Performance Parameters

Stiffness

A windsurfing mast must have an overall stiffness within the range specified by the sailmaker for any given sail.

Stiffness is measured by supporting the mast at each end and applying a 30kg weight to the middle. The mid-point deflection figure is applied to a formula that takes account of the mast’s length to give an IMCS number (Indexed Mast Check System) The bigger the number the stiffer the mast; IMCS numbers typically range from about 11 (soft junior mast) to 36 (stiff race mast).

Bigger sails need stiffer masts to support the extra forces so it’s normal for longer masts to be built stiffer with higher IMCS numbers.

For the technically minded, the formula for calculating IMCS is:

IMCS = Length(cm)3 / Mid Point Deflection(cm) x 216225

 

Bend Curve

Stiffness does not tell the whole story. The shape of the bend curve is also important; we’ve all heard terms like ‘constant curve’ & ‘flex top’ but what do they mean? In fact, these are quantifiable terms. When we measure the half height deflection for the IMCS test, we also measure deflection at the quarter and three quarter height points. We express these as a percentage of the maximum deflection, for example here are the test results for a ‘constant curve’ 460cm IMCS 25.

  • Maximum deflection at ½ height = 181mm (which calculates to IMCS 25 on a 460cm length)
  • Deflection at ¾ height = 138mm which is 76% of maximum deflection 181mm
  • Deflection at ¼ height = 116mm which is 64% of maximum deflection 181mm

The bottom half of the mast is always stiffer than the top. If we subtract the ¼ height percentage from the ¾ height percentage we get; 76 – 64 = 12. This mast has a bend curve number of 12

The industry standard terms describing windsurfing mast bend curves are:

  • 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 higher the number the more flexible the mast is in the upper half relative to the bottom

The mast in our example just falls into the ‘Constant Curve’ range and can be described as a 460/25 Constant Curve with a bend characteristic of 12.

 

What is best?

How do these different bend characteristics relate to my windsurfing?

We need a basic understanding:

A Constant Curve mast is over 10% softer in the head than in the base, historically it’s what the industry considers a middle of the road shape for all round windsurfing. Our testing indicates that a slightly softer head will improve the feel of most sails. We consider a bend characteristic of about 15 (Constant Curve-Flex Top) to be optimum for most freeride and slalom applications.

The advantage of a slightly stiffer bottom/softer head set-up is that it locks the power in at the bottom of the sail whilst allowing the upper part of the sail to react and breathe with variations in wind and chop. This characteristic becomes more important as sail size increases and the sail becomes harder to handle. We therefore, move towards a slightly more flexible top as the mast length and sail size increase.

The requirements for a wave or freestyle sails are slightly different. These sails are in any case smaller but they also benefit from a more ‘lively’ feel. This requires a mast that’s relatively softer in the middle and lower section which means a lower bend characteristic, a number in the range 13 to 14. The rig will feel more powerful when you sheet in but will not be as stable when overpowered.

 

RDM or Standard Diameter?

We are building our RDM’s with softer bottoms because this delivers the best performance in wave and freestyle sails where the durability of the RDM is a distinct advantage.

If we consider a crossover sail such as the Storm we can change its character with mast selection. The RDM will deliver a more ‘springy’ wave style feel, whilst the SDM with its stiffer bottom and more flexible top will promote a more locked in slalom style feel. Those windsurfers who blast back and forth across a windy lake may feel more comfortable with the SDM, whilst the RDM may be the preferred choice for freestyle or chop hopping at the coast. Of course, these are not hard and fast rules; personal preference is a significant factor.

 

The Need for Speed!

The highest speeds are achieved in maxed out extreme conditions when sailing well off the wind and very overpowered. In these conditions extra flex in the topmast will magnify the advantages of locked in low down power with the reactive topmast dealing with gusts and the high level of twist required for this extreme discipline. This is not a set-up that would work in normal conditions but an example of how a sail like the X-15 can be tuned either for slalom or speed by using the appropriate mast.

The ‘Speed Pro’ with its bend characteristic of 16 to 19 is ideally suited to speed sailing in maxed out conditions whilst the ‘Ultimate Pro’ with its lower bend characteristic will deliver a more balanced slalom performance across a wide range of conditions.