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What is the Best Rope to Use in a Mast Extension?

Tim Reinieren at

Finding that your line does not fit a new or borrowed, unfamiliar extension can be frustrating - especially if the wind is blowing and you're in a rush to get out there!

So, is there a thickness of line that will do the job across-the-board?

ANSWER: Yes. 3.8-mm. Formuline™ is the best - if your extension cleats are compatible. Here's why …


Most windsurfing mast extensions have wheels with a 23-mm. diameter.

This makes them mostly compatible with the standard, 4.0-mm. polyester line supplied with most extensions. (Unifiber supply 4.0-mm. polyester on standard models, 4.0-mm. pre-stretched polyester on HD models and 4.0-mm. Dyneema® on the Élite line.)

Standard 4-mm polyester windsurfing downhaul line

Some extensions and cleats will simply not accept anything less than 4-mil rope. 

Marlow Formuline™ 3.8-mm. as sold by Unifiber

Others work best with the 'pro' favorite of  3.8-mm. Dyneema® or Marlow Formuline™. 

The distance between the pulleys and the block on various models also makes a difference on what rope works without having to be jammed through.

Formuline™ is popular because it has no outer cover for the teeth in cleats to wear and stretch and is famed for it's 'slippiness' and for how easy it is to tension with.

How to thread 0 and 90-degree sail pulleys

For sure buying the best quality line, such as a Spectra, Dyneema®, is best and, over time, the most economical - not to mention safest option too.

Sea water is savage as the dried salt crystals act as tiny shards of glass or miniature knives to cause significant wear in all fabrics or yarns that are not rinsed or soaked through enough after immersion in the ocean.

For that reason alone you should remove the basic line supplied with many models and replace it with premium quality rope.

4.0-mm. Dyneema® on Unifiber Élite Stainless Steel Mast Extenions Line

We recommend 4.0-mm. line to be the best overall solution if you are buying in bulk and expect a stock of rope to last you over some time and for outhaul and inhaul use as well.

Whatever the purpose, good quality rope will always pay dividends over time. 

But, if you are specifically wanting the best downhaul solution - and your extensions cleats will happily grip 3.8-mil. rope, then buy the Formuline™ for the best effect. 

One other easy downhauling tip is to use 'dry' lubricant silicon for the pulleys on your extensions and booms to aid tensioning and prolong their lifespan too. (Winches also help of course!)

Note: Beware how you tie stop knots with Dyneema®, as that slippy quality means they can easily come undone and cause a potentially dangerous or painful situation should they loosen while rigging, or out on the water! Always use a 'figure-of-eight' knot if you can - and pre tighten it well.)

Read more equipment selection advice guides

Post by Brian McDowell, Sales & Marketing