North Sails -- MC Scow Tuning Guide
Updated Spring 1995
Superior boat speed depends on constant and consistent adjustments to your rigs and sails. The following measurements are those we have found to be the fastest settings for your new North sails. After reviewing this guide and setting up your boat according to the instructions, we are confident that you will find the superior speed that both the North Big Boy and AP MC Scow Sails have demonstrated in various conditions both upwind and down. If you have any questions about your speed or sails, please don't hesitate to give us a call. We're anxious to help and are sure glad to have you on the North Team!
Mast rake is measured by hoisting a tape measure on your main halyard to the top of the mast and locking it in position. The measurement taken to the center of the transom at deck level should be between 28' 3 1/2" - 28' 4".
We have not found an advantage to adjusting the rake for various
conditions.
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We have found the MC performs best with the rig tension just "less than taut". Basically, a good guide is that when the mast is stepped it takes very little pressure forward on the rig to allow the headstay to be easily attached at the bow.
While sailing, it is not unusual for the leeward shroud to just begin to go slack in winds as light as 2 - 3 mph.
In light winds, too tight a rig will impede the boat's ability to accelerate. In heavy winds, too tight a rig will force the rig to sag to leeward in the middle. However, too loose a rig in heavy winds will allow the mast to overbend and make it quite difficult to power the boat up in lulls and to maintain pointing ability. In addition, too loose a rig in a lot of breeze will allow the mast to bend a great deal while sailing downwind, overflattening the sail and compromising the boat's downwind speed.
We have found this type of rig tension to be appropriate for all
wind conditions.
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Most boats come from the factory with the spreaders set at the proper length and cant (the fore and aft "sweep" or tip to tip measurement). The measurements should be close to the numbers indicated on the diagram below. The longer your spreaders the more lateral support your mast will develop. Spreaders that are too long will push the mast to leeward resulting in a bow to leeward at the spreaders. This will make the mainsail appear fuller.
Spreaders that are shorter than the proper length will allow the
mast to bow to windward at the spreaders, overflattening the main.
Spreaders that are spread further apart (spreader tips farther forward when the mast is up) will result in a stiffer mast and a fuller mainsail. Spreaders that are closer tip to tip will result in a bendier mast and a flatter main sail.
The best check for the proper spreader length would be to sight up the back of the mast, up the tunnel, while sailing upwind in an 8 - 10 mph breeze. The mast should be very close to straight (perhaps leeward sag of only 1/2"). Remember that rig tension can also affect leeward bend in the mast as mentioned above.
An excellent check for the fore and aft sweep of your spreaders
is that when sailing upwind in an 8 - 10 mph breeze look for overbend
wrinkles (oftentimes called "speed wrinkles" or inversion
wrinkles) in the lower third of your mainsail. These wrinkles,
while slight in winds of less than 8 mph, will be approximately
3' - 4' long angling from the mast below the spreaders down towards
the clew of your main. In breezy conditions these wrinkles will
be quite distinct, especially when the vang is applied to help
bend the mast and flatten the sail. The wrinkles are an excellent
guide to be sure that proper mast bend has been achieved. If
these wrinkles do not appear, it is a good sign that either the
spreader tip to tip measurement is too wide, your rig is too tight,
or your mainsheet/boom vang is not tensioned enough to make the
mast bend.
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We have found that the MC performs best in winds when the boat is not overpowered with the leeward board nearly all the way down. This will position the head of the board approximately 3/4" above deck level.
In heavier winds, when the boat is overpowered (usually associated
with the need for boom vang tension) the leeward board should
be slightly higher in the trunk with the head of the board nearly
2 1/2" up above deck level. Raising the board in a breeze
helps to balance the helm and allow the boat to accelerate better
in puffs. On the other hand, if the leeward board is pulled up
too high, the boat may actually develop "lee helm" (the
boat's desire to turn away from the wind when trimmed properly)
thereby compromising the boat' s ability to point.
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The mainsheet should be tensioned so that the upper batten is set parallel to the boom. This is determined by looking up from the boom towards the upper batten and aligning the two in a lateral plane. Easing the mainsheet will allow the outboard end of the upper batten to fall away to leeward from parallel while trimming it in tighter will bring the outboard end of the batten up to windward of parallel.
In conditions where power is needed or where the most important goal is to accelerate, ease the mainsheet slightly so that the upper batten angles outboard from parallel to the boom about 10 15 degrees. This would obviously be in chop, accelerating off a starting line, out of a tack, or after sailing through a big shift when the boat needs to regain its speed.
However, it is possible to under trim the mainsheet, especially
in lighter winds, which will result in too full of a mainsail
and the boat's inability to reach top speed and top pointing capabilities.
Remember that the mainsheet has a direct effect on the bend of
the mast and therefore the fullness of the sail. More mainsheet
tension will mean more mast bend and a flatter sail which will
result in a higher top end speed and higher pointing capabilities.
If your boat seems to be lacking in pointing ability or speed,
oftentimes it is that the main is undertrimmed. On the other
hand, if the boat seems to point fine but not have the proper
forward speed or acceleration in puffs, it is most probably a
good indication that the main is overtrimmed. However, perhaps
most important of all, the mainsheet should never be cleated for
any length of time. It should be constantly adjusted to help
achieve the upper batten parallel to the boom position.
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Downwind the boom vang is set to position the upper batten nearly parallel to the boom. In light winds it is certainly easy to overtrim the vang and hook the leech of the main up top, thereby stalling the sail. In a breeze, especially when sailing downwind, too loose a vang can exaggerate the boat's tendency to want to roll over to windward. Again, the goal of setting the upper batten parallel to the boom is best. There is a tendency to "over vang" in light to medium winds and to "under vang" in heavier winds when sailing downwind.
Upwind the vang is usually not tensioned until the skipper is in a full hike and the boat is slightly overpowered. The vang is then tensioned quite hard allowing the skipper to play the mainsheet more like a traveler allowing the boom to move more laterally when eased and trimmed, rather than up and down. This amount of boom vang tension is important to maintain proper mast bend while easing the mainsheet in puffs. A quick ease on the mainsheet when the boat becomes severely overpowered will help maintain proper heel and therefore proper speed. As soon as the boat is balanced and under control the mainsheet can be brought back in. However, with the vang tensioned properly the mainsail will maintain proper shape as the sheet is eased and trimmed.
As described earlier, when the vang is tensioned and proper mast bend is achieved, overbend wrinkles will be developed in both the Big Boy and AP mainsails. On the AP mainsail, the overbend wrinkles will be noticeable but stop mainsail approximately a third of the way back on the boom. On the Big Boy, in breezy conditions the same overbend wrinkles should be quite severe and stop nearly halfway back on the boom. Since the Big Boy, is a fuller, more powerful sail, when it is flattened out in heavier winds, the overbend wrinkles will become more distinct.
NOTE: Be sure to ease your vang as you bear off around the weather
mark!
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The traveler is usually positioned in the center of the boat unless
the boat is extremely overpowered and difficult to hold down.
In this condition, easing the traveler down as much as a foot
can help maintain the balance of the boat. Rarely does it help
to ease the traveler farther than a foot off of centerline.
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In light winds with no Cunningham tension applied, there should be small horizontal wrinkles (approximately 8" - 10") from tack to head (on both the AP and Big Boy sails). If you do not have these wrinkles the draft will be positioned too far forward in your sail which will harm the boat's ability to point and accelerate in light to light-medium wind conditions. Some older North sails (pre-M4) have been slightly short on the luff and the bolt ropes have been tight enough that it has been difficult to develop these wrinkles. In some cases it may be necessary to add some type of extender at the head of your main to allow these wrinkles to develop. Proper luff tension in your main in light winds is important! If you have any questions, please don't hesitate to call on us.
Once there is enough breeze for the skipper to sit on the deck and maintain proper heel, Cunningham tension should be applied to minimize the horizontal wrinkles along the luff of your main to just the lower half. If the wind lightens up, be sure to ease the Cunningham.
In heavier winds when the boat is overpowered, the Cunningham is tensioned until nearly all the wrinkles are removed. Unless it is blowing very hard and maximum boom vang tension has been applied, it is quite easy to over-tension the Cunningham and pull the draft too far forward. Only in the heaviest winds will the Cunningham be pulled very hard where there are virtually no wrinkles along the luff of the main (except the overbend wrinkles as previously mentioned).
Again, the Cunningham adjustment, like the mainsheet adjustment,
is important to proper speed and proper sail trim. While it is
not necessary to adjust as often as the mainsheet, it is important
that the Cunningham be set so that the wrinkles guides as described
above are achieved.
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AP Mainsail - In light to medium winds (before boomvang tension is applied) the outhaul should be pulled just tight enough so that the foot of the main is very smooth. The bolt rope tape on the bottom of the sail will just begin to stand up proud from the boom, but there will not be a crease in the sail from the tack to the clew. Once any boom vang tension is applied, the outhaul should be tensioned to the point where a hard crease develops from the tack to the clew and the clew should be nearly at the black band. Downwind, ease the outhaul until the bottom of the sail is smooth, but never loose enough that there are vertical wrinkles in the foot of the main.
Big Boy - In light to medium winds, the outhaul should
be tensioned so that there is a slight crease in the foot of the
main from the tack to the clew. In heavier winds, with more outhaul
tension, the crease will be quite severe. The crease indicates
that excess fullness in the lower 3' - 4' of the main has been
removed by tensioning the outhaul. Rarely, except in light/medium
wind conditions (where the skipper is just barely sitting on the
deck) and a great deal of chop will the outhaul ever be eased
to the point where the bottom of the sail is smooth. Sailing
in normal conditions with an eased outhaul upwind will result
in a fuller than appropriate sail shape which could stall more
easily. Downwind, as with the AP, ease the outhaul until the
bottom of the sail is just smooth but without vertical wrinkles.
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1 . Maintaining proper boat heel is important for the MC. The boat should consistently be heeled upwind approximately 25 degrees where the leeward rail is just barely above the water. In light winds this may require the skipper to not only sit to leeward, but hike out quite hard to develop the proper heel. In heavy winds, excess heel will result in not only reduced speed, but lower pointing ability as the leeward board becomes less effective.
While reaching, the same heel guides apply. However, while broad reaching and running in light to medium winds, heel the boat to the point where the boom is just clearing the water.
When sailing dead downwind in medium to heavy winds, heeling the boat to weather can be quite effective. Actually, anytime the skipper can sit to weather, when sailing downwind and still heel the boat to windward, it should be attempted. Weather heel downwind helps the boat sail faster and lower. Pull the leeward board up all the way and drop the weather board only 1/4. Sit next to-the traveler and hike to windward, if necessary.
2. Oftentimes downwind, more board is left in the water than is necessary. When sailing with leeward heel, position the leeward board (the windward board should be all the way up) so that the helm balances. When the boat is sailing a straight course there should be no load, no feel on the tiller and the boat should want to travel straight ahead with no tendency to bear off or head up. Lowering the board will give the boat windward helm (the tendency to want to turn closer to the wind) and raising the board will give the boat more leeward helm (turning away from the wind). Remember that wind velocity changes may affect the proper positioning for the board for the same course angle.
3. The importance of proper and constant mainsheet trimming has already been described. Upwind in breeze, especially when sailing into a puff, it is helpful to ease the main quickly (perhaps as much as 12") to "unload" the boat's excessive windward helm that will develop and thereby allow the boat to accelerate forward. Quickly, after the boat has rebalanced and is at top speed and proper angle to the wind, trim the mainsheet in to its proper position. Mastering this technique of sailing the boat upwind in puffy conditions by constantly playing the mainsheet to keep the boat on a constant heel and a constant angle to the wind, will definitely result in top boat performance.
GOOD LUCK!
We wish you good luck and fast sailing! Don't hesitate to give us a call if you have any questions or problems. Please consider us your personal sailmaker!
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