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| The mast is the backbone of the rig, it gives the sail its dynamic performance (how it rides). Think of a mast like a shock absorber. Having too stiffer a mast can result in a very powerful sail, but one that has next-to-no ability to automatically react to variances in the wind and water conditions. Use a mast that is too flexible and often the sail will lack power and exhibit stability issues. |
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| Manufacturers reference stiffness by what's called the IMCS rating (Indexed Mast Check System). To measure this a 30kg weight is hung from the mid point of a mast, the amount of deflection at that point is noted and a calculation is made. The lower the number the softer/more flexible a mast is...as an example a 430cm IMCS 21 mast will flex more than a 430cm mast with an IMCS 23 rating... |
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| The shape a mast takes when loaded varies depending upon how the structure has been specified. It is possible to have many different physical curves even if the IMCS number on each mast is the same (see the diagram below). Because IMCS is only measured at the ½ way point, the amount of bend at the ¼ and ¾ points can vary greatly. Masts are often referred to as constant curve, hard top or flex top, but there are other terms all referring in some way to the shape either side of the midway point. |
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Bend Curve & Flex - How Important? |
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| Both the static bend curve and the manner in how the mast flexes in use are vital factors in achieving perfect rig synergy. |
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| First off the static bend effects how a sail will set - it is what a sail designer works with to precisely engineer foil shaping and the pre-defined degree of twist. Twist is the in-built ability for a sail to exhaust and provide some consistency in unstable conditions. |
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| So we know that when rigged a mast bends in its first dimension, that's fairly obvious. In use however as the rig is loaded by both the power of the wind and shock forces coming from the board (as it impacts chops etc) the mast actually bends back and forth in a second dimension, this is what's called 'mast deflection or cycling'. There is no way to easily pre-determine how a mast will actually deflect as there are too many variables involved such as the stiffness, bend curve, physical structure, sail type, downhaul & outhaul tension, rider power and many more...so it really is a case of experimenting to see what works most favourably. |
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| Most sails will physically function on any mast of the correct length, but the shaping and twist will often be incorrect if the stiffness and/or curve is wrong. Compatibility issues can be allowed for by varying the degrees of downhaul and outhaul tension, but by correcting either the sail shape or twist by adjusting these settings there is always a knock-on effect to the other characteristic. Worse still, changes like this effect the in-use mast vs sail loadings resulting in an inability for the rig to cycle correctly. |
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| Because of the importance of the sail and mast relationship, and the complexities involved, it is highly advisable to always use the mast type the designer intended. This ensures you can benefit from the full wind range on offer and spend minimal time tuning. |
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