By Steve Thompson
Rigging and Tuning a Sportsboat by Steve Thompson
Rig tuning tips from one of the world's foremost designers
The Modern sports boats achieve a high sail area to displacement/righting moment and must be able to meet the requirement of carrying this sail area from zero to 30 knots
The design and setting up of the rig is an essential part of being able to effectively carry this sail area.
In a sport boat there are three main requirements of the rig.
1. The first is that the rig is strong enough to withstand the loads
of the different upwind and downwind configurations.
2. Secondly we need flexibility to enable the boat to exhaust the
wind velocity not required for forward drive.
3. And thirdly a straight forestay for jib shape.
It is important that when designing a rig for a sport boat that it is set up so that all of these requirement happen automatically without the need for a crewman to tweak the mast as we sail. In larger boats the use of topmast and running backstays help with this process. These systems are not practical in a sport boat mainly because of the reaction time involved, basically by the time a crewman has adjusted the mast bent mechanically its too late, you are already in trouble.
How do we achieve all of this?
Carbon fiber in the construction of masts has helped a lot. Most people when talking about carbon fiber in masts assume that the biggest speed gain is from the reduced weight. While weight is a big factor when the total rig with mast, spreaders, and rigging is taken into account a weight saving of 25% can be achieved. While this may be significant the added stiffness gained over aluminium for a similar weight is a real bonus. Carbon fiber has allowed the designer to vary the stiffness of a mast in each panel so that the bend characteristics required are achieved. This allows more mainsail roach to be carried without the use of a topmast backstay. The mast tip above the forestay can be a little higher and more flexible to bend off and open the leach of the mainsail to exhausts unwanted air.
The most common setup for the rig in a sport boat is a one or two spreader configuration at around 25-30 degrees angle with another spreader or jumper at the hounds to take the masthead spinnaker loads. Often the topmast backstay is not used, however this does load up the mast considerably when sailing downwind in a fresh breeze and requires the support of a very strong boat to counter the added compression from the mast.
The Thompson 7 rig is common for a sport boat with two sets of spreaders at around 25-30 degrees angle with another spreader or jumper at the hounds to take the masthead spinnaker loads.
The first consideration when setting up the mast in a boat is rake. The amount of rake depends a lot on the individual boat but in general a 7 meter boat will have a rake of around 600mm at the mast tip above the deck. Often the will be reduced to around 400mm in heavy air to help reduce the helm load. I have found that it is best to use two rig setups, one for light to medium air and one for medium to heavy air, this way it is a lot harder to be caught out with the wrong setup for the wind you are sailing in. It is essential that once you have achieved the right setup for the conditions to record the measurements. I find that simply measuring the settings of the rigging screws or their adjusted length is good enough to achieve the same settings each time. As you find a new setting record them to allow you to progressively tune your boat.
In general the mast for light to medium air can be set up with less stay tension than for heavy air. The mast itself will have more rake and less pre-bend. The pre-bend is very dependant on the mainsail shape but generally in the lighter to medium air a fuller mainsail is required and therefore less pre-bent in the initial setup. The jumpers, if aft facing need to be tensioned so that the jumper stays are tight when the masthead spinnaker is flown and the topmast straight, and loose when going upwind so that the topmast can fall off in the wind puffs. This is a delicate adjustment and care should be taken so that the jumper stays remain in support of the topmast when the masthead spinnakers are flown.
The heavy air setup requires a tight forestay and a mast that can bend below the hounds in order that the jib will have its center of effort forward and the mainsail will be flattened. Just taking on the forestay from the light air setup will largely achieve this with the added load going into the cap stays and compression adding more mast pre-bend. The use of additional mainsheet tension in the heavier air helps keep the forestay tight. Rig tension cannot be achieved if the boat is soft so care should be taken to keep an eye on the loads used.
The most important thing when setting up a rig is that the mast and sails fit each other. I have always found that it is important to first get the mast settled in the boat before blaming the sailmaker for cutting the sails wrong. It does take some time for the mast to settle down and to achieve the desired setup on a new boat. If the boat is a class boat it’s a simple matter of initially copying the setup from one of the more successful boat in that class.
I have always found Sailmakers are happy to listen and make adjustments to sails after the rig has been tuned, a few photos of the sails while sailing are always helpful. After a while you will find the best setup for your particular boat and the way you like to sail it, measure and record everything you can so that it is easy to achieve that same desired rig setup again.
Steve runs Thompson Performance Design and he has many sport yachts sailing around the globe.
His contacts are: PO Box 34-540, Birkenhead, Auckland, NZ
Phone (09) 419 6032.