The former U.S. Secretary of Defense, Mr. Donald Rumsfeld, once made a statement that earned him a dedicated Wikipedia page on the subject. It goes like this: “Reports that say that something hasn’t happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don’t know we don’t know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.”
I think Mr. Rumsfeld would have made a fine weight engineer because, if you think about it, the quote above could just as well be related to weight engineering. We have the same concepts: Known knowns, known unknowns, and unknown unknowns. Let us break it down:
The known knowns are the easier ones. When you model a pipe and you know the material (thus density), thickness, and length, you know the properties that make up its weight. You also know where the pipe will be located on the vessel. So this, and all similar cases, you should be able to handle quite well.
The known unknowns are a bit trickier, but still not too bad, as we know they are there and we have methods to handle them. In an early phase of the design, the modelled pipe as mentioned above may be just that – a model of only the pipe itself. However, you know that attached to the pipe there will be fittings, valves, and other weight items, not currently modelled but definitely something to account for. These are the known unknowns, as we know they will be there, but we don’t know yet exactly how much and where. Many weight engineers label these weights as “allowance” or ” contingency” and add a percentage to the known weight (in this case the pipe itself) based upon experience and model maturity.
The unknown unknowns represent the weight we know will be there, but we don’t really know what it is or where it comes from. It is sometimes described as a result of the development of the project from contract to completion. When you have accounted for the pipe and the known unknowns, your risk analysis or experience is used to transform this weight estimate to a 50/50 weight estimate (equal chance of final result being higher or lower than estimate). It is another form of ” allowance/contingency” and it may be added as a percentage to the base weight in a similar way as the known unknowns.
Beyond the above (un)known (un)knowns variations, we have the discrepancy that appears when you compare your complete and detailed weight report to the result from the displacement and inclining test report. This deviation is the “dark matter” of weight engineering (science tells us that dark matter in the universe constitutes about 84 % of the total mass; hopefully the results are not that bad for your vessel).
How do we handle this? Ahead of the discovery of it, the way to handle it is to add a security margin to your results. But what to do when you know the size of this mass? That depends on the size of the matter (pardon the pun). Do keep in mind that the inaccuracy of a displacement and inclining test is often given to be around 1-1.5%, and if the deviation is in this range, you really don’t need to deal with it at all. If it is larger than this, the first step is to look at the center of gravity of the missing mass. This can, in some cases, give you an indication of what has been left out from your weight report, or at least some area to look into further. But you may find that you can’t do too much about it except add it to your weight report as a remainder mass – and if so, do spread it into the various weight groups – do not add it as one chunk! Finally, add it to your experience as a weight engineer.