Sail Design Basics I
Categories:
Builder’s Corner | Sails
by Dave Acree
This article is for those skippers who are starting to design and make their own sails. There are several good reasons to do this; the first is a cost savings and the increase enjoyment of building your own sail boat. This article is an answer to a question on how do you calculate your sail plan. The main point to start with is this article centers on the 36/600, 1 Meter, and Marblehead boat classes.
Before I go any farther, I need to point out one item you must know if you are designing a sail suit for the Marblehead. Here is the rule:
5.2.2 "The height above the deck of the lower edge of the upper mast band shall not exceed 85 inches. The height of mast crane is no more the 85 inches.
5.2.3 "The height above deck of the lower edge of the middle mast band, shall not exceed 80% of height for the relevant record rig."
This is important in the design of the jib as you will soon see. As for the 36/600 and 1 Meter, this rule does not apply.
For all three classes there is a sail area rule which must be enforced. For the 36/600 and 1 Meter, the total sail area is 600 square inches. For the Marblehead, the total sail area is 800 square inches. These square inches are very important when designing a suit of sails.
To calculate a suit of sails, the formula for all three boat classes are the same: LENGTH times WIDTH divided by 2. The sums of two sails added together for the total of square inches for that class. Now lets apply the formula in an example.
To start the task of sail design, the process starts with the length of the mast minus the length for the gooseneck, boom and masthead fitting. For example if your mast is 65 inches and you subtract 3 inches for the gooseneck and boom, and 2 inches for the masthead fitting, that leaves you a main sail length of 60 inches for a 36/600 (any boat for that matter.)
With 60 inches main sail length, the next step is the width of the sail's foot. The average width of the foot depends on the class of the boat. For a Marblehead, the average width is 11 inches to 13 or so inches. 36/600 or 1 Meter it can be between 10 to 12 or so inches. We will choose 12 inches for the foot of the main sail. The basic idea is finding the area of a rectangle by divided by 2 which is the area of a triangle. See the diagram and look at the drawing of the main sail. You will see the triangle when you look at the luff, foot and dash line from the head to the tack. Here is the math: (60 x 12) = (720 / 2) = 360 sq. inches.
Now lets go to the jib sail. The jib offers a special problem that we don't encounter with the main sail. Because of the jib unique shape, we can't use the same method as we did for the main. To start we get a total measurement of the jib stay (don't forget that 80% if this is a Marblehead's jib sail). At this point it is just a guess for the length of jib luff's length by subtract it from the total jib stay length. For our jib sail, we will choose 48-inch luff.
Next is the special problem I mention before, you can not measure 90 degree square off the end of the jib's crew, it won't work for an honest calculations. What you need to do is choose a width and plot a 90 degree line off the luff 2 to 3 inches up from the jib's crew, see the jib sail drawing. For our width, we will choose 10 inches. Here is the math: (48 x 10) = (480 / 2) = 240
Now we add the two sail totals together: 360 + 240 = 600. If we used 11 inches for the width of the jib, here will be the jib's total 264 inches. If you add 264 to 360 the sail area will be 624 sq. inches, too much. Lets go in the opposite direction, we will use a 9 inch width for the jib for a total of 216 inches. Add 216 to 360, you get 576 sq. inches, far too little.
The main idea here is add or subtract the jib width until you find the correct jib sail area which when added to the main's sail area, it will equal 600 sq. inches or slightly less, 597 to 599 sq. inches.
For the foot of all three boat classes can be straight or curved up to one inch rounded. As for the roach length for the 36/600, it is 2 ½ inches for both jib and main sails. 1 Meter's jib roach length is 1 ½ inch and 2 ½ inches for the main. Last, the roach length for the Marblehead's jib and main is 2 inches. All these lengths are up to but not exceed the correct length.
One point I want to bring out before we are done, that is always good to check the current American Model Yachting Association (AMYA) Rules for the boat class you are working on before you start. This way you will have the correct rules and your work won't be a waste in time and your sails illegal in measurement.
Once you have gotten your sails measurements, the next step is to put them down on a paper pattern. Once you have got them down on paper, cut them out, and tape them up on a rigged mast on you boat. By doing this you can test you design, and check your shape before you start cutting your sail cloth. If there is a correction to be made, now is the time.
Well, I hope this helps your sail designing, and I wish you success on your work!
This article is for those skippers who are starting to design and make their own sails. There are several good reasons to do this; the first is a cost savings and the increase enjoyment of building your own sail boat. This article is an answer to a question on how do you calculate your sail plan. The main point to start with is this article centers on the 36/600, 1 Meter, and Marblehead boat classes.
Before I go any farther, I need to point out one item you must know if you are designing a sail suit for the Marblehead. Here is the rule:
5.2.2 "The height above the deck of the lower edge of the upper mast band shall not exceed 85 inches. The height of mast crane is no more the 85 inches.
5.2.3 "The height above deck of the lower edge of the middle mast band, shall not exceed 80% of height for the relevant record rig."
This is important in the design of the jib as you will soon see. As for the 36/600 and 1 Meter, this rule does not apply.
For all three classes there is a sail area rule which must be enforced. For the 36/600 and 1 Meter, the total sail area is 600 square inches. For the Marblehead, the total sail area is 800 square inches. These square inches are very important when designing a suit of sails.
To calculate a suit of sails, the formula for all three boat classes are the same: LENGTH times WIDTH divided by 2. The sums of two sails added together for the total of square inches for that class. Now lets apply the formula in an example.
To start the task of sail design, the process starts with the length of the mast minus the length for the gooseneck, boom and masthead fitting. For example if your mast is 65 inches and you subtract 3 inches for the gooseneck and boom, and 2 inches for the masthead fitting, that leaves you a main sail length of 60 inches for a 36/600 (any boat for that matter.)
With 60 inches main sail length, the next step is the width of the sail's foot. The average width of the foot depends on the class of the boat. For a Marblehead, the average width is 11 inches to 13 or so inches. 36/600 or 1 Meter it can be between 10 to 12 or so inches. We will choose 12 inches for the foot of the main sail. The basic idea is finding the area of a rectangle by divided by 2 which is the area of a triangle. See the diagram and look at the drawing of the main sail. You will see the triangle when you look at the luff, foot and dash line from the head to the tack. Here is the math: (60 x 12) = (720 / 2) = 360 sq. inches.
Now lets go to the jib sail. The jib offers a special problem that we don't encounter with the main sail. Because of the jib unique shape, we can't use the same method as we did for the main. To start we get a total measurement of the jib stay (don't forget that 80% if this is a Marblehead's jib sail). At this point it is just a guess for the length of jib luff's length by subtract it from the total jib stay length. For our jib sail, we will choose 48-inch luff.
Next is the special problem I mention before, you can not measure 90 degree square off the end of the jib's crew, it won't work for an honest calculations. What you need to do is choose a width and plot a 90 degree line off the luff 2 to 3 inches up from the jib's crew, see the jib sail drawing. For our width, we will choose 10 inches. Here is the math: (48 x 10) = (480 / 2) = 240
Now we add the two sail totals together: 360 + 240 = 600. If we used 11 inches for the width of the jib, here will be the jib's total 264 inches. If you add 264 to 360 the sail area will be 624 sq. inches, too much. Lets go in the opposite direction, we will use a 9 inch width for the jib for a total of 216 inches. Add 216 to 360, you get 576 sq. inches, far too little.
The main idea here is add or subtract the jib width until you find the correct jib sail area which when added to the main's sail area, it will equal 600 sq. inches or slightly less, 597 to 599 sq. inches.
For the foot of all three boat classes can be straight or curved up to one inch rounded. As for the roach length for the 36/600, it is 2 ½ inches for both jib and main sails. 1 Meter's jib roach length is 1 ½ inch and 2 ½ inches for the main. Last, the roach length for the Marblehead's jib and main is 2 inches. All these lengths are up to but not exceed the correct length.
One point I want to bring out before we are done, that is always good to check the current American Model Yachting Association (AMYA) Rules for the boat class you are working on before you start. This way you will have the correct rules and your work won't be a waste in time and your sails illegal in measurement.
Once you have gotten your sails measurements, the next step is to put them down on a paper pattern. Once you have got them down on paper, cut them out, and tape them up on a rigged mast on you boat. By doing this you can test you design, and check your shape before you start cutting your sail cloth. If there is a correction to be made, now is the time.
Well, I hope this helps your sail designing, and I wish you success on your work!