I remember a 52 ft ferro yacht that sat on the hard
North of Perth in the late 70's for some years with a large "wet spot" on the hull. I believe the wet spot was actually fuel rather than bilge oil.
I understand that the hull was cleaned up and the internal source of the fuel leak was fixed. The hull was washed thoroughly and after drying it had epoxy
applied to the outside only, then the usual antifouling etc.. (There is generally no need to paint
the hull inside plus paint on both sides may cause one paint surface to blow off).
That boat has been continuously used now and is still in use.
With Ferro Cement you can test for void problems by rolling or dragging a metal tool over the surface. Areas with voids make a distinctively different sound. Voids are generally a consequence of "two shot" plastering where air is actually entrained between tow cement layers. The "avoidance' of "voids" (a truly terrible pun) is a consequence of considerable care during the plastering events
. Some voids can be fixed by drilling a number of small holes and pumping in a laminating epoxy
resin. If water
enters a void it migrates causing armature rusting.
A good test of oncrete quality is the compressive strength. The higher the compressive strength (usually) the lower is the porosity. In old units, 15 to 20 000 psi is good. Ordinary domestic cement mixers usually can't produce this quality as too much water
is required to make the mix hydraulic.
Ferro cement boats were once very cheap
to make and there are still many sailing around. The cost of cement and mesh and labour has now made them uneconomic. The two oldest working boats are ferro cement and were built in the 1830's by George Lambot in France
. Ferro cement vessels, if built properly do not rust, can be immensely strong, do not burn and any damage tends to be localised. However it is dense and does not lend itself to smaller vessels. One Ferro Cement vessel ("Helsal", designed by Joe Adams) won the notoriously competitive Sydney
Hobart yacht-race against a large field of international and local boats.
What I can't understand is why anyone would buy a boat on the hard
as there are so many floating second hand vessels around that sell for a fraction of replacement and can be trialled prior to purchase
Nearby our vessel a 56 ft steel
vessel that has circumnavigated and has everything needed on board sold for under $50 000 (AUD). Yes, it needs work, but it has everything!
Brand new boats usually require more expenditure than this to make them ready for serious cruising.