In-Mast or In-Boom Furling Mainsail for a Catamaran?

Question from a Catamaran Guru Subscriber:

I was wondering if you have ever written an article about in-boom and in-mast furling for catamarans? Any experience with those systems? What are your opinions about it? If a boat did not come with such a system, can you retrofit it even though the boat was not designed with that in mind in the first place?

Answer: The Mainsail on a catamaran is the main source of power.

– by Stephen Cockcroft

The mainsail on a cat, unlike a monohull which relies heavily on the fore triangle for power, is the primary power source. The jib or genoa are less important for powering, but are good to balance the sail plan. Remember most catamarans have a fractional rig which means that the foresail is smaller than a top rigged monohull for example. So, considering the importance of a powerful mainsail, the object is to have a large roach or flat head main with a high aspect rig so that you can take full advantage of the winds aloft. Full battens help to keep the sail shape and ensure an effective and efficient sail.

In-Mast Furling Mainsail

When you have to cut a sail to be able to roll into a mast, the leech has to be cut concave, so you loose a lot of sail area. You also loose the fully battened sail, so the sail shape is never as good and the performance of the mainsail is more like a jib or genoa than a main sail. While this might be a good option for a monohull where the power source is both in the fore triangle and the main, on a catamaran this would be my absolute last choice. Some manufacturers have in-mast furling mainsails with horizontal battens so they can roll into the mast, but if this was so great then why do we not see a lot more of this? Then there is always the risk of the sail jamming in the mast and then you have a whole new set of problems. To change to an in-mast furling system, you would basically need to replace the entire rig. There were some furling systems in the early days that were tacked on to the existing mast, but it was very industrial and I advise against this.

*Picture courtesy Harken

In-Boom Furling Mainsail

If you are going to go with a mainsail furling system, the in-boom furler is a better option. When you are rolling the main into the boom it means that you can keep the full battens and have a small roach, so the sail shape issue is addressed. Consider that if there is an issue with the furler then, unlike in-mast furling, you would still be able to drop the main onto the deck like a conventional system. To change from a standard rig to in-boom furling you would not need to replace the entire rig. The boom can be replaced with the furling boom and then a track would be attached to the aft of the mast to take the bolt rope luff of the mainsail. I would say that a new main built specifically for the system would be a must so the standard main would not work.

My Personal Preference for a Cruising Catamaran Sail-Plan

My personal preference would be a fully battened mainsail with a good batten car system and a large roach with a standard head. This system, together with a set of lazy jacks and a zip up bag, is the most reliable and functional in my opinion. While I really like the flat top mainsail, there is always the issue of getting the head and top batten back into the bag since it sits at an awkward angle. There are now systems that allow you to unclip the top batten so it lies flat but it is another step that is required when stowing or deploying the mainsail.