Ferro Cement Hull Repair - Curing in the Ocean?

[popeyecrew]

hi all
we have to do a repair job on our ferro hull under the waterline, and have been told that it is actually possible to launch the boat straight after, hence letting the cement cure in the saltwater. the repair job is over a fairly big area on the keel. has anyone got any experience with saltwater curing?
All sincere advice is appreciated and again if you are a ferro hater, don't worry commenting!
thank you

[mesquaukee]

There should not be a problem as long as the concrete has set (not cured).
I assume you have painted it.

[popeyecrew]

hi there
thanks for your reply. we haven't done the job yet, because we want to be sure to do it right before we start. someone actually told us that it can be painted after it has been in the water again, like a year after..... what is your take on that?
thanks for your help

[mesquaukee]

Type V Portland cement is better than the more common type III. It is much more resistant to saltwater (akaline) than the type III. The commonest mistake is adding too much water to make the mix more workable, this results in a low strenght concrete with a lot of surface problems. The stiffer the better. Adding PVA to the mix makes it bond better.
Putting it in the water right after setting up is a good idea as it prevents any surface cracking due to drying out (evaporation not curing). I am not so sure about the salt though, some type of barrier would be an idea to prevent salt from leaching in to the surface layers. You may want to slap on multiple coats of PVA before in goes into the water. It drys fast as the water in it will be absorbed by the concrete. One source of cheap PVA is primer paint for sealing new drywall, it comes in 5 gallon buckets (it should say PVA on the label).
I must add I am not a ferrocement boat person, I have just been around a lot of concrete.

[popeyecrew]

hi there,
thank you so much for taking your time to reply to my questions. i am amazed how helpful everybody on this forum is. i have already learnt a lot, and once i feel a bit more competent i will happily share my knowledge as well!
happy sailing!

[Martinini]

Cement will cure just fine in water, if it didn't all those bridges wouldn't be possible. It actually strengthens with age.

[meanda46]

IMHO If its a structual fix keep it wet with fresh water ie. dont launch it for a week or more. If its cosmetic I would go at least 24hrs and paint it before it goes back in.

[popeyecrew]

thanks for your help. it was structural and we fixed it two weeks ago and have a sprinkler system on it that wets it for 5 min every hour. would you mind elaborating anyway? i am just really keen to learn as much as i can about ferro boats and would like to know why you think to wait and paint with structural repairs before launching again. thank you very much