What is the difference between ‘In Mast’ and ‘Slab’ Reefing?

Reefing a sail is simply the name we use for the process of reducing sail area. Not that many years ago, the sailing sloop or ketch would have reduced sail by either reefing the mainsail or changing the size of the headsail. It was usual for yachts (even relatively small ones) to carry three headsails of varying sizes and a storm jib. 

Nowadays, many yachts, especially cruising yachts and single handed racing yachts, have a furling mechanism on the forestay which allows the headsail to be reduced in size rather than unclipping and removing the entire sail so as to allow another smaller sail to be ‘bent on’. In effect, this was a new way of reefing the headsail rather than changing it.

Of course, it didn’t take long for sailmakers and designers to realise that this convenience was popular amongst shorthanded sailors and cruisers. The principle of reefing by use of a roller furling mechanism has now been incorporated into the mainsail on some craft too. The sail simply rolls around a central spine that runs through the mast, or even the boom. 

A reefing system that relies upon roller furling is, one could argue, simpler and easier to do when shorthanded. The alternative had been to slab reef the mainsail. The sailmaker would cut the mainsail with preformed stainless steel rings in the luff and leech at fixed points up the sail. When you wanted to reef, you simply depowered the mainsail by easing the kicker and mainsheet and lifted the boom with the topping lift, then a crew member would ease the halyard until the first strong point (known as a cringle) was at the boom. The cringle would then be hooked onto a ramshorn (a stainless steel hook at the front of the boom) by another crew member standing at the mast and the halyard re-tensioned. The leech of the sail would then be pulled down to the boom by the reefing line running through a stainless steel ring in the leech and then the boom would be lowered and the mainsheet powered on again.

Of course, slab reefing could take time and/or be very labour intensive, so it is not surprising that the shorthanded family cruiser is tempted by in-mast or boom reefing. However, it is worth bearing in mind that the need for the sail to be furled means that the leading edge of the headsail will become thick and this will disrupt airflow and make the headsail less efficient. Also, a mast furling mainsail can’t have horizontal battens which will make the mainsail less efficient. Furthermore, furling systems can become awkward to use in heavy weather and if a system seizes or a furling line snaps, you can end up in the unenviable position of having half your sail out and no way of reducing sail or dropping it to the deck. 

At this point, the shorthanded sailor armed with a sharp knife, a bosun’s chair and the unpleasant task of scaling the mast and cutting away the mainsail, might be regretting the decision to abandon slab reefing in favour of something simpler!

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