Spar engineering is an interesting combination of numbers and (hopefully) real-world experience.

Because weight aloft is such a critical factor in comfort, performance, and range of stability, there is a continuous force pushing the sparmaking industry toward ever more efficient engineering solutions.

Most of this push comes from the racing set. While you never want to lose a rig in an important regatta, if you aren't dropping rigs now and then, you are making them too heavy.

The racing rig failures tell the engineers where the edges of the structural envelope are located.

A "gravity storm," as losing a rig is called in the business, is a good opportunity to find out what went wrong (not to mention a chance to sell a new rig).

Of course in a cruising context the issues are somewhat different.

While we certainly want performance and a good range of positive stability, we don't want to consider losing a rig. In addition, where racing rigs have to be carefully tuned and used with a degree of caution, and where an accidental jibe is expected to result in a dismasting, some abuse tolerance is necessary in a cruising context.

Still, unnecessary weight aloft in a cruising rig is a huge hindrance to performance. In severe weather, perhaps trying to beat off a leeshore, unnecessary weight and windage in the rig might make the difference between success and failure.

How do you know if the engineers are on the mark for your own needs? It's very difficult to tell. You also have to beware of the spar-extrusion inventory on hand. It may be that they'll try to sell you a section that isn't quite right, but has been sitting around for a while. The best approach is to work with a company you trust and to get several quotes comparing the various factors discussed in this section.

Before going on, let me tell you about an experience I had years ago with some of the engineers at Sparcraft. We were sitting in their offices in Southern California, looking at a huge computerized spread sheet with all sorts of data. It was a scientific-looking piece of work, something on which a programmer had spent many weeks. As we were looking at rigs for two large sisterships, and I wanted to get them as close to optimal as possible, I had the engineers walk me through the various factors in their spreadsheet, showing me how they were applied. What I was looking for was the logic of their engineering approach. When they had apparently finished, there was a pause. At the bottom left-hand corner of the spread sheet was a cell named "B-Factor" that had not been explained. This cell was a variable affecting the results of the entire spreadsheet. I pointed to the cell in question and asked what it was for and how it was used. The engineers exchanged uncomfortable glances. "Come on, guys," I said, "I'm not going to consider you for this order until you tell me what the 'B-Factor' is all about." After some hemming and hawing, the answer came back. "That's the Bozo factor."

Bozo?

After all of this scientific analysis, the guys in engineering would input a number that reflected their analysis of the capabilities of the crew. This could have as much as a 60-percent impact on overall design, if memory serves me correctly. Obviously, spar "engineering" is very much based on guesswork, experience, and an evaluation of what sort of crew-induced loads the rig will see (although today some spar builders are starting to use finite element analysis for rig design).

While the details of rig engineering are quite complex, the basic principles are not. What follows is a basic overview of the process engineers go through to determine the correct rig size.

Stability

The stability of the hull as it fights to keep the boat upright against the force of the wind is what puts most of the load on your rig. So rig loads are directly proportional to the stability of your boat. If stability goes up, say by increasing payload in the bilges, the rig loading goes up in proportion.

Generally, most spar engineers will take the stability of the boat at 1 degree of heel, then multiply this figure by the angle of heel at which they expect the boat to sail most of the time. If you have a righting moment at 1 degree of 2,000 foot pounds and you are assuming the maximum heel angle would be 30 degrees, the total load on the spar is assumed to be 60,000 foot pounds.

With many cruising yachts, stability increases more or less in a linear fashion with heel angle. This being the case, the approach just outlined above works fine. But if you have a hull shape with high initial stability, and maybe water ballast, the stability at moderate angles of heel, say 18 degrees, will be much higher than what you find at 30 degrees. In this situation the rig engineering loads must be modified.

On most yachts stability continues to increase up to around 60 degrees of heel before it starts to taper off. If you suffer a wind-induced knockdown, your rig must cope with the higher stress levels coming from these greater heel angles.

In the olden days, before computers, everyone guessed at the stability curve of a given yacht after testing for initial low-angle stability. Today, however, most designers can develop a stability curve to use in conjunction with an inclining test to determine just what the loads on the rig will be. If your sparmaker does not already have this data, it is worth calling the builder or designer for the curve. If this fails, go to the U.S. Sailing Association for a performance package on a sistership, or contact Peter Schwenn at Velocity to develop this data from a set of your hull lines.

Factors of Safety

An initial factor of safety is placed on the righting moment at the maximum normal heel angle. This allows for an increase in load from heavy weather, and also accounts for the crews' abilities (or lack thereof!), as well as the desired longevity of the rig and rigging.

This factor of safety, multiplied by the real righting moment, creates a basic load that is used throughout the rest of the formulas. In many cases engineers will take the actual righting moment at 30 degrees, then multiply it by 1.5 or 2. This basic number is then used to determine the compression load on the mast and tension load on the rigging. In each of the subsequent steps, additional factors of safety are added on for each component so that you have factors of safety multiplied by factors of safety.

In real-world cruising you normally only sail at, say, 20 degrees. The righting moment at 20 degrees is much less than at 30. Thus, the normal factor of safety that your rig really has is usually more like 2-to-1.

It is important to keep an eye on the various factors of safety and how they relate to the way the boat is to be (or has been) used. You don't want to be too low or too high. Be realistic.

Shroud Angle and Beam

With a conventionally rigged mast, where side loads are taken by standing rigging, the tension on the shrouds is proportional to the cosine of the angle that the shroud makes with the mast.

As a result, the further out your chainplates are located, the lower the rigging loads. When the tension in the standing rigging goes down, the compression load on the mast is reduced. Both the rigging and the mast can be lowered in size, reducing weight and windage aloft. Having a wide staying base almost always pays dividend for cruising. The only negative comes with sheeting overlapping headsails when sailing hard on the wind. However, in a cruising context this is not such a critical issue.

Mast Compression Load

With the righting moment established and factors of safety allowed for, you now have a load that can be applied to the mast. Each shroud puts its load into the mast on a basis of the cosine of the angle it makes with the spar, so the more open the angle, the lower the loads, as we've seen. Spreader width affects this, so within certain limits (discussed later in more detail under spreaders), the wider the spreader, the better in terms of mast loading.

Engineers add up the total load each wire puts onto the rig. This includes side shrouds, head and backstays, runners, and cutter stays. In addition, if they are doing things carefully, allowance will be made for halyard loads on each sail flying.

Finally, a figure is sometimes added for the amount of pre-tension in the rigging wire, which is present in addition to the actual sailing loads.

The sum total of these, factored by the cosines of their angles, adds up to the total compression load on the spar.

Ambient Loading

How a rig is used has a big impact on how it should be designed. For example, a racing yacht, sailed by an experienced and skilled crew, can be successful with a rig that has very low factors of safety. If someone makes a mistake, the rig comes down, but while it is up they are very fast. Besides, some folks get an adrenaline rush from sailing at the edge of the envelope.

A cruiser who spends his time on Long Island Sound or sailing between Southern California ports will be able to use a much lighter rig than someone who sails on San Francisco Bay, in New Zealand, or in the English Channel areas.

It's important to be realistic about how you plan to use your vessel when designing or evaluating the rig. And be sure this is clearly communicated to the folks with whom you are discussing your rig.

Cyclical Loading

Cyclical loading is the most difficult factor to analyze. As loads on your rig rise and fall, as the boat rolls back and forth, everything on the rig goes into varying degrees of tension and compression. Constant loads would be better for the rig in terms of longevity, but this is not the world we sail in. Throw in environmental issues like temperature range and salt-water corrosion, and you end up with a very difficult brew indeed Ñ something that could be impossible to engineer on a numbers basis. All you can do is establish the type of sailing likely to be done, then understand that the higher the factors of safety, the longer a given structure will go before fatiguing.

What is important to recognize in all of this is that the lower the stress level at which a given item operates, the longer it will survive. This is more of an issue with tangs and standing rigging than with spars.

So, starting out with a very scientific numbers-oriented approach to spar design, we are reduced to a verbal description of how and where we will use our rig, hoping that the engineer listens and uses the correct factors of safety for what we need.

Obviously, real-world experience is a critical part of this entire equation!

Worst-Case Loads

With all of the basics now covered, the next step is to look at how the mast will be loaded in a variety of conditions, then choose the appropriate forces to use on the mast extrusion.

Take the cap shroud going to the masthead. Typically, the worst-case scenario for a cap shroud is a spinnaker knockdown. The load placed on the cap by a flogging chute trying to refill, then knocking you back down, are typically far higher than anything a jib would generate.

Intermediate stays are another example. With the full sails, the intermediate on a double-spreader rig carries only a fraction of what the cap and lower shrouds carry. But what happens when you reef the main and fly a storm staysail? The intermediate is now feeling almost as much load as the caps had felt, while beating with a full jib and main. So, on a cruising yacht the intermediate shrouds must be sized for reefed sailplan loads.