Ferrocement! Why not?

If you're going DIY then it'll never get done. The rebar/mesh matrix will be well rusted before you'd get it all tied together and with regards to the plastering it all has to be done in one day so regardless how good you are, you'd have to hire help in. Cheap materials wise yes, but labour intensive. It would work well where I am, where skilled labour is very cheap. Hire a dozen plasterers on the weekend, make it worth their while, invite their families, and throw a bloody big BBQ when the sun goes down.

 

Not to get off topic here but my own boat is a 63ft Wood & Plywood sailing catamaran. In the sea it is fine as the salt water acts like a pickling brine. Come rainy season it's a different story though. Warm fresh rainwater gets into every pinhole and hairline crack on the deck ringing the dinner bell for fungus and bacteria and then the fun starts. I am still chasing rot accumulated during the pandemic.

 

As aforementioned to DogCavalry where I am in SE Asia the cost of skilled labour is very cheap and their housing construction method is the same so they're pretty good at, it and all materials are available down at the local hardware store. If I were to go down the steel route I'd have to start importing Lloyd's ship-grade steel from Singapore at a great cost, far more than what I can purchase rebar and mesh for locally.
I know of China's endeavours, haven't heard much about Vietnam, and I know India builds a lot of fishing boats using ferrocement, but where I am it has never been heard of, and where I'm from, they still have ferrocement barges on the Thames left over from WWII.

 

It worked so well here the few left when I came of age are gone now. It was probably the least adopted material from the building booms of the 70s.

Maybe it works with your isolated systems, if so more power to you.

 

With cheap labor and reasonably skilled plasterers, you can do it in one shot. However, you will need to build a structure to hold the metal framing from deforming. The weight of plaster is considerable. If you build upside down, adding legs to the inside and patching the holes later is probably the easiest.

 

@MarkOHara , definitely a different economic regime. Here in Vancouver, even unskilled labour is expensive. Skilled labour is precious.

 

Concrete Ships: McCloskey Ships of WWII https://www.concreteships.org/ships/ww2/
a bit of history of some WWII concrete ships

SS Peralta - Wikipedia https://en.wikipedia.org/wiki/SS_Peralta
1924 and still floating

 

It does look a bit anomalous to feel a need to import Lloyds ship grade steel or to feel obliged to use local labour and ferrocement.There almost certainly won't be any Lloyds oversight of the ferrocement building process and just about any old steel can be welded into a watertight vessel.The challenge with steel might be finding a way to shape it,I suppose.

 

Totally off topic here. I ran across an enzyme cleaner called Sporicidin for $40 per quart, but used at a few % concentration, so a quart is a lot. It will remove mildew stains from wood in a week. At first I thought I wasted my money, but after a week or so the wood looked much better. I applied it scrubbed, and let it dry. Tried again next day when it didn't seem to be doing much. It was slow to clear up, but did the job eventually. Supposedly it has a residual anti fungus effect for 3-6 months. I'm talking window frames in houses not boats, damp at times, only wet when the temps are below 0. It might help on a boat, I would try it. I think the manufacturer would give a straight answer if you were to ask. I called with a question and they were helpful. It is more geared to the professional/industrial market.

Ferrocement was studied in detail by a boat builder in Maine back in the 60s. They measured properties as influenced by steel to concrete ratios. All the performance curves pointed to 100% steel, so they settled on steel and forgot the concrete. As others said, a skilled crew can do a good job, but the bad reputation is set in stone.

There is a fiber reinforced grade of concrete for dry laying concrete block with a cement coating to hold it together that you can buy at any big box home center. The tds says it has a MOR of 700 psi. First time I saw tensile strength listed for a concrete consumer product. Good for repairs if not the entire hull.

 

The interesting thing about ferro cement is that as it ages in seawater the various oxides at play tend to preserve each other so neutralize if anything improving the bond. If you left a sound ferro boat in water for years unprotected it would likely cope better than an all steel hull. Ferro is less dependent on paint integrity but once the steel armature is open to oxygen through damage or poor workmanship, it becomes vulnerable to rust, which isn't that hard to repair.. but needs locally to be removed . There are products now that can chase cracks in the ferro cement that sort of "heal" that type of impact damage, ferro with not too much cement ratio hull can absorb impact without cracking.. a knitted shell.. so some better qualities .. . An all steel hull can also flex and will likely just dent , so with a good paint schedule .. steel..but with non damage neglect...or very long term..ferro ..it depends on the likely use. 2 cents worth from a backyarder.

 

That is used in thick slabs and works well for the application. It would be impossible to use it as grout to fill small wire mesh.

 

The fibers will wick moisture, jeopardizing the encased steel reinforcement.

 

Yes, I should have been more specific. I'm new to the forum and didn't expect the level of technical interest and understanding.
The product is Crack-Resistant Surface Bonding Cement, which is a bit different from typical concrete.The fibers are fairly short, but you would have to test it to see how well they can be worked into the mesh. I am fairly confident that it could work and even if the fibers tend to be concentrated near, but not on the surface it might not be a bad thing if you are working from both sides. The guys to talk to are the concrete canoe crowd. They have a handle on thin concrete if anyone does.

I doubt that, but it is a valid concern. What can I say? I don't really trust anyone. I don't even think you should trust me until you independently verify what I say. So yes, your criticism is welcome.

There is a pretty well accepted hydraulic cement based coating called Xypex that is supposed to penetrate and seal that might be better. This is according to my daughter, which may sound lame if you don't know her. She is building a house. The architect selling the kit over booked and dropped the ball on the blueprints. She had to learn how to do the blueprints for the permit, including learning enough to make an accurate 3D model of a geodesic dome. Building inspectors think that an amateur's work has to be scrutinized in detail, and rightly so. The bank requires a detailed BOM down to the last screw, and bundle of quotes, even for the work she will do herself.

Looking at the SDS, it is basically hydraulic cement and may have some extra sodium silicate. I am more of a metals and polymer guy, this is not in my wheelhouse, and I prefer to learn more before recommending it. Trust but verify. Waterproofings for buildings and boats may have very different requirements.

 

I am very familiar with concrete. The product you refer to is commonly used in driveways and sidewalks. It is hard to finish. The fibers tend to bunch up. It is like the old Bondo tiger hair. The fibers prevent cracks, but the product is designed for slabs that at least 3 inches thick and have no mesh.

 

Xypex can be added directly to the concrete as an add mixture , better than the coating . Delvo Master Set is another add mixture that will retard set time for about as many hour's as you want , adding work time as well as expanding pour area , also adding to psi strength . Lots of modern mix additives to consider if doing a ferro cement boat today , material,s such as carbon or fiberglass mesh / rebar could replace steel mesh .