Capricorn F-18 Tuning Manual

Capricorn F-18 Tuning Guide

1 General aspects

The Capricorn seems to have advantages in medium to heavy conditions with respect to other F18 boats. This holds
especially upwind, where the Capricorn shows sparkling performance. We therefore propose to bias the basic set-up
of the boat slightly towards lighter conditions. Thus a small portion of the heavy wind superiority is sacrificed in order
to get an advantage in light winds.

The rig of the Capricorn is derived from latest A-Class developments and is quite different from traditional F18 set-ups.
This shows especially with the adjustment of the mast rotation. In contrast to basically all other F18 boats, the mast
rotation on the Capricorn is reduced with increasing wind speed The main sail has a special cut. The panels have
already a built in twist. Thus the sail opens nicely even when sheeted relatively hard, which helps to maintain a flat and
twisting sail in strong winds. This means also that the sail must be sheeted very hard in light to medium conditions to
compensate for the built in twist and to close the upper part of the leech.

2 Boat setup

The Capricorn is a very fast high performance F18 with impeccable handling qualities. When you first sail one you are
aware that every thing happens very quickly. This can also include getting into trouble. As a result it is important that
the set up of your boat is kept clean, simple and efficient. Things that tangle or are difficult to adjust should be
eliminated. Almost any system can be made to work in light winds but if it doesn’t work in strong winds
then you would be better not having it on the boat.

2.1 Platform

It is important to have your boat as stiff as possible. Make sure that the beam bolts are tight. The beam pads are
accurately moulded to your beams. After a couple of years sailing it may be necessary to reseat the beams as it will
improve the boats stiffness. Keep the tramp laced up tight as this makes it much easier to move about on the boat.

2.2 Rudder alignment

The rudder alignment is checked by measuring the distance between the leading edges of the rudders and then
measure the distance between the trailing edge of the rudders. The rudders need to set up so that they have 0 - 2mm
of toe-in. The leading edges of the blades are closer together. Adjust the length of the tiller crossbar if necessary.

2.3 Rig tension

The rig tension is in general high to avoid forestay sacking. The limit is passed if the mast no longer rotates easily, in
this case the tension should be slightly reduced. For light winds set the side stay tension at approximately 50kg. In
strong winds set the side stay tension at approximately 100kg

2.4 Mast rake

The mast rake is measured with the forward trapeze wire. The distance to the attachment point of the forestay in the
hull is measured (trapeze has to be extended with a rope). This distance is than taken to the stern of the boat. The end
point of the point taken should be about 10cm below the upper edge of the transom. Light crews (140 kg or less) can
go all the way to the lower edge of the transom; heavy crews should sail with slightly less mast rake.

In general the mast rake on the Capricorn is quite substantial compared to other F18 boats. If the boat is sailed with
too much mast rake the boat feels a bit heavy on the rudder, and the acceleration out of the gust becomes a bit slow.

2.4 Spreader Rake, Diamond Tension.

These are the primary controls for tuning your rig to the sailing conditions and to your crew weight.

Spreader rake.

Spreader rake is the method of tuning the fore aft stiffness of the mast below the hounds. The ideal amount of
spreader rake is dependent on the fore-aft stiffness of the mast and to a lesser extent, the amount of luff curve cut into
the sail. Stiff masts require more rake to make the mast bend sufficiently, soft masts require less rake. The spreader
rake is measured by placing a straight edge or string line between the diamond wires at the spreaders and measure
the distance to the back of the mast. Values between 30 mm for heavy crews and 75 mm for light crews are usual.

As the basic angle we use 45 mm. To answer how much is sufficient rake can only be determined by sailing the boat
and knowing what to look for. If you have excellent height, but lack boat speed up wind and the boat does not want to
accelerate in the wind gusts, then you need more rake. This helps the mast bend fore and aft which allows the sail to
flatten and the leech to open in the wind gusts. If you are lacking height and "grunt" in light weather, then you have too
much spreader rake.

Diamond Tension

This primarily controls the side bends of your mast. Loose diamonds allow the middle of the mast to bend to leeward
and the top of the mast to hook to windward. This tends to cause the boat to heel very easily in wind gusts. Very tight
diamonds do the opposite. Downwind, tight diamonds keep the mast bent reducing camber and power. On our boats
the diamond tension is set at 30 - 34 on the Loose gauge.

Pre-Bend

Prebend in a mast is the result of diamond arm rake, diamond tension and mast stiffness. These days we do not really
measure prebend; as the same prebend can be achieved with varying degrees of spreader rake and diamond tension
on any given mast. This is even further confused when the mast stiffness varies.

2.5 BATTENS

Battens should be shaped to match the general curvature of the sail. The battens that I recommend and use are the
Fibrefoam battens. These are a fibreglass foam sandwich construction that is light, strong and have excellent bend
characteristics. All battens need to be looked after and stored so that they do not become permanently bent or
twisted. Battens should be tied firmly into the sail to remove creases along the batten pocket when sailing.
(See attached Batten Tying Guide). Battens 2 & 3 (counting from the top) need a lot of tension.

Batten stiffness can affect the camber and twist characteristics of a sail. They are an important aid for tuning your sail
and rig. Stiff battens hold the sail flat and help the sail to twist more easily. Soft battens allow the sail to develop more
camber and reduce leach twist.

2.5.1 General Sailing Philosophy of Catamarans

A number of years ago I heard the following quote applied to catamaran sailing. “It doesn’t matter where you are going
as long as you are going there fast.” The element of truth in this statement is that boat speed is ultimately important.
Go fast. Look for pressure then angles; opposite priority compared to dinghies.