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Old 30-11-2020, 05:09   #1
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Concrete Keel in Steel Boat

Hey all!
back again with more questions concerning my steel boat. thanks to everyone for all your answers and help! You guys are a wealth of information!
I was underneath the water tanks cleaning some rust and taking a general look around, and as I looked in the keel area and did some chiseling with a hammer and chisel, I saw that it was poured concrete .
I've read around and heard bad/good things about concrete. It looks well poured, and only in part of it was the seal starting to flake. the other parts (further aft) is sealed really well with an epoxy and is almost mistakable for lead. The old owner said that he was told it was only lead when he bought it.
He didn't pour the concrete so I assume there are lead punches in the concrete with the concrete sealed around it. Concrete wouldn't be my first choice when pouring a keel but I have to deal with what I have. I plan on doing a lot of offshore sailing with this boat. Is this a major problem? I've already been in gale force winds across Lake Ontario with her and had her over to 50 degrees so I know she is balanced well at least. lol
Thanks!
-Captain Zay.
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Old 30-11-2020, 05:26   #2
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Re: Concrete Keel in Steel Boat

You're probably looking at cement, not concrete.


Cement is a really common way of securing ballast in an encapsulated keel. This is true of steel boats. Cement is a decent corrosion inhibitor in steel, which is why reinforced concrete is popular. However, you probably know from seeing deteriorating reinforced concrete structures that it's not a perfect protector. Cement is porous and can absorb water and salts. Cracks can also form at the interface with the steel where water can intrude.


The "gold standard" for a steel boat is to weld a plate over the top of the cement, checking carefully for pinholes in the weld, and then epoxying over the top of the plate to ensure that water in the bilge can't work its way into the keel. However, if there's a good, thick, well-adhered layer of epoxy over the cement, that's probably nearly as good.
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Old 30-11-2020, 06:36   #3
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Re: Concrete Keel in Steel Boat

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Originally Posted by jordanbettis View Post
You're probably looking at cement, not concrete.


Cement is a really common way of securing ballast in an encapsulated keel. This is true of steel boats. Cement is a decent corrosion inhibitor in steel, which is why reinforced concrete is popular. However, you probably know from seeing deteriorating reinforced concrete structures that it's not a perfect protector. Cement is porous and can absorb water and salts. Cracks can also form at the interface with the steel where water can intrude.


The "gold standard" for a steel boat is to weld a plate over the top of the cement, checking carefully for pinholes in the weld, and then epoxying over the top of the plate to ensure that water in the bilge can't work its way into the keel. However, if there's a good, thick, well-adhered layer of epoxy over the cement, that's probably nearly as good.



How about leaving an air space , maybe 1/2 inch, between the plate and the cement and after welded in place make a threaded hole in the plate, fill the air space with oil, then plug the hole.
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Old 30-11-2020, 07:12   #4
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Re: Concrete Keel in Steel Boat

I can give you my 2c on the topic.

I built a steel boat and launched it 1981.

Though the boat was sand blasted inside and out, I could not get into the keel voids...no worries, I thought....as these will get filled with ballast...and sealed over with coal tar epoxy, which they did...

All but the most rear void. The plans did not call for this to be filled with ballast, additionally it was located under the engine, but I had to do something about protecting the steel inside that void, so I filled it with cement.
As I was still in the build process, I drilled a small 1/4" weep hole in the keel right at the very bottom rear to allow water to escape as the concrete cured. That weephole oozed water for quite some time...I can't recall how long anymore, but it was several weeks. When I noticed the weephole had dried up, I welded it close.

After the concrete had dried, I slathered the top with coal tar epoxy as was the rest of the inside.

In the 15 odd years I had that boat, I never had a single problem with that keel cavity, despite having salt water in there from time to time.

Interestingly, that boat is now about 40 years old, and is still around, but with a new owner, who stripped it down, wanting to make some interior modifications. But the inside of that hull has remained rust and corrosion free.

Had I to do it again, I would consider using the two part expandable insulating foam to fill that void, and would coat it with coal tar epoxy as before. I don't recall the weight of the concrete, but seem to recall it was around 700 lbs...about 320 kg...which affected the trim of the boat a little....

hope this helps...
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Old 30-11-2020, 07:29   #5
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Re: Concrete Keel in Steel Boat

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Originally Posted by DomPerignon View Post
How about leaving an air space , maybe 1/2 inch, between the plate and the cement and after welded in place make a threaded hole in the plate, fill the air space with oil, then plug the hole.
I designed a bolt on lead filled steel keel for a 40ft motorsailer some years ago and this was what I specified, I suggested using fish oil so if it somehow got out it would not pollute. We have a commercial smoked fish processer in the area where I figured we could get the oil. The builder cut corners and did not cap the keel and the lead did shrink away from the steel a little as expected so I feel this was a necessary step for a long lasting keel.
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Old 30-11-2020, 07:30   #6
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Re: Concrete Keel in Steel Boat

An alternative to welding over the top with steel is to coat it with the stuff they use in airplanes to deal fuel tanks. Boat Life sells it but is expensive. Flame Master and 3M varieties are also available. It is a 2 part goo and comes in 2 different viscosities. Sort of like 5200 on steroids and resists diesel.

Anyway, that will seal the keel pretty well, next best to welding.

Incan hive more info if requested.
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Old 30-11-2020, 10:45   #7
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Re: Concrete Keel in Steel Boat

On steel yachts and barges it is common practice to coat the bilge/floor with a thick layer of grease and then pour in the concrete. I have done this on 2 x 39m motor barges by heating up a 20kg tub of grease over a portable stove and then applying the molten grease with a broom to a level of 2/3mm. Then add the concrete or ballast on top. It is a good idea to coat the sides of the hull up to the waterline with grease as condensation will be present to this level.
I have had 2 Scottish fishing boats with wooden hulls which had concrete in the bilge as ballast and those were 60 years old! I expect that the base under the concrete in their case was tar.
So I see no issue with adding concrete to hulls as long as regulations allow and the substrate is protected in some way.
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Old 30-11-2020, 10:52   #8
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Re: Concrete Keel in Steel Boat

It was common in commercial fishing boats. It is poured wet into steel, then cures, but must leave dampness or wetness in there. so one wonders about corrosion for sure. I dont have an answer.... I do know there is enough moisture that on some boats the keel swells up big time.
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Old 30-11-2020, 11:06   #9
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Re: Concrete Keel in Steel Boat

I remember what we did in the 70s... We used led and poured polyester resin with shredded fiberglass... Worked for a fiberglass hull, and would probably work well for a steel hull too...
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Old 30-11-2020, 16:22   #10
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Re: Concrete Keel in Steel Boat

Cement/concrete? It is a variable material. The first feature already mentioned is that the plastic (non set) material is highly alkaline (due to the cement powder) and thus affords chemical protection to the steel (with reservations). Mixes that have excess water (i.e. above that required for the hydration process that sets the cement) usually have higher water permeability so minimizing the water content of the mix can be important. The size distribution, quality and % of the aggregate used has a major effect on shrinkage during curing and later, compression strength and thermal expansion which may affect adhesion to the steel. It is our most common building material by far and it is well understood.
However it can work well. I would definitely seek the advice of those who have had success. The idea of repairing a rusting keel filled with concrete sounds horrific.

If used and built properly the material can last forever. George Lambot built a ferro cement boat in 1834 that is still functional (I understand), but there are many concrete structures in salt water such as bridge foundations that are totally successful.
The Romans at Ostia used cement with pozzolano in it for jetty structures that has stood the test of time. So if made and applied correctly it can be essentially inert and last forever. And of course, it's certainly cheaper than lead.
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