RIG SELECTION GUIDE FOR MULTIHULLS

This is not an easy one to pull apart. Not so much because it's complicated but more because the advantages of one rig type over another are difficult to quantify and whichever way you go there will be trade-offs.

It involves weight saving, efficiency of flow over the sails, how much control you have over the shape of your main, your budget, your budget, your budget, and a whole lot of personal opinion that is not always easy to back up with science.

Let's look at some of the basics and see if we can provide some guidelines for choosing between carbon or alloy, rotating or non rotating, wing section or conventional wing profile.

This article is not going to tell you which rig is right for you - but it might help to point out some of the options and what the compromises are.


Rotating or Non Rotating

In theory a rotating wing section lines up the mast profile with the curvature of the luff on the lee side of the main, and by all logic of fluid dynamics this should increase lift. It almost certainly does on high performance multihulls. On cruising boats; that is boats with a lower power to weight ratio there may be a performance benefit as well but in my opinion it has often been overstated. The fact that the OMR governing committee recently revised downward the handicap factor on rotating wing masts would seem to confirm this; however it is a difficult thing to quantify where a boat's performance is influenced by so many other factors.

So in making the decision whether to rotate or not we need to look at the other factors in effect.

• Because the mast rotates the halyards need to be cleated on the mast. If they are lead to the deck they need to pass through the foot of the mast rather than exit out the side as this will inhibit rotation. Another option that is sometime employed on trailer boats is to exit the halyard out the side of the mast, tension it using a deck winch and then cleat it off on the mast rather than on the deck. However this makes it complicated to adjust tension while sailing and requires additional rope.

 

 

• With a rotating rig the spreader rotates with the mast and will interfere with the jib if the diamond spreader is too long. The solution; short spreader, a double spreader rig, or a spreaderless rig. Whichever way you go the design of the rig and sails needs to account for the rotation factor.

 

• Because the mast is fitted to a ball rather than securely bolted to a deck plate there is potentially a lot more movement and there can be more noise generated from the banging around of any play in the fitting at the mast foot, or the side to side motion of the rotation spanner if it is not locked off securely.

 

• Fitting masthead instruments and nav lights is more complicated because they will rotate with the mast.

 

• If you are rotating a conventional mast section it may not be too critical to get the rotation angle exactly right, but if it is a wing section it is important to rotate the mast to the right degree to ensure clean flow across the luff of the sail. This is one more set of control lines and fittings required and one more adjustment to be attended to when tacking.

 

 

 


At right: detail of the mast plate and ball on trimaran Venom


Carbon or Alloy rig

Carbon or Alloy Rig

The carbon rig is lighter and stiffer, and significantly more expensive. If you're serious about racing and have a light boat and an ample budget then the carbon rig is probably a no brainer. But even once you've chosen to go carbon it's not that simple. There are different methods of making carbon masts, different grades of carbon, and different ways of supporting the mast that affect the weight, the quality of the laminate and the price. This is a field for specialists.

My advice: If you're seriously considering a carbon rig choose one or two of the carbon rig manufacturers who have plenty of experience with your type of boat and start the discussion.


Wing Mast or Conventional Mast Profile

Just because you decide to rotate doesn't mean you have to use a wing section, although logically a wing will provide maximum gain from the ability to rotate. A conventional mast profile will still provide some benefit when rotated, it will be lighter and less expensive, and it will be less detrimental if you simply decide to lock it off to prevent it from rotating while you are cruising.

One benefit of the rotating mast, quite apart from the performance factor, is that when you are raising and lowering the main the mast will tend to remain head to wind even though the boat is swinging in the breeze. This makes it easier to get the main up and down, an important factor with the large square tops we commonly deploy in our rigs these days.

 

At right; Carbon wing on Venom rotated


Above: Mast spanner and gooseneck on trimaran Venom

Rig Configuration

There's multitude of ways to support the rig sideways and every rigging company has their own methods of engineering the rig support to complement the standard sections they have available, or are able to manufacture.

The sail plan in combination with other factors; like whether the rig is rotating or not will be important factors in determining the number and the length of spreaders and whether the rig needs lowers or mast head support for the gennaker.

The ideal solution for the boat owner is to work with a rigging company and a sail maker who regularly work in cooperation with each other. This is not essential but mutual understanding and experience can help to streamline the process of setting up a successful sail and rig combination.

 

At Right; Single spreader carbon rig on trimaran Venom.  


Above; Joel Berg (upper, Allyacht Spars) and Ben Kelly (North Sails Brisbane), setting up the rig on Venom. Ben and Joel have years of experience sailing together and cooperating on projects. The mast and boom were supplied by Lorima in France and set up by Allyacht Spars in Australia.

Trilogy wont he Australian Multihull Championships several times with a small profile non rating carbon mast built by Omohundro in the USA.

A quick and dirty tip for assessing costs and weight saving.

1. Figure how much you are investing in your boat and how much you expect it to weigh.

2. Figure out how much you are saving in weight by selecting one option over another, and note the cost of the weight saving. 

3. Compare the percentage of weight saved to the percentage of additional cost.

 

Example: You are investing 400k in your boat. It's going to weigh 4 tonnes at launch

The basic rig is 20k and the weight is 200kg  . 

The expensive rig option is 30k and the weight is 150kg

So you're spending an extra 10k which is 2.5% of your investment.

And you're saving 50kg which is 1.25% of your total weight.

This in itself may not give you the answer you are looking for because there are many factors apart from the cost and weight but its a simple way of assessing the cost of weight saving for all kinds of materials and equipment right through the boat.