Inside the Numbers

Henry McCray


There have been many sailors in the Fireball fleet who have found one good setting for their rigs and gone quite fast nearly all of the time. This so- called 'magic setting' is usually found by copying someone's rake, rig tension, and spreader length and angle. The problem is that once you have that setting, you are almost never tempted to change it, and miss out on major advantages as the breeze and sea states change outside of the groove you have found. Also, differences in crew sizes and styles can account for significant changes in how you would set up your rig. Using someone else's settings at the wrong time can make for serious helm imbalance, poor pointing, low boat-speed, or all three.

By learning the effect each control has and how to one reflects on another, you can begin to optimize the boat for you and your crew. Once you understand the relationships involved between the key adjustable factors in a Fireball rig, you can learn to set up your mast for the conditions you are most likely to face that day, or customize for your crew or style.

There are three major factors in properly tuning your rig. First, you must know each of the three major settings- rake, rig tension, and pre-bend. Second, you must learn how to measure each one. Lastly, you must learn how to calibrate your rig so that each setting is repeatable.

Rake

Rake is the vertical setting of your mast. It controls the balance of the sail-plan, and effects the helm, pointing, and boat-speed. A mast would be 'up-right' if it were set at dead vertical. An upright setting in any boat would produce the best pointing and boat-speed options in light air. Then, as the breeze builds you would rake the mast aft to move the center of effort of the entire sail-plan aft. This effect will ease the weather helm, and balance the boat.

Rake is measured on a Fireball by pulling a 25' tape up on the main halyard to the uppermost black band. Then, you should pull the tape taught and measure to the top edge of the transom. The Fireball will perform best with a rake between 22' and 23', depending on the breeze and sea state.

Rig Tension

Most Fireballs tension their rig by either tensioning the jib halyard or the forstay. Rig tension, along with the length of your shrouds determines the rake. The more rig you pull, the more upright the mast becomes, and the tighter the tension on both the halyard and the shrouds. Tension helps controls the amount of pre-bend in the mast, as well as the luff sag- the amount the luff of the jib is allowed to sag to leewrd from a straight line under way.

A tighter rig for a given shroud length in flat water or light air will be more upright (powerful). It will also have more pre-bend, flattening the mainsail and allowing better flow, and minimal luff sag which helps to keep the slot open and flowing. As the breeze builds to trapezeing the rig is eased some to rake aft and allow some luff sag for more power and boatspeed. As the breeze builds to heavy the rig is eased more, inducing yet more rake and making the boat easier to steer and keep on a plane. The also induces some leeward mast bend, further de-powering the rig and allowing footing to keep on a plane.

You can measure the amount of rig tension on your shrouds and forstay with a Loos gauge. To determine the proper amount, select a target rake by choosing a shroud length, usually by selecting a hole in your chainplates. Then pull on the desired amount of rig tension. Ignore the for-aft bend for now..Check to make sure there is no sideways bend. If there is, use a lower chainplate hole for the side that mast tip is bending away from. Check the rake in the manner described and record the rake for that set of chainplate holes. Then repeat the experiment for each set of chainplate holes, moving down one incrament on each side at a time. Each time shoot for the same amount of rig tension by using the loose gauge. As you use lower chainplate holes the rake will decrease.

Pre-Bend

Pre-bend is the amount that your rig will bend fore and aft if the strut is neutral (loose up and down) and the rig tension pulled on to the desired rake and tension combination. It is determined by the length and angle of the spreaders. For any given rake and tension combination, the effects of moving the spreaders will always be the same. Raking the spreaders aft will induce more pre-bend, moving them forward will cause less bend. Lengthening the spreaders will always cause more bend, and shortening them will always cause less. In addition to the effects on bend, length also controls the amount of lateral (sideways) bend. Shorter spreaders will allow more lateral bend, longer ones will limit the lateral bend.

Knowing that more pre-bend will cause a flatter main and that more lateral bend will depower the boat you have your first key relationship to you and your crew. Lighter crews would want to depower earlier and therefore use more pre-bend and shorter spreaders. Heavier crews would use less pre-bend for a fuller main and longer spreaders for more power (less lateral bend.) Also, crews that have slow or less aggressive boathandling can benefit from shorter spreaders- it produces a more forgiving setting that will help minimize the effects of sailhandling or boathandling errors. In a real world comparison between two identical boats and crews in medium conditions, longer spreaders and less pre-bend will allow higher pointing while the opposite would allow easier handling and better speed. Practically, the differences would equal each other out.

Pre-bend is measured by tensioning a piece of string from the top black band to the lower black band and measuring the distance from the string to the back of the mast at the spreaders. To get an accurate read make sure to tie off both ends with a long enough tail to sinch the line right each black band. The spreader length is measured from the center of the shroud at the tip to the side wall of the mast, along the back edge of the spreader.

Repeating Your Numbers.

Being able to repeat your setting is at the heart of boatspeed. If you were really fast one day in a particular condition there is no way to repeat that setting unless several systems are calibrated. You can do this by putting marks, or better yet 'trim strips' (adhesive calibrated scales) at your rig tension. Also, record the shroud pin for each rake. Lastly, know how far to move your spreaders forward or backward to accommodate for a rake change.

What is the 'Magic Setting?'

As a rule of thumb, the base setting for a Fireball is a 22'6" of rake and 2" of prebend, with 400lbs of rig tension measured on the forstay. Crews near 320 pounds use spreaders 410mm long. Heavy crews go as long as 430mm, and light crews go as short at 390mm. With this one setting a full range of adjustment can be made with the rig tension alone. Have marks for 350, 400, and 425 pounds on your rig tension tackle.

In light air, the rig tension is increased to 425lbs which rakes the mast upright and induces more bend. This will flatten the main, and deepen the jib. The rig tension stays on until the crew is consistantly sitting on the windward bildge. Then, the rig tension is eased back to the 'nominal' mark and you return to 22'6" and 2" prebend.

In medium air, from about 8-15 knots you sail with this one setting. As the breeze builds towards the top of the range ease about " of strut, and rake you centerboard aft 10%. This will ease the windward helm and allow a little more bend to flatten and depower the mainsail

Once the breeze build consistantly above 15 knots you will need to depower further. Ease the strut another " to allow more bend. Then, ease the rig tension to 350. This will rake the mast aft, allow more lateral bend, and ease the helm even more. After you ease you will need to pull on HEAPS of boom vang, as well as cunningham. The main should be trimmed at of just off the leeward quarter, and the jib should be as tight as possible without backwinding the mainsail.

As you can see, the tendancies for even the magic setting follow the basic rules of rig tuning. More rig tension in light air with the same shroud length will cause the rig to rake upright, more pre-bend to flatten the sail, etc. Also, easing the rig tension in heavy air induces more rake (aft) and allow lateral bend- this is also a go fast secret for heavy breeze sailing. So how do you choose your magic number?

There are two factors which should lay most heavily on your descision. First, the amount of weight of the skipper and crew. Successful Fireball crew weights at the last several Worlds have ranged between 280 and 380 pounds. Fireballs can be handled by crews as light as 220 and can provide excitement for crews as heavy as 420 pounds. With this wide range of crew weights it is necessary to adjust your magic setting to your crew weights.

The crew weights effect the way the boat goes through the water, when it planes, and how stable it is in the big breeze. It is all a tradeoff. A heavy crew of 375 pounds may be dominant in the breeze, but have trouble in the lighter air. A light weight crew of 275 might have awsome boatspeed in medium air, but fall off the pace in heavy air. In order to combat this, we must play our magic setting as described as above.

While a median crew at 320 pounds might use the 22'6" median setting with 2" prebend and 400lbs on the rig, a heavier crew might have a slightly higher base setting, and a lighter crew might be slightly lower. Worlds competitive "one setting boats" set as high as 22'8" and as low as 22'4".

So now you know basically where to set your rig for your base setting. This is farther than many crews going in to tuning their boats. But now, you also know how to use that setting to improve performance as the breeze builds and drops. This will give you a major advantage in your local fleets, without having to spend countless hours tuning your rig!

Here is a copy of my "Baseline" settings from the 1997 and 1998 season.
controlLightLight TrapMed. TrapLow Trap Heavy
Rake 22'10" 22'8" 22'6" 22'4" 22'2"
Rig Tension 450 400 400 350 300
Pre-bend 2.5" 2.00" 2.25" 2.75" 3.0"
Spreader Length 415mm 415mm 415mm 395mm 395mm
Strut Up on in drift off on reaches on reacheslocked
Strut Downoff off on ease " ease 1"
Cunningham snug off snug tight v. tight
Jib Luff snug snug tight v. tight v. tight
Outhaul tight snug tight v. tight tight
Vang off off snug tight v. tight
Jib leads out 7" 9" 11" 13" 15"
Jib leads up 6" 5" 4" 5" 5"
Centerboard 10 forwardvertical10 aft 25 aft 40 aft
I would modify each control according to sea state and altitude.

My magic setting was generally 22'8", but at midwinters I used the 22'6" setting almost all the way throughout the regtta. This years North American circuit saw no real breezy days, and I only used my two lowest settings in Australia in January. At the US Nationals we set up for two of the three days at 22'10" because of the altitude, extremely flat water, and little breeze. At CORK last summer we used only the 22'6" setting and played the strut quite a bit to account for the changing conditions.

Henry McCray
North Sails South Carolina