Contrary to Internet rumors, it is not weak
Issue 79: July/Aug 2011
Recently, in online sailing forums, I have seen self-proclaimed pundits advise sailors new to yachting that single-spreader rigs are not as strong as double-spreader rigs. This is utter nonsense, and since most good old boats have single-spreader rigs, it’s time to set out the simple truth: a properly designed single-spreader rig is every bit as strong as a double-spreader rig and even has some advantages in reliability.
Several methods are used to determine spar requirements, based either on wind speed or the boat’s stability. I have used both but I usually preferred the long-established Sparkman & Stephens system, which takes the righting moment (stability) at a 30-degree heel angle as the basis for the calculations. The compression load on the mast is then determined by the righting moment and the distance from the centerline of the hull to the chainplates.
Once the mast load is calculated, the next factor is the length of the unsupported mast panel from the deck to the lower spreaders. The distance will be greater for a single-spreader mast than for a double-spreader rig, of course. Given equal mast loads, the formula will require that the single-spreader mast tube have a higher athwartships moment of inertia (Note: In layman’s terms, a beefier cross section. –Editors) than a double-spreader mast. Naturally, that is what the designer will specify and will consider when selecting the mast tube.
The calculation for the fore-and-aft requirement is based on the total distance from deck to masthead, modified for a 7/8 rig. I do like to see a bluewater cruiser fitted with a staysail stay and running backstays. But, even without runners, the higher athwartships moment of the single-spreader mast tube usually ensures that the selected tube will have a high fore-and-aft moment as well.
In essence, the single-spreader rig will be given a spar that is wider athwartships, longer fore and aft, and heavier than a similar double-spreader mast. So overall, the single-spreader rig will be equally capable of handling the expected athwartships mast loads and will often be stronger than required fore and aft.
Single-spreader advantages
In addition, the advantages of the single-spreader rig to the cruising sailor are several:
- The single-spreader rig is easier to tune than a double-spreader rig.
- The single-spreader mast with double lower shrouds has six rigging terminals per side. The double-spreader mast has eight, unless there is a connection at the lower spreader tip, in which case it will have nine. Many mast failures are actually the result of a rigging terminal failure so the fewer terminals to fail, the better.
- The heavier spar of the single-spreader rig increases inertia to motion in light air and beam seas and can slow the roll.
Of course, there are advantages to the double-spreader rig on a racing or performance yacht. Given the same angle of the shroud to the mast, the distance from the centerline to the chainplates will be reduced. That allows the chainplates to be moved well inboard and the headsails to be sheeted more closely for improved weatherliness. Still, on a long, bluewater voyage, the overall advantage is often very slight indeed. In any case, I would not make a choice of a cruising sailboat based on whether it has a single- or double-spreader rig. There are too many other factors that affect a vessel’s offshore capabilities and all must be considered.
The main purpose of this short rant is to emphasize that you should not believe everything you see on the Web, especially in sailing forums. Many of the writers have considerable sailing experience and can offer good advice. Others may be opinionated but relatively inexperienced, as well as uneducated in the engineering and design of sailboats. Unless you know the experience, qualifications, and design expertise of the writers, take their opinions on rigs — and all other aspects of design, handling, and cruising — with a very large grain of salt.
Ted Brewer is a contributing editor with Good Old Boat and a well-practiced and respected authority on the art of yacht design.
Thank you to Sailrite Enterprises, Inc., for providing free access to back issues of Good Old Boat through intellectual property rights. Sailrite.com
