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This article documents a series of common points that are a useful pointers on yacht rigs today. It is by no means advice on how you should design your mast or how we would design your mast. But in any case there are a number of common traits shown on many masts today and it helps to recognise them and look at the 'big picture'.

Most rod rigs today use a cap shroud angle of 10 degrees to the mast wall. This is normally considered a state of the art angle and it usually permits a reasonable compromise between sail sheeting and rig stiffness. Similarly, the chainplate angle for an overlaping 140% genoa on an inline spreader rig is normally about 13.5 degrees from the forestay (forward end of J). This defines the most common chainplate position. With the V1 shroud vertical and the cap shroud coming from the hounds at 10 degrees it then remains to obtain a modest spreader envelope to generate even spreader pokes on each spreader. For 110% genoa and swept spreaders on the gunwhale a whole different approach is typically seen. However, it rarely pays off to have a cap shroud angle greater than 14 degrees and a cap shroud angle that is too large can actually lead to some torsional instablity which has been seen on carbon rigs. Carbon rigs have a fairly low shear modulus anyway, and therefore are prone to excessive twisting if this is not managed carefully at the design stage.

A lot of designers design their sail area based on the sail area to displacement ratio. Whilst that is their business. As rig designers we think that they would be better off to consider a ratio of (mast height x sail area) to righting moment. As this is a better way of making a comparison. Given a constant windspeed and lift coefficient on the sails in question this ratio is a rough balance of overturning moment and righting moment. The problem of sail area displacement ratio is that it fails to take into account stability generated by beam or form, draft nor aspect ratio of sail plan (distance from CLR to CoE). Clearly a beamy yacht with low aspect sail plan can handle a lot more sail area for its displacement than a narrow yacht with a tall mast. So in conclusion, why not calculate a second ratio of sail area x CoE / RMC. This ratio will soon start to give some feedback on whether the yacht can handle its sail plan and we think it is a better form of comparison when comparing two similar yachts. I was recently informed that the ratio I am discussing is an upside down dellenbaugh angle, sounds ok to me.

Factors of Safety on rigging vary greatly from design office to design office and there must also be variance on the type of vessel itself. As commentary on our industry here are some good rules of thumb. Forestay 2.0/2.5. Verticals 2.25/2.5 and Cap Shroud 2.75/3.0. The D1's vary between 2.75/3.5 and other D's 2.5/3. One issue is that whilst many designers are using RM30 the righting moment at 30 degrees as a basis for maximum safe angle of heel and therefore maximum safe working loads, current practise varies widely on analysis under reefing and if reefed loads are not applied to the D's individually, then it transpires that much heavier D's are needed or that higher Factors of safety are required. There are a lot of yachts that do not reef at all (race yachts) but if they have a delivery mainsail of cut down dimensions then it is important that these sails do not overload the adjacent D. Otherwise a safe move can turn around and bite you.

Nearly all rigs today are designed on the basis of their vessel's righting moment and nearly all engineers designing rigs and the loadings on hull and sails want to know what that righting moment is. Most hydrostatic programs give this output and it is a common element of stablity important to the design of any yacht. FIND OUT what it is if you want a professsional rig design. HINT; the easiest way to find it out is to obtain from your local measurer (usually at minimal cost) the rating certificate (IMS) of a yacht the same class or similar design. However a number of naval architects will do the work for you.

Rigs (modern) generally come in four flavours. There are masthead and fractional rigs and there are swept spreader and inline spreader rigs. Combine these and you have four possible combinations, I would like to propose to you an interesting set of generalisations one can make about these four flavours.

Masthead Sail Plan with Inline Spreaders

Racer IMS /  Cruising
Downwind Racing Isp=I (sleds)
No Runners (unlikely but possible)
Large yachts, Large Crew
traditional cruiser

Masthead Sail Plan with Swept Spreaders

Full on Cruising
No runners (probable, quite possible)
MPS or no Spinnaker
Yankee, furled reacher
Large yachts, small crew

Fractional Sail Plan with Inline Spreaders

Full on Racing PHRF, IMS
Masthead Spinnaker 'ideal'
Always have runners
fully crewed racing
small yachts, large crew

Fractional Sail Plan with Swept Spreaders

Racer / Cruiser
Masthead Spinnaker for Racing
Fractional Spinnaker/MPS for Cruising
No runners (possible)
short handed cruising, short handed racing
small yachts, small crew

These comments about four basic configurations are obviously just rules of thumb, and at first it looks fairly confusing, but it works for us and whilst there are exceptions to these rules due to a wide or narrow chainplate width and genoa overlap considerations, these observations are things that you might find useful. Now, speaking about chainplate width in particular, not enough people seem to realise how important this is. For example if you have chainplates a 1m off centreline and you move them out to the gunwhale at say 1.5m from centreline that is a 50% increase. The loads in the side rigging will therefore drop by 33% and in a lot of cases, the savings in rigging weight and mast weight will be far greater than building your mast in carbon fibre!! The savings in cost of a light alloy mast and light side rigging is also something to be considered, you save money and you save weight. A carbon rig with chainplates on the gunwhale might be lighter again, but cost can be a factor. We have to say that self tacking headsails make a lot of sense for cruising or short handed sailing or to just plain keep costs down. This does not mean that you must have less sail area, though for a racing yacht the rules kind of mess this up (we know). The point is that you can not put on wide chainplates if you have an overlapping headsail because it ruins the sheeting angle. A properly designed sail plan with a self tacking jib can allow very close sheeting angles and no loss in power with the right size of sail plan. So, whilst nonoverlapping sail plans with wide chainplates are not suitable for everyone, the saving in cost and weight is worth considering in many yachts.

Spreader rake is an interesting subject. If you have read the above and determined that you are interested in an inline rig; skip this. Once a swept rig is decided on the next question is what angle of spreader rake. If you have a spreader rake of 30 degrees on a fractionally rigged yacht, the chances are that you will need little or no runner loads. Indeed many yachts with 30 degree rake have no runners what-so-ever. The cap shrouds are effective at tensioning the forestay and provided J the jib foot is not too long considerable forestay tension is developed without runners. On a masthead rigged yacht the backstay is effective at tensioning the forestay and again without any runners you can tension the forestay using only 20 degrees of spreader rake. As you reduce rake to only 15 degrees the need for checkstays and runners starts to eventuate, sooner if you have an inner forestay/staysail of course. At spreader rakes of less that 10 degrees our best advice is DON'T. We say this because you obtain the worst of both worlds with spreader rakes between 5 and 10 degrees. You need runners and checks because if you gybe with 5 degrees spreader rake or fly a spinnaker you might snap the sidestays if you have no runners. At the same time you have a swept rig and can not modify mast bend / luff curve as in an inline rig. So, you have not been able to get the benefit of either configuration. The cliche 'in for a penny, in for a pound' holds true albeit poorly paraphrased. If spreader rake is between 0 and 5 degrees this is to all intents and purposes inline and would normally just be treated as 0 degrees/ inline.

At time of writing this the IR2000 rule has been mooted and it remains to be seen what will become of IMS. However, an interesting aside is the issue of spreader length. Most IMS yachts are restricted in terms of genoa sheeting by the length of the top spreader. As stated above the rule of thumb is a cap shroud angle of ten degrees to mast wall. HOWEVER, for IMS yachts a few wrinkes are evident. Let us assume that you develop a cap shroud angle of only 8 or 9 degrees. The stiffness and rod strength requirement in your cap shroud shoots up very high (so does cost and weight) and similarly compression and inertia and weight in your carbon mast also get bigger. But for all this you are fairly or more than fairly compensated by the IMS rule you will in fact be given a nice bonus in this respect. The truth is that you will now be able to sheet your genoa much closer in the higher regions which are normally forced out quite wide. So, you might potentially make a big improvement in your sail trim, sail shape and in the driving force of the yacht. Well, that's the theory.

The other wrinkle at time of writing is the oversize mast issue. Most IMS yachts have oversize masts, or at least fairly big masts. This is due to IMS compensations on mast windage. Mast windwage is a tricky one. We know flagpoles develop drag. We know that rotating wing masts generate driving force. A plain old yacht is probably closer to a flag pole. So, the IMS rule gives a compensation which most think is fair or more than fair. As a result a number of masts have been made with core in the side walls so that large sections with very thin walls have been made. It is rumoured that IMS will be modified to clamp down on this and to herd designers/ builders to a more sensible size of mast generally speaking. Of course IOR pushed everyone towards a very small very heavy mast with a thick wall, this was equally silly and I won't bother explaining that unless you are interested. At the end of the day it is unlikely that IMS will penalise large masts, probably just fair things up a bit. The large masts and surplus inertia issue does actually dovetail very nicely with the narrow cap shroud angle / short spreader length issue discussed above, so in effect each encourages the other.

IOR Masts were small and heavy. Requests have been made for me to discuss why I think this was so. The IOR rule gave no windage allowance for a large diameter mast, so clearly a small mast would have less windage. At the same time IOR yachts developed (became type-formed) with lots of internal ballast. This internal ballast raised the yacht's CoG and reduced the righting moment. This was done on purpose because whilst designers found heavy yachts to be an advantage they were penalised for a high righting moment and so hence CoG (centre of gravity) was raised with internal ballast to compensate. "Why" I hear you ask. Well let's face it a rating rule such as the IOR or the IMS is not about designing fast boats; what it is about is designing boats which are faster than the rating rules thinks that they are. So designing a slow boat with poor performance is GOOD if the rule thinks that the performance is worse than it actually is. So, anyway given the high CoG environment it is not impossible to have a heavy mast because there is still lots of internal ballast to play with. The small sections with heavy walls were able to bend a long way without breaking and as a result a degree of safety was built into the masts. That is to say the crew had the good sense to become frightened before it was too late. This inherent safety let a number of mast designers and mast builders play some really interesting games the extra safety allowed them to take some extra risks. I think it is fair to say that Bruce Thompson and John Green at Sparcraft UK had this figured out better than anybody. Since the yacht designers were still able to achieve their target weight, CoG and Righting Moment using a small low drag section made sense. Very narrow sections with five or six spreaders were shut out with the low rigging penalty so overall if you look back at mast extrusions designed about that time they were both heavier, smaller and thicker walled than the IMS extrusions most commonly on the market at the moment. Whether this is good or bad is hard to say; it is definitely disruptive and costly to owners that's for sure. It took a while and a lot of investment in new extrusion dies before IMS mast sections came on-line. For this reason many early IMS yachts were rigged with IOR masts. Similarly when the rules are next changed it will take a little while and yet more money to create even more extrusion dies. This may prove to be the end of aluminium masts at Grand Prix level. If the IMS rules simply modify the 2:1 aspect ratio, then the carbon mast builders will have a big problem too. In regards the IMS windage compensation it remains to be seen if the compensation really is "too good" it might actually be perfectly fair. The state of computational fluid dynamics makes this a very hard problem to solve. The reality is that as long as everyone 'thinks' that the compensation is over generous, then they will continue to make oversize masts; some of which are dangerous in respect of thin wall buckling. The IMS rule may have to generate an unfair windage allowance merely to herd mast sizes back to a sensible size regardless of what is technically right or wrong. From our perspective when designing the mast for a non-rating fast cruiser we generally recommend masts which are lighter than IOR and heavier than IMS in trying to create a safe conservative mast. This keeps costs down while maintaining performance and therefore value.

Mast Jacks are a total nightmare for a rig designer, the problem is that too few sailors have a degree in hydraulics (just kidding). There are a couple of things that should be borne in mind. Firstly if you have a load from your rig designer for jacking up the rig, then he should work out the pressure for you. If not this load must be converted into a pressure that you can read off the hydraulic gauge on the pump. In order to work out this pressure you need to divide the load by the area of the cylinder bore. As a result, pressures usually come in psi (pounds per square inch) or kPa (,000 Newton per square metre). On occasion the situation is confused by the use of Bar, one bar is another way of saying one atmosphere of pressure. Secondly, if you have two jacks, one on either end of the jacking bar, you better make sure that you divide the load by two times the area of a jack. This is where it gets messy, the same pressure on two cylinders gives twice the load. Not everyone believes this, but I assure you it is true;). Moving along to more complicated issues is the orientation of the jacking bar. Our office prefers to have the jacking bar run athwartships. This causes the least problem with mast rake and mast bend. We recommend a self-leveling top to the piston; these are also called 'tilt saddles' in some quarters. If a mast has a lot of rake the jack pushes on one corner without a self-leveller; a colleague of mine on the Il Moro di Venezia team was hit in the face by a self-leveller which flew out from the jack/bar due to slippery oil. MAKE SURE THE SELF-LEVELLER IS PERMANENTLY ATTACHED TO THE JACK. We try to set up the jacks so that if the jack is pumped to full height it can only lift mast a maximum of 10mm above the wedges. Otherwise if you use (say) a 6" jack with 4" wedges you could set it up to so as to run the risk of causing real damage if somebody is not paying attention and they overjack the rig 2 inches by mistake. If the jacking bar runs fore/aft the load sharing between the two jacks will not be equal. ideally. The two jacks will supply equal force, so this develops bending moment in the mast, the bar has to be longer, sometimes making insertion difficult and finally affects of rake and bend are harder to deal with on a fore/aft bar. Lastly, they tell me that oil at only 2000 psi is injected into your blood stream if you are nearby a burst hydraulic hose. But at 10,000 psi it will blow a hole right through you. In your eye you will be blind, plus you have a good chance at death from the oil poisoning you. So, never treat hydraulic hoses badly, don't stay near them longer than you need to. Just cause they rarely burst does not mean that they wont ever. Be especially wary of mast jacks because they usually operate at up to 10,000 psi which is a pressure far far higher than probably any other equipment on the boat. Make sure an expert checks your hoses and gear if it is all rusty and has not been used for a while. Here ends today's lecture.....