ok this is for anybody who has attempted to make there own set of sails.
what did you use and how did you do it? my first attempt, i used rip stop nylon and made paper template . then sew them along the seam. it made for long battens. lol but the boat moved. I used grommets, for the clews
now I am looking at build a suit using trispar 25. talking with other people . it looks like i dont have to sew. but what glue should i use?
keep the storys coming . maybe others can get some good information. and some help

This is what I’m working on.


Over here we use a thin double sided tape, it can be bought in several widths from 3mm up to 19mm. Talk to some sail makers of small dinghy sails and they may be able to help. Over here we buy it a large newsagencies(stationary suppliers etc).

There are several brands, perserve until you find one that works well for you. The advantage of using this kind of tape is that you can pull the seam apart a few times if the sail shape seems wrong, also you don’t need to sew the seams.


MYRC Sailmaking

Sail making with blocks by Lester

my 2 cents or whatever, if you want fast sails, buy them from sail maker!

don’t use glue, it will make a real mess. all you need is this tape.

don’t buy it hear though, get it from hang-em high, because they have it in 14/" wide rolls instead of 3/8". Once the panels are joined they are never going to pull apart while sailing. I have used this stuff for years, i did a tension test using it. basically I clamped one pannel to the work bench and then pulled on the other. The mylar deformed before the tape let go.

for sail material i personally prefer 2 mil mylar.

if you want fast sails, buy them from sail maker!

Wis…Just how do you think those sailmakers started???


I made a few sets myself, they looked good but I sure don’t have the knowledge “they” have…so my “good looking” sails were actually crap!

I am sure they tried and tested many different materials, ideas or what-so-ever!

They do have the knowledge and the knowhow!

I will not spend MY time making sails anymore, I’ll ask the pro.

Making sails is easy, making GOOD sails, not at all.

my 0.2 yen

Thats my point…keep making them (or anything for that matter) and you gain knowledge and know how.Thats how the pros became pros!!
They arn’t born with this knowledge.
Nothing in this life is easy if you want to be the best at it.(not even the best just OK)
my 2 cents

I totally agree, but sometimes “you” just don’t have the time:bag: nor the skill(s).

But, again, you did make a point!

SAILSetc Sailmaking notes

Reading Graham’s sailmaking notes again (thanks Wis) I am pleased to see that everything he talks about is exactly how he makes his own sails. Except for the shaping method. Getting a curve of around 1 or 2 mm per 1000 mm of seam is pretty difficult; getting that curve to stick down onto a straight line neatly and without puckers is more or less impossible. That’s why Graham doesn’t use that method any more, he uses the “sail blocks” method that Larry Robinson shows in his book and that he says is the “better” method.


A year or two ago I derived a formula very similar to the R=h/2 +c^2/8h formula from your web page (
The problem I hade was when you multiply h by sin() of the bevel angle, to calculate how much draft you are actually building into the seem, the value you get back doesn?t jive with what I have learned from real world trail an error.

Consider the following:
If I have a seem with a chord of 4 inches, and I want to build 10% draft into the seam, and my blocks have a bevel angle of 6 degrees. Then you would need a block with a radius based on the following calculation.

4*0.1 = 0.4 = draft built into seem

h*sin(theta) = draft

draft/sin(theta) = h

0.4/0.104 = 3.846 = h


R= h/2 +c^2/8h = 3.846/2 + 4^2/8*3.846 = 2.443

From my personal experience a block with a radius of 2.443? is way too small.

My theory is that the sin(theta) approximation is to simplistic, and fails for small radiuses. I?m clueless though as to what a better approximation would be.

Your comments???

Hi Dan

A radius of 2" or so is certainly a little on the small side (smile).

The formula I give for radius is the radius of a circle whose geometry matches that of the sail draft you are trying to achieve. If you are looking at a 4" sail chord with a sail draft of 10%, then the radius of curvature is 5.2" for that sail shape.

I probably did not explain this clearly enough, but this is not, in itself, the radius you want to give your sail shaping block. The reason for this is that much of the necessary draft to a sail is given by simply moving the outhaul forward. (Might be worth saying that some draft is also naturally caused by downhaul, and by the pressure of the wind.) You only want to sew in “extra” draft in a seam to make adjustments to how the draft is distributed from head to foot. When the outhaul is set to, say, 0.5" in, you might get 8% draft in the sail. If you wanted 9%, you’d sew in the extra 1% at the seam. Note that you are not making a seam with 9% draft!

On the 4" seam you mention you might want to sew in 1% of draft, 0.04". Using the formula, the radius of curvature of a sail with 1% draft on a 4" chord is around 50".

Now the bevel angle only allows a fraction of the block curvature into the seam. A 6 degrees bevel allows something like sin(6) or about 10% (0.10) of the curvature in, so you want the block “h” to be around 0.4" instead of 0.04" and hence to have a radius of curvature of around 5.2". Though you are sewing a rather small seam, this is a rather small block, and perhaps this is indeed where theory departs from practice. I think that the departure is the “something like sin(bevel)” idea, which is not precise.

Perhaps the bevel angle in fact allows in 2sin(bevel angle). The reason for saying this is that the bevel applies to each panel separately, and so when applied to both panels the amount is doubled. (Imagine a block where just one side was beveled, and the other side had no bevel. The sail on the bevel side would have some draft built in to its part of the seam, while the sail on the flat side would have a seam which was a straight line and therefore no draft built in on its side.) If we use 2sin() instead of just sin(), this suggests the much more reasonable radius of curvature of the block of around 10.1" if it has a bevel angle of 6 degrees on each side …

What block curvature do you use to get your 4" seam to have “good” draft?


No need to explain you self more clearly, I understood your formula just fine. What I was trying to say and I don’t think I did a very good job of; was using known values I reverse calculated h.

If I know how much draft I want to built into the sail, and I know the bevel angle of my block, then I can use the “draft = h * sin (bevel angle)” equation to calculate h.

I can then feed my known c and calculated h into the “R= h/2 +c^2/8h” formula to find R.

I think I should clarify that, the seam I’m talking about is the upper most seam on the sail. Thus since the outhaul’s affect on draft diminishes linearly the further up the sail you go, you need to build more draft into the upper seams. As the top of the sail twists off, it losses draft, thus again you need to build extra draft into the seams.

For example if I wanted 10% draft at the upper most seam, through trial and error, I have learned I need to build 8% or 9% of the draft into the seam.

I have looked at 3 different versions of the equation.

  1. h = sin (bevel angle)
  2. h = 2*sin (bevel angle)
  3. h = sin (2* bevel angle)

All 3 failed when I was trying to build a large amount of draft into a seam. My hypothesis is that the formula needs to be related to R in some way.

Dan - on my big boat sail, the top is almost flat, with little if any draft. Also battens (top two) are awfully stiff, so that the power in a gust gets twisted off, and doesn’t increase the heel of the boat. Perhaps I am wrong and Lester can clarify, but I would think power up top just adds to the heel of the boat, and it winds up getting lost when sail twists off.


I?m at work at the moment so I don?t have a great deal of time to get technical, but hopefully Lester will give a comment here while we are sleeping (don?t you just love the time zones).

In order to make sure your sail maximizes its power output, the entire sail needs to maintain the proper entry angle (think wind gradient). One way of doing this is through twist, another way is by increasing draft as you go up the sail. However this is based on the assumption that you?re not overpowered.

Hi Dan

OK, got it! I think your method is fine.

since the outhaul’s affect on draft diminishes linearly the further up the sail you go, you need to build more draft into the upper seams


For example if I wanted 10% draft at the upper most seam, through trial and error, I have learned I need to build 8% or 9% of the draft into the seam

Wow! OK, it is clear I’m not a sailmaker! I think the block bevel in combination with the radius of curvature is doing something that the formula does not take into account. Will get back to you.

In practice, what radius do you use on your block to get the top seam draft you want?


I don?t use circular blocks anymore, because I was never able to come up with a reasonable set of equations that would at least get me close. Today I use blocks that are based on a spline style curve.

However the recent discussions about sail shape has peaked my interest in trying to figure out a more precise equation for using circular cross-section blocks.

A little progress… The factor by which the bevel feeds the block curvature into the seam is not “2sin(bevel)", it is "2 * SQRT(2(1-cos(bevel)))”…

I’ve tried to present the data I have for an unshaped sail better here than I have at

Drafts with outhaul, wind pressure, & leech tension (ie no twist)
Foot: 13%
Bottom seam: 11%
Middle seam: 9%
Top seam: 7%

Draft with outhaul & leech tension only (ie no twist)
Foot: 10%
Bottom: 7%
Middle: 5%
Top: 3%

Draft with outhaul only (ie twisted)
Foot: 10.0%
Bottom: 5.5%
Middle: 2.5%
Top: 0.0%

Wind pressure (“gravity” when the sail is held horizontally in a room) and leech tension (vang or kicking strap) can introduce around 7% draft into an otherwise perfectly flat sail at the head. I’m thinking that, if I wanted 8% draft at the head of a sail when flying, I’d not want to sew in much more than 3% or 4%. Sewing in 3% on a 140 mm seam with a block with 6 degree bevel each side, its local radius of curvature of 130 mm isn’t too much off the scale. Push that to 4% and a bevel angle of 8 degrees gives a similar result.

Hi Dan

Understood. What is the local radius of curvature, then, somewhere in the middle of the spline curve block you would use to shape a 140 mm seam?