Malcolm Tennant of Malcolm Tennant Multihull Design in New Zealand <http://www.tennantdesign.co.nz> sent me the following thoughts on displacement where he concludes, sort of, that the most meaningful displacement figure is the one of a vessel at full load. --Georgs DISPLACEMENT: Recently I was reading some reports on a number of new boats. There was nothing unusual in this, as I like to keep abreast with what is happening in the field. However there was a particular feature of all of these boats that caught my eye. Their displacements were all exactly the same! There were variations in the length, in the beam, the power and the speed of all of these boats. Enough differences that I questioned how could they all displace exactly the same amount? However it is important to realize that there is not any commonly accepted standard for reporting the displacement parameters for pleasure boats. So, perhaps the builders/designers did not all mean exactly the same thing by the term displacement. That would certainly explain the strange coincidence and would make any meaningful comparisons very difficult, if not impossible. But surely displacement is displacement and that is it. Well no, it is not quite that simple. The classic definition is "When a vessel floats it displaces a volume of water whose weight is exactly equal to the weight of the vessel". That seems simple enough and explains why a vessel will float slightly deeper in less dense fresh water than in the denser salt water. But it is a mistake to assume that this value is some unchanging, god given figure, which is an absolute. The definition as it stands does not say anything about the condition of the vessel when it displaces that particular amount of water. It needs a corollary that says "-at that particular moment in time, in this load condition". What that means is, that in fact, the displacement of a vessel is a continuum, or spread of values, from when it is "empty" to when it is "full". Depending on the vessel and its purpose, this continuum can be quite broad. Obviously for a day boat, either sail or power, the range of the displacements is quite small. Very little is added on to a vessel of this type from its empty condition. At the opposite extreme is the commercial cargo vessel where the range of the displacement from "empty" to "full" is very large. So much so that ballasting is required to preserve its stability when it is empty. So generally, when discussing the characteristics of a vessel, it is much more meaningful to talk about three different displacements: light ship displacement, the half load displacement and the full load displacement. Generally the light ship is the vessel complete and with such items as anchors and chain etc on board. There would be no fuel and no water. Nor would the crew or their food and personal effects be included in light ship. The full load displacement is with all the water, fuel, crew etc added. The half load is the median value between the full load and the light ship. The half load may be used to describe the vessel in some cases because, it can be argued; that this is the condition that is the most common for a lot of vessels. This may be true for trawler yachts or similar vessels that are designed for long range but that actually spend a lot of their time traveling relatively short distances, relatively lightly loaded. However this may be totally misleading for a day boat. Caution is advised. Some times the light ship displacement will be used. I tend to think of this as a "salesman's" displacement because "everybody" knows that a lighter displacement boat is faster for a given sail area/horsepower than a heavier displacement one. This tends to give a false idea of the vessel, which may have very different characteristics in full load displacement to what it has in light ship. It really would appear to be of very little real use. Particularly in comparing one boat with another. There is also a slight peculiarity with the term light ship that means it may need to be defined quite carefully. Some Classification Societies actually require the removal of things such as the cooling water in the engines and lubricating oil in all the machinery before the resulting displacement can be called light ship. The light ship may sometimes be called weight, particularly when describing the smaller cruising yacht. But it is probably better to avoid this usage even though weight generally means more than light ship to most people. When it is used to mean weight it is necessary to be very clear about what this "weight" consists of. So is the more realistic displacement figure to use that of the full load, is this the figure that should be given when describing a boat? Well yes and no. If you look at commercial vessels you will find that for them there is an accepted standard for giving the displacement. They will usually have the light ship, half load and full load displacement given. Then the Dead weight, and possibly the Gross tonnage, may be given. The deadweight is basically the weight of cargo/people/trucks etc that the vessel is designed to carry. Gross tonnage is really not a weight at all even though it sounds like it is, but is a volume that is used largely for the purpose of calculating duty due on cargo. These multiple values are probably more complex than is necessary for pleasure boats and it is made even more complex by the use of "Plimsol" lines which define the loading conditions allowed at various places in the World at various times of the year. Obviously if the boat is floating higher as required at that time, and that place, it displaces less. Even in full load. There is another complicating factor in all this, as if it wasn't already complicated enough. This is the requirement by the designer/naval architect to know what the full load displacement of the vessel is going to be during the design stage ie: before it is afloat. We talked about the displacement of the vessel being the weight of water it displaces when it floats. So how does the designer determine what the displacement is going to be if the boat is not floating? The answer is, with great care. He must know precisely what every component of the vessel weighs and where it is located in all three dimensions. This includes the hull and all parts of the structure. Once these calculations were all done by hand, and a very tedious job it was too. Today this data is all fed into a computer spread sheet, which calculates the moments of all of these components and gives a total mass and the positions of the LCG, VCG and TCG [longitudinal centre of gravity, vertical centre of gravity and transverse center of gravity] of this mass. Upon all these calculations will ultimately depend whether or not the vessel floats where it is supposed to with the correct trim and, most importantly, whether or not it performs as expected. One way designers can check on the accuracy of their mass calculations is to have the boat weighed, on load cells or something similar, at fixed stages in the construction when the actual weight achieved can be compared with that estimated. You will notice that the mass of the hull is included in these calculations. But how does the designer know the size of the hull required, and therefore what it weighs, until he knows the final displacement? What he has to do is make an estimate of what the vessel is going to displace and size the initial hull accordingly. The accuracy of this estimate will determine whether or not the hull will need to be changed once the "real" displacement is calculated. Sometimes the builder will run with this original estimate of the displacement if he is time constrained, or he is trying to pre sell boats. This can be a little dangerous as it depends very much upon the experience of the designer as to how accurate this estimate is. This is particularly true if he has not designed similar vessels. It also contains a basic assumption. This is: that there will be no changes made, during construction, to anything that impinges on the final displacement. What this means in reality is that this assumption is usually flawed. The designer will often have to adjust the displacement and, as a consequence, the hull as a design progresses because changes have been made. There is a final displacement that should be mentioned. I don't know what we could call it but basically it says "if the vessel happens to float at the water line as drawn then it will displace this amount". This is similar in concept to the designer's guesstimate and may, or may not, represent the true displacement in any real condition. This was largely a function of the amount of work required to calculate hydrostatics and mass by hand. Hopefully this is something that no longer occurs now it is simple and quick to do hydrostatic calculations on the computer. Of course even after all this I am still none the wiser as to which displacements were actually being used for the vessels that I had originally looked at. Because, in only one case, was it stated that it was the full load displacement that was being given. In some cases it might have been the designer's original un-corrected "guesstimate". It might be light ship, half load or even full load. However, if all three displacements were to be given, along with the definition of what light ship, or "weight" actually means in that particular instance. Then you would know [1] what it "weighs", you would know [2] what is hopefully the most common loading situation and you would know [3] what it is supposed to displace when fully loaded. The difference between the light ship and full load is its "payload". If you can't get all of these then the full load displacement is probably the most meaningful single figure that can be used to compare the displacements of various vessels. This is because it is the worst-case scenario. The others, by themselves, can be misleading if not carefully defined. This is certainly true if you do not actually know which value is being used. Malcolm Tennant Multihull Design Ltd PO Box 60513 Titirangi, Auckland 1007 NEW ZEALAND e-mail malcolm@tennantdesign.co.nz www.tennantdesign.co.nz www.catdesigners.com

>Malcolm Tennant of Malcolm Tennant Multihull Design wrote: >Of course even after all this I am still none the wiser as to which >displacements were actually being used for the vessels that I had >originally looked at. Because, in only one case, was it stated that >it was the full load displacement that was being given. In some >cases it might have been the designer's original un-corrected >"guesstimate". It might be light ship, half load or even full load. >However, if all three displacements were to be given, along with the >definition of what light ship, or "weight" actually means in that >particular instance. Then you would know [1] what it "weighs", you >would know [2] what is hopefully the most common loading situation >and you would know [3] what it is supposed to displace when fully >loaded. The difference between the light ship and full load is its >"payload". If you can't get all of these then the full load >displacement is probably the most meaningful single figure that can >be used to compare the displacements of various vessels. This is >because it is the worst-case scenario. The others, by themselves, >can be misleading if not carefully defined. This is certainly true >if you do not actually know which value is being used. As a follow-up, Malcolm sent me the following note to help make his point: >Incidentally my NY-51 design which is being built by Brazil Boats has a >light ship displacement of 44,092lb and a full load displacement of >67,419lb. This is a difference of 23,327lb. More than the total >"displacement" of the Endeavour 44! This illustrates the importance of not >just using a single displacement figure unless you know exactly what it is >refering to. Alas, in my experience, it is difficult enough to determine whether the displacement figure shown in sales literature is light, half or full load. Never mind trying to get all three numbers from a builder. --Georgs -- Georgs Kolesnikovs Power Catamaran World http://www.powercatamaranworld.com