Much like the adage about air travel that states ‘so long as you have your wallet and your passport, you’ll make it,’ successful bluewater sailing relies on two of its own simple principles: keep the rig up and the water out. Understanding yacht rigging, from an engineering perspective, should be of utmost importance to anyone heading offshore.
A good yacht designer designs the rig around the hull itself – they work in concert to create a particular sailing experience. The implications, of course, are that there is usually a reason a particular boat has a particular rig, and there is usually no good reason to change that.
Fundamentally, a sailing boat rig begins with an understanding of the forces at play on the hull. Righting moment – the resistance to heeling, or the amount of energy, measured in foot-pounds, needed to return a heeled boat to an even keel – is the key measurement in rig design. RM30 refers to the amount of force exerted on a hull heeled thirty degrees, generally considered the most that it will undergo in normal circumstances. This number is derived from a formula based on waterline length and hull displacement. A safety factor is added to RM30, resulting in more or less the largest loads that the rig will have to endure (this safety factor is dependent on the boat’s intended use – offshore cruisers will generally have a higher safety factor than an all-out racer, for example). A boat with a Max-RM of 20,000 ft-lbs. (including the safety factor) would then require a rig able to withstand those forces.
Basically, a yacht’s rig consists of a few separate pieces working together – spars, spreaders, the wire that supports them, chainplates and the hull itself. Like a chain, a rig is only as strong as its weakest link.
Yachts can have a deck-stepped mast or a keel stepped mast. Designers will argue about the merits of each, but the differences are quite simple – a deck-stepped mast must be of a larger section, and is therefore heavier (and stiffer) as it is supported at less places along its length. Keel-stepped masts can be bent more easily by a backstay adjuster, and are lighter, as the section can be smaller (the partners, where the mast goes through the deck, essentially act as an additional set of spreaders).
Similarly, for spars of the same length, single-spreader masts must be of a heavier section than masts with two sets of spreaders. The issue is unsupported length – single-spreader spars have two longer unsupported lengths, while double-spreader rigs have three shorter ones.
Double-spreader rigs allow for a narrower footprint – their chainplates can be mounted further inboard on the hull, allowing for tighter headsail sheeting angles. It is all about angles. Where a shroud terminates at the mast, an angle of less than about twelve degrees would exert too much compression on the spar itself and provide too little lateral support for the mast. This is why spreaders are used in the first place (and why they are often not seen on old schooners – those boats, often gaff-rigged, were wide enough at the deck, where the chainplates were attached outboard, and had masts short enough so that their shroud angles were in fact greater than twelve degrees even without spreaders).
A logical question now presents itself: Why not ‘modernize’ a classic yacht with a narrow, multiple-spreader rig that would allow for tighter headsail sheeting and subsequently closer tacking angles? The short answer: simplicity. More spreaders mean more connections and more potential problems. The long answer brings us full-circle. The older hull designs of those yachts would not be able to maintain those sailing angles anyway. The rig must fit the boat – the original design, having taken all of those factors into consideration, is usually the best. That said; removing weight aloft from any rig will improve sailing performance, regardless of design.
Certain design elements are lost in translation when in the hands of a (usually) cost-conscious builder. This is where the sailor with a keen eye can notice deficiencies in both yacht and rig design.
Take for example chainplate placement. Shroud angles at the deck determine where chainplates should go. Chainplates, usually of stainless steel, should be designed and installed to remain perfectly in line with the pull of the shroud they support. Fore and aft lowers, for example, meet the deck both at an angle fore and aft, and also at an angle athwartships. The chainplates, therefore, should be tilted inboard and either fore or aft to compensate. However, most chainplates, especially on older boats, are installed vertically on an inboard bulkhead. The shroud then exerts force sideways to the metal, causing undue stress and shortening their useful life. My own Allied Seabreeze suffered from this basic design flaw, which we rectified before sailing her across the Atlantic, by mounting new chainplates outboard on the hull, angled correctly fore and aft and pre-bent inboard at the proper angles. A Pacific Seacraft, on the other hand, is a good example of a boat with proper chainplate design straight from the drawing board.
Next issue we will take a look at rigging in practice, in particular inspecting a rig prior to an ocean crossing.