[Public-list] Draft vs depth

[edward schroeder]

Regarding Draft (Draught) and Depth, the following is from the Dixon Kemp Nautical Dictionary. Clear as mud, except on Leap Year. Ed. Schroeder #303   
   
Depth, Moulded.-- The terms used in ship and yacht building and relating to the depth of vessels are numerous and occasionally confusing. For instance, there is draught of water aft and draught of water forward, extreme draught and mean draught. In a merchant ship, draught aft and extreme draught would most likely be the same, but in many yachts, the extreme draught is amidships, or nearly so, and the draught at the sternpost is frequently less than the extreme draught. The draught forward in most sailing yachts would be a purely fanciful quantity, on account of there being no straight length of keel forward of amidships to measure the draught from. Beyond this, formerly depth or depth of immersion was used to denote draught; and then there was moulded depth, that is the depth from the load line to the rabbet of the keel ; after this came depth of hold, which in a man of war meant depth from the lower deck, or orlop deck, to the ceiling above the kelsons, and in a merchant
 or carrying ship, or yacht, the depth from the upper deck.   
  
The term "moulded depth" is now never applied to the depth of immersion, and when the term is used it is always understood to mean the depth as defined by Lloyd's, as follows: "The moulded depth of an iron or steel vessel is the perpendicular depth taken from the top of the upper deck beam at the centre at the middle of the length of the vessel to the top of the floors, except in spar and awning deck vessels, in which the depth is measured from the top of the main deck beam. In wooden and composite vessels the moulded depth is also taken to be the perpendicular depth from the top of the upper deck beam at the centre of the vessel amidships to the top of the floor frame." It will be seen that, even with this excellent definition of moulded depth, it may mean a great many things in the case of yachts with very hollow floors and great dead rise, or in the case of yachts with box keels the same as Vanduara, Galatea, and Wendur have. However, there is one definite point to start
 from in all cases, and that is the "top of the upper deck beam at the centre."   
  
Depth of a Yacht, to Measure.-- Very frequently it is necessary to know accurately the external depth of a yacht from rail to keel, or her draught from load line to keel. The following simple plan is a ready means of obtaining such depth and draught:   
  
To obtain the depth take a straight-edged bar of wood (see e e, Fig. 35) which will be placed across the rail, at right angles to the keel. A small chain, f f, will be passed under the bottom of the yacht, and one end will be made fast on the bar at g, so that the chain just touches the bilge; the chain will be drawn tight, and the other end made fast to bar at h. The distance g h must be accurately measured on the bar, as also, when removed, must the length of the chain which passed from g under the yacht to h. (To obtain the points for the measurement of the chain, it would be found convenient to fasten a small piece of cord or yarn at the points g and h, immediately under the bar, before the chain is cast off.)   
  
Having obtained these measurements, it will be an easy matter to find the depth i j. The distance g h can be laid off to scale, divided in the centre by a perpendicular, i j: half the length of the chain will then be laid off from g and It to intersect the perpendicular, as at j; the distance from i to j on the bar, measured by the scale, will be the depth required. The draught of water of the yacht will of course be found by subtracting her height out of water, from load line to rail, at the points where the depth was taken. If no scale be at hand, the depth can readily be found by calculation. Take half the length g h, which call k l (Fig. 35), and half the length of the chain, which call k m; subtract l from k m; multiply the remainder by the sum of k m and k l added together; the square root of the product will be the required depth. Expressed in algebraic language:   
  
[formula]   
  
Say k m is 10ft., added to k l 7ft., make 17ft. ; next 7 subtracted from 10 leave 3 and 3 multiplied by 17 make 51. The square root of 51 is 7.1, which would be the required depth. The mean draught would be found by taking the actual draught at several (say 4) equidistant intervals, commencing at the heel of the sternpost and ending at the stem; add these draughts together, and divide the sum by the number of measurements taken, including those at stem and sternpost. If the forefoot is very much rounded away, the measurement at the stem will be 0, but in counting the number of measurements, that for the stem must be included. The Barrow Corinthian Yacht Club formerly included mean depth in their tonnage rule, and adopted, on the suggestion of Mr. R. S. White, the following plan for obtaining depth at any point without calculation. (See Fig. 36.)   
  
  
A is the keel batten, graduated from centre, in feet and tenths, with slots marked C, at each end, to slide the side or depth battens to the exact beam of yacht.   
  
B B. Side or depth battens, graduated at upper part in feet and tenths from top of keel batten, and secured to keel batten with thumbscrews marked D.   
  
  
The manner of working is as follows:   
  
Having obtained exact beam of yacht, set the depth battens B B at this distance apart on keel batten A, by means of thumbscrews D tightly screwed up. Dip the keel batten under keel until opposite marks on gunwale, where depth is required to be taken; then bring it close up to keel, and take readings off depth battens B B, until they correspond on each side--this being depth of yacht, keel to gunwale, in vertical line, as shown in sketch.   
  
If the measurements have to be taken in a tideway, the batten A must be kept close up to keel to prevent its driving aft.