Jan 4, 2009
If you enjoy windsurfing, you eventually become knowledgeable in technical terms you never dreamed you’d have interest in. There’s just no getting around it. When you look at a bird in flight and no longer just take pleasure in its aerial acrobatics, but start to notice small nuances in how the wings move and feathers flex, you have windsurfing to thank. When purchasing a sail, it’s impossible to suppress your self-taught understanding of aerodynamics as you make a decision based on what “looks right.” With masts, this purchasing technique doesn’t work so well as they all pretty much look the same sitting in the sales rack. Let’s look at why there are so many options and what exactly is the difference between them.
When you start looking at price tags on new masts, you’ll immediately notice that the more money you spend, the more carbon you get. Masts are generally made up of fibreglass and carbon-fibre strands held together with epoxy resin. Few masts these days are made with less than 30 per cent carbon, while the majority of performance masts have over 60 per cent. Carbon is the perfect material for masts as it has incredible compression and tensile strengths. In other words, it can’t be squished or stretched. This means that masts with higher carbon content can be lighter, since less fibreglass is needed, without any sacrifice in durability. As a bonus, a higher carbon content also makes for a snappier mast that moves through its flex cycle faster, allowing your sail be more responsive to the wind.
Most windsurfers think that durability decreases as carbon content increases. However, this is only true in the parking lot, so don’t drop your mast on concrete or other hard surfaces. Once on the water, strength is dependant on wall thickness rather than carbon content.
RDM versus SDM
The advantage of the skinnier reduced-diameter mast (RDM) is greater strength, as the wall diameter of an RDM is greater than that of a standard-diameter mast (SDM). The RDM rig’s feel is also slightly different, offering a softer, more elastic and responsive feel versus the more immediate pull of the thicker SDM. One is not better than the other, only different, so it’s advisable to rig at least all of your smaller sails on the same style of diameter mast to maintain a similar feel. In recent years, some manufacturers have been designing larger sails around longer RDMs, such as 460- and 490-centimetre models. Many of these longer masts attain proper performance through the use of expensive high-modulus carbon fibre. This higher-quality carbon has greater tensile strength, allowing for the same strength and snappiness as found in shorter masts.
Stiffness and Bend
Manufacturers try to make “how much carbon you can afford” all you need to consider when purchasing a mast, but the truth is, there are more factors. On every mast you’ll a find info referring to its bend and stiffness. This info is the result of a Mast Check System (MCS). The basic idea of the MCS is this: A 30-kilogram weight is hung at the mast’s midpoint, allowing the manufacturer to check how much the mast bends at three places, the quarter, mid and three-quarter points. At the time the MCS was created, most masts were 460 centimetres long. When different length masts became popular, the MCS formula was tweaked to account for length and named the Indexed Mast Check System (IMCS). The number associated with the MCS or IMCS informs consumers of the mast’s stiffness: the higher the number, the stiffer the mast. Today, nearly every mast of the same length is rated at the same stiffness.
There is also a standard for the bend in masts. Misnamed the constant curve, this term refers to a bottom section that is 10 to 12 per cent stiffer than the top. A mast with a stiffer bottom section is termed a flextop, while a softer bottom results in a hardtop.
With regard to sail tuning, you’ll find a deeper draft and more perimeter tension wherever stiffness is added to a particular section of the mast, either the top or bottom. This means if you use the same amount of downhaul tension to rig identical sails on masts with similar lengths and IMCS rating, a flextop bend will result in more shape in the lower part of the sail compared to a hardtop, which will exhibit greater tension in the leach. On the water, a flextop mast will lower the sail’s pull and make it feel lighter, while the hardtop’s tighter leech will help you get find more power. A mast that is stiffer overall will be both deeper down low and tighter in the leech but, with more downhaul, can be tuned to look similar to the softer mast. When it comes to performance, the flextop generally offers more stability at top speed but is less responsive to gusts when accelerating.
The MCS is not an exact science, though, and with every manufacturer using different recipes and cooking techniques, there can be slight differences from one mast to another. In particular, the MCS does not account for the mast’s diameter. Any sailor who has tried the same sail with both an RDM and SDM will tell you there’s a noticeable difference. Some manufacturers simply design a sail to rig on either an RDM or SDM, and they strongly recommend you stick to one or the other. Others tweak the bend of their RDM to account for the reduced diameter and ensure that their sails rig well with both types of mast.
A Perfect Fit
The easiest way to know if your mast will fit your sail perfectly is to buy the one recommended by the sail’s manufacturer. This is the mast that the sail was designed around, and it will reward you with a better tuning range and performance than any other mast. If the recommended mast is not available, you may still be able to find a perfect match. Companies like Powerex make masts for a number of different sail lofts; often only the label is what’s different. However, this is not always the case. Some brands have masts made in the Powerex factory but built to their own individual design specs. A quick call to your local shop or the sail manufacturer should be all the legwork needed to find out what your best options are for any particular sail.
(Words by Derek Rijff / PWA-Carter photo)