Interesting that your labour-efficient method wasn't successful, Jim. Windboats and 'Mick-the-Brick', in the UK, both had a successful business building in ferro, more conventionally. I think it was the unfairly-bad reputation that ferro gained that put them out of business more than anything.
No photos this week. I think I've finished filling, but am still working hard on longboard and random-orbit sander. Thank god the boat is only 26ft long and isn't round-bilge!
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.
Interesting that you mention ferro-cement, Arne. There were a quite a number built in New Zealand and many of them were pressure grouted. the Ferro-Cement Association owned a hand operated grout pump that hull builders could borrow. When we came to grout our hull, which was plastered using the one shot approach as you described, we couldn't get any grout into the holes that were drilled half way through the hull. We drilled in a bit more and the grout suddenly went in, maybe skipping a hole or two and coming out through another hole. Whilst working the pump you could feel if the grout was going in. At one hole on the bow the grout actually went in and came out around the other side of the bow spraying into a bush. After the grouting was finished we waited a few days and then repeated the process but we couldn't get any more grout into the holes. The boat is still looking good and sailing some 40 years later.
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.
I have encountered this problem too. I wondered if a solution in some cases is to use a technique and engineer told me about called "vacuum grouting". It is used to fill voids in steel reinforced concrete bridge girders. Holes are drilled in appropriate places. Inlet and outlet holes. A vacuum pump at the outlet holes draws epoxy grout into the inlet holes. Provision would have to be made to prevent the grout from entering directly into the pump. Similar to vacuum bagging laminates with cloth and epoxy construction.
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.
Scary stuff Annie - as Befur is strip-planked I have avoided these problems for the most part, but was somewhat shocked when an off-cut of a laminated ply bulkhead that I threw out of the boat, delaminated when it hit the floor! and this was a flat panel that I had laminated on the floor using Epoxy and bricks on top to ensure it was OK :-(
Clearly this is a significant area of concern - I think your "pump the voids" solution is the right one - I resorted to the same thing when I wound up with some air bubbles in the glass/epoxy cladding inside the hull.... but it still is something else to worry about when staring at the ceiling at 4am!
:-) Keep up the great work and blogging ...
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.
However, the keel is nearly finished. It just needs sandblasting and coating now, and will be ready for SibLim to be dropped on top.
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' !!
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