Proa Rig Options: Overview

18 July 2014    Editor    1 Comments

The proa has unique requirements for a sailboat rig, the main one being that it is reversible fore and aft. Since no Western craft have this ability, we who are developing the proa for use here in the West are truly in uncharted waters.

The obvious place to look for inspiration is with the traditional proa rig: the Oceanic sprit, developed over thousands of years of ocean voyaging by the Pacific Islanders. This rig, often called the crab claw, is an ingenious blend of clever engineering and powerful aerodynamics, that is ideally suited to the great voyaging canoes and their sailors. The rig is very powerful for its area (with perhaps the highest lift coefficient of any rig), it is close-winded, has a low center of effort to keep heeling moments low, is structurally robust with low loads on the rigging and spars, is easy and forgiving to trim, and when combined with the asymmetrical Micronesian canoe hull, creates a very well balanced sailing machine that can be steered without rudders or oars on most courses.

Sounds perfect, you say, where do I sign? Not so fast. There is one big problem with the Oceanic sprit: it is difficult to sail short-handed, and impossible to single-hand, except in the smallest of sizes. During a shunt, the heel of the yard is carried from one end of the canoe to another by a crewmember, while other crew alternately slack and tighten the fore and aft running rigging. More crew are needed for larger canoes, because of the increasing heft of the spars. It is an operation that requires skill, timing, and muscle, and if not performed properly, can lead to the canoe being caught aback. This "shortcoming" is no shortcoming at all to the islanders, who sail with plenty of well-trained crew. Canoe sailing is an important part of island culture, and it is always done communally.

In contrast, sailing in the West has developed as a hobby and a sport, often with a single sailor or his family in control of the yacht. This has led to a proliferation of labor saving and power amplifying devices such as winches, block and tackle, roller furling, etc. Western sailors need rigs that are safe and pleasant for 1-2 people to sail; something the Pacific islanders need never take into consideration.

In a nutshell, this is the goal of Western proa experimentation: to combine the economy of materials and performance of the Pacific proa with the ease and safety of short-handed sailing that we desire in the West. To that end, proa sailors are on two distinct paths. One group is starting with the Oceanic rig and then making modifications designed to ease shunting for one or two crew, while the other group is starting with the already labor saving Western rigs, and adopting them to the peculiarities of the bilaterally asymmetric proa. Both groups have the same goal, though they are starting from opposite ends of the cultural rope.

Oceanic Inspired Rigs

The Oceanic rig must be considered in context with the platform (canoe) it sits on. The deep and narrow asymmetrical V is the canoe’s only lateral plane. The center of lateral resistance (CLR) of this type of hull is about 30-40% of LWL (or, length of the canoe) behind the bow. The rig is set with the tack in the bow, and the center of effort (CE) of the rig aligns with the CLR of the hull. This alignment keeps the sail and hull in balance, so that the boat is capable of steering itself for long periods, and course changes may be achieved with mainsheet trim and/or weight shift of the crew.

If viewed from the top, the force vector of the sail travels directly away from the outrigger log, or ama, which functions as the canoe’s ballast. Lining up the ballast and sail CE in this fashion removes any downward pitch moment that the ballast might otherwise contribute to the hull, so that the very narrow bow is free to rise to oncoming waves, and to avoid pitchpoles.

Another feature of the Oceanic rigs is the backward sweep of the yard, which has lately been recognized as a valuable aerodynamic attribute, and one that likely contributes to the traditional proa’s performance. The backward sweep allows the sail to develop upward lift as well as drive as the sheets are eased, which is just what is desired in off-wind courses where the bow is more likely to be depressed by sail force. The backward sweep also contributes to vortex generation at high angles of attack, which may explain the crab-claw’s amazing power on a reach.

The geometry of the mast/windward stay/outrigger creates a strong yet flexible truss that utilizes the specific engineering attributes of the materials to their best advantage. The mast and outrigger poles are in near perfect compression, (no bending loads) while the stay is in pure tension. This arrangement allows the lightest possible structure, and savings in weight and cost that are not possible with catamaran or trimaran configurations, which always put heavy bending loads on their connectives. As you can see, the Oceanic sailing canoe is a *system* of rig, hull, and sailor. It is an elegant and effective solution that has been refined over many generations. The goal of the Oceanic inspired rigs is to maintain the balance of that ancient system, while mitigating the handling problems for Western sailors.

Shunting Western Rigs

For the past 100 years, Western sailing rig development has been in the hands of the sportsman. Sailing as a serious means of transport ended with the age of steam. Consequently, serious scientific study into sailing has been sporadic at best, and has largely depended on the vast amount of aerodynamic research done on behalf of the aeronautical industry.

This is not to say that advances have not been made, however they have almost always happened in the context of racing rules or some such *artificial* constraint, so that the research and development has mostly happened along narrow corridors of inquiry, focused on how to achieve a better rating under a certain rule. For most of the past century, that corridor has been the Marconi (Bermudan) sloop rig. The Marconi has been tweaked and tuned into a formidable aerodynamic weapon indeed, and its performance, especially to windward, is virtually unchallenged. The rig is powerful, very close-winded, can be easy to tack if designed correctly, and modern systems make it quick to reef, furl, and get under way.

The rig unfortunately is also highly stressed, and expensive because of it. The attraction of using a modern Western rig on a proa is its aerodynamic sophistication, its ease and familiarity of operation, and the fact that much costly and/or time consuming development time can be avoided.

One constant of modern fore and aft rigs is the fact that more sail area lies aft of the mast than before it. So if the mast of a sloop rig is placed in the center of a proa hull, (as it must) then the CE will be aft of center, not forward like a traditional proa. This would ruin the balance of hull and rig of the Oceanic canoe, so as with the traditional boat, both hull and rig must be considered together. If the CE shifts aft with every shunt, then so too must the CLR. This is done a number of ways, twin fore and aft daggerboard/rudders that are raised and lowered to adjust the CLR is one of many approaches.

Other Approaches

Some original thinkers have been developing rigs for proas that do not necessarily belong to either the Oceanic or the Western tradition.

The iconoclast small boat designer, Philip C. Bolger, designed a proa rig that makes too much sense to be ignored. The basic concept is a rig that echoes the peculiar geometry of the proa; it is symmetrical fore and aft, and asymmetrical athwartships. This means the sail need not be turned around 180 degrees for every shunt, since the luff becomes the leech, and vice versa.Theoretically, this rig could greatly simplify and speed up the shunting process. So far however, practice has not equaled theory.

Traction kites and kite boarding are the hot new watersport, replacing windsurfing as the latest buzz. Some think the proa will make the best platform for kite boats. When the kite sailors get all the bugs worked out, be ready for a true revolution in sailing!

Originally published 2001. We are re-publishing some archived articles in order to present them to newer readers.

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1 Comments

03 Sep - 05:22

Ben

I’m very curious about the idea of a junk rig on a proa. it seems like you could have two sets of ropes and alternate between them depending on the direction. it’d also allow for a single sailor to manage a bigger amount of sail. i don’t have experience with junk rigs or shunting craft so I might be overlooking something. the idea just appeals as a cheapish build option.