Rig Setup for Boat Balance

The J-24 has an inherent problem of leeward helm. Leeward helm is evident when the center of effort above the waterline is too far forward in the boat, meaning the boat is not well balanced.

The problem of leeward helm can be detrimental to boat speed and pointing ability, especially in light air. Leeward helm will make it difficult to steer the boat to windward where a balanced

Helm or a slight amount of weather helm will help the helmsman keep the boat in the grove,

While sailing very flat. The flatter you can sail a J-24 to windward while staying in the groove,

the faster the boat will go! By shortening the mast to the class minimum; and lengthening

the headstay and "J" measurement to maximum, you have moved the sail plan as far back as

the class rules allow. Raking the mast aft will move the center of effort aft in the boat, which will induce more weather helm in the boat.

If possible, we recommend having a certified J-24 class measurer mark your minimum mast

length, maximum headstay length and maximum J measurement. To cut your spar as close to

class minimum as possible, you will need a class jig. There seems to be some discrepancy

from one boat to another to exactly where the stem/sheer line point is at the bow. This point

can make a big difference in your headstay length and J dimension. It is crucial to your upwind,

light air performance to have the mast length as close to minimum and the "J" measurement

and headstay length as close to maximum as possible.

The mast should be cut to within 3mm of the minimum class allowance (see class rules

3.52, 3.53). It is important to note that most J-24 spars come long from the factory;

therefore they must be checked to class rule measurements and shortened.

After cutting the spar, the shrouds and backstay may be too long to tension the rig

correctly. This must be checked. You may have to shorten the shrouds and backstay as

well.

The headstay must be cut or lengthened to the maximum class allowance (8670, see

class rules 3.52, 3.53). This measurement is taken from the fixing point on the mast

bracket to the intersection of the stem-line and the sheer-line.

J Dimension

The mast should be blocked at the deck so the J Dimension is the maximum class

allowed (2925mm). The J measurement is taken from the forward part of the mast at the

lower edge of the band to the stem/sheer line intersection at the bow (see class rule 3.5,

2.b)

Factory mast chocks generally need to be altered to acquire the maximum "J"

measurement.

Spreaders must be cut to the minimum length of 760 mm (see class rule 3.5, 3.F).

Spreader rake should be set between 155 mm and 160 mm (see class rule 3.5,3.F).

Note: There are spreader through-bar kits sold today which determine the spreader rake.

They are generally close to the 155mm-160mm mark, but many still need to be altered to

acquire the proper rake.

Spreader rake is directly related to the fore and aft stiffness of the mast and indirectly to

your headstay tension. The more spreader rake you have, the easier the spar will bend

for a given amount of backstay and the harder it will be to tension your headstay with that

same given amount of backstay. The opposite holds true with a smaller spreader rake.

A smaller spreader rake will give you a stiffer spar, which will make it harder to bend the

spar with a given amount of backstay and easier to tension the headstay with that same

given amount of backstay.

Mast butt placement should be positioned for 1’’ pre-bend at the base shroud setting.

Tighten shrouds to base setting at (21) on the upper shrouds and (20) on the lower

shrouds. Have one person climb a ladder facing the front of the spar with a tape

measure and a second person hold the ladder in place while holding the main halyard

firm against the mast and as close to the gooseneck as possible. Have the person on

the ladder measure the maximum pre-bend, which will be approximately one foot

below the spreaders. The measurement is taken from the back edge of the spar to the

nearest part of the main halyard. If your pre-bend is not 1", you should move your

mast butt accordingly. Move the butt forward to reduce pre-bend and aft to increase.

An adjustable mast step can make this process quick and easy.

Note: Be sure the backstay is loose and there is no tension on the spinnaker and

genoa halyard or you may get a false reading.

The upwind sails are designed to interact with each other throughout the wind range by

changing the mast bend and headstay sag offsets to accommodate different conditions.

With correct spreader rake, pre-bend, and following the upper and lower tensions from

the tuning chart, you can be sure that your mast bend and headstay sag will match the

luff curves that are designed into your Ullman upwind sails. The result will give you

maximum performance throughout the entire wind range.

First, measure back from the bow approximately 10’6" on each side of the boat, and mark

the toe rail with a permanent marker. Then attach a tape measure to jib halyard and

raise just a few feet. Measure to the mark on the toe rail on each side and adjust the

uppers so that measurement is the same side to side.

Once the spar is centered at the hounds, tighten each upper shroud to (20) on a Model B

Loose Gauge with no tension on the lowers. Next, sight the spar making sure that the

mast is relatively straight side-to-side. If the spar hooks to one side you will have to

customize your mast chocks, allowing the spar to move closer to the partners on the side

that the mast is hooking to. Adjust the mast at the partners left-to-right until the spar is

straight, sighting up the mast groove. Chock permanently in that position.

It is important to note that the side-to-side spar placement at the partners is critical to

ensure that you can sail with even lower tension, keeping the spar in column side-to-

side. Having to tension one lower more than the other to keep the spar in column will

cause you to sail with different fore and aft mast bend and headstay sag from tack to

tack. This makes it impossible to acquire consistent speed and trim setting from tack to

tack.

Tension on the upper shrouds is mainly critical to get the correct upwind shape of the genoa

and jib. There is also some affect on the mainsail. For maximum speed, it is important to

aggressively adjust the tension on the upper and lower shrouds depending on wind and sea

conditions. Remember, you can only adjust shrouds before the preparatory signal.

The lower shrouds control the side-to-side sag or bend of the mast, and to a smaller degree,

how much the mast can bend forward in the middle. Because of this, the lowers have a lot of

control over the shape of the mainsail and it is very important to be sure they are adjusted

correctly.

You will have to determine how many turns it will take to get from one setting to another. This

depends on the types of turnbuckles you have. We recommend making your own chart

counting turns from the lowest setting to the highest. This will allow you to change setting

between races easily.

When the wind velocity is 10 knots and above, your leeward uppers and lowers should be just

taught. If they are really dangling or seem very snug, your shroud tension will be incorrect.

Above 15 knots, keep the sheave block and tackle just bottomed out against the connector plate

when the backstay adjuster line is slack.

Below 15 knots, keep the sheave block and bridle 6 to 8 inches below the connector plate when

the backstay adjuster line is slack.

Backstay: As your shroud tensions are increased or decreased, your backstay turnbuckles

should be adjusted accordingly.

Understanding how boat balance and rig tune affect the weather helm and sail shape

throughout the wind range is an important aspect of getting the most performance out of your

boat and sails. Proper tuning can make your racing much more rewarding and enjoyable. If

you have any questions about this tuning guide or your Ullman Sails, please contact your local

Ullman Sails loft.