Poured concrete for ballast in a sailing hull..a no no??

Im considering purchase of a steel hull ketch, homebuilt and 47'6" LOA. She is reportedly constructed entirely of 6MM shell plate (which seems a bit thin to me in the keel and bottom areas). Her ballast/displacement ratio is slightly less than 25 % (i.e. 6 tons ballast and 25 tons vessel displacement fully tanked). She has integral bilge tanks. The ballast reportedly is steel and lead set in concrete.

I have seen concrete in bilge areas both in steel and wooden construction. A number of the old wooden snapper schooners on the Gulf Coast used boiler punchings set in concrete for ballast. Back in my merchant marine days an elderly harbor tug berthed adjacent to our company docks had concrete in her after void.... occasionally a little water would be found back there and someone would pour in another couple of bags to seal up where she pitted through. Not only the shell plating but the aft w/t bulkhead to the engine room was corroding. One day there was rising water in the engine room and they barely got her on a railway before she sank...to re plate the offending areas...

Bruce Roberts seems vehemently against concrete against steel or when used in the ballast mix..... In doing a Google of "Concrete Ballast in steel hulls" there were some 12,900 returns. A number of them are horror stories of varying degrees while more are either in favor or against from a conceptual standpoint. In the case of the boat which I am considering, it appears that the lead and steel were loaded in the keel and then concrete was poured in to more or less bind things together. It does not appear that any plate was welded over top of this in the deep areas and it also appears that the concrete is where it can be wet by whatever water sloshes around in the sump either from a dripping packing gland or what ever other contributors of water might exist.

From my somewhat elementary understnding of concrete and from a vague rememberance of the materials sciences courses I took in college many years ago it would seem at first blush tht concrete due to its inherent porosity and due to the difficulty or impossibility of removal to get at the inside of shell plate and framing for maintenance, would make this a bad idea. However again, I have seen concrete used in steel hulls not only for weight but also to fill up areas that would be difficult to drain properly-- i.e where the shell plating meets the keel between the frames. I ve seen concrete poured in these areas to keep water higher where it will drain easily aft to a sump for pumping overboard.

So I am cautious. Partly because of the home built nature of this hull, partly because of the published shell plating thickness and partially because of the potential maintenance problem due to the concrete.

Any information or opinion would be deeply appreciated


Carr

 

Personally, the only concrete filled steel hullls I have worked on were serious problems.

I am sure that, like all materials, if done correctly, the end results will be good, but as most "cheap" boats are slack, I would expect more problems to arise.

Where sea water, concrete and untreated steel are mixed, it is only a matter of time before the steel rusts through.

If you don't own it, let someone else have all the fun.

 

I would assume what you are calling concrete would more properly be called cement. For a long time cement was the best available coating for the inside surfaces of steel or iron vessels. Have you ever tried to clean a wheelbarrow that was used for concrete work and didn't get washed out? The term well bonded is not adequate to describe it. At present there are better but more expensive coatings. Try a search on this site for cement coatings or some such. I think there is a good thread here on the subject.

 

Oh, and 6mm should be fine for a 50 foot boat.

 

NEVER !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!concrete!
A thin film of cement is a very good coating on mild steel.
A massive cement (concrete) block in your bilge is a friendly invitation for corrosion of the worst sort....invisible until to late........
I owned two ships which had been converted from steam to Diesel Propulsion in the 60ies. To compensate the weight reduction, both were ballasted by concrete, and both had massive problems with corrosion under the concrete.
It was the plain joy to remove the crap from the bilges, before replating.
Another one of my old steamers is a former water supply steamboat, with a tank coating of cement on mild steel surface. The coating was applied in 1923, partly restored during the years, and the metal almost completely free of corrosion at inspection 1989. A completely different subject.
So, be careful, very, very careful. And then ..............................................................................leave it.
The 6mm plating should do the job IMHO.

Regards
Richard

 

I know little about naval application of concrete but I do know something about reinforced concrete for civil structures.
The main enemies of steel rebars are: humidity, atmospheric carbon dioxide (CO2) and chloride ions (Cl-). The humidity is necessary for the formation of electrolytic environment. Normally, in absence of the latter two elements, a concrete will create a highly alcaline (pH > 12) electrolyte. Therefore, in absence of carbon dioxide and chlorides, the concrete/steel interface is electroliticaly passive and no corrosion occures.
But as soon as carbon dioxide penetrates from the atmosphere all the way down to the interface layer with steel (thanks to the porous nature of concrete), the pH of the electrolyte drops to less than 9 and the steel starts to corrode, due to the breakdown of the protective oxyde film.
Something similar happens when chloride (Cl-) ions penetrate to the interface layer. Chloride ions react directly with oxyde layer, destroying it and hence enhancing the corrosion process.
Knowing that 2 of these elements (humidity and chlorides) are particularly abundant in marine environment, it seems logical to me to expect big problems when using concrete-steel composite structures for naval applications.

 

Thank you daiquiri, there is a much simpler factor working against the poor boater: the different expansion value of the steel and the relatively massive concrete, when temperature changes. That opens a gap sooner or later, open for water intrusion. It´s sooo simple.
Regards
Richard

 

Hi Richard,
I hate to counter you (I find myself so many times perfectly agreeing with your views) but this time I have to, for the sake of... numbers.
Concrete and steel have very similar coefficients of thermal expansion. That's one of the reasons for their successful application in reinforced concrete structures. Just to make an example of typical values, take a look here:
http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Concrete: 14.5*10-6 m/(m K)
Steel: 13.0*10-6 m/(m K)​

Those are, for any practical engineering use, the same values.
Maybe you were reffering to the thermal inertia. But, again, if the steel skin and the adjacent layer of concrete are bonded together well the thermal flux will not have big discontinuities and the two materials will expand in a nearly same manner, due to identical expansion coefficients. And then, the thermal excursions in the underwater structures (if we assume all the concrete located in the keel) are not significant anyways.
I see the water (and salt) diffusion due to porosion as the main problem here, but it's all just my guessing which might easily be wrong.

 

As mine, mate, as mine.
We always THOUGHT it was that simple, due to the sheer mass of the concrete. And I did never doubt that.
But being not overly stubborn, I can accept your explanation too. It is just harder for me to understand, why is a thin layer of cement a good preservative and a massive block of the same stuff a destroyer? lassiare perplesso?
Maybe both of us are on a wrong way, and just the different mechanical properties crack the bond?
How so ever, the result is what counts in this case, and following my experience, concrete ballast is a nono.

Regards
Richard

 

I was wondering the same thing. Guess we'll have to wait for someone else to give a reasonable explanation.
Cheers!

 

I´m patient......................................................................................sometimes

 

Here are my .02 worth. I have never poured concrete in a boat. I am a marine contractor... I throw 1000 of yards of concrete down the hole along with rebar every year.

Concrete like steel have a lifetime depending on design. Make it thicker it last longer... But it will disappear over time no matter what.

There are special mixture of concrete that will be tighter or completely moisture proof. Anything against a steel hull with the exemption of zinc will help corrode it. The concrete does not let oxygen into steel thus prevent corrosion. We are now using alot hard galvinized or epoxy rebar. There is a special epoxy binder that we order in concrete. Also 5000 psi concrete is less porous than 3000psi.

So here is my suggestion. Sandblast steel, hot galvanize it, Epoxy it several layers...Then pour then highest psi concrete perhaps with additive you can... The seal it in epoxy some more.... it should last a few years.

The issue with thinner better than thicker might relate to type of concrete vs use of perhaps pure portland. So it boils down how it is done...

 

Notice the time stamp................post 11 and 12........................ thats what I call real patience! I´m so proud.......

Mydauphin said:
"Sandblast steel, hot galvanize it, Epoxy it several layers...Then pour then highest psi concrete perhaps with additive you can... The seal it in epoxy some more.... it should last a few years."

and I agree!..................................................
Leave the concrete and it may last a few centuries.

 

Well first a very heartfelt thanks to all for excellent data here.
Also I apologise for taking so long to get back with input. Ive been out of pocket for the last several days

I did contact the broker for clarification. He says that the ballast was put in the keel and then cement (not concrete) was poured in to bind everything together. Again, it appears that in the way of the ballast area, the cement is exposed and forms the bottom of the bilge sump.. Depending on how and what shaped pieces there either could be a lot or a little cement. I want to apologise to everyone here by possibly having given the impression that she was ballasted in concrete. According to the broker the 6 tons of ballast consists of 2 tons of lead and 4 tons of steel. Cement is poured in on top and around all of that.

Comments on the thread (and a lot of you know a WHOLE lot more about material science than I do and thanks very much for sharing your knowledge here )and my own observations. I ve had problems with ballast keels before. My last boat, a 48' aluminum ketch.....had lead inside the ballast keel. The lead was bedded in epoxy. There was also a diesel tank in the keel. What was not readily apparent was that, there was no separation between the lead area and the tank area i.e. no floor plate in the tank. We found this out, in process of the first bottom job I did in her, when we discovered (and we did not discover this during the initial survey) an area about the size of the hemisphere of a golf ball on the lower leading edge of the keel (she was fin keeled) filled with what looked like an old epoxy misture called Red Hand. I decided to chip the Red Hand out (better to know than not , right??) and well the worst you can imagine happened, diesel started to ooze from a small crack. Apparently she had been grounded or sailed into something at one point before I purchased her really hard....to put this dent in the keel. You can imagine what a job it was to get that area clean enough to weld that crack up in the aluminum. The crack was below the tank, in the ballast area. What ensued was a difficult time of draining, cleaning, welding and pressure testing to finally get things tight and it took a LOT of work......so I have reservations about what can happen (this was the only boat I ve ever owned that was owner built). The worst part of it was when we discovered that we didnt have enough containers to catch what might ooze out of the hole before we could get someone to pump the tank to empty.... we were only about 30 ' from the water (downhill from where the boat was of course) and in a yard that had a somewhat hostile attitude toward do it your selfers. Fun times.

On the back side of this I ve seen and sailed in some perfectly beautiful owner built vessels, some much prettier and with much better workmanship than I ve ever seen a yard produce.....in both aluminum, steel and ferrocement. I think someone who builds with the idea of going cruising is going to do things carefully because after all ones life depends on it right??

Many thanks for the input here and for making me welcome. Were I a little younger (Im 57) I might attempt to build from scratch. I say this because after having owned 5 different boats, I find what I consider good designs for passage making are absent in what a lot of commercial designers (even of metal boats) incorporate. I did spend 5 years refitting a 42' ca. 1936 Alden Cutter (ribs, floors, keel bolts, interior &c) but have never built from scratch. I dont know that I would attempt such a project at 57. Were I to do so and could that I afford it, I probably would attempt it in aluminum with steel as a very close second.

Finally where I live here in Veracruz Mexico, there are problems with concrete porosity. Typical construction is vertical column with brick infill and a thin layer of concrete over the outside. Here by a warm ocean, its typical in a new home to have sweating walls, concrete cracking away from the rebar in the columns and mildew stains on the inside of exterior walls no matter whether the outside is above or below grade. It s a mess. Older construction is worse. Quality control on concrete here is poor to absent. I ve seen many people build good sized homes by mixing the concrete up, a bucket at a time, on trhe edge of the street in front of the construction site. Ready mix or trucked concrete is by far the exception in this part of the world.

Again many thanks to every one here for the information and interest.

Carr

 

If the keel cavity was treated with tar, epoxy or other good coating, it doesn't matter if the concrete gets wet. If it wasn't and it is still dry, you can either weld plates to cover everything, fiberglass/epoxy or use a flexible coating. A coating of tar-epoxy or chlorinated rubber would work well.