Tuning of Nacra F18 catamaran.

There are many controls for adjustment in a modern catamaran, these noteswill detail each one and the effect that it as on performance. They must beread as a general guide, as each sailor will have their own preferences. It isimportant that you measure your catamaran and know where your settingsare, so that they can be reproduced in any given wind range.

The mast is the critical factor to achieve performance:

a. Rig tension.Rig tension, slacker in lighter winds and tighter in heavier, slack is with, say70 to 80 kgs pulling back towards the stern of the catamaran. At this tension,the leeward shroud will appear slack when going up wind. This will alsoallow the forestay to “fall away” causing loss of pointing ability. Sailmakersnow cut jibs allowing for this to happen, but don’t let it happen too much. Aswind increases then increase the rig tension, but remember with a rotatingmast, it will always want to stay “in the middle”, which means that the rig isgoing marginally tighter each time the mast turns hence wanting to return tocentral “neutral” position. Now if you are too tight in light winds, the mastwill not rotate enough, so slacker rig in light winds.Slack, as a general guide, is when you can hold the shroud and rotate yourhand through 45 degrees.If you go too tight, then the mast will not rotate freely due to excess pressureon the mast ball, and the catamaran just won’t feel right.Don’t forget, pull with the same person in the same conditions, then you canre-produce where you started.

b. Mast Rake.Take the forward trapeze and add a short piece of line, then stretch tightdown to bridle fastening to hull. Measure that distance, with trapeze wiretaught, then take trapeze wire to stern and length ought to be in the middle ofthe hatch cover for lighter winds, to the “back end” of the hatch housing forheavier and finally half way from back of hatch housing to the transom forseriously heavy wind.Moving the mast aft, decreases power and makes the catamaran easier tohandle, hence further back in heavier winds. Also the lighter the crew, thefurther back the mast and visa-versa.Also moving the mast aft, allows the center of effort on the sails to movefurther aft and so will encourage the boat to point higher. BUT this mightmean that you feel “weather helm” through the tiller, if so, “tuck” the ruddersfurther underneath the catamaran by adjusting the screw thread on the ruddercasting. When sailing upwind, with two on trapeze, you ought to be able torest the extended tiller on the top of your index finger and the catamaransshould slowly climb up into the wind.

c. Spreader rake.The third crucial factor in tuning masts. Broadly the rake is set for the weightof the crew, and the tension for the wind conditions, when sailors feel thatthey are single and double trapezing in the right conditions. With the NacraF18, the settings are further back that usual, with lighter crews going to 45 to50mm, heavier from 35 to 45mm. (Peter Vink who makes the sails suggests50 to 60 mm for lighter and 40 to 50mm for heavier!!!) Now, when thespreader rake is increased, it means that the tension of the spreader wires“bends” the mast aft, hence flattening off the sail therefore de-powering theboat. This adjustment is made on the land by turning the bottle screw at thebase of the diamond spreaders. ALWAYS know this measurement!! Then asthe wind increases you can increase a set number of turns, remember thissetting and then the catamaran will reduce power in strong winds and increasepower by reducing (slackening the spreader wires) the number of turns inlight breezes. The technology is the new Nacra F18 mast is considerable andthe mast is lighter but stiffer in the “sideways” bend, but achieves morevariation in the “backwards” bend. This stiffness sideways allows the main tostay fuller at lower levels, hence giving more upwind drive and less heelingmoment.

(The medium (middle) setting is 36 on the Loos gauge. ( about 175 kgs)When practicing your sailing, always try and get accurate wind strengthmeasurement, I know it isn’t easy but try, that will help you reconstruct thesettings again on another day.)

d. Sail batten tension.Do no more than fit the battens and tie in snugly. Do NOT try and overtighten, all you are doing in stretching sailcloth that doesn’t want to stretch!When you have put them in “snugly” just feel the tension in the batten cordswhen you have down hauled and sheeted in the main.There is wisdom in lighter breezes to increase sail shape by increasing thebatten tension, but it is the natural curve of the batten that gives most sailshape, not tension.You will find that the top two are very stiff and short, but they are also highup, and it is windier up there!! So in lighter breezes, or with heavier crews,you may want more shape at the top of the sail.

e. Cunningham (Downhaul).Another critical setting to learn and to be able to re-produce. Increasingdownhaul “flattens” the mainsail and decreases power, as the mast is “bentbackwards” the bolt rope takes up that mast curve, flattening the main, butcritically “opening” the top of the sail and so reduces power as the head ofthe sail “falls away” from the wind.With modern 8:1 downhaul systems, it is important to learn the subtleties; itisn’t a question of “full on or full off”.Remove all Cunningham tension on downwind legs when spinnaker is flying.

f. Mast rotation control.Another factor influencing on sail power. To achieve maximum power, themast rotation control yoke wants to be pointing at the shrouds, (or in lightwinds marginally infront). This will yield the perfect airflow over the mastthen on to leeward side of the mainsail, hence most power. In strongerbreezes, by moving the yoke further aft, that perfect air low will beinterrupted, leading to a reduction in power. So maximum power with yokepointing at shrouds and pointing at “red traveller stop” on back beam forreduced power. (All this is assuming that the boom is in the middle of theback beam.)

The Nacra F18 is equipped with two lines that hold mast rotation ondownwind legs when spinnaker is raised. These are invaluable on longdownwind legs especially in light breezes, BUT BUT make sure that they areremoved BEFORE gibing.

g. Out haul controls.The next important control. In normal circumstance try and get a “handspan”between the boom and the loose footed sail. ie about 10 cms. In lighter windsespecially if there is swell, increase as this will give increased drive. Whenseriously windy, reduce sail shape so that the mainsail loose foot runs parallelto the boom.The Nacra F18 is fitted with a “limiter” line to stop sailors over out haulingthe mainsail, as there is a risk that if the mainsail is out hauled severely, then8:1 down hauled, the bolt rope might be pulled out of the mast track. If it isdone in the reverse order then all is well.

h. Mainsheet tension.Sailors will notice that if they “oversheet” then the mainsail “hooks” towindward and the boat sails appallingly, over sheeting is one of the mostcommon causes of poor sail performance. Ensure that the telltales areflowing, indicating the even air flow over both sides of the mainsail. Thegeneral guide is: ease mainsheet tension and make sure that leeward tell talesare flowing smoothly, then increase tension and get the windward telltales tofly evenly!Don’t forget that you MUST keep some mainsheet tension when you fly thespinnaker. Your main sail acts as your backstay. Failure to do this could wellfinish with a damaged mast.

i. Traveller positioning.Upwind, keep traveller as close to the center of the back beam as possible, ifit gets windier then allow the traveller to move out, and at the same time keepmainsheet firm, as this will keep a more even sail shape throughout the heightof the sail.Most F18 sailing is done with traveller in the center for both upwind anddownwind spinnaker legs.

j. Dagger boards.As a “Golden rule” dagger boards are down going upwind and 50 to75% upwhen going downwind. However, they can serve as a very useful depoweringadjustment when over powered going upwind. Raising the boardswill allow the catamaran to “slip” to leeward and this will give the feeling ofless power coming from the mainsail. This will be most noticeable in gustyconditions.

k. The jib.Like the main, it is important to have the jib tuned to the winds. If strongerbreezes, then increase downhaul tension on the luff, and visa versa, so in lightwinds it is acceptable to have slight “creases” around the luff, that will meanthat the high aspect sail stay fuller. Be careful to ensure that you don’t puttight luff tension of the jib if you have slack rig tension on the boat as this willmean that when you go sailing ALL weight is held by the jib cloth!!The positioning of the jib is critical to give the “slot effect”, the jib is veryhigh aspect and not very large in its own right, but “deflects” airs over themain, and so massively increases the speed of those airs. There are threestandard settings, but try with the jib traveller car at about 38 to 40 cms. fromthe center of the mast. This will give an ideal point to start.In lighter breezes bring the jib carriage in and out when wind picks up. If it istoo tight then the air can’t flow “through” the catamaran, causing her to heeland not drive.Next comes the Jib clew position. There are three positions, use the middleone to start with, but the bottom one will tighten the foot and free the leechand the top one will close the leech and open the foot of the jib. If windy thena free (open) leech will reduce power and reduce boat heeling, as the air canflow “through” the boat more easily, whereas in light conditions, closing theleech will increase power as will increasing the sail shape in the foot bothachieved by using the lower fitting. The “slot” effect of jib and main arecrucial to boat speed, they must be working in total unison.

l. The rudders.Run the rudders parallel. Some used to think of marginal toe-in, but it iseasier to have them parallel. With the boat on the land, a simple devise thatdoes work is point the rudders direct down the boat so that they are in linewith the hulls. Then fasten in the grub screws into the flexible rubber jointsand the job is done ensuring that the tiller cross bar is equi-distant from therudder arms. ie there is equal rubber jointing showing at both sides. Rudderrake depends upon mast rake, further back the mast, the further forward therudders and visa versa.

m. The trampoline.The trampoline wants to be as sensibly tight as you can get it. There arethose sailors who get bars of wood to gain even greater tension. I am not oneof those, but don’t have it slack either. The Nacra F18 has deep bows andthat keep sailors well off the water, but water hitting the underneath oftrampoline is cold, wet and slow!! You will find it very slippery to start withbut that only lasts a few days.(Leave the cover off for the first two weeks andit will be fine then!)

n. The Front beam.Ensure that the front beam has about 5 mm of upward bend when the mast isnot on the boat. Also make sure that the stainless steel threads are fullygreased and tight. (you may have to get them undone one day!)Ensure that the eight hulls bolts are firmly tight, about 17/18 ft.lbs. Make surethat if you ever take them out, then re-grease on subsequent assembly.

o. Catamaran balance.Try and make sure that the catamaran is balanced both fore and aft and also“side to side”. You want the boat level with the water fore and aft, so keepingthe bows well down when going upwind. The Nacra F18 has very fine entrybows and points very high compared to others, this will mean that you arewell forward going upwind therefore using water line length and move weightback when reaching BUT sailing with spinnaker means that the buoyancy inbows presents little problem, and the boat can be driven very hard downwindwith low levels of concern.Catamarans have a low wetted area, for it is wind that drives and water andweight that slow!! So try and keep one hull in the water and one hull just“kissing” the surface, this is on all points of sail! At this point, you willachieve the compromise of a mast that is upright and a reduced wetted hullarea therefore achieving maximum speeds.

So record all your settings, so that you can set to your breeze conditions.Hours and hours of work have gone into getting the catamaran to this stage.She is seriously very quick. We want sailors to get the best out of her!!

Be happy on board. I have always found that “happy” catamarans are usually quite quick.

Summary of the 12 commandments:

For more power and opposite for less power:

1. Mast more upright.

2. Spreaders with less rake.

3. Diamond wires less taught.

4. Less downhaul tension.

5. More sail shape from battens.

6. Dagger boards down going upwind.

7. Mast rotation yoke pointing at shrouds or just in front.(light breezes)

8. Outhaul controls allowing shape in foot of mainsail.

9. Mainsheet tension firm but not excessive.

10. Traveller in central position.

11. “Jib slot” allowing airs to flow “through” the catamaran.

12. Catamaran balanced port/starboard and bow to stern, one hull in thewater and one just “kissing!”Have a Great Season!!!!!!!!!!!!Fair Winds!!!!!!!!!!!

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