light displacement
I hope this doesn't get too complicated. Based on my experience loading ships, displacement when it is referred to in ship's documentation is usually lightwight (sometimes called lightship), i.e. the weight of the hull, tackle, machinery and fixed equipment only. Displacement is the weight of the water displaced by the vessel when it is floating. It is equal to the total weight of everything onboard. The difference between lightweight displacement and loaded displacement is known as deadweight. Deadweight is the weight of variable loads. For large ships the designers will publish an approved stability book which has all the stability data in it including 'displacement curves' which will give data for various loading conditions, e.g. tonnes per centimeter immersion (TPC). TPC tells you how much weight you can load to change your draft one centimeter. The vessel I am on now will sink approx. one centimeter if I add 7.8 tonnes on deck. TPC will change varying with the shape of the waterplane area at the waterline. Thus it changes with displacement. The stab book gives a lot of other data which I won't go in to. This is all way to complicated for pleasure boating! I rather believe that the design waterline in a production sailboat is likely similar to a plimsoll line on a cargo vessel. If this is true (and I am not sure whether it is or not), the sailboat shouldn't be loaded so that this line is submerged to maintain an adequate margin of reserve buoyancy (safety). I would contact the manufacturer and get their take on the figures. No doubt they could tell you. Interestingly enough, although the weight displaced would be the same, the volumes displaced will differ between fresh and salt water. Is the design waterline for fresh or salt? Interesting topic for Sunday afternoon. Regards Chris
I hope this doesn't get too complicated. Based on my experience loading ships, displacement when it is referred to in ship's documentation is usually lightwight (sometimes called lightship), i.e. the weight of the hull, tackle, machinery and fixed equipment only. Displacement is the weight of the water displaced by the vessel when it is floating. It is equal to the total weight of everything onboard. The difference between lightweight displacement and loaded displacement is known as deadweight. Deadweight is the weight of variable loads. For large ships the designers will publish an approved stability book which has all the stability data in it including 'displacement curves' which will give data for various loading conditions, e.g. tonnes per centimeter immersion (TPC). TPC tells you how much weight you can load to change your draft one centimeter. The vessel I am on now will sink approx. one centimeter if I add 7.8 tonnes on deck. TPC will change varying with the shape of the waterplane area at the waterline. Thus it changes with displacement. The stab book gives a lot of other data which I won't go in to. This is all way to complicated for pleasure boating! I rather believe that the design waterline in a production sailboat is likely similar to a plimsoll line on a cargo vessel. If this is true (and I am not sure whether it is or not), the sailboat shouldn't be loaded so that this line is submerged to maintain an adequate margin of reserve buoyancy (safety). I would contact the manufacturer and get their take on the figures. No doubt they could tell you. Interestingly enough, although the weight displaced would be the same, the volumes displaced will differ between fresh and salt water. Is the design waterline for fresh or salt? Interesting topic for Sunday afternoon. Regards Chris
But nobody has addressed just what exactly is the displacement weight of a production sailboat. Is it the actual weight of the bare bones boat with spars, rigging and engine or does the displacement weight consider the addition of a theoretical weight of "goods" that the owner loads into the boat, along with crew. I'm guessing that it varies even among manufacturers. For instance, when I was shopping for boats with 27' length, there was a considerable difference in the stated displacement of similar sized boats between about 5,000 pounds to 7,500 pounds. Granted, boats with a shoal keel typically have more lead in the bulb than the deep keel versions. But the difference in the keels for these boats was only about 300 pounds. So I wouldn't be surprised if the differences in displacement were mainly differences in what constitutes the theoretical additional load (above the actual weight of the boat). BTW, Phil, it doesn't make any sense that the load on the tractor trailer should be 6,500 pounds more than your displacement weight. A water soaked hull couldn't add more than a hundred pounds at most. Something is definitely screwy about that scenerio, and I would suspect that the trucker is way off. He must have had a lot more than just his thumb on the scale! 
