I am building a small ship (or very large sailboat) that will have a concrete ballast at the base of the keel. I am looking to increase the weight of the concrete (lb/ft3), but I also want to maximize the impact resistance of the concrete to resist cracking/shattering should the boat hit a rock hard vertically due to waves.

The simplified dimensions of the concrete portion of the keel will be 11.5" W x 18" D x 40' L. This will essentially be bolted on to the bottom of a laminated wood (fir) keel which is 11.5" W x 12" D x 40' L (simplified dimensions).

How can I determine the best concrete recipe and reinforcement scheme for this particular usage?

I need to determine:

  • optimal size of rebar to use;
  • optimal size(s) of aggregate;
  • how many parts of cement, sand, and aggregate to mix;
  • and the optimal amount of rebar to include.

In general heavier is better; I am not looking to lighten the concrete. I know concrete recipes are a topic all on their own; what I need is a recipe to start with that will produce a strong and heavy concrete that resists cracking from a heavy impact. The boat will displace 100 gross tones of water and may be subjected to waves with crest-to-trough wave height of about 6' in areas where there is an unlikely yet possible danger of striking underwater obstructions. It doesn't need to support being continuously jackhammered, but support get hit perhaps say 5 times over it's expected lifetime of say 20 years (just to give you an idea of infrequency). I am not planning to strike bottom, but it can and does happen despite all cautions taken.

  • $\begingroup$ One important piece of information is what is meant by "hit hard". What would be the height of the drop, or what would be the vertical speed of the boat at the moment of impact? $\endgroup$
    – Wasabi
    Commented Apr 10, 2016 at 20:09
  • $\begingroup$ @Wasabi Not looking for indestructible solutions, nor any that target a specific PSI of impact force or ductility or whatever, I am just looking to figure out what size and amount of rebar and concrete recipe would provide the maximum strength of the concrete as far as impact resistance goes. I imagine technically, all rebar with cement as a bonder would be "strongest" but wouldn't the thinness of the cement lose its bonding strength and therefore be counter productive? So what is the right amount of rebar? Say 1x half inch bar a square inch (viewed end on), or what? $\endgroup$
    – Escoce
    Commented Apr 10, 2016 at 21:06
  • 3
    $\begingroup$ The question remains too broad. Concrete mixes are a huge topic in and of themselves, highly dependent on the specific situation. Other than the issue of impact, which hasn't been quantified yet, there's also the issue of oxidation of the rebar. If the steel is permanently submerged, this actually won't be an issue, but if you intend to take this in choppy weather (as perhaps implied by your worry about vertical drops due to waves), then its something else to take into consideration. $\endgroup$
    – Wasabi
    Commented Apr 11, 2016 at 14:12
  • $\begingroup$ You ca consider it permanently submerged. This will be 5 feet below the surface while the boat is at rest and I didn't think the keel will ever come out of the water except in extreme circumstances. I know concrete recipes are a topic all on their own which is why I am asking a group of engineers to give me a recipe to start with that will produces a strong and heavy concrete that resists crack from a heavy impact. We aren't chipping away at it with a hammer. Talking about a boat that displaces 100 gross tones of water being lifted and dropped by a normal sea wave of lets say 6ft of amplitude $\endgroup$
    – Escoce
    Commented Apr 11, 2016 at 14:44
  • $\begingroup$ I agree with Wasabi in principle that the problem statement seems too broad in its current form. I suspect that with just a little research you will be able to find a good marine concrete mix/recipe to use. Given that recipe, the reinforcement problem is still a rich and interesting one. $\endgroup$
    – Air
    Commented Apr 11, 2016 at 16:31

1 Answer 1


You intend to submerge a rectangular concrete section in a fairly corrosive environment (ocean water). The section is 11.5" wide and 18" deep. During the 20 year life span of the boat, you anticipate 6 heavy impact events.

The concrete keel is intended to act as a beam which is to spread the impact load evenly onto the laminated wood hull to such an extent that the hull does not sustain any damage. This means that under full impact load the concrete beam (keel) can deflect only a limited amount, before it causes damage to either the points where it is fixed to the boat, or the boat itself. This deflection is a function of load as well as geometry and design of the beam. The beam will require structural steel (the amount can be calculated, based on maximum impact loads).

The steel mentioned above needs to be protected to avoid corrosion. This means it needs to be encased in min. 2" of concrete. Should an impact occur, this concrete cover most likely will be compromised due to spalling of the the concrete due to the impact. No matter what you do, you will have a 100 ton load dropping 6 feet onto a very small area during impact. This is in the realm of large breakers which are used to demolish concrete buildings. Once the steel is exposed, rust will form and cause further spalling of concrete.

The only sustainable solution is to take a 16-17" steel pipe with a fairly generous wall thickness and fill it with mass concrete to get the ballast. This way your keel is impact resistant, and you have a long beam which spreads the load evenly to your wooden hull.

  • $\begingroup$ I don't think there's really much risk of corrosion, actually. In the comments below the OP, they state that the concrete will always be submerged (except under stormy weather, where it might be momentarily exposed for a few seconds at a time). And without exposure to air, oxidation won't occur. $\endgroup$
    – Wasabi
    Commented Apr 12, 2016 at 11:34
  • $\begingroup$ @Wasabi, the chemistry in ocean water is a corrosive to most ordinary cement types. This means the cement paste chemically corrodes over a period of time. As long as the environment around the reinforcement remains alkaline, things should be fine. As soon as the concrete is compromised, you have a scenario where the steel is exposed and you are left with unprotected, bare steel. This would be in a submerged state most of the time, so less exposed to oxygen than in the atmosphere, thus the corrosion process is delayed. $\endgroup$
    – SlydeRule
    Commented Apr 12, 2016 at 12:19
  • $\begingroup$ @NamSandStorm you aren't quite getting the gist of the question. The concrete is not meant to serve to protect the wooden keel. Although it does have that happy consequence, it is secondary. It is simply meant to provide ballast, that is in nautical terms means dead weight at the bottom of the keel to produce a few effects by lowering the center of gravity helping keep the boat upright and therefore afloat and moving faster through the water. I simply want a concrete recipe that will provide the most durable concrete keel ballast. Weight is not an issue, in fact heavier is better in this case. $\endgroup$
    – Escoce
    Commented Apr 12, 2016 at 13:41
  • $\begingroup$ Don't worry about the effects of ocean water. I know very well what it is, what it does and how to minimize the effects of galvanic corrosion and oxidation. That's not the issue, the issue is the best more durable and impact resistant recipe there is for concrete. $\endgroup$
    – Escoce
    Commented Apr 12, 2016 at 15:47
  • 3
    $\begingroup$ The proper term for "concrete recipe" is concrete mix design. It is an aspect of structural engineering. Concrete is strong in compression but weak in tension and bending, no matter what the mix design is. So whatever happens to a concrete member, it will be subjected to a combination of tension, compression, bending and shear. The reason we provide steel is to mitigate the weaknesses of the concrete. So your approach to just attach mass concrete with some steel will not work, sin you are not addressing the weaknesses of the concrete. A fibre reinforced concrete may work however. $\endgroup$
    – SlydeRule
    Commented Apr 14, 2016 at 7:27

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