Thistle Tuning Guide

  • Last Updated: May 26, 2022
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This tuning guide is for the DSD design, VS design and the Proctor design Thistle sails. These designs are the result of hours of boat on boat sail testing and racing experience.

VS (Fisher) Quick Tuning Guide

DSD Quick Tuning Guide

Proctor Quick Tuning Guide

Northwest Quick Tuning Guide

The following tuning guide is meant to be a comprehensive guide for setting and trimming your North Sails. Please read it thoroughly before using your sails for the first time.

We urge you to read the section on sail care in order to prolong the life of your sails.

Boat Preparation and Rig Tuning


The Loos tension gauge is a relatively accurate instrument that will help measure the tension of your standing rigging. The newer Loos Pro gauge is spring loaded and some sailors have found it to be slightly more accurate and reliable than the Loos Model A. Following is a comparison of the numbers between the Model A and the Pro gauges. We have broken down the numbers for both 1/8” forestay tension and 1/16” diamond tension.

1/8” Cable Forestay Tension


























1/16” Wire Diamond Tension With North Proctor model sails














1/16” Wire Diamond Tension With North Fisher model sails















Before you step the mast, set the diamond tension to achieve proper mast bend.

Diamond tension, not only has an effect on the sideways bend in the mast, but also on fore and aft bend. The suggested numbers that we offer here will, again, get you very close so that your mast and North mainsail will work well together. Due to the nature of the aluminum extrusion and in some cases the way the masts are rigged, some masts are stiffer and some masts are softer than others. This is not a problem; it just requires slight adjustments to your diamonds. Always be sure to check your diamond tension and straightness of the mast while it is supported at both ends with the sail track upwards.

If while sailing in marginal hiking conditions (10 – 12 mph breeze) you notice slight diagonal overbend wrinkles in the upper part of your mainsail, your upper diamonds are most likely too loose which is allowing the upper part of your mast to bend too much. On the other hand, if your mainsail in the upper third appears fairly round and is difficult to flatten out in a breeze, your upper diamonds are most likely too tight.

For Proctor’s main when sailing above 490 pounds of total crew weight, set the diamonds close to 18 (number scale on the Loos gauge—the numbers do not correspond to pounds) top to bottom.

Set them progressively looser for lighter crew weights to a minimum of 10 on the Loos gauge, e.g. top crew weight of 490 pounds, scale 18; to crew weights less than 430 pounds, scale 10.

For Fisher’s main, set your diamonds at approximately 6 – 14 – 9 (not pounds and measuring from bottom to top). The lighter the crew weights, the lighter the tension on the top diamonds. Set the top diamond at 4 or less when sailing below 430 pounds in crew weight.

On wood masts, the diamond tension should be slightly less than the standard aluminum mast because of possible compression problems that could develop from excessive tension. Never tension any of the diamonds on a wood mast to more than 16.

On the older, stiffer, gold aluminum masts the diamond tension should be considerably looser to allow the mast to bend as easily as possible. You should drop the tension on all the diamonds approximately 4 numbers from top to bottom.

While the Thistle mast looks fairly complicated with three sets of diamonds, these adjustments listed above will make it fairly easy to properly tune your mast to your mainsail for all crew weights and all wind velocities. If you have any questions about the proper bend in your mast, please don’t hesitate to call us.

(Note: The numbers above relate to the Model A gauge. Refer to the conversion chart under Loos Gauge section for new Pro Gauge.)


Position your mast butt casting so that the mast will be positioned at the minimum “J” —or 4′ 9″ measured from the forestay to the front of the mast on an aluminum mast or 4′ 8 1/2” measured from the forestay to the front of the wood mast.

Position the mast on the step so that it is even fore and aft. In other words, the mast butt does not protrude out over the front or back of the mast step. We have found that it works well to have the step positioned at minimum “J” dimension when the mast is set squarely on the step.

The mast should rest on the mast step in such a way that it has nearly 1/2” of positive prebend will develop when the rig is properly tensioned (we’ll discuss this later). Measure prebend by pulling the main halyard down all the way to the gooseneck so that the wire is up against the back of the mast. You will notice the prebend as the gap between your main halyard wire and the mast as it bends forward.

There should be a slight gap (nearly 1/8”- see picture) between the casting on the mast and the casting on the grating at the forward edge. The gap at the front will help the mast bend easier because it rocks slightly aft of the central axis of the mast. If the mast prebends more than 1″ (forward at the middle, aft at the head) shim the front of the step between the step and the grating. This is better than putting thickness (coins, stainless tangs, etc.) between the casting on the mast and the casting on the grating because it still allows the mast to remain flexible.

It is important that the mast butt casting be securely fastened to the bottom of the mast. Any slop whatsoever can greatly affect the mast’s ability to bend properly. A quick check to verify if the mast is set up properly is to push the mast forward from behind, halfway between the spinnaker pole eye and the lower diamonds. If the mast feels stiff or it’s bend restricted, most likely the mast butt is not set up well.

A telltale sign that the mast is bending correctly is that when all the crew weight is positioned on the weather rail (about 8 mph breeze), there will be evidence of overbend wrinkles developing in the mainsail running from the area of the lower diamond diagonally down towards the clew of the main. In heavy winds (over 15 to 18 mph) or in flat water, these overbend wrinkles could be quite pronounced and should actually run back to the near the aft edge of the windows. If not, check your mast butt and diamond tension as there is most likely not enough mast bend developing. On the other hand, if the overbend goes past the aft end of the window, then you have too much pre- bend and should add another shim under the front of the mast step.


Mast rake is measured by hooking a tape measure on the main halyard and hoisting it all the way to the top of the mast until the shackle just hits the sheave. The measurement point on the boat is located at the back edge of the bottom of the tiller hole (not the traveler).

For those boats still rigged with the smaller 3/32″ forestay the reading should be 5 numbers lower. Too much rig tension will tend to increase prebend. Too little tension will tend to reduce it.

(Note: Refer to conversion chart under Loos Gauge section for new Pro Gauge.)

Rigging the Sails and Sail Trim

your shrouds and forestay so that your rake measurement (with the rig properly tensioned and the correct amount of pre- bend) is: 26′ 11 1/2” to 27′ 1/2” for both Proctor’s and Fisher’s mains. The major consideration with both design mainsails is to maximize rake and still be able to get sufficient mainsheet tension for all conditions.

The rake mentioned above positions the rig for proper boat balance. On some boats, however, because the traveler or boom block arrangement is not set as low as ideal it may not be possible to develop enough mainsheet tension in heavy air. To check this, hold your boom up so that the mainsheet is block to block and measure to the 11′ 11″ point at the back of your boom. On most boats this should be just in front of your outhaul sheave. Again, with the halyard pulled to the top of the mast, the measurement to the top of your boom should be 26′ 2″ minimum. If you have difficulty achieving this number, check to see if you can arrange your traveler or mainsheet blocks.

Note: In order to allow sufficient sheeting with a long leech and rake, the mainsheet needs to be properly arranged at the end of the boom. Do not dead end the sheet on the becket on the bottom of the boom block; oftentimes this can reduce available main sheeting by nearly 6″.

Instead, run the mainsheet from the boom block through the block on the traveler from front to back and dead end it on the boom, behind the block. You can either tie the mainsheet to the same slider that the block is attached to, or preferably a whole separate slider or boom bail. You can even drill a hole in the bottom of the boom and tie a knot inside the boom.


For a 1/8″, 1×19 wire forestay (which we suggest), tension your rig to the point where your Loos tension gauge reads: 30 to 35 (260 – 360 lbs) for Proctor’s main

and 28 to 32 (240 – 300 lbs) for Fisher’s main. (Important: Measure the forestay, not the sidestays).



The halyard should be pulled all the way up to allow for correct leech and luff tension. When hoisted, the top of the sail must pass the bottom of the sheave box. You may check your main halyard lock position with the mast down to be sure the shackle is tight against the sheave when fully hoisted. Stretch in the wire will allow it to come to the proper position when the sail is pulled up.

In lighter winds, using the lower ball will help reduce the tension and allow wrinkles to develop from head to tack.

An aid in hoisting the sail to the top, it helps to let the tack out of the tack pin and leave the outhaul looser. After the halyard is locked, pull the sail down with the Cunningham to help make in the tack pin (the boltrope is attached into the luff of your main sail into the luff of your mainsail under considerable tension which will help make the Cunningham snap up when released). Some Thistlers like to feed the rope back into the groove below the sail entry slot on your mast.

To do this you must put the rope in the lower groove before putting the boom on the gooseneck. Then, with the outhaul, mainsheet, and vang eased all the way

and the mainsail tack attached, you can get the boom onto the gooseneck fitting. This will help the Cunningham work better but can do harm to the sail if not treated properly. If you do feed the rope into the groove below the entry, be sure you release your outhaul before you drop the mainsail. If the outhaul is on tight, you can easily tear the bolt rope away from the sail above the Cunningham.


The Cunningham flattens the sail and moves the draft forward as it is tensioned. When under-powered, ease the Cunningham and when over-powered pull it tighter. Generally it is better to leave the Cunningham too loose than too tight. In light winds, if your main halyard is positioned properly, there will be wrinkles all the way head to tack. Little or no Cunningham will be required. In marginal hiking conditions the Cunningham would be tensioned slightly so that the wrinkles will be limited to the lower half of the main. In heavier winds, pull the Cunningham harder so that wrinkles are just barely showing in the lower one-third of the luff of your mainsail.


The outhaul adjusts the depth in the lower part of your mainsail. As the outhaul is eased, the shelf on the bottom of the sail opens and the seam that attaches it to the sail moves away from the boom. To gauge outhaul tension, judge the distance from the seam to the side of the boom at roughly the center of the mainsail foot.

Proctor’s main upwind

In 4 to 8 knots of wind and medium chop the seam should be 4″ to 5″ from the boom. In smooth water 3″; when fully hiked 1″. When overpowered and you’re dropping the traveler to leeward, pull the outhaul as hard as possible—until the clew slug hits the stop at the end of the boom.

Fisher’s main upwind

The outhaul will be pulled tight enough so that there is no more than a 2″ gap between the side of the boom and the shelf foot seam in the middle of the foot. In breezes above 10 – 12 mph when the boat becomes overpowered the outhaul will be pulled tighter until the seam is pulled up snug against the side of the boom (maximum outhaul).

When reaching, to find the most eased position, ease off the outhaul so the shelf is open 4-5” from the side of the boom. When overpowered on a reach, with the spinnaker, leave the outhaul set as you had it upwind. For downwind sailing, also leave your outhaul in the upwind tensioned position for maximum projected area.


The mainsheet controls the powerful upper leech of your mainsail. It is the single most important adjustment. Therefore you must judge proper mainsheet trim by two factors: angle of the top two battens and feel.

In ideal pointing and boat speed conditions, the top batten will be angled considerably to windward of parallel to the boom (nearly 15 degrees). It is not unusual that the upper batten telltale will be stalled most of the time when the sail is trimmed properly when sailing upwind. In most sailing conditions the mainsheet (and/or the traveler) should be played all the time in order to gain top speed and pointing ability. A good rule is that as long as the boat feels good and is going fast, keep pulling the sheet harder! More mainsheet tension usually relates to higher speed and higher pointing but the sail becomes easier to stall. When the boat starts to feel slow, it’s correct to ease the sheet, but for things like waves, you should try to anticipate and ease the sheet before you hit them.

In heavy wind be careful not to overtrim. If the mainsheet is trimmed too tight the boat will develop a great deal of weather helm and become more critical to steer. In very light winds, where the weight of the boom hangs on the leech and hooks the top batten greatly to windward of parallel to the boom, ease the mainsheet so that the end of the boom is nearly over the corner of the transom. You may experiment with pulling the traveler to weather (but not farther than the windward edge of the tiller hole) which will help allow the boom to rise slightly easier and the upper leech to open up quicker when the velocity increases.


As mentioned above, in light winds and when you will not be tacking a great deal, some Thistlers have experimented with pulling the traveler to windward. If you have the proper rake, 6″ is the absolute maximum; 2″ to 3″ is normal. Never pull the traveler to weather where the boom is actually positioned to windward of centerline.

In moderate conditions (8 – 10 knots), and especially with the Proctor main you can keep the boat in the groove with the crew hiking with a little help from playing the traveler. The goal is that your helm should always be balanced (nearly neutral). In heavy winds (above 15 mph) the Proctor Thistle main will perform best with the traveler nearly completely to leeward and the mainsheet trimmed quite hard. Your mainsheet will control mast bend. As the sheet is eased the mast will straighten up and the main will, therefore, become fuller.

In extremely heavy winds (above 20 mph) you may experiment with positioning the traveler only slightly below centerline and easing the sheet to allow the top of the main to angle outboard more.

With the Fisher main, pulling the boom vang on quite hard (see below) helps in these breezy, puffy conditions upwind. Just be sure to ease it before you round the weather mark!


The vang is primarily used downwind to help keep the upper batten parallel to the boom (telltale flowing off the top batten). A common mistake in light to medium conditions is to over-vang and tighten the leech too much. In puffy conditions when reaching, the vang is often “dumped” to help keep the end of the boom from hitting the water and creating a disaster!

In the middle of your groove, the windward telltales should lift indicating a stall. At the “top end” of your groove (when in a high pointing mode) both telltales will lift and oftentimes the luff of your jib will actually break.


To properly set the jib lead, use the trim line that is drawn on the clew of the jib.

This pen line runs from the clew grommet directly out into the body of the sail.

When your lead is set properly, your jib sheet should be a direct extension of this line. You should find that on most Great Midwest boats the lead would be nearly at the forward edge of the thwart. In heavy winds, move your lead aft 1″ of the sheet extension to the trim line position.

Be sure, though, to ease the vang in the lulls!


On both jibs, at the lower end of your steering groove (when you are looking to accelerate) the windward and leeward telltales should both stream straight aft.

On the Fisher jib, the lead should be set laterally so that the sheet runs through the jib lead block and intersects the thwart at 15-16” from Centerline. The Proctor jib trims best at 16″ to 17″ from centerline.


For Proctor’s jib trim the jib sheet so that the leeward side of the jib at the bottom is just barely touching the rail about 2’ aft of the bow. With the foot set this way and the distance between the end of the spreader and the leech of the sail should be 1 1/2 ” to 2″ (depending on the length of the spreaders). If while the bottom of the jib is just barely touching the rail, the distance of your jib from the spreader is more than 2″ then move your jib lead slightly forward. If the leech of the jib is closer to the spreader, then move the lead aft.

For Fisher’s jib in medium winds and flat seas, we suggest trimming so that the leech of your jib is 1″ off the spreader, at the middle diamond. In very heavy winds and very light winds, ease the sheet so that the leech of the jib is nearly 2″ to 3″ off the spreader. Do not be surprised to find that the leeward side of the jib at the bottom lays inside the rail approximately 2″ to 3″ at its middle. Fisher’s jib is flatter down low at the very bottom so that the sail will lay inside the rail when trimmed properly it will just touch the rail when eased for power to accelerate.

For both jibs remember to quickly ease the sheet if the boat needs power, after tacking, before hitting a series of waves, or after a large header. Ease the jib out to nearly 3″ to 4″ off the spreader.

Note: We have found that in many cases there are wide variances in spreader lengths. Of course, this would make it very difficult to use the spreader as a guide in trimming your jib. We are assuming 9 1/2″ spreaders (positioning Fisher’s jib at 10 1/2 ” and Proctor’s jib at 11″ – 11 1/2″). Some Thistlers have put a mark that is the same distance inside the spreader tip that you want to trim outside. Some even use a straw taped to the spreader to provide a reference!


On both jibs, tension the halyard so that there are always slight horizontal wrinkles off of each snap tab. This is especially important in light to medium winds, while in heavier breezes the halyard is pulled tight enough so that the wrinkles are nearly removed. The wire in the jib luff is usually slack unless it is blowing very, very hard. In fact, the only reason to have the wire in jib is that the class specifications require it.


Sailing your North spinnaker is fairly easy as long as you follow a few basic guides. First, always fly your spinnaker so that there is a 5″ – 6″ curl in the luff. It is much better to sail your spinnaker slightly eased and slightly under trimmed so that the slot between the spinnaker and the main will not be choked. It is important that the sheet be constantly played for maximum performance. Constant and smooth 1′ trims and eases work better than rapid 3′ jerks. Work smoothly with both the sheet and the guy to try to keep the pole basically perpendicular to the wind.

Adjust your pole height with the topping lift so that the clews are roughly equal. When you can’t view the leeward clew (because it is behind the main), set the pole height so that the center seam of your spinnaker is parallel to the mast. At this point the lower luff of your spinnaker above the pole should be nearly vertical. We recommend a pole ring height of 32″ – 33″ from the butt of the mast (this is the standard Great Midwest position).

We also suggest tying your halyard to your spinnaker with a long bowline so that the head of the spinnaker will always be 4″ away from the mast. This will also help open up the slot between the spinnaker and the main.


Sail Care

Your North Sails are constructed out of the best materials on the market today. We make sure of this by testing every roll of cloth we use. Through proper care and maintenance your sails will give you the performance you have come to expect from a North Sail.

The most important factor for a long life for your sails is to watch them for signs of wear and tear in high load and chafe areas. Be sure to wash the sails off with fresh water and dry the sails thoroughly before storing. A dry, mild climate is best.

Excessive heat can cause problems with the sails due to the possibility of shrinkage. It is best to roll the mainsail and jib.

When hoisting and lowering the sail try to minimize the amount of creasing or wrinkling of the sail. Every time the sail gains a crease the cloth breaks down that much faster. Always have someone contain the leech and luff during these procedures. The battens can be left in the sail without any problems. Be sure to roll the sail down the leech so that the battens will not twist. This could cause damage to the battens.


The spinnaker is fairly straightforward. Be sure to repair all tears and pulled stitches. Folding the sail when storing is best.

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