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Posted by on Feb 19, 2020 in Equipment |

Which mast diameter is better? SDM vs RDM

The choice between a fat (SDM) or skinny (RDM) masts remains a big source of debate in the windsurfing community. Mast diameter affects far more than just compatibility of your equipment! The change in this core element of your rig heavily affects the handling and characteristic of your sails. Because of that, choosing the right one can make a difference between a better or worse windsurfing experience. But how to know which mast diameter to choose? And is one inherently better than the other? Let’s find out!

Masts over the years…

When windsurfing was invented in 1948 in Pennsylvania, they were essentially shrunk versions of already small sailboats. Made from wood, with booms tied on with special knots, the triangular sails were heavy, but lacked any of the rigidity of a modern rig.

As the sport got more popular, more and more manufacturers started innovating their designs. However, despite great improvements in weight and handling, the mast diameter varied wildly from company to company. Que in the 1980’s and the explosion of popularity of windsurfing and suddenly the lack of compatibility became a problem.

Original mast diameter: SDM

The Standard Diameter Mast standard came about around that time to remedy the situation. How did all the manufacturers actually agree to it remains a mystery. Whatever the case, the standard for SDM was established as 48mm internal diameter. All major companies stuck to it, with no real competition in sight until the early 2000′. That’s when the new kid on the block gave SDM it’s new nickname: fatty.

RDM’s rise to power

Reduced Diameter Masts were first introduced as a stronger, more durable alternative for wave sails. The standard traded off some of the mast diameter (down to 38mm) for wall thickness resulted in a much more durable mast. It is fairly self explanatory why this would be beneficial in the shore surf conditions. Because of their thinner walls, SDM s was snapping like a matchstick, where RDM endured. However, at least at first, the heavy-duty nature came at the cost of increased weight.

But what’s the state of the mast diameter rivalry today?

Which mast diameter to choose?

TL;DR
Choose RDM unless you have a reason to go with SDM.

To understand why, we need to have a look at each options pros and cons.


RDM

Pros
  • More manoeuvrable and forgiving than SDM
  • Better suited for stronger winds (flexes to ease off gusts)
  • Curves more under downhaul, lowering centre of effort
  • Much more durable than SDM
Cons
  • Lower low-end power due to flatter sail profile
  • Less immediate transfer of power
  • Slightly heavier than SDM, especially in bigger sizes (>6.0m²)

SDM

Pros
  • Stiffer and more sensitive than RDM
  • Better suited for early planing in low winds
  • Stiffer, provides more support for bigger sails
  • Lighter than RDM, especially with big sails (>6.0m²)
  • Generally thought to work better with camber rotators
Cons
  • A lot less forgiving than RDM, harder to control
  • A lot less durable (less impact and bend resistance)
  • Can suffer from heat damage when used with cambers (rare)


Although SDM with its lower mass might seem like the easier to handle option, that is not the case because of the sail profile it favours. The wider mast diameter creates a deeper profile in the lower section of the sail. While that provides more power, it also makes the sail feel a lot heavier, cancelling the weight advantage. Add to that the vastly more durable nature of RDM and you’ve got a recipe for a clear recommendation.

Conclusion: which mast diameter to choose?

Anyone from a beginner to a freestyle champion can benefit from using a sail that feels lighter and better in the hand. That’s why at Poole Windsurfing we exclusively use RDM masts on all our sails. The lightness of the sail makes it much easier to intuitively learn the best positioning and control. In general, if you use sails smaller than 6.0m² and you’re not into racing, RDM is the way to go.

As for SDM, it definitely still have a place on the market. However, they certainly have fallen from being the dominant standard to more of a niche market. Similarly to the seat harness, SDM is now mostly found on large, cambered rigs. Optimised for maximum performance, they are a lot trickier and less convenient to handle. Hence the recommendation: don’t buy SDM masts unless you really know you need them.

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