Concrete ballast in steel hull

[HVYJimmy]

Concrete ballast in steel hull ?
Pros and Cons ?

[ItDepends]

Pros: It is cheap.

Cons: Everything else.

It is not all that heavy, so it is not very good ballast on a weight/volume basis. If the hull was designed for lead, or other proper ballast material, concrete will leave the boat very under ballasted.

In a steel hull it will hold moisture against the steel leading to corrosion that you can not see or fix.

But it is cheap.

[Lepke]

It's what is done in fishing boats. Usually a good epoxy coating on the steel before pouring the cement. And there's a foam layer over the cement. In fishing boat the fish hold is the lowest point and the foam is the start of building the fish hold.
I've repaired or remodeled boats that had the cement as much as 30 years and the steel was in fine shape. It's a lot of work to remove the cement.

[Uncle Bob]

Originally Posted by HVYJimmy

Concrete ballast in steel hull ?
Pros and Cons ?

If the concrete is made using steel punchings, very little sand and plenty of cement, laid into the keel after being well painted with coal tar epoxy or similar and sealed on top then there should be no problem. But as others have said, not the ideal material for the job.

[HVYJimmy]

Thanks Guys

[RaymondR]

I used steel punchings cemented in with concrete I made from cement and monel metal cuttings I sourced from a machine shop which carried out a lot of SS and monel steel fabrication. One of the few parts of the boat I did not have corrosion problems with. Had the boat for thirty years.

[GordMay]

“A Quest for Keel Integrity” ~ by Darrell Nicholson [PS, 2016/2020]

”... Many production-boat builders opt for iron ballast and galvanized, high-carbon steel keel bolts. This is a cost-saving shortcut that, down the road, leaves an owner coping with rusting ballast and the ticking time-bomb of corroding keel bolts. Those buying a decade-plus-old, pre-owned sailboat with high-carbon steel keel bolts and iron ballast should consider having a good boatyard lower the ballast and carefully inspect the bolts. Simply torquing up the fasteners and heading off on a long ocean passage on a boat of this vintage is courting an unnecessary risk ...”

➥ https://www.practical-sailor.com/sai...keel-integrity

[MicHughV]

When I built my steel boat, the plans called for ballast in the first two keel cavities, but not the last one at the trailing edge of the keel.

I was not able to get in there to paint it, so being concerned about rust, I filled it with concrete. Prior to doing that, I drilled a 1/4" dia. hole at the very back of these cavity, so excess water from the concrete could drain out, as the concrete dried.

For about two weeks, this little drain hole would weep some water, then it stopped, at which point I welded up the hole.

Finally, I coated the top with coal tax epoxy similar to the rest of the hull interior. I never had a problem with this area for as long as I owned the boat.

I'm guessing at the extra weight in there, and I'm guessing about two cubic feet or about 300 lbs.

If I were to do it again, I'd consider filling the void with the pour-in expandable foam, and then sealing with coal-tar epoxy.

[Dockhead]

Unlikely to matter much on a long keel boat, but the density of typical concrete is only 2.3 tonnes per m3, or only 2.3x that of water, or much less than cast iron at 7.1 tonnes/m3, not to speak of lead at 11.3 tonnes.

So it's a very inefficient ballast material from that point of view -- requiring a lot more volume underwater (= a lot more drag) for the same righting moment. You'll want to add as much iron, steel, or whatever as possible, to the mix, to get that density up at least some.

That's why lead bulb keels are so good for sailing performance.

But unlikely to matter much in a long keel boat which already has a lot of wetted surface just because of the hull form. You'll just get less righting moment for the same amount of ballast, as you'll have to fill it higher in the keel, 3x higher than cast iron.

[MicHughV]

It should be noted that a concrete mix usually includes stone. Just buying cement, is just like putting water in the keel.

As a retired structural marine engineer I can tell you that adding steel to the concrete mix is of itself not a problem, as long as you provide sufficient " cover" over the steel. For marine work, I always specify a minimum of 3" ( 75mm) concrete cover over steel, to prevent salt water/moisture from wicking to the steel.

Finally, one last tip, concrete should dry slowly. This prevents surface cracks. While it may go against the grain, placing a wet or moist towel over the concrete for the first few days will go a long way to avoid this. The towel needs to remain wet or moist for the entire drying period. About a week should do it.

[RaymondR]

Originally Posted by MicHughV

It should be noted that a concrete mix usually includes stone. Just buying cement, is just like putting water in the keel.

As a retired structural marine engineer I can tell you that adding steel to the concrete mix is of itself not a problem, as long as you provide sufficient " cover" over the steel. For marine work, I always specify a minimum of 3" ( 75mm) concrete cover over steel, to prevent salt water/moisture from wicking to the steel.

Finally, one last tip, concrete should dry slowly. This prevents surface cracks. While it may go against the grain, placing a wet or moist towel over the concrete for the first few days will go a long way to avoid this. The towel needs to remain wet or moist for the entire drying period. About a week should do it.

Aggregate in concrete serves two purpose: it increases the compressive strength and reduces the quantity of expensive, manufactured, cementitious material required. Including stone will just serve to displace far heavier 490 lbs/cu ft steel and consequently reduce the ballasting effect. A water cementitious slurry will better flow and fill voids and create a single mass which will both exclude moisture and provide a basic chemical environment.