2005 Mylar Sails

Development and Trialling of Mylar Sails - A Progress Report from Frank Bethwaite

The background:

1. At the Japanese worlds I was asked to trial an asymmetric spinnaker. I did so, found that it didn't work and analysed it and realised that it couldn't work, and canned the idea.
2. From the following ferment I distilled the Tasar Two idea. Had that run to trial and proved as successful as I believe it would have been, the twin classes would by now be growing strongly with the expectation of a forty to fifty year future, because there is nothing remotely like it even now on the market. Those who were so selfish as to oppose even a civilised trial, look in the mirror.
3. A Melbourne group inspired by Alistair Murray promoted the Tasar locally to such good effect that interest and demand from Victoria has risen from zero to twenty plus, and as a follow-on I have been asked to re-image the class with mylar sails. The hope here is that this will spur the renewed interest beyond Victoria and beyond Australia.
This sets the scene for a fourth try.

It is time to explain why I am doing what I am doing.

My starting point is a relatively small, hard, manually adjustable rig of extraordinary efficiency in steady air. Its area is right for 300lb crews. It is too small for today's heavier crews.

My brief is to do the best I can with Mylar sails which will fit on the existing spars.

My intention is to produce the fastest sails possible with present knowledge within that brief for crews now averaging say 320 to 330 lbs.

In the years since 1975 when we signed off on the Tasar sails we have learned much. Of key importance to this project are three factors which we did not know thirty years ago

1. The wind speed fluctuates within gusts by about 18 to 20% every 5 to 8 seconds. This makes a mockery of manual adjustment, because manual adjustment for the gust and the lull every minute or so can access only a fraction of the potential advantage. This understanding has lead to the automatic rig. The Tasar spars are too stiff to do the automatic job by themselves, but I will try to create a rig which will be efficient when sailed by crews who steer and trim sheet with a light and springy touch like a fisherman with a big fish on a light line.

2. The turbine blade effect. In considering this let us look at three factors:

a. The first is that most races are sailed in winds of less than 12 knots.

b. The second is that what we want from our sails when sailing to windward in winds of less than 12 knots is higher force, not best lift/drag. (for the purist, optimum power factor.)

c. The third is that a sail which is set to exhaust into the area of lowest pressure of the sail behind it acts like a turbine blade. The pressure at the leading edge of the jib is greater than the pressure at the trailing edge (the jib leech) which is just to leeward of the mast where flow speed is highest and pressure lowest. So the flow around the lee side of the jib is accelerated and the sail pulls harder and develops greater force. Julian put the first cuff on the Eighteen footer "Looney Tunes". His object was to stop the spinnaker from tangling around the mast base. The boat bolted. So did all the other boats to which we fitted cuffs. It was only some years later that I realised that the logic had almost nothing to do with the mainsail. What was happening was that the cuff was extending the area of lower pressure just to leeward of the mast down to the deck level of the lower jib, and this lower jib was suddenly beginning to pull much harder than it can when there is no cuff. And this low level is exactly where it serves best.

So I now look at the present Tasar rig as three levels

d. From the deck up two feet to the boom, there is eighteen inches of jib which is doing half the job.

e. From two feet to about 11 feet the sails are efficient and powerful.

f. From 12 feet to the masthead the aerodynamics are good but the dynamics are too stiff to yield properly.

3. Smoother transition from light air (0 to 5 kts at height of 5m, laminar flow, with glassy water surface) to breeze (6 kts plus at 5m, turbulent flow, with rippled water surface.) In the lab and in the aerodynamicists' wind tunnels the boundary layer flow trips abruptly from laminar to turbulent. At the time I designed the Tasar rig I thought it always tripped abruptly over the water too, and proportioned the rig accordingly. I now believe that there is a blending which is dependent on both surface roughness and temperature. Over a hot rough beach the flow will trip turbulent abruptly, Over cold smooth water the transition will be smoothed to the point where it may need up to 8 knots to develop turbulence to masthead height. The practical effect is that I now think that sails with more area higher will be faster not just to 6 kts, but often through to 8 kts.

My object with the new rig is:

1. That the whole jib from deck to 11 feet and the mainsail up to 11 feet should be efficient and powerful.
2. This two feet of greater force at the lowest level should enable all crews to sail faster and higher in winds up to 12 knots, and determined crews to windward plane as a routine in strong conditions with the upper sails twisted off.
3. The upper mainsail will look like the top of a 49er mainsail. Because the topmast is too stiff we will have to fudge the flattening with battens, but we have done this before.
4. When vang sheeted with light sheet tension the broader upper sail will yield if it is given a chance with light springy muscles. It is possible that an internal halyard will make a detectable difference. I am advised that it often does - we will never know if we do not try.
5. Offwind and downwind the extra area up high will give more speed particularly in light unsteady conditions. It will of course benefit heavier crews.

All positive suggestions welcome. If they are negative - stick them onto your mirror.



1. This development project is line with the motion passed at the World Council meeting in Victoria in 2003, which endorsed the establishment of a development committee and investigation of possible changes to the Tasar, followed by a report to the World Council.  Any changes to the class rules will go through the normal approval process, which now includes approval by ISAF.
2. Franks thoughts on how to modify the rotation lever to work with the cuff are here.

Richard Spencer


Report #3 - Shane Guanaria


The day we test sailed the "Tasar X" was in variable winds, 6-14 knots. It was only the second time in 8 months that I have sailed a Tasar (the first being 2 weeks earlier in 20-25knots with the test sails). I sailed with my fiancée, Jeanette, who had only sailed twice before. Our combined weight was 145kgs.

It was agreed that we would forfeit any right of way and started at the un-favoured end of the line (the line was heavily biased to the boat end). We did not interfere with any other Tasar during any race, which in retrospect was a considerable disadvantage to us.

We thank the other Tasars for letting us sail with them, it was definitely worth while.


• The new sails look great on the boats. Even the younger 29er sailors made positive comments regarding the new more modern look.
• The extra visibility was fantastic, both for safety and tactical reasons


The initial performance of the boat on the weekend was very impressive, if you include us in the results for the 4 races our places were 4, 1, 3, and 2 for a total of 10 points. Top boat for the day was Tasar 170, his places were 1, 2, 1, and 7 for a total of 11 points and next was Tasar 2710, 2, 5, 2, 6 totaling 15. It is worth noting that there were 4 boats that finished in the top ten at the last Australian titles competing (one was Tasar X). I feel if we were in a standard Tasar we would have finished 4th or 5th.

Upwind the new sails performed very well, we were consistently faster than the standard rig through the ranges. I can't recall a boat going past us up any work. The mainsail leech was a lot more mobile in the fluctuating breezes, hence I was adjusting a lot less sheet and traveler. The jib, with the extra roach acted in a very similar way. The jib was also easier to steer to in the down range conditions even with the shrouds all the way back.

Cross wind our results were hindered by the protocol above, we couldn't pass when I thought we could and couldn't defend when we had too. We also surrendered inside positions at mark roundings. The jib trim was only ever a median setting, straight through the fairlead and cleated. In the race where we were clear ahead we initially put a couple of boat lengths on 2nd place before another breeze line filled in and they closed the gap, the gap stayed the same on the following leg. In summary the sails would have been done more justice in more experienced hands.

Downwind, on sheer sail area I felt we were a little quicker, but it was hard to tell on a day like that. Gybing the jib was not hindered at all by the battens. The sails set well, and I'm sure, as above they would have performed with fewer restrictions.


I guess only time will tell how long they last but if the 49er's are any indication the competitive life of the sail will match, possibly exceed the Dacron sails. The first set of 49er sails we built are still being used on a weekly basis, they have been on the boat for 5 years now. The first time I used the sails it started of at around 10 knots, but before long had increased to 25 knots. At the time I was sailing single handed and the jib was doing a fair amount of flogging, to the extent where the jib sheets came off!! The jib wrapped it self round the forestay while I was trying to reattach the sheets but still did no damage, not even a broken batten. I had a similar experience with a Dacron jib, and that jib suffered a worse fate. Both mylar sails are in perfect shape. It was interesting to note the way the main reacted when my crew joined me, no inverting of the lower parts of the main, just the head. It acted in the way a skiff main does, laying open before inverting and flogging. The bottom of the sail set properly, making the boat well balanced and easier to sail.

In summary, I was surprised at how well they performed. If the top standard boat on the day had used these sails with no restrictions the results would have been much more one sided.

Shane Guanaria
March 7, 2005


Development and Trialling of Mylar Sails

Report #2 - Frank Bethwaite
As a youth Shane Guanaria sailed with his father Ian and routinely finished top ten in major Tasar regattas. (Ian was for many years Chief measurer for the Tasar World Council.) Shane then sailed Lasers and represented Australia in the Laser World Youth Championship in 1999. He works with Ian MacDiarmid, and made the new mylar sails. These were finished a few days ago. I now have a hull but have not yet rigged the spars.
During the week Shane telephoned and asked if I would agree to his trialing the new sails in the Sunday races at a weekend Tasar regatta. I agreed.
Venue: Bethwaite Design regatta at Speers Point, Lake MacQuarie. Fleet: 25 Tasars, 12 29ers, 6 B-14s Saturday program: Several races completed in severe frontal conditions. Sunday program: Five back-to-back races. Course: St-W1-G1-L1-W2-L2 Fin, St/Fin line 200m upwind of L Wind: The cold post-frontal southerly air was about 1 to 2 degrees colder than the 24 degree water temperature of the lake. The wind pattern was of constant, small, sometimes severe, "bubbles" of unsteadiness which occurred at random within a much larger pattern of half kilometre patches of stronger and lighter air. Waves: Up to 1 ft. Crew: Shane G and Jeanette Russo, Shane's fiancee. 145kg total. This was Jeanette's third sail and first regatta. Protocol: Shane started with the Tasar fleet, sailed to clear air without adversely affecting any competitor, did not slow any competitor, conceded at marks, and did not cross the finish line. Shane and Jeanette returned ashore after the fourth race. Measurements: I timed parts of Races 1, 3 and 4 from shore, and all of R2 from a rescue boat.
"Exp" denotes the experimental boat, "T" the first Tasar to round the mark.
The sequence of each line is - Mark; time difference for leg, Wind/velocity a + time difference indicates Exp boat got to the mark first

Time of rounding
Race 1      
W1 11:13:44 11:12:51
L1 11:17:42 11:16:22
Race 2      
St 11:40:30 11:40:30  
W1 11:50:24 11:50:40
G1 11:55:02 11:55:10
L1 11:58:33 11:58:40
W2 12:07:15 12:07:40
L2 12:13:28 12:13:44
Race 3      
L1 12:44:16 12:43:20
W2 12:51:40 12:50:44
L2 12:58:09 12:57:31
Race 4      
G1 13:27:21 13:26:24
L1 13:32:24 13:31:28
W2 13:38:35 13:38:01
L2 13:44:50 13:44:22

Summary To windward X was about 4% faster on 3 of 4 legs timed, no diff on 4th. Downwind X was slower in early races; faster in later races. Crosswind Average difference about zero. X sometimes trapped, sometimes unable to pass, due protocol.
Observation Leeches of X rig noticeably more mobile than any adjacent T rig.
Frank Bethwaite


Mylar sails

Notes to test sailors from Frank Bethwaite.
17th March 2005

The design of the Tasar rig of 1974 was driven by -
• The then-new ability to adjust sail shape and the awareness that crews who adjusted on a gust by gust basis sailed faster than those who did not adjust.
• The belief that while the wind speed within gusts was stronger than in lulls, the wind speed within gusts and within lulls was relatively steady.
A group of us researched mast and sail shapes in wind tunnels ashore and in the steadiest winds we could sail in on the water. The resulting rig is flexible enough and controllable enough to be manually adjustable over a wide range of camber. It is too stiff to yield significantly to change of wind speed. It is probably as good as any and superior to most contemporary rigs in steady air.

In the years since 1974 we have learned -
• That the wind speed changes within gusts and lulls every few seconds.
• That the wind direction also changes frequently.
• That the changes in wind direction are relatively small when the air is cooled, and much greater when it is heated. (Water or adjacent land warmer than air.)
• That rigs which are flexible enough to begin to yield automatically to the changes in wind strength and changes in wind direction sail faster than more rigid rigs.
• That rigs which yield and flatten from the top down sail faster - sometimes much faster - than rigs which yield first in the middle - and
• Both children and adults eat more and exercise less than they did, so weigh more than they did.

My response to the request to re-image the Tasar with mylar sails is to move as far toward the 59er or Byte C2 rig as is possible with sails which will bend onto the present spars.
• I have put together a mast which is non-standard in that I have put nyloc nuts and washers on the diamond stay adjuster screws inside the mast just above the mast base fitting so that the diamond stays can be run as slack as desired (within reason - say diamond stays to mast 6 to 9 inches above whisker pole fitting) without fear that the screws will drop and project below the mast base fitting and foul the screw heads of the mast step.
There is nothing I can do to make the topmast as flexible as I would wish, but allowing the lower mast to "work" rather than be held rigid may make a significant and favourable difference to the flexibility of the whole rig. We will see.
• I have put together a boom with a rotation stop notched so that the cage will remain in the slot when the mast tries to anti-rotate, which it always does with a slack sheet.

I would encourage crews who trial the new rig to start by sailing as they always sail, say for the first race. (Immobilise the lower mast by tying the diamond stays together.) Then progressively try sailing with a more flexible mast and the traveller further to windward, a slacker sheet and sufficient vang (constantly adjusted) to control twist and to trim the upper leech, and to hold the sheet more lightly on springy muscles.
The leech of the mainsail will become much more mobile - like a 49er. Trialing will establish whether there is performance difference, and if so what is its degree, in unsteady and rough winds.
Comments about ease or difficulty of handling, as well as about relative performance, will be welcome.
If there is a consistent performance difference, it will presumably be associated with an optimum lower mast bend (diamond stay tension.) This, once established, may call for adjustment to the luff curve.
Once we have these factors optimised and handling pleasantly, it will be time to trial the cuff. This, I believe, will primarily affect windward-going performance, particularly in stronger winds.

Trial Kit.

I have put together -

Mast and topmast and halyard with vang and downhaul as above.
Boom with whisker pole ears aft and shock cord forward.
A fixed forestay.
Jib halyard arrangements.
Mainsail, jib, both battened.

Tester provides -
Shrouds (may need to adjust)
Jib sheets, mainsheet.
Whisker pole.
Pencil, paper.

Frank Bethwaite

Report #4 - Brad Stephens
Report on Tasar Trial Rig, March 19, 2005

Frank has put together a trial kit:

Therefore, unlike Shane Guanaria's test sail, this sail testing session was with the trial kit that Frank supplied.

The Sails

The jib now has roach ( maximum at top batten ) and three full length battens. The battens are known as "split jib" being half width of a standard batten style and are only 7mm/.25" wide. As a consequence the furler has gone and the jib is now hanked to the forestay with small plastic clips. The clips are a two-part system; one part webbed to each side of the sail and clipped together such that the forestay rides on the webbing. The halyad block was rigged inline with the forestay at the hounds.

The halyard was lead from the head of the jib up through the inline block and returned down the forestay inside the plastic clips. It was then lead through the forestay takeoff on the bow to a floating cleat that Frank had supplied for the trial kit. The cleat had a line attached at the rear which we lashed around the forward top of the dagger board case. This line ran up the bulkhead with the cleat sitting flat on the deck at the mast.

On hoisting the jib we tied a hitch in the line and lead it through the cleat so as to give a 2:1 purchase before cleating.

At least one club member did lament the passing of the furler but it was pointed out that the new jib leech profile required battens to support the roach and that this roach would produce a more responsive leech plus some additional area.

The mainsail now has only 5 battens with the first batten at approximately 25% of the luff length above the boom as opposed to just above the cunningham on the existing sails. Foot length appears to be a little shorter and as evidenced by the first photos of the new rig there is substantially more roach in the upper half.

The mainsail battens featured a threaded leech end fitting that could be turned by hand to change tension with a snap down plastic cover. Very neat.

Mast and Boom
As noted above Frank has put together a test kit including mast. On this rig the diamond wires had initially been set so as to touch the rig at about 30cm/12" above the gooseneck. However, as presented to me the wires had been lashed together around the front of the spar to in Frank's words "Immobilise the lower mast …" as the initial approach to sailing with the new rig.

We experienced some difficulties with the rotation lever setup ( still standard ) but Frank has corrected this minor problem.

On the water.
The test sailing was conducted on the upper reaches of the Parramatta River, which is the major tributary of Sydney Harbour. Tasar's sail locally from the Concord and Ryde Sailing Club on a stretch of water that is characterised by wind conditions that are extremely variable in strength and direction. The wind strength changes alone require constant movement from the crew.

A number of regular Tasar sailors from the club sailed with the new rig in the morning and the first impression of all was how flat the lower half of the mainsail was setting up on the trial rig. It was noted that both upper leech's responded significantly more in the gusts then the standard sails. This was noted both by those onboard and observers in other boats. All appreciated the extra visibility offered.

During the afternoon race time the local Olympic Park site recorded SSE 10 knots wind speed gusting 17 knots for most of the race dropping to SSE 9 knots gusting 15 knots towards the end of race time.
The race course was essentially long upwind, short reach, long run run, short reach, 4 times around.
For the race I sailed with my son Nicholas who combined with me weighed in at 133 kg in measurement trim. In a fleet of 9 for the day we placed 2nd which would have been our typical placing against the fleet.

Given the wind conditions for the afternoon we would typically just manage to stay ahead of the much heavier ( 150kg/330lb + ) crewed 3rd place getter for the day upwind and skip away downwind. To the heavier ( 150kg/330lb + ) crewed 1st place getter of the day we would loose upwind but close up again downwind but with the overall gain during the course of a race being to them.

Overall, we seemed a lot more comfortable upwind in the gusts then usual ( this is centreboard up to the 87% immersed setting for us ) and faster downwind. Overall, our finish was closer to the first placed boat then I would have expected in the conditions.

I know that the above seems a little against the grain of Frank's stated aim of benefiting the 145kg/320lb + crew weight but the rig as presented to me on the day was well suited to a light crew. I am sure that rig tune will make a huge difference to the potential benefit for the heavier crews.

I had been concerned how as a light crew we would handle the additional sail area but the flatter lower half of the mainsail combined with the more responsive top half of the mainsail and same with the upper jib leech actually provided an easier rig to sail then the existing rig.

As a guide to how flat the mainsail was setting up there was virtually no interaction between the jib and the mainsail all day, even with the jib sheeted on quite firm.
Trim wise with the mainsail I initially sheeted the mainsail in a fairly normal manner and progressed to more vang and looser sheet with the traveller at times ending up fully to windward with the boom on the centreline.

This seemed to provide a good balance of sheet tension and gust response in the upper leech of the mainsail.
We had provided our own shrouds for the test rig and had ended up taking them up a couple of positions but in hindsight could have done more of this as the new jib design seemed to require or would benefit from more rig tension then the existing jib. The other consideration is that with the hanked luff ( there are eight "hanks" with three spread over the luff area where the lower tufts are ) it's going to look a little different with low luff tension and will tend to sag between the attachment points.

The new jib did seem to fit the tracks a little better with us sheeting off the middle clewboard position but as noted above we had to take our shrouds up a little so that might just have reflected a change in rake!

Concluding Comments
I think that we benefited from an initial rig setup that whilst great for us as a light crew was not suitably tuned for the target 145kg crew. That is, as a starting point for the next sailing session I would change the diamond tension to enable the lower part of the mainsail to set a little deeper.

Brad Stephens
March 21st 2005

With thanks to Frank Bethwaite for letting us play with the new sails, Tony Keevers the Class Captain at CRSC for organising with Frank and all the Tasar sailors at CRSC for their enthusastic involvement.

More information on mylar sails, from Frank Bethwaite

Re Jib

At Concord I arranged a three to one purchase through a clam cleat for the 2mm spectra jib halyard, but the 2mm spectra did not bite properly in the bottom of the cleat, and slipped during the race. Nicholas tied it off as well as he could during the race, but the jib luff was sagging about two inches from the forestay between several of the lower hanks at the end of the race. This would not be fast.

At Northbridge the next day I found I had inadvertently left the cleat and tail at Concord, so we arranged a truckies' hitch around the forward end of the centrecase. The tie-off knot in the 2mm line was much too awkward to be adjusted during the race, so again there was jib luff sag in the puffs.

A point needs to be made about jib luff tension (ie halyard tension) with battened mylar jibs which are hanked to the forestay. The hanks allow about two inches of movement fore and aft with respect to the stay. If the tension is set too tight, the battened jib luff moves forward of the forestay in light air, which makes the jib flat, which is slow. If it is not tightened in the breeze, the luff sags (as above) which again is slow.

I have now put together an arrangement in which a strong plastic ring lies on the foredeck by the mast and is tied by a tail to the centrecase. A short 4mm adjuster line connects ring and a trapeze cleat with a 2-1
purchase - this is the adjustment, The halyard is tied to the cleat. End of jib problems, I hope.

Re Mainsail

At Concord, to show what was possible I had slackened the diamonds to the point where they could be pinched to touch the mast eight inches above the whisker pole fitting, then tied them together at the WP fitting which left them still quite loose. I suggested to Brad that he fiddle the tie if he wanted to experiment.

The rotation handle bolt loosened as it always does on its first day out, so the handle drooped and Nicholas had trouble with rotation. The end result was that the mast rotated a long way, bent forward further than normal because of the slack diamonds, and this abnormal bend made the sail very flat as he reports.

At Northbridge the next day I tightened the diamonds until they could be pinched to touch the mast 4 inches above the WP fitting - ie still very slack, and this is how they sailed, so the mainsail would have been flatter than when Shane sailed it on his stiffer mast. The rotation bolt had been tightened, so rotation was not an issue.


The ability of the combination of slack diamonds and degree of rotation to control mainsail fullness so easily is something I had not anticipated. This is an exciting advance. I will trial a system by which I can set rotation to the degree desired and see what happens to the speed when this is used intelligently.

Frank Bethwaite