Weight Control

Weight control is not sexy, but the consequences of ignoring it can be very scary. If we ignore weight control, we risk a potentially unusable ship. That is why proper weight control starts with the right attitude: understanding the risks and the necessity of a weight estimate.

The Silent Killer

1.0 Introduction

Human intuition is a great asset; it keeps us alive.  As humans, we rely on our intuitive understanding of the world, and we extend that intuition into shipbuilding.  But every so often, ship science produces a concept so bizarre that our intuition breaks down.  Weight control is a prime example.  It defies our understanding to believe that a few extra coats of paint make a ship inoperable.  All too often, what we can’t understand, we just ignore.  And that is why lack of weight control is the silent killer for a ship.

2.0 Weight Consequences

BE AFRAID.  To understand weight control, we need to instinctively recognize the need for it.  I knew a passenger ship that underwent major modifications.  The owner invested everything they had into this ship.  When it came out of the shipyard, the ship looked beautiful.  And the vessel was not allowed to operate, because improper weight control rendered the ship unstable.  USCG would not allow an unstable ship to operate.  The owner had very limited engineering solutions, since everything was already built.  Even worse, they already invested their money into a currently useless ship.

Lack of weight control can even kill a new ship.  I joined project for new construction where the owner had originally required weight control from the shipyard, which was the right move.  But the shipyard never did their part, and the owner didn’t press the issue. 

Fast forward 20 months to the end of construction.  We discovered that the vessel only achieved 1/3 of its planned cargo capacity.  The ship was too heavy.  Our owner invested hundreds of millions into this ship.  Financially, they already committed to accepting that vessel.  But the vessel missions were greatly restricted, and the ship looked far less attractive as a business investment.  I wouldn’t be surprised if that owner tried to sell their new ship within the first year.

Stories like this happen more frequently than you think.  They all have a few things in common.  Weight control was ignored, until after construction.  The vessel capabilities were seriously limited from being overweight.  And very little could be done to fix the problem after construction finished.  All the possible options were expensive, with limited effectiveness. 

This emotional context is why we talk about weight control.  It isn’t about spreadsheets and boring margins.  Weight control is about the fear of being overweight, with nothing you can do about it.  That fear motivates us to control the vessel weight.

3.0 Weight Limits

The process of ship design starts with making a guess at our ship weight, and then refining that guess.  At the beginning of the design, we pick a hull shape and size.  The hydrostatics of that shape dictate certain limits on maximum weight and the center of gravity.

We then build inside that shape.  (Figure 3‑1)  We fill it with all the components required for a ship:  structure for the hull, engines, propellers, beds, tables, paint, etc.  All these components add weight to the ship.  But there are no guarantees that construction weights match the limits of the hull shape.  We need careful weight control to keep everything within a narrow margin.

4.0 What is Weight Control

Weight control means different things, depending on who you ask.  The offshore industry has a whole ISO standard for weight control. [2]  In that context, it means specific standards for reporting and communication.  Shipyards exercise different levels of weight control, varying with the contract.  For design offices, weight control focuses on creating an accurate initial estimate. 

Weight control starts with an attitude more than a technique.

1. Create an accurate estimate of the weight changes BEFORE starting construction.
2. Maintain consistent responsibility for tracking and monitoring the evolution of the vessel weight. 
3. Regularly check on the evolution of the vessel weight.
   1. During construction.
   1. After construction, during vessel operation.
4. Ensure that key decision makers at all levels recognize the importance of weight control.

The key element of this attitude is to understand that without weight control, your ship runs very good chances of going overweight.

4.1 Weight Estimate

Weight control starts with a weight estimate, where we detail the weight and location of every single item on the ship.  Very boring, and very important.  A good weight estimate includes several features:

1. Margins are added to the estimate, artificially increasing the weight and vertical center of gravity.  This provides breathing room during construction.
2. Weight items should be divided and grouped into logical structures.  The estimate needs to be organized so that anyone else can review it and determine if each item seems reasonable.
3. Provide clear supporting documentation for all weight items.  Vendor information should be readily available.  Any calculations should be easy to understand. 
4. Revision control.  The weight estimate is a living document, frequently updated.  You need a way to track these changes.

Weight estimates can also be combined with project management techniques to track the evolution of the vessel weight during construction.  Just like a construction cost budget, the weight estimate becomes your weight budget.  Track its evolution to predict the final weight at completion.  Using these methods, you shift from a reactive to a proactive strategy.

5.0 Conclusion

Weight control is not sexy, but ignoring it leads to very dire consequences.  Intuitively, we have a hard time believing that little changes across the ship add up to huge penalties.  That is why proper weight control needs a champion, someone to remind us of the hidden danger.  They stay vigilant to the evolving ship weight, armed with their weight estimate.  It sounds overly dramatic, because weight control is not about the process.  Instead, focus on the risk.  Think of the consequences if we ignore weight control:  a potentially unusable ship.

6.0 References