[Public-list] Draft vs depth

Thu Aug 31 09:03:04 PDT 2006

All this pettifogging nitpicking detail exists because MONEY is
involved. You'll find the Gross Registet Tonnage caclulations are just
as Byzantine. These numbers are used to determine shipping fees and/or
ship structural requirements and therefore affect the operating costs
and capital cost of a ship.  These rules are complex for the same reason
IRS rules are complex.

For a yacht these considerations are usually moot. A simple number for
depth is the maximum draft + minimum freeboard.

Mike

Michael J. Meinhold / SAIC
2111 Eisenhower Ave, Suite 303, Alexandria, VA 22314
703 842 2606 / michael.j.meinhold at saic.com

-----Original Message-----
From: public-list-bounces at alberg30.org
[mailto:public-list-bounces at alberg30.org] On Behalf Of edward schroeder
Sent: Thursday, August 31, 2006 11:47 AM
To: alberg30
Subject: [Public-list] Draft vs depth

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. 
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