Through the years there have been several techniques to make broad seams when
sewing two panels together.
There are three types of seams that can be used. When making sails using light cloth the round seam was used. and in heavier cloth the double round seam sewn from left to right. the flat seam was used later on in history.
The “false seams” was used for bias control when using wider cloth where bias stretch would be excessive. This was mainly used where the lighter cotton cloth was for yacht sails.
The seam that we are using is the flat seam or English flat seam,
sewn overhand and from right to left.
For the gaff mainsail, the broadseam technique is only used in seams that run into the gaff or the head part and that run into the foot. The purpose of this method is to get the proper shape in the sail. With the correct curve, or camber, the airflow will move faster on the outside, leeward side, that produces a low pressure. On the inside, or windward side of the curve the airflow will be slower producing a higher pressure. With the low on the outside and the high pressure on the inside you get lift. You can discover this with an umbrella on a windy day.
How do you place broadseams in a sail?
If you make seam's in the top part and in the foot with no broadseams, the sail will be perfectly flat, but if you begin to taper the seams, wider at the edge of the sail to normal seam width at some terminating point deeper in the sail, a curve shape will develop. Now if we make the seams lets say double, this develops a seam with a significant curve.
Using this method creates depth or camber into a sail.
Let's say that we normally make the seam 2,5cm., but on the line tack/clew and on the line throat/peak we will double this seam up to 5 cm width .this seam will be even wider on the edge of the sail in both the up and lower part. This depends on the length of the broadseam.
In the foot I use the length between the straight line tack/clew and the line that runs from the tack up to the leech in a 90 degree angle called clewcut so the length between these two line's is then the length of the broadseams.
How width it will end on the foot round edge and on the head edge is not important.
You can see that all seams starting with the first one (the leech cloth is always the first cloth), are now getting a little shorter as we move toward the throat, but on the seams on the edge are getting a little wider. This means that if we imaging the sail parallel to the mast the fore section will get more shape and more camber than the after part.
In the upper part of the sail it is an average length of 1 meter below the line throat to peak and how much width the seams will have at the edge will depend on the position of the gaff.
For getting the same results when a sail is cut on the floor of a sail loft, it wise to first make a rough calculation of the number of panel's in the up and low part of a sail. If the seam is made wider in the up and low part of the sail the risk is there that the sail will be to short in these lines, so we have to put in more cloth.
Use the length of the peak cut and add 2,5 cm to this length for each seam that runs through this line. The diagonals will change a little bit but we all know Mr. Pythagoras who
stated that (A*+B*= C*) or C is equal to square root of A square + B square.
If you should draw this sail in this way you will see that the sail is made to big but with all the extra cloth for tabling's and the broadseams taken out it will be just right.
This is the one of nice things on an old fashion method of cutting sails, you can tell that the sailmaker wanted it to last forever
there was no big rush if the sailmaker thout that the seams were to wide,he just chanced them in a way that he new or in a way that he thout was more reliable
You could say “ The sailmaker's relied on judgment for adjusting the shape of a sail.
This was the ART in sailmaking.”