Measuring the mainsail
Measuring the mainsail will be a little more difficult because you will be
working with two lines. To make it easier you can use one line and a tape measure.
The tape measure is tied to the peak of the gaff and a line is tied at the throat of the gaff.
This line will be used to measure the luff length.
Using this method you will be able to measure the diagonal length
from the peak to tack and the throat to clew. These measurements are necessary for giving the gaff and the boom the correct position and for double checking our calculations later.
Before we begin, we must hoist up the boom to the proper or desired position. To get the correct boom lift we stand at the helm then add about 50 cm. above the head of the helmsman, secure the halyard. ( By doing this we allow for stretch of the sailcloth letting the boom sag. The boom must have enough elevation so that in a quick tack or accidental short gybe the boom misses the helmsman's head.)
Now we can hoist up the gaff and set it in the good position,(the blocs high as possible) and belay this line on its cleat.
Pulling the tape measure a little bit the you can see the outline of the sail which gives you an idea as to how it will look. You can alter this shape by changing the position of the gaff. When you feel the gaff position is good, you can then measure the length of the leech and the length from the peak to the tack. Remember to write these measurements down as soon as possible after they are taken, since they may be forgotten or entered incorrectly. Measure the offset, mast to tack and mast to throat, note, the two measurements should be the same.
Take the line that represents the luff length and pull it down, but not to hard, to the top of the boom, the same distance from the mast to throat measurement, which generally is about a few inches, marking this position with a knot in the line. Now move the line to the clew and at the clew secure another knot, this will give the measurement from the throat to clew diagonal.
Remember that the first knot is the position of the tack and the second knot is the position of the clew. We now have the length of the leech and the diagonal length of the peak to tack and a line with two knots in it that represents the position of the tack and also gives the length of the luff. and the diagonal length from the throat to the clew, which is the second knot.
The remaining measurements are the length of the gaff and the length of the boom.
It is important to write the measurements down, do not leave anything to memory.