Why worry about masts?


The season has just started, and some of us are considering a new sail or a new mast. However, most of us can't afford to buy everything at the same time, and consequently we are facing problems with mixing new and old (or old and old!) equipment in a new way. "Can I buy a used Severne 520 race mast, shorten it and use it with my new Neil Pryde RS6 7.8?" - "Can I buy a new Maui Sails TR2 and use it together with my Neil Pryde X6 mast?" - "I've got an old Fiberspar Reflex 6000; can I use it with my new Gun sail?" ...etc.


Another reason for having a closer look at the questions on masts, is that as usual at this time of the year the companies are pretty busy using their  internet sites and foras for spreading fogs (or perhaps mis-information?) about their products. All in a not especially helpful attemt to tell us that not only are their masts best for their own sails - but also the sails from the competiters can benifit from their masts. And furthermore also once in a while to convince us, that it is not a good idea to shorten or to lengthen our old masts in an attemt to get a result, that you (according to the masts producers) can only achieve in buying an all new mast in the right length.


Below you'll find some guidelines for choosing masts and for modifications on masts. Trying not to make things too complicated, lots of interesting discussions are not persued. Nothing else mentioned we are talking about the most expensive (race) masts.


N.B.: There has been a lot of arguing about "demonisation" in this country for the last months - and the following shall not be misinterpreted as an attempt to "demonise" the mast companies. However, it is evident that there are at least different (perceived?) interests between the consumers and the mast producers. That these differences are not real, we can only hope for  the industri to realize soon.



The most used method of mast measuring (IMCS).


Currently one method of mast measuring is commonly accepted - and that's the so called IMCS (Index Mast Check System). If you IMCS-test a mast, you simply support the mast at the ends, and the you hang a 30 kg weight from the midpoint of the mast. Then you measure 2 things:



Measuring how much the mast deflects related to the length, you have a figure for the stiffness of the mast. When a mast is imprinted with say IMCS 32, it is the stiffness that are referred to. The bigger the number, the more the stiffness.


Bend Curve.

If you measure how much the mast deflects at the 1/4 point of the mast, at the midpoint, and at the 3/4 point you get some figures telling of the way the mast are bending under load (called bend curve or bend characteristics). If we for instance say that a mast deflects 13 cm at the 1/4 point, 20 cm at the midpoint, and 15 cm at the 3/4 point, you find your calculator and do as follows:

First you calculate how much the mast deflects at the 1/4 point in % from the deflection at the midpoint. In our example it's (13 x 100 / 20) = 65 %.

Next you do the same calculation for the 3/4 point - that is the deflection of the mast at the 3/4 point in % from the deflection at the midpoint. The calculation is (15 x 100 / 20) = 75 %.

Now all you have to do, is to subtract the two figures - that's 75 % minus 65 % = 10. To be correct, the denomination is not 10 % but 10 %-points. Anyway, you say that the mast has the bend curve of 10 *).

To make it easier to understand the bend curve figure you can translate the figures to these IMCS terms:


  0 -  6: Hard top.

  7 -  9: Hard top - constant curve.

10 - 12: Constant curve.

13 - 15: Constant curve - flex top.

16 - 18: Flex top.

19 - 21: Flex top - super flex top

22 -     : Super flex top.



The misleading use of bend curve characteristics of (some of) the companies.


It is evident from the scale that  "Constant Curve" doesn't mean that the mast is equally soft (or stiff) from the bottom to the tip. Constant Curve means that if you hang a weight of 30 kg from the midpoint, the mast deflects 10 - 12 %-points more at the 3/4 point than at the 1/4 point. No more - no less. You can't say that a mast is "relatively Constant Curve" - as heard from a well known sail designer in an excuse of his top stiff masts.

In spite of this lack of room for mistakes, almost all masts from the companies are called Constant Curve - wether they are or not **). Perhaps this is so, because there is most sale in telling that the masts follow a middle course. The underlying message of the companies is that the mast has a standard bend curve, so that it fits all modern sails. But often it is misleading *), and the buying of a mast according to the inaccurate or perhaps immoral playing with names that - as we saw above - is in fact precisely defined, has cost many a windsurfer lots of money for no good.

For instance, Gaastra, Fiberspar (the longer masts) and Maui Sails masts are all very stiff in the tops (most often placed between 5 and 9). These masts are specifically made for the companies own Vapor (former Nitro and Neutron) and TR sails. The masts are not Constant Curve - in spite of the pretence - and they mostly only fit to the sails mentioned ***).

A couple of years ago Neil Pryde had a pretty sofisticated way of spreading fogs: They published some "Bend Offsets", which (when you calculated on the figures) placed the masts in the middle of the Constant Curve scope. The figures obviously more reflected  a kind of  "calculating backwards" to get the wanted results, than they reflected genuine measurements. A more recent Neil Pryde trick is to indicate approval to the IMCS system by telling the IMCS stiffness (say IMCS 21), whereupon they - without any defintion at all - name the bend curve "Progressive Flex". Are you confused? No need to be - just remember that the Neil Pryde masts are pretty soft on the tops (most often placed between 14 and 17 ****)), and that they are specifically made for Neil Prydes own sails. The masts are not Constant Curve, and you can't use them in many sails from other brands.



What's most important - stiffness or bend curve?


What to look for - stiffness or bend curve? Unfortunately we get no help from the companies that (besides  their empty way of using the phrase "Constant Curve") only inform us about the stiffness of their masts (say IMCS 32). And - as we saw - the stiffness is far from the whole truth about the masts. In fact you could pretty easily argue that information about the bend curve might be more important. Since the stiffness tells something about when the mast starts working (at what load), the bend curve tells something about how the mast shall work when loaded.



How does the soft and the stiff areas of the mast influence the appearence of the sail?



Working in the sail, the mast of course deflects the most in the softest areas of the mast - and deflects  least in the stiff areas. Where the mast deflects the most, it pulls away from the leech - in this way straightening the sail (with little depth) on level with the soft area. If the sail has a camber inducer at this place, it often rotates easily - or perhaps it is even not pressing properly on the mast.

Conversely, the stiff areas of the mast shall not move a long way from the leech - thereby not straightening out the sail-cloth on level with this stiff area (creating lots of depth). If the sail has a camber inducer at this place, it most often presses hard on the mast - often to a level at which it's difficult to rotate. And when it rotates, it is often accompanied by a loud "camber-bang."



The same sail rigged with a Flex Top and a Hard Top mast respectively.


If we think of rigging the same sail with the two mast-extremes respectively (for instance a Neil Pryde mast with soft top and a Maui Sails mast with stiff top), we shall try to predict the differences. It's important to note that this is not science - just a kind of common "knowledge"  among the gang of old windsurfers from the local beach:


With the Neil Pryde mast the sail shall have a big depth (much profile) in the bottom and in the middle of the sail, while the top of the sail shall be pretty loose. Often the change from the area with big depth to the loose area shall be very "sudden" with a lot of wringles. The center of effort shall probably be placed forward (and deep) in the sail, and the camber inducers shall have a lot of pressure. Actually it could be necessary to remove a couple of spacers - or to file off some of the plastics - to make them rotate. As the mast is pretty stiff in the middle sections, a lot of downhaul is probably needed.

With its loose top the low-end pull from the sail comes from the deep profile in the bottom and the middle sections. And often the angle of incidence around the mast pocket shall be pretty rounded, so that the air flow over the sail shall be very attached and make it easy and steady to sail.


With the Maui Sails mast the sail shall appear pretty flat (little profile) in the bottom and middle sections, and the top shall not be very loose. The change from the flat bottom to the not-especially-loose top is spread over a large area, and the sail shall have a rather harmonic appearence - gradually twisting towards to the top.

The center of effort shall probably lie a way back, and the camber inducers most often shall not press very hard on the mast - in fact sometimes you shall have to place some spacers between the battens and the inducers. As the mast is relatively soft in the middle, you shall probably not use a lot of strength to downhaul the sail - and using too much downhaul force might almost collaps the sail  in the middle.

With the flat bottom and middle sections of the sail, the low-end pull are caused from the relatively tight leech high up the sail - and from the fact that the center of effort is placed a  way back.

Often there's a tendency that the angle of incidence around the mast pocket shall be rather "sharp" (the sail is almost cutting its way through the air), and in this way it is pretty easy to over-sheet the sail. As the air flow over the sail is always at risk of being turbular, it is easy for the sail to loose its power. A sail with a sharp angle of incidence might be exellent for very good sailers, but for us mortals it might be a little nervous.



Some findings when measuring masts.


Here is a link to some measurings of masts. Even if the measurings shown are selected, the figures are still mostly raw data - but here under you'll find some of the simliplified and generalized conclusions, you can make from the material:


  • The mast neither changes stiffness nor bend curve, as the temperature changes. Even though you by means of the IMCS test can't find any difference, if it is 0 degrees celsius or 25 degrees celsius, of course you might speculate, if the mast perhaps gets a little "slower" in the lower temperatures - that is if the "flex response" is dampened.

  • If you shorten the mast (from the bottom!), it gets softer and doesn't change its bend curve. Of course it's not wise to cut off so much that the mast foot/extension doesn't fit the mast - or that the boom is fixed above the extra strong boom area. And as indicated - never shorten the mast from the top.

  • If you extend the mast it gets stiffer (most with Flex Top, a little less with Constant Curve and least with Hard Top masts), and it almost doesn't change bend curve. Of course you shall not extend the mast so much that you clamp on the boom below the strengthened area.

When you try to find a way to calculate, what approximately happens to a mast that you extend, you'll have to decide what you'll understand with "extending". For instance, how long extension are we talking about (in numbers or relatively)? How much extension in the mast are we talking about (in numbers or relatively)? What is the stiffness of the mast to extend? What is the bend curve of the mast to extend? And so on and so forth ...

If we make a fast decision and say that we are talking about a 30 cm extension of the mast by means of a 50 cm extension device (that's of course 20 cm up in the mast), then - based on numerous measurements - you can (very) roughly say that ...:

  • All masts keep (approx.) their bend curves when they are extended.

  • All masts become a little stiffer (according to the IMCS standard) when they are extended.

  • Hard Top masts become relatively least stiffer (around 2-3%).

  • Flex Top masts become relatively most stiffer (around 6-7%).

  • Constant Curve masts become stiffened up between Hard Top and Flex Top masts (around 3-6%).

This can roughly be "translated" in this way ...:

  • If you extend a Hard Top mast with 30 cm, so that it reaches the length of the next mast in the line, the extended mast is supplied with 1/6 - 1/4 of the extra stiffness necessary to get on level with the next mast in line.

  • If   you extend a Constant Curve mast with 30 cm, so that it reaches the length of the next mast in the line, the extended mast is supplied with 1/4 - 1/2 of the extra stiffness necessary to get on level with the next mast in the line.

  • If you extend a Flex Top mast with 30 cm, so that it reaches the length of the next mast in the line, the extended mast is supplied with 1/2 - 3/4 of the extra stiffness necessary to get on level with the next mast in the line.

The more mast with or without extension that are measured, the more precise the ability to predict what happens as to change in stiffness - and the less likely it is that mentioned general rules have to be changed.

  • If your mast has become "excentric", this doesn't change the bend curve significantly. However, as the mast now is a little "pre-bend", it equals it has grown a little softer.

There is a tendency that the increasing softness that follow heavily use, are most noticeable in the areas that work the most - that is the top of the mast. But - you have to use the mast very much, before you face any problems in this respect.



In short, paring mast with sail (race sail/race mast).


  • If you use the mast the sail producer recommends, you don't risk anything.


  • Never use a Neil Pryde mast with a Gaastra (from before 2007) or Maui Sails sail.


  • Never use a Gaastra (from before 2007), a Fiberspar (the longer masts) or a Maui Sails mast with a Neil Pryde sail.


  • Think about it, before you use any of the obove mentioned ("extreme") masts with other sail brands (North, Severne, Gun, Naish, Sailworks, Aerotech etc).


  • If you choose a  "detuned" Gaastra or a Maui Sails mast (for instance the Maui Sails SRS 75 % mast), there is a tendency that these masts fit in the Constant Curve scope. The same is true for the older and longer Neil Pryde Freeride Pro masts.


  • A lot of 100% carbon masts are manufactured in two Italian plants. Even if the sail producers say that they order their masts from their own specifications, it is a well known "secret" that you might be lucky enough to buy a a cheap mast (for instance a GUN Dominator) with IMCS numbers that is hardly distinguishable from the numbers associated with one of the much more expensive masts from one of the well reputed (and expensive) sail producers (Neil Pryde for instance). Of course this is just an example!





*)       There is a little further information to get in the calculation for bend curve than just the figure ("10" in the example). If you compare the 1/4-deflection and the 3/4-deflection mentioned (65% and 75 %) with for instance deflections of 63% and 73 %, you'll end up with the same bend curve figure. However, intuitively you might see that the 65/75 mast is a little more "U-shaped" than the 63/73 mast, that is a bit more "V-shaped." Admitted, this is perhaps a little nerded.


**)     There are exceptions - Sailworks seems to be honest about their masts.


***)   The new sail designer for Gaastra (Dan Kasseler) came from Naish Sails, where he designed sails around some (probably) traditional Constant Curve masts. At Gaastra he inherited the top stiff masts from Barry Spanier and Phil McGain, and he has built the first Gaastra race sails (the Vapors) around these masts. From the start Kasseler said that he'll continue to design sails around those original Gaastra (former "Ignition") masts - but from 2007 Gaastra without any announcement changed their race masts to Constant Curve and presumably changed their sails accordingly. At Naish Sails he also had a great success building sails around such more traditional masts - for instance the sail that helped Finian Maynard making his World speed record in the autum of 2004.

In an all new video at the Maui Sails homepage, Phil McGain one more time tells us that his SRS 100% masts are Constant Curve. But the truth of this statement doesn't grow  with the reiterations of the falsity. Maui Sails race masts are and have always been Hard Top - Constant Curve!


 ****) The bend curve of the longest (and seldom used) Neil Pryde race mast (the X9 580) is placed around 12.