UK-Cherub Class

Get Your Heart Racing


The Rig Design

Rig Tension

Most modern Cherubs use loads of rig tension - enough for many SMOD boats to just fold up! 200Kg plus on the shrouds is not unusual and often a lot more. The primary reason for using lots of rig tension is because the crew is on the trapeze. If you have 45Kg of rig tension on the windward shroud, the leeward shroud just slack, and the spreader holding the mast straight - as might be the case on a sit out boat. Then when your 70Kg crew gets out on the trapeze the windward shroud will go slack and there will be 30kg tension on the leeward shroud, with its spreader pushing the mast mid section up to windward, de-powering the whole rig. This is slow! Consequently, most boats start with 130kg plus of rig tension. On the other hand you have to be sensible with older boats. If I put the 180kg odd tension I use on the foam sandwich and carbon reinforced modern boat on a 3mm plywood topsides 1972 Farr then the poor old lady would just bend up and probably break. A good clue is to watch the foredeck as you put the rig tension on. If the boat is distorting the gunwales will spread between the mast and the forestay, and the foredeck will distort and go into humps and hollows. You have no foredeck? Another reason why its a bad idea to lose it! Obviously extra rig tension also tightens the jib luff etc, but I suspect that by the time you've got enough tension to make the spreaders work right with the crew bouncing on the wire the behaviour of the jib luff has ceased to be an issue. Its good to have the shroud anchorage’s as far outboard and aft as possible to reduce the peak loads when nose-diving etc, although obviously you also have to be able to let the boom out.

Standing Rigging.

Most boats have all the standing rigging made from stainless steel wire suggested sizes are

  • 3mm dyform for main shrouds
  • 2.5mm 1×19 for uppers
  • 3mm 1 x 19 for lowers
  • 2.5mm dyform for intermediates ( D2 to lower spreader root )

The advantage of dyform over 1×19 is that because it is made up from a smaller number of strands that are carefully shaped to fit to each other it will stretch less under tension leading to a stiffer rig unfortunately it is more expensive.

Exotics

There has been some experimentation with exotics such as “Vectran” and PBO which have the potential to give significant weight reduction over stainless steel and there will probably be more in the future. However the cost and difficulty in attaching them to the rig in a way that avoids any sharp corners has put many people off. One other disadvantage of composite standing rigging is creep. This is a property of the material and means that it will slowly elongate when left under load. This means that it will be difficult to keep the rig tension constant over the course of a day and repeat rig settings over the course of a year. Time will tell and as the cost of the material comes down and the terminating methods mature they will probably start to work their way into the boats.

Gooseneck Level Control.

Cherub rigs need to be strongly supported at gooseneck height. The short boom and big roach means there will be a lot of kicking strap load which will try very hard to bend the lower mast sideways and forwards dumping all the power. All fittings must be stronger than you think. Everything should be bolted on, and attached to something secure. If you have the slightest nagging doubt about something, it will break. Cherubs seem to put phenomenal loads on fittings, and most especially anything to do with lower shrouds. T terminals are the only choice in an alloy spar. Any kind of riveted fitting will either snap or pull through the mast. A plate round the mast front like an RS800 is one alternative. Another is to use wire loops around the mast threaded through the gooseneck. On a carbon mast T terminals are also OK. Another good alternative is stainless steel rings bonded on with a lot of Uni.-directional carbon wrapped around the mast. This has the advantage of not cutting any holes into one of the most highly stressed parts of the mast, however it must be done carefully to ensure that there is enough layers of carbon over the ring where the loads are highest. The other end should be absolutely rock solid, bolted, and ideally putting its anchorage in compression - e.g. running round the gunwale. A well-mounted plate loaded in shear will also be fine, but any trace of tension and it will just peel off. A stainless steel ring firmly located with a lot of unidirectional carbon is a good solution used on many foam sandwich boats.

One of the problems seen with older single spreader rigs especially when using a pole kite was that the mast would invert when going downwind. This inversion was caused by the tension from the kite halyard pulling forward well above the hounds To combat this many boats had what we in the UK call a prodder - a strut to gooseneck height from the foredeck. They do, of course, preclude self-tacking jibs. An alternative you will see is to have the mast supported by “rigid” lowers - struts to the gooseneck replacing lowers, which at the cost of a bit more windage lock the mast very rigidly. These rigid lowers can even be integrated into the boat by attaching them to gooseneck level stump that the mast sits on. The twelve-foot skiffs use a similar arrangement without the struts - just a post of truly massive construction integrated into the hull. This weight goes into the hull weight total and is towards the middle of the boat so its quite acceptable on a new boat. Unfortunately, the loads on the mast step at gooseneck level are magnified using this arrangement as all the kicker loads now go through the mast step without any help from the lower shrouds.

How Spreaders Work

Spreaders are used to support and control the bend in the mast they do this by deflecting the shroud away from the straight that it would like to take up. It is this shroud deflection that puts the force into the spreader that bends the mast. The greater the shroud deflection the greater the force that can be applied through the spreader to the mast. This means that the longer the spreader is the more effect it will have on the shroud and hence the mast.

If you look along the length of a shroud when it is under tension at where it passes through the spreader you can see what effect it is having on the shroud and so the mast. If it seems to be pushing the shroud forward then it is being used to help keep the mast straight and more rig tension will mean a straighter mast, if the spreaders are too far forward there is even the possibility of the mast inverting under excessive rig tension. If the spreaders are pushing the shroud back then they are trying to bend the mast and an increase in rig tension will only bend the mast further, which will work to de-power the rig

Single Spreader Rigs

This was the most common set-up prior to the extra sail area introduced in 1997 and will still be found on older boats. Two shrouds, one set of spreaders, lower shrouds to gooseneck height, and prodder - used as much to induce pre-bend as anything else, so needs to be strong in tension as well as compression. The lowers don't need to be adjustable on the water, but you need an easy way to tension them. A “Spectra” or other high tech rope lashing is as good as any, very cheap, and very effective, however they do make it difficult to repeat settings like you can when using standard chain plates. Although in theory raked back spreaders lead to better gust response. In practice on a older Cherub you are hunting for power much of the time, especially with a pre 94 rig, and so spreaders would be neutral or slightly angled forward against the neutral line of the shroud. This keeps the lower mast straight in conjunction with the prodder and lowers, leaving the top mast to flex and provide the gust response. This is probably still the best solution for a boat with an alloy mast. Tall alloy masts with twin spreader rigs seem generally to have too much weight aloft and performance suffers upwind. I think Andy Paterson was the only person to manage a competitive two spreader alloy rig. The spinnaker halyard can be appreciably above the hounds. You can probably go to about two feet or 60% safely, although this depends some on the mast section.

Twin Spreader Rigs

With the greater sail area since 1997 and the much lower weight of carbon spars these are pretty much universal since the sail area increase in 2005. The twin spreader rigs are required for the use of masthead spinnakers, without them you would need to use a tree trunk of a mast to keep it in one piece. A second advantage of the twin spreader rigs is that they support the mast sideways, helping to keep the power in the rig. When used with a lot of pre-bend the amount of sideways support given to the mast is adjusted using cap tension, and fine tuned using the Cunningham. As the Cunningham and kicker bend the mast the caps become looser, allowing the top to bend off sideways spilling the wind. There are a few different ways in which the mast and shrouds can be setup.

Diamonds

This rig configuration uses conventional spreaders to a conventional height. These lower spreaders are angled forward against the shrouds to lock the bottom of the mast fairly rigid. The masthead shrouds are actually diamond stays, normally running from the tip of the mast down through the top spreaders just above the hounds, through the main spreaders and then down to the base of the mast. There is a lot of variation in the staying arrangements. Many rigs use shorter upper spreaders, with the caps attached to the lower spreaders about halfway from the end to the root so that the main spreaders do not deflect the upper shrouds. The idea behind doing this is to isolate the control of the topmast bend from the control of the mid mast bend. The cap spreaders are generally heavily raked aft, introducing greater or lesser amounts of pre-bend, according to your sail cut. Some rigs have the caps pulling the mast forward at the lower spreaders and the shrouds pulling the mast aft and acting against the pull from the caps. This has the effect of locking the mast to a certain extent at spreader height and can mean that you can do without the D2’2 and all their extra windage.

C-Tech

By contrast the New Zealand designed C-tech rig has a lot of aft rake on both sets of spreaders and full length spreaders at the hounds. Typically, these are setup with the mast locked very straight up to the hounds with the D2’s trying to straighten the mast and the lower spreaders trying to bend it. Above the hounds they have a lot of prebend which is used to control the gust response of the mast.

49er style

With this setup the cap shrouds run from the tip of the mast through the upper spreaders, which tend to be quite long, and down to the shroud points. The main shrouds run through the main spreaders and down to the shroud points, and the lowers go from either the gooseneck or the top of the Gnav out to the shroud points. This setup allows for good sideways support for the top of the mast and it is also easy to adjust the tension in the caps when rigging. However as an increase in cap tension will decrease the main shroud tension, it can be difficult to control the middle of the mast and is likely to need quite long main spreaders set quite forward. This setup results in a high forestay tension and a large amount of the rig load passing through the boat, and would not be recommended for an older boat.

Attaching the trapeze wires

The trapeze wires should attach to the mast at about the hounds, and this gives a problem as you do not want to go drilling more holes in the mast than you absolutely need to. One option is to get stainless steel rings welded to the shroud T-terminals this is neat but getting someone who can do it without reducing the strength of the T-Terminal is more of a problem. Another solution is to drill a small hole through the upper spreaders and thread the trapeze wire through, chafing can be a problem with this method.

Gnav’s

A number of boats have recently started using strut kickers or Gnav’s 1). These add some extra windage and weight they also disrupt the shape of the mainsail on one tack. However they do open up a lot of extra space in the cockpit for the crew and in a 12 foot long boat that can make enough of a difference to be worth it. When using the Gnav the forward kicker loads act further up the mast and so the lower shrouds need to attach at the point where the Gnav fits the mast rather than at the gooseneck. As they constrain the bend of the mast about mid way between the

Over Rotating Masts - Wing Masts

It hasn't been done in this country for many a year, but in Australia a couple of boats have recently been seen with over-rotating semi wing masts, based on NS14 sections. This is a classic example of the wheel turning round, as such rigs were used in the early 70s, although the masts were spruce and balsa then. Such spars ought to be effective if you get them right. Supporting the bigger UK kites, especially masthead ones, could be a challenge, but it can be done - Alex Vallings of C-tech Spars in New Zealand has recently won an R Class Championship with a mast which has the tip supported by rotating spreaders. He tells a story of spending some time with a model made from broomstick and string working out how everything needed to be set up to avoid wires clashing as the mast rotates. In 2002 we permitted a mast with a greater chord than previously provided the area is measured in, and there is probably a performance benefit to be gained for a project with a considerable development curve. Bethwaite is good on the subject in “High Performance Sailing”.

1) ‘Gnav’ is ‘Vang’ spelt backwards which is what the rest of the world call kicking straps

tech/rigging.txt · Last modified: 2013/06/25 15:55 (external edit)