilder, presently building a Gary Underwood 'Shoehorn'.  He gave the same advice, together with the reassuring comment that he'd had to do it quite often when cold-moulding boats. 

Being a methodical little soul (to say nothing of being concerned that I might not have located all the voids) I drilled 8 mm holes at roughly 300 mm centres and then drilled 1.5mm holes diagonally below them.  Starting at the bottom row, I forced resin down the large holes.  In some cases it came out of the holes further down and if I noticed quickly enough, I put some masking tape over them.  If not, I drove in a toothpick which was a perfect bung.  One or two holes on the starboard side (the first piece of ply fitted) took quite a lot of resin, but most of them filled up straight away.  When they overflowed, I drove in an 8mm dowel.  Then I did the same thing along the next row up and so on.  It took quite a long time on the starboard side, which had several substantial voids, but on the port side, I only found one of any size and one which filled up almost immediately. 

After the glue had cured, I went back and thumped the whole area on both sides, very heartily.  It seems absolutely solid.  I suspect there may be one or two voids left, but they'd be no larger than a teacup, I'm sure.  While it was all a bit upsetting at the time, I feel very confident now, that the two laminates are stuck together.  As I'd not been that happy about the whole process of gluing those big panels on in the first place, it's probably all to the good that I discovered the voids when I did and didn't find my resin disappearing down the same spot when I came to glass the hull.  It would have been awful to have doubts.

Knowing what I know now, I should have cut the second layer of plywood into strips, say 250 wide and glued it on in that way.  I would suggest that method to future builders.

This is the price you pay for trying to have a curvy, plywood boat - and for being the first to build it.  Looking at that gorgeous bow, and seeing how strong it all appears (now!), I feel it's a small price to pay. 

David Thatcher wrote: Not so Annie, chines are very 'in fashion' these days. Many of the new production boats have a chine incorporated in the hull, supposedly to allow more interior volume but I think it is really all about the look. So our boxy plywood boats with noticeable chines are right up there with the current vogue. A round hull form is 'so yesterday' !!

Well, of course.  I should have guessed, my reputation as a trend-setter being what it is!!  Annie the fashionista is known far and wide :-P