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How you walk on water? (Block Coefficient)

A posting related to the belated holiday season’s greetings here. Something to ponder for the New Year:

Block Coefficient:  The ratio of the volume of water displaced by a ship to the product of the  ship’s length, breadth, and draft.

A couple of weeks ago, I was in our Cannes headquarters meeting with our president, Mr. Paul Bezzi. We were looking at some quotations, and Paul said to me, “John, I’ll bet you don’t know this, but I can tell you exactly how much Jesus weighed.”

“Huh?”, I replied.

It is simple. You know, in the Bible it is mentioned how Jesus walked on the water. Well, the water density of seawater is well-defined: A known variable. But remember, Jesus was walking around on the Dead Sea, which is a much higher density. In order for Jesus to walk on water, his displacement must be higher than the density of the seawater. So, if I know the size of Jesus’ foot, then I can use calculations to determine how much he weighed.

A simple demonstration that can help make clear the importance of a block coefficient in high speed boat design. Think of the block coefficient as a representation of displacement. In a cube, L x W x D is an easily calculated volume. However, a ship’s hull is not cubic in shape: It has curves, angles, and symmetry… Its’ own shape. As the weight of the vessel increases, the draft increases, displacing more water from the cube. Therefore, the block coefficient takes this added displacement into account.

Naturally, different hull shapes yield different block coefficients. Planning hulls, designed to skim over the water, and typically very light, have low block coefficients, normally around 0.4. Frigates or corvettes, with semi-displacement hulls, are heavier and displace more water. Therefore, the block coefficient on these vessels tend to average around 0.5. Finally, barges or cargo ships are designed to carry large amounts of weight. They are much slower, and more stable. These vessels also displace much more water, and the block coefficient averages around 0.8. Therefore, we always ask very specific questions about the hull shape and dimensions when quoting our systems.

Why is this important?

Well, as mentioned frequently on this site, France Helices measures, and compensates for, hull resistance in designing our surface drive systems. It is what makes us stand apart from other manufacturers, and why we make performance guarantees. Quite simply, in order to maximize speed and performance, we need to minimize resistance. By their nature, surface drives produce much less drag, and therefore some resistance is compensated. With planning hulls, as speed increases, the drive provides life, moving the hull out of the water, decreasing resistance by applying a higher opposite force.

In the example above, Jesus had some Divine Interference. However, in the world of boat design, the application of sound engineering principles can certainly suffice. It becomes a question of balance.


Comments

  1. Angeles Quezada says:

    Great information :)

  2. RANJAN KUMAR SAHOO says:

    Dear Sir/Madam,
    How to ascertain cargo weight of loaded barges by block co-efficient factor?

    Kindly suggest with suitable reasons.

    Thanks & best regards.
    Ranjan Kumar Sahoo.

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