How to design a reinforced concrete ship keel?

Question

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.

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.