Proa Rig Options: Bolger Rig

07 August 2014     Editor    0 Comments.

John Dalziel gives a summary of his series of experiments on his proa C. L. Brock with a Bolger-type rig. Note: Frequent references are made to Philip C. Bolger’s proa "cartoon", which first appeared in an issue of the late Small Boat Journal, and was later published in Boats with an Open Mind; International Marine, 1994.

The Bolger proa rig echos the unique symmetry of the proa: the airflow reverses direction during a shunt, just like the water flow on the hull. The rig has many powerful advantages that have led to a 50-year series of attempts to develop it into a useful sailboat rig. It is a wonderful example of the maxim: "In theory, theory equals practice. In practice, it doesn’t." Though the rig has become known in proa circles as "The Bolger Rig" due to the fact that it was Bolger’s proa cartoon that became widely known, the rig in fact was invented in the 1960’s by members of the Amateur Yacht Research Society (AYRS) who thought so highly of its potential to christen it the "AYRS Sail".

Great Potential of the Bolger Rig

The very high potential is obvious at a glance:

  • With the mast out of the way, airflow to the sail is smooth.
  • Permanently curved battens give ironclad control over foil shape, allowing precise tuning and twist control.
  • The elliptical shape provides optimum span loading, meaning that unlike conventional rigs the top third of the sail can operate at full power.
  • The balanced tack provides low sheeting loads and excellent sail shape off-wind.
  • The sharp leading edge provides near-perfect flow separation and in combination with the curved battens grants unheard-of ghosting ability.
  • Reefing is quick and well-controlled.
  • Lift/drag ratio is exceptional, about the same as a full-battened wing-mast.
  • It has no directional weaknesses, performing excellently when beating, reaching, and running.
  • In theory, shunting is dead simple; harden the sheet, release the tack; the old sheet becomes the new tack, the old tack the new sheet.

The Downside

Collapse in the upper third of the sail is caused by the actual line of rotation (red dotted line) being well aft of the theoretical line (blue). This puts the line of rotation aft of the lift line, and the upper third of the sail becomes over-balanced. »

Unfortunately, during the entire developmental history of this rig, nobody has ever fully tapped its potential, for along with the above advantages there come a host of disadvantages, and the fact is that these relate directly to the concept of the rig.

Though the sharp leading edge provides the best L/D ratio, it is also extremely sensitive to angle of attack: Theoretical best angle for highest L/D on this sort of single-surface sail is all the way down at 4-5 degrees. But at such shallow angles, the circulation bubble on the windward (windward is not a typo) side of the foil, just behind the leading edge, would grow enough to distort the sailcloth between the battens, reducing drive. In addition, the lowered pressure on the windward side of the leading edge leads to an early and severe collapse of the sail to windward. 6-7 degrees was as close as it was possible to go. But note that there is only a one or two degree difference here! In real-world conditions, it is impossible to control the sail well enough to keep it on that knife-edge, and the sail routinely "bluffs," or collapses to windward.

The top of the sail can become over-balanced: When not under load, the sail pivots neatly along a line between the halyard and the tack. But once the sheet is hardened, this line moves aft to approximately mid-chord, at least for the top third of the sail. Now the lift line is ahead of the pivot line, and when the boat is hard on the wind the top third can to pivot to windward in a manner similar to but not as drastic as a gybe on a catboat. This further reduces the sailor’s control over the rig, and is extremely annoying to boot!

Stresses through the sailcloth are high: This rig lends itself better to Mylar-Kevlar sails than to Spectra or Dacron, as the latter two show substantial amounts of creep and the distortion will reduce sail efficiency rather quickly.

As you would suspect, rigging loads and mast compression are high as well. My 170 sq.ft. sail could put an 8" bow in a solid 3" diameter Douglas Fir mast in as little as 10 knots of wind. Both running and standing rigging must be very low-stretch steel or Kevlar. Sail efficiency and control both depend on keeping things well tensioned.


Shunting the Bolger is a mixed bag. In light winds the sail performs as advertised, changing ends with ease. But in moderate or higher winds it is dangerous to shunt in this manner for several reasons:

  1. Since the sail shunts power-on the proa will stop and restart with tremendous acceleration, potentially injuring the crew or knocking someone overboard.
  2. There is simply not enough time during this power-on process to accomplish all the tasks needed to reverse the boat, so one is left with finishing up the shunting process while the boat is screaming along at a mad rate.
  3. During the power-on shunt, the sail swaps ends with enough force to place damaging impact loads on the rigging and fittings.
  4. If the wind catches the sail parallel to the hull, the boat can be knocked down.

In order to shunt the rig in higher winds, it is necessary to drop the sail, shuttle the boom by hand, then re-raise the sail and tension the luff. This is not as much of a handful as it first sounds, since the sail is fairly light, but both standing and running rigging must be designed with this task in mind.

Heaving to, or simply slacking the sheet, can cause unexpected grief; the permanently curved battens mean that the sail, when at zero drive, is actually balancing a "negative" force (towards the nominally windward side of the sail) located just behind the leading edge against a "positive" force farther back on the sail chord. The effect is that at zero drive there is a strong and consistent force rotating the boat to windward!

This is more apparent when the sail has been moved forwards in the Oceanic arrangement. In the case of heaving to, this means that the boat will quickly rotate into a backwind. In the matter of slacking the sheet for a gust, the boat will attempt to turn into the wind unless strong windward helm is applied. Neither effect lends itself to a healthy, confident attitude on the part of the sailor.

There is one final problem with the Bolger, and it relates to having the sheet and tack simultaneously handled during every shunt. If both lines get out of control at the same time- and this is guaranteed to happen at some point- then the sail in effect becomes a huge and unstable wing tethered from the masthead; it can - and will - fly up, turn sharply and come screaming back at the rigging and crew with tremendous force, easily enough to cause serious injury or death. This all happens so fast that there is literally no time to react once it begins.

Possible Solutions

« When a dual halyard system is used, the actual line of rotation stays forward of the lift line, and the collapse does not occur.

Some of the problems presented by the Bolger can be addressed as follows:

1: the three-point suspension of the sail, causing loss of control over the leading edge and "bluffing," is one of the chief culprits. This can be addressed by adding a short yard and turning the Bolger into a four-sided sail. The yard needs two halyard locations, one for each tack, as shown in the illustration. This unfortunately adds considerably to complexity. The alternative, leaving out the roach, works but leads to a poor sail shape of low L/D and little power.

2: I have tried a number of ways to control the luff, none entirely successful. The very best was to use a Chinese Junk-type sheeting arrangement to each of the lowest three batten ends. This stopped the bluffing completely and allowed the highest pointing angles of all arrangements I tried. This works for two reasons: the tension to the front end of the batten keeps it from diving significantly to windward as the angle of attack decreases, and the vertical line of rotation is moved forwards. The downside is that the sail is no longer balanced, and therefore twist control is much hampered and the off-wind shape of the sail is effectively ruined. Tacking also takes much longer due to the additional length of line that has to be handled. The battens tied into must also be heavier, and are more apt to break.

Unsolved Problems

1. I have yet to come up with a solution to the problem of the sail spinning the boat into a backwind when at zero drive, and I suspect that the only real solution is to invent some way of immediately straightening the battens when the sheet is released. Alternatively it is possible to use a double surface airfoil, which would not show the same sort of action. It may be worth looking into. Spoiler flaps have also been suggested, but not tried; these may prove to be the simplest possibility.

2. Some solution must be found to the problem of both sheets getting out of hand at the same time, or else the rig is simply too dangerous to use. We have tried a couple of schemes without success:

  • using one centrally located sheet and two dedicated tacks allowed the boom to be controlled sufficiently, however we had a knockdown when the unemployed (aft) tack got tangled and wouldn’t allow the sail to be eased.
  • We also tried a tether to mid-boom, but if the tether was long enough to allow the sail full range of normal movement it was also long enough to let the boom thrash wildly.

Rigging Details

Details of sail showing approximate positions of battens, reef cringles and straps, lazyjacks (red), parrels (blue). »

For the benefit of those who would like to experiment with this sail, here are a few of the gory details. The sail size in question is 125-175 square feet (12 to 17 square meters). It is possible to use the sail at a stationary point at the middle of the vessel, as is shown on Bolger’s cartoon, or to move it forwards on each shunt in the Oceanic manner. There are advantages and disadvantages to each scheme.

Sail and Running Rigging

Halyard: Should be double if handed or single if led to a winch. It is necessary to have some leverage to tighten the luff after raising the sail.

Parrels and lazyjacks/ topping lift: Some sort of topping lift is necessary, or the boom will come crashing to deck every time the sail is lowered. And some sort of lazyjack is necessary or the sail will be blown into the water as it comes down. My solution: run two or four hard-twisted (for longevity) 1/8" (3mm) lines from a masthead deadeye or block to spots on the boom as shown in the drawing. On the windward side of the sail, sew parrels of the same cordage so that the sail can slide up and down under control of the lazyjacks. Make sure the parrels are long enough to compensate for the widening between the lazyjacks as the sail is lowered. The top panel of the sail does not need to be controlled.

Battens should be made straight and sprung into the appropriate curvature with 50-100# Dacron braided fishline. Make several loops in the ends of the line so the batten curvature can be easily adjusted to three or four preset curves. The battens should not be square or round, but should be flat/rectangular to prevent them from reversing when the sail is down. Rotating them to the right direction every time you raise the sail is quite the bother. The fishline serves the added purpose of protecting the battens and sailcloth somewhat in backwinding situations.

Dispense with batten pockets, as Bolger did on his sketch. Sew loops or ties onto the windward side of the sail about every 6-9" and run the battens through these.

Batten Curves: For beating I found the most effective curvature for my typical conditions- 5-8 knots wind- to be 6% at the boom and 8% at the top batten. For a smaller sail on a heavier boat, increase to about 8 and 10. The top batten must be more deeply curved than the boom or the top of the sail will stall prematurely. For off-wind work, curvature can be increased to as high as 15-18%.

Reef points: add cringles 3-4" below each of the lower 2-3 battens at both ends of the sail; these will tie onto the boom when the sail is reefed. Add two or three loops of nylon strap- one each at the 25% chord location, another if desired at the 50% point- also slightly below the battens to tie to the bottom batten.

Boom: If you build the rig according to Bolger’s sketch, the boom will be too stiff to change the curvature by much. Simpler is to use a heavier bottom batten and a straight boom of wood or aluminum. The boom is provided with several fastening locations so the bottom batten¡s curvature can be adjusted.

It is necessary to locate the tack/sheet mounting points in between the straight boom and the bottom batten, bearing on each, in order to provide sufficient tension to the luff and to the belly of the sail. This can be done with a short wood beam lashed on to the boom and batten at the appropriate distance aft of the luff (determine by experiment). Alternately, two lengths of line making an X and knotted in the middle can be led from the ends of the boom to a point about 3/4th the way to the other end of the bottom batten. The tack/sheet block can be lashed onto this line at the appropriate point (found by experiment again).

Sheets/tacks should be 2-1 purchase. They should NOT be a continuous line. Provide jam or cam cleats for the tack; when the wind pipes up, shunts happen fast.

When used with a stationary mast as in the Bolger cartoon: Follow Bolger’s deck mount locations for the sheets/tacks exactly. The tack (for the direction you are travelling) must be to windward of the mast base, or else the sail will overpower the rear board when the boat is being shunted. The Bolger sail is never sheeted so hard as to bring it in line with the hull, so the mast will not interfere with sheeting properly.

If the sail is moved forward using a raked mast, consistent with Polynesian practice, you have more freedom in locating the mounting pads for the deck blocks, however it is still preferable to mount them to windward if the bow allows. Make sure you have enough rudder area to keep the boat tracking straight when the sail is let out to 90 degrees on a run; the turning force is considerable.

At this point I do not recommend building the sail with only one halyard attachment, as this is guaranteed to produce sail handling problems. Supply two as shown in the sketch. The "yard" can be little more than a long sturdy headboard with two halyard attachment points located near the respective luffs.

Standing Rigging

At the masthead, the standing rigging should be fastened below the halyard blocks if the mast is to be raked, but above if the mast is stationary and the fore /aft stays will clear the edges of the sail leading-edge roach (both conditions must be met, or else the tangs should be below the halyard block). Tangs must be sturdy and able to take high impact/ shock loads. For a sail this size, 1/8" (3mm) stainless steel wire, or the Kevlar equivalent, will suffice. Use the widest staying base possible to reduce compression loads on the mast.

You will need either three stays (fore/aft plus windward) plus a backwind support strut, or four stays (the fourth is to leeward of the mast). I doubt that the two-stay system in Bolger’s sketch will suffice. I recommend three plus the backwind strut as being superior.

Mount the windward stay to a four-part tackle as far out on the outriggers as is practical; this adjusts masthead cant and also serves as a shock absorber. A lee stay, or a backwind support, should be set up with enough slack that the windward stay may be adjusted 2 feet or so.

Raking mast: Set up the fore and aft stays inside the distance between the sheet/ tack block pads. Set up to long lanyards, and run a bungee cord at right-angles to the lizard or block. This will serve as a shock absorber and also will pull the slack forestay out of the way.

Stationary mast: use lanyards to fix the backstays and tie in a bit of tension to hold the mast still. If the stays will clear the sail edges (as noted above), run them out as far on the bows as possible.

Add a tie-down between the mast heel and the step, so that if (when) the sheets get away the mast will not be forcibly pulled out of the step.

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



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