The following articles will always talk about measuring points and measuring marks.
They are about 10mm wide, but at the same time they should have a size that fits the mm. This is only possible if a certain point on the measuring mark or point is to be decisive. Which are they:
Measuring marks that delimit a gap:
The decisive factor here is the edge of the measuring marks that limits the dimensions. We have measuring marks on the mast for the upper and lower limits of the mainsail luff. So once the upper edge of the lower mark (in the area of the Lümmel fitting) and the lower edge of the upper measurement mark (in the area of the trap rollers).
Measuring marks that mark a position on the boat:
The edge that is closest to the keel is decisive here. At the bow (about 1/3 of the bow tip - keel approach) and stern (in the area of the rudder stick) we have measuring marks for the waterline. The lower edge of the marking is decisive here.
Ideally, the center of the marking is decisive here. The hulls should have permanent markings at these points (depressions, colored embedding or similar). The measuring point only serves to make it easier to find these points and is ideally located centrally above the fuselage marking. However, these permanent hull marks are often not available, especially in older boats.
Measuring tapes and rulers ("rulers") are available in different grades. The surveyor always uses the highest quality class (Class I).
What does that mean:
a 10 meter class I has a deviation of + -1,1mm for class III (which has a yardstick) it is 4,6mm. The quality class of the measuring equipment is printed in an oval at the beginning of the scale.
For normal private use, people also like to use giveaways or cheap hardware store items, which are typically without any standard guarantee. If we want to make a cabinet for ourselves and always use the same measuring equipment, that's perfectly fine. It only has to be accepted that the meter is only 99,9 cm or 100,1 cm.
If you want a measurement to be “to the millimeter”, you must of course also measure exactly. It will often be helpful to work with two people, one mooring, the other reading. Place on the edge EXACTLY and read. Make sure that the measuring device actually takes the straight path. (So, for example, there are no traps in the way of the mast. The slide ring has to be bypassed somehow at the boom ...) A measuring tape likes to have a movable hook at the front, which is also a source of errors.
There are of course the class rules. However, they describe the general conditions that a ship must comply with so that it is “a 2.4mR”. In detail, the cracks of Norlin, Rohde and Södergren are different. Much of what the technically interested sailor wants to know is not described or is only described in relation to another dimension. Details in my other articles.
It gets really interesting in the Equipment Rules of Sailing (ERS).
These are the equipment and measurement rules of World Sailing (formerly ISAF). It says where the tape measure should be stopped and what needs to be done to determine the weight of the mast tip, for example. In contrast to the class rule, there are also plenty of pictures. My articles repeatedly refer to the appropriate section of the ERS. A look at the site will help you understand it, even if you are initially afraid of the foreign language.
The ERS can be found here: http://www.sailing.org/tools/documents/ERS20172020UpdatedPrintVersion-.pdf
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