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Windsurf Mast Bend Curves

Jorren Eggenkamp at

Mast bend curves are a frequently discussed topic among windsurfers. Are masts from different brands interchangeable? Is it true that all brands are suddenly using Constant Curve? Why have Unifiber changed their masts? And the big question: is it really that important?

This article brings you up-to-date with the latest developments and explains the underlying theory.

What are Bend Curves?

Let's start with the basics.

A sail obtains its three-dimensional shape from the tension applied by the downhaul, outhaul, mast and battens. The mast pushes against the sleeve and pulls on the luff of the sail. The sail designer adjusts the outline of the sail so that the mast pushes and pulls in the right places, giving the sail it's desired 3D shape. 

So it's important that you use a mast that takes on the same shape as the mast that the sail designer used when they designed the sail. If the curve of the mast doesn't match the curve of the sail, the sail is loaded in the wrong places and it cannot obtain the 3D shape that the sail designer had in mind. 

To know if a mast has the same curve as the 'original' mast, the masts are measured and categorized. If you want to know how this is done, you can read all about it in our article about MAST MEASUREMENT.

The bend characteristics of a mast is defined by four variables:

  • IMCS: this represents the average stiffness of the mast
  • Base%: this defines the deflection at the base (relative to the midpoint)
  • Top%: this defines the deflection at the top (relative to the midpoint)
  • Bend Curve Number: this number - in an approximate way - describes the mast curve. It is calculated by subtracting the Base% from Top%.

The IMCS is more or less standardized for each mast size and doesn't say much about the bend curve. With the Top%, Base% and BC Number combined, you have a very accurate description of the bend curve of the mast. 

For example, let's compare 2 hypothetical masts:

Blue mast - Base%: 63%  Top%: 73%  BC number: 10

Red Mast - Base%: 61%  Top%: 77%  BC number: 16

Below image shows a visual representation of the bend curve of these masts. The image is exaggerated. In real-life the differences are much smaller. 

You can see that the blue mast flexes more in the base (higher Base%) and less in the top (lower Top%).

Categorization of bend curves

All these different numbers can be quite confusing. To make communication about bend curves a little easier, the bend curves are categorized. This categorization is based on the Bend Curve Number: 

BC Number:Category:
0-6Hard Top
7-9Hard Top - Constant Curve
10-12Constant Curve
13-15Constant Curve - Flex Top
16-18Flex Top
19-21Flex Top - Super Flex Top
>22Super Flex Top

But these above categories are outdated. In the last 15 years no brands have used bend curve numbers less than 10 or greater than 17. Accordingly, the categories that Unifiber have used since 2011 are:

BC Number:Category:
<12Hard Top
12-14Constant Curve
>14Flex Top

The problem of Hard Top - Constant Curve - Flex Top

This categorization has simplified mast choice for many windsurfers, but it doesn't tell the whole story. 

The difference between Hard Top - Constant Curve - Flex Top is one-dimensional: it only uses the Bend Curve Number, which means that it fails to capture some differences between masts. We'll demonstrate the problem with an example. 

We take two new masts:

Green Mast - Base%: 61%  Top%: 75%  BC number: 14

Pink Mast - Base%: 64%  Top%: 78%  BC number: 14

The BC number of both masts is the same: 14. So based on the categorization by BC number, these masts should be a perfect match and you could exchange them without noticing any difference. However, as shown in the following image, they clearly have a different curve. They will tension the sail in a different way and the feel and performance will not be the same. It may be that one mast is a good match but that the other is not.

Latest developments

The bend curves of the windsurf sail brands are not set in stone. Just as they develop and update the design of their sails over the years, there are also developments in the masts. We frequently measure masts from many different brands to monitor these developments. In this way we're always certain that our Unifiber mast range is compatible with the sails on the market and that our MAST SELECTOR is up to date. 

It is true that most brands keep using the same bend curve over the years. But over the last few years some of the major brands have been making changes. The overall picture is that most brands have moved towards Constant Curve, a few brands still favour Flex Top, and Hard Top is virtually extinct. 

In other words, the entire market is now operating within a narrow range of bend curves. The range is so narrow that the old categorization in Hard Top, Constant Curve and Flex Top no longer captures the differences between masts.

In theory it should now be possible to make one bend curve that works 'good enough' for all sail brands. At Unifiber, we considered this, but decided that 'good enough' wasn't good enough! The result is that we developed 3 specialised bend curves to provide a perfect match for all sail brands.

UNIFIBER 2021 Bend Curves

We carefully analysed the largest sail brands, which represent approx. 80% of the market. Based on our findings we developed new bend curves that offer the best possible match for these brands. We also compared these new curves with the data we have on all the other brands and this analysis is the basis of our new MAST SELECTOR.

  • Constant Curve- This is the most commonly used curve profile
    • Base%: 62,5%
    • Top%: 76%
    • BC number: 13,5
  • Constant FL Curve- This is similar to the old "flex top" mast, but with more flex low down (around the centre) of the mast. FL = Flex Low.
    • Base%: 62%
    • Top%: 77%
    • BC number: 15
  • Constant FH Curve- This is also similar to the old "flex top" mast, but there is more flex high up. The centre section is relatively stiff. FH = Flex High.
    • Base%: 64% for RDM, 63% for SDM
    • Top%: 79% for RDM, 78% for SDM
    • BC number: 15

Take note that the above images are exaggerated to show the differences more clearly. The bend curve numbers are based on our own measuring machine. It is possible that if the same mast is measured on another, the numbers will be slightly different. This 'language problem' is explained in our article on MAST MEASUREMENT.

The influence of a bad bend curve

So what happens when you put the 'wrong' mast in your sail? The interplay of tensions in a rigged sail are complex, so there is no clear 'cause and effect'. But there are a some general rules that apply most of the time. 

Top% of the mast is too high

  • Looser leech -> loss of power
  • The sail's centre of effort moves downward

Top% of the mast is too low

  • Tighter leech -> less control, less drive
  • The sail's centre of effort moves upward

Base% of the mast is too high

  • Flatter profile -> loss of power
  • The sail's centre of effort moves upward

Base% of the mast is too low

  • Fuller profile -> more power
  • Batten tips come closer to the mast -> flipping the battens will be harder
  • The sail's centre of effort moves downward

For planing and powered up windsurfing, a little bit too much flex top is usually easier to live with than a little bit too much hard top.

Be aware that these rules only apply when the differences between ideal specs and the actual mast are small. If your sail is rigged with a mast that has a very different bend curve - whether too much flex top or too much hard top - the sail's intended profile will be lost and it will perform in a substandard way.

To see if a mast matches the desired specifications, Unifiber compare the Top%, the Base%, and the Bend Curve Number. See below table. If all three specs are in the green area, the mast makes a good match: even a pro windsurfer would have a hard time to notice the difference. If one of the specs is in the red area the mismatch will likely have a clear negative effect on the sail's performance.

Difference between actual specs and desired specs

BC number<-1,5-1,5-1,25-1-0,75-0,5-0,2500,250,50,7511,251,5>1,5

A hunger for data

We can never obtain too much data on masts. So if you have doubt about our findings, you're always welcome to bring you masts to our warehouse so we can measure them. We don't mind being proven wrong. We take all opportunities to improve our products.