A Fast Offshore Cruiser - 13. Rig design.

On a sailing yacht with a very small number of crew the rig has to be designed carefully so that one member of the crew will be able to do all normal jobs required to sail the boat. The requirement that this boat is to have a good turn of performance should not decrease the ease of which the rig can be controlled.

There are many possible choices in the configuration of the rig. Some can be ignored because of the differing styles between rig and hull, e.g. gaff etc. Also some are not going to be considered on the grounds of over complication for a yacht of this size, e.g. ketches etc. So the choice of rig has been reduced to the different types of single masted modern rigs.

Aerorig (9).

This rig has a free standing carbon mast with the jib foot on the end of a fixed boom extended forward past the mast. This is a very good rig, it allows very clean decks and gybing is much less stressful. Its downside is that all control lines have to be at the mast (mast rotates), the rig has no ability to be tuned or trimmed and the use of spinnakers will need extra rigging to stop extra bending.

Bermudan sloop.

This rig is the norm for modern boats but it is still being changed and refined slowly with new designs. There are many choices within this rig alone, especially the distribution of the sail area between the two working sails and the rig construction.

Bermudan cutter.

This rig differs from above in that it has twin smaller foresails. This rig is quite commonly chosen for offshore cruisers because of the reduced sheet loads and stronger rigs.

Rig choice.

The chosen rig configuration is the bermudan sloop. The ‘Aerorig’ (9) was not chosen simply on the fact that there is very little design work needed to fit one as they can be bought as a complete unit, off of the shelf almost. The cutter was not chosen because although it is the traditional cruising rig it does require extra work during tacking, double jib sheets etc.

Distribution of sail area.

The choice here is weather to have a large or small foresail and the corresponding mainsail. A large genoa (LP=135-150%) will provide plenty of power, more stability in the sail to power into waves but is hard to tack, produces high sheet loads and is difficult to handle. A small jib (LP» 100%) has the option of becoming self-tacking, therefore no work during tacking and minimal sheet loads, clear decks. It is obvious that with a small it has to be a small jib distribution. This instantly brings up the question about the handling of a large mainsail; this will be answered later on in this section.

Jib.

As it has already been decided that the jib will be small it seems sensible that it should become self-tacking. This will make tacking, gybing the sails an almost non-existent operation, allowing you to operate the water ballast system? There are various methods of making the jib self-tacking, the normal curved track and sheet up the mast and the latest one being a solid boom for the jib that rotates about an axis parallel to the forestay, keeping leech tight. This boom can be seen on the ‘YW H42’ (8) and the ‘Island Packet 380’ (10). This boom has the advantage that it will keep the leech tight as the sail is let out, bearing away, and the power will still be delivered, were as before the top would twist off losing much of the power. This is a good looking system but it does require a lot of space and weight right forward including the boom, bearings and the extra reinforcement required in the deck etc. This system would have been used is the spinnaker system was not also being developed, see later in section, for easy use. The chosen system of curved track on the deck does have a problem when it is reefed, the sheet angle is far to far back so a second track will be fitted forward of the first, in the position for about 70% area.

Mainsail.

The mainsail has to be large; this will cause handling difficulties, such as reefing, that can be overcome by the use of modern systems. These include single line reefing and lazy jacks, plus a catch bag. To make this system even easier the sail will be fully battened, minimising flogging of the sail. Now the sail is fully battened the size of the roach can increase; this will allow greater area with a lower heeling arm, this will also produce a more efficient sail because it is nearer to an ellipse plan form. These all add up to an easier handled sail that operates more effectively.

Spinnaker.

Spinnakers and cruising chutes are sometimes ignored on cruising boats because of the difficulty and high demands on crews to use them well. This is because the all three edges are unsupported and only the head has a truly fixed position. This all makes the sail very unstable, requiring quick and constant attention, not what you really want to be doing on your own. To make the sail steadier there needs to be more control over more edges or corners of the sail. This can be achieved by moving the spinnaker towards becoming a baggy jib or genoa, having the foot fixed to a point on a bowsprit or deck will control the luff. The sail could be hoisted and dropped in a snuffer or is a tight luff is used on a roller. Using a bowsprit moves the foot forward allowing the sail to work more effectively at deeper angles to the wind.

The lead is based on the method outlined in ‘Principles of Yacht Design’ (2). The CLR of the hull and keel is taken at the 25% chord line of the keel, extend up to the DWL, and 45% of T down from the DWL. The sail area and the centre of area are based upon the base triangle of the main and the jib.

It is recommended that for a fractional rigged boat a lead of 3-7% of the Lwl, on a masthead rig boat it is 5-9% of the Lwl.

These numbers are for the traditional style of hull with distinctly more overhangs, both fore and aft, giving a smaller Lwl. The example 40 footer has Lwl that is only 83% of the Loa; "7seas" has a Lwl of 96% of the Loa. Using these factors as a guide the lead should be readjusted to about 2.5% to 6% for a fractional rigged boat and 4% to 7.5% for a masthead rig.

The need for a different lead for a fractional or a masthead-rigged boat is that on a masthead rig the foresail e.g. genoa will make up a larger proportion of the total sail area. The centre of effort of the sail will be further aft from the centre of the geometric area of the base triangles; the genoa will have a greater overlap across the mainsail.

The chosen lead is close to the largest recommended. This will counteract the effect of the deep roach on the mainsail. The roach has the effect of bringing the centre further aft.

The construction of the rig will be based upon the ‘Nordic Boat Standard’ (11), NBS, outlined in ‘Principles of Yacht Design’ (2) along with principles outlined in 2nd year notes. If, in the text, there is no direct link to the NBS it can be assumed that it has been satisfied and the author believed the method shown has more relevance.

Rigging arrangement.

The general rigging arrangement is a fractional sloop with twin sets of swept spreaders. The transverse rigging will be arranged discontinuously. This method increases the number of individual lengths of rigging and fittings but does allow the upper lengths to be reduced to a diameter that is more corresponding to the load they are actually carrying, rather than the same load that the lowest vertical is carrying. This will reduce windage and weight aloft. The fore and aft rigging is limited to a forestay and a backstay. Only a single forestay is possible with a self-tacking jib. The swept spreaders and stays longitudinally support the lower panels of the mast from deflection from the main sail. A set of forward swept diagonals is rigged up to the lower diagonals to resist aft deflection of the mast.

Rigging wire.

The standing rigging will be exclusively of ‘Dyform’ (12) 1*19 wire. This form of rigging allows reduced diameters and weight for the same strength as conventional rigging. As it is still made of strands it should be safer than rod in that you will still be able to see if there is any wear or breakage’s, hopefully before the rig falls down. ‘Dyform’ (12) can also be coiled, which rod can not.

Mast.

The mast is here being made of aluminium, an off the shelf section from ‘Selden’(13). This will reduce the cost but does not have the weight advantage of a carbon mast that would lower the CoG height. A carbon mast has not been included in the design because of the lack of data on carbon spars sections but it would be very easy to change with little design work required with this data available.

Spreaders.

The spreaders are distributed more evenly between the deck and hounds than on boats with narrow shroud bases. There is no necessity for close sheeting genoas around the upper spreaders. This will produce a more stabile mast with less unsupported panel length.

Boom.

The boom is a ‘Selden’ (13) boom section. The boom is similar to the mast in being aluminium when it should really be carbon and the same reasons for none inclusion are repeated.

The construction deign of sailing rig can be very complicated and time consuming, so, in the authors current situation some simplifications have been used to save time.

These assumptions are: -

The mainsail acts as a UDL along the length of the mast.

Heeling acts at vertical centre of this.

Jib acts as point load at top of forestay.

Shrouds and spreaders act through similar points.

Spreaders have the same sweep angle.

Ignore pre-tension in rigging.

Forces act perpendicular to centre line of hull.

Lower diagonals take equal forces.

A rig of a cruising yacht will come under a variety of loads from a variety of sail set-ups during its working life. So to find the maximum loads that will be applied to the rigging the worst case situation for each item of the rigging should be found.

Based upon the approach of taken in ‘Principles of yacht design’ (2) 3 situations have been found. The first two situations are from the mentioned source and the author created the third to include the possible arrangement that could arise on this particular rig.

Load cases.

Case 1. Jib only. Modelled as a point load at top of forestay, the worse case for the cap shrouds as the entire load, heeling the boat is transferred through them.

Case 2. Deeply reefed mainsail only. Modelled as a UDL along lower panel of mast. Entire load is being transfer through the lower shrouds.

Case 3. Storm jib only on inner forestay. Point load acting at upper spreaders.

The heeling force that is needed in each case is calculated from the maximum righting moment at 30° of heel at full load with full water ballast effect. This value is dived by the heeling arm of the particular case in question and the heeling force is found.

The forces in the individual stays are found by force resolution, including the effect of having swept spreaders.

Longitudinal stays.

The assumptions used to calculate the shroud dimensions do not include any pre-tension in them, which with swept spreaders will increase the load on the forestay. So to find the breaking load forestay the NBS will be used. The NBS method shall also be used for the backstay and temporary inner forestay.