Fireball Mast Tuning

David Landsberg, Spring 1997
If you have recently acquired a Fireball or mast, and are starting from scratch, the following mast tuning guide will be helpful for getting the mast set-up. Before starting your work, be sure to view some competitive boats and decide on how you want to configure your shroud adjuster and jib halyard rigging systems. You should ideally have a 12:1 purchase system on your jib halyard.

Basic Setup

The mast rake is measured from the tip of the mast to the top of the aft transom, amidships. Rake is one of several tuning variables critical to boatspeed. Rig tension is another. Rake is measured by attaching a measuring tape (25 ft, minimum length) to the main halyard, and hoisting it as far as it can go. Do not cleat the main halyard in the normal spot - pull until the shackle is jammed against the top mast sheave, then tie it off or cleat it anywhere. Pull the tape tight to the transom spot referenced previously, and read the rake. Rig tension can only be determined by using a tension gauge. It is essential to buy or borrow one in order to set up the mast properly.

Rig tension should be 350 lbs (as measured on the shroud) for older boats that do not have boosted stiffness between the chainplate area and the centerboard case, or just generally decrepid hulls. If the boat is new or stiffened in this area, select a rig tension closer to 500 - 550 lbs (as measured on the shroud). Once you decide your tension, stick with it all the time. Do not worry is your hull restricts you to 350 lb. Many older boats carrying this amount of tension go very fast.

The basic objective is to adjust the length of the jib halyard (or forestay) so that you can easily set it to achieve the basic rake setting: 22 ft., 8 in. at your chosen rig tension. It is essential that this setting is achieved with the shrouds pinned at one or two holes down from the top-most set of holes on the chainplate adjusters (or shroud levers, or whatever system your boat has. When you have accomplished this, set-up reference marks (tape, magic marker, wax pencil) on the mast aside some distinguishable feature of the jib halyard that correspond to the following settings:

Note that the mast reference mark, and corresponding rake as a pair correspond together with a chainplate hole for shroud attachment, for a given rig tension. Before (or during) sailing, you now simply adjust rake by deciding which value you want, placing the shrouds at the corresponding chainplate holes, and pulling on rig tension (via jib halyard) to the appropriate reference mark. Choose rake as follows:

If you go racing in over 25 knots, bottom out your rake as far as you can take it!

This is of course a rough guide (probably good for light crews) the best values for your combined crew weight must be determined through experimentation - the best part!


Before you get out your drill and finalize your hardware for jib the halyard arrangement, a note on spreaders is required. These adjust the mainsail fullness in the upper half of the main. The spreaders should be the right length for your crew weight and swept aft a somewhat. Note that the spreader bracket should be the type that blocks the spreaders from pivoting forward. If this is not the case, modify your spreader bracket so it has adjustable stops. Better yet, throw it away and get a modern set of spreaders with matching bracket.

Spreader length affects the sideways stiffness of the mast. The longer they are (i.e. stiffer mast arrangement), the less the mast will automatically de-power your rig in the stronger winds by bending off. Shorter spreader length will make the mast less stiff, and therefore it will readily bend in a blow - de-powering the mainsail. It stands to reason then, that heavier crews will benefit from longer spreader length while the lightweight teams would be better off keeping the spreader length a little shorter. A good starting point for average weight crews (~320 lbs combined) is 395 mm from the spreader pin at the mast to the shroud eyelet at the tip.

Spreader angle affects the amount of pre-bend, or fore/aft bend of the mast. The pre-bend is measured by applying your chosen standard rig tension at base rake (22 ft., 8 in.), releasing the mast ram so it is free to float in it's track, and drawing the main halyard tight against the aft face of the mast at the step. Looking up along the mainsail track on the aft face of the mast, it will be possible to view the fore/aft curvature of the mast with respect to the tightly drawn, (therefore) linear, main halyard. The gap width between the back of the mast and the halyard at the widest point is the definition of pre-bend. Ideally, your sailmaker will specify the required pre-bend for his artfully crafted sail. For getting started, any of the good sails available will tend to work well with about 2 in. (50 mm) of pre-bend. Increase spreader angle (i.e. sweep spreaders more aft) to increase pre-bend. Note that increasing spreader length acts to increase pre-bend as does rig tension - so the higher your rig tension, the less spreader angle you will require.

To summarize spreader tuning, consider the example of team A - the heavyweights, in comparison to team B who are very light. Let us suppose they have the same mast, but team A have a new stiff hull and are carrying 600 lbs of rig tension. Team B on the other hand have a soggy, flower-pot of a hull and can safely carry only 350 lbs of tension without destroying their boat. Team A love to blast around in 20+ knots of wind and find the screaming spinnaker reaches are more intense with long spreaders - they have settled on a spreader length of 415 mm. Team B on the other hand want to somehow cope with any breeze over 18 knots, and they find the boat easier to keep flat with the top section of the mast bending off in the puffs - this works for them with a spreader length of 385 mm. Both boats have been set up properly with 50 mm of mast pre-bend at base rake and ideal tension (600 lbs for team A and 350 lbs for team B). Observing these boats side by side, team A's boat would appear to have very little spreader angle; by contrast, team B's boat would appear to have radically swept-back spreaders.

To ensure you spreader setup is symmetrical try the following trick. Dial on some rig tension (any amount over 300 lb), and tip the boat on its side. At the spreaders, tie a string tightly from shroud to shroud. Viewing from the tip of the mast, visually align the reference string to some feature of your boat that is parallel to the beam (i.e. the thwart). Fine tune your spreader set-up until this string is parallel to this feature, i.e. perfectly perpendicular to the centerboard - and all your variables are achieved.

Mast Ram

Basically, the ram should be thought of as a mainsail lower half fullness control. Generally fullness equates to power. If the car is mounted on the mast, then lowering the car will push aft on the mast, removing pre-bend. Straightening the mast in this way pushes the sail in on itself making it fuller. Raising the car up flattens the sail.

In really light air, the fullness of the main presents a challenge to the low energy air (wind) trying to flow across the sail. In this case you need a flat main just to keep the boat moving, so raise the car and add some pre-bend.

In light air, when the crew is required to sit on the tank to keep the boat flat, the ram should be neutral (pushing neither up or down). Induce some fullness by dropping the car a bit only to develop power for punching through chop, otherwise use no ram.

In medium air (approximately 8-10 knots), the objective is develop all the power available from the rig. Drop the car sufficiently to straighten the mast for power.

When the wind has built to the extent that you are over powered and cannot keep the boat flat, raise the car to flatten the main and reduce power. Do not release the car altogether however, as in this weather one tends to apply a lot of vang. Without the car pinned in an upward position the mast would bend freely and possibly break.