Rig & Sail Fine Tuning

Hi John,

I understand that there are two schools of thinking 1) is bendy masts where people talk about pre-bend and all sorts of reasons which mainly seem to related to IOM class I think (I’m not a follower) rules limit this class to “aluminium masts” which I read many employ a forward pre bend in the mast to help forestay tension (with high backstay tension) and use only a single top shroud, which I would love to have ](second lower shroud is not permissible ?) [/i]and 2) straight stiff masts.

For a IACC 120 fractional rig you can use any mast material you wish (with two shrouds) I have three different masts:

  1. Straight Aluminium foil shaped groovy 16 x 10 mm (this was my first problematic mast setup)
  2. Straight Carbon foil shaped groovy base 20 x 10mm taping to upto head to 15 x 8mm
  3. Straight Carbon round 10 mm dia stiff (matrix winded layer unidirectional inner & bidirectional outer)

As I said earlier, the applications & theories for rigging & sail tuning I’m finding are considerably different between these two classes. (IOM vs IACC 120) … but I not sure, hence reason for asking for experienced advice.

Cheers Alan

Hi All,

Please note that I have just changed illustrations in post # 18, after reading John’s response where he put me on the right track (Thanks John!) …my orginal rigging problems were the first two pics (Heavy & light Air) with “new rigging setup” looking for more main control shown in last three pics.

Still open for further comment ? before move onto next question about upper & lower shrouds positioning.

Cheers Alan

John, I just read IOM Nordic post on what Graham Bantock says

Quote [i]"On flat water with inland small lakes, best performance to windward in a breeze can often be got sailing very close to the wind. Keeping the boat high on the wind reduces the heeling moment and keeps the rig working at a high lift/drag ratio and this is good for best speed made good to windward. Sailing high like this means that small heading wind shifts are relatively easy to spot as the luff of the headsail will lift immediately it happens. Then tacking is simple as there is only a small angle that the boat needs to tack through before it is heading off on the new, correct, course.

On more open waters (seaside) where there will be large waves in stronger wind, this technique will not work. The boat will not have good speed to windward if it is sailed high on the wind. It will pitch a lot and the large momentary changes in apparent wind angle will keep the sails shaking. To keep the boat driving it will be necessary to use fuller sails sheeted wider than would be used on flat water.

A large amount of twist in the sails will keep the boat more upright and will also make the rig more forgiving to trim well. Even if there are large changes of apparent wind angle as the rig heels, pitches and yaws in the waves, there will always be some part of the rig driving well. At the upper end of the range of each rig I would expect to have the backstay tightened so much that the top seam of the mainsail is quite straight.

This will also lower the heeling moment by lowering the side force on the rig. To keep the boat balanced well it may be necessary to rake the mast back. This is opposite to what people often do in strong winds. Frequently they decide that the boat is luffing too much. Instead of twisting off the sails and sheeting slightly wider they try to cure the luffing tendency by raking the mast forward. Now the boat will luff less and so the rig will develop even more side force and heeling moment.

The situation just gets worse and when it is time to tack it is nearly impossible. However, once the rig is eased and the boat is easy to sail through the waves it should respond well when it is time to tack.[/i]"

This describes exactly the problem I was having in Italy recently with my old rig setup, I do sail in inland lakes here and not used to the more open conditions which I experienced in Italy …Thanks again, my muddy thinking slowly becomes a little clearer :slight_smile:

Cheers Alan

Alan -

I was looking for mast rotators, bends, diamond wire adjustments, etc. - and probably is no longer relative to your questions about an “auto-sail flattening/bending mast” - but visit the thread on wings, wing masts, etc. on this site. May answer questions yet to be asked.


I found this whilst doing some research for myself… http:////c_r_y_a.tripod.com/Sterne%20How%20to.htm#3%20Sail%20Trim
Nice, clear, methodical approach to achieving a good sail tune…

I know you said that you had enough info, but a bit more can’t hurt!

Hi Jim, this link come up with “address not valid” tried different variations but can’t find it, can you post URL again please, never said I had enough information did I ? if so I was wrong, there is never enough.

Dick I’ve read just about everything I can find on this site over last few days and come to the conclusion (as I said earlier) two schools of thinking on rig setup, one is based mainly on “round” bendy masts which seem to be 99% of RC yachts and the other is for “foil” stiffer type masts which there is very little about (especailly carbon) … still researching and learning :graduate:

Cheers Alan


Four ////'s probably was the cause, I will try this one and check it works!


Yarp, She works now!
My apologies…

Hey thanks Jim …more reading & thinking :stuck_out_tongue:

Take a look at Matthias latest video’s posted …love the mast cam in video # 4, took a snap shot & compared it to yours posted here earlier, no rotating mast there :cool:

Figure on small RC rigs regardless of mast shape (thinking that the wind does no scale) that mast rotation is may not be as critical as it maybe on larger boats :rolleyes:

Cheers Alan

Hi Alan,

Great videos to watch as you can do some good re-con work and see where the SUI 100 is working best.
She is an upwind weapon, sailing higher and faster than the other two…
I have a hunch that Matthias is running A LOT of forestay tension… The key to pointing well.
He also has the rig set up soft athwartships, so the centre of the rig sags to leeward. That might not be deliberate though?
I have just glued in part of the mast track for my new mast, I will see tomorrow just how stiff it is. With a 20mm chord to the top on this one I am very grateful for a rotating setup!

Poor me!!! :slight_smile:
I done a big error to share this video!!! All my secret discovered! :wink:
Jim we will sail together next year in Ravenna?


:lol::lol::lol: all your secrets discovered … I think not Matt :)… can give us the weight of SUI 100 keel & bulb and have a look at your bulb profile and then there is the jib deck sheeting guide (great idea) and now the cat is out of the bag about SUI 100 higher pointing ability …can you explain to us how the second Jib stay works, I noticed that you photo-shop this feature it out of lots of your posted photo’s, but it can’t be a secret can it ? :rolleyes:

Cheers Alan :wink:

P.S If you interested in my wing mast construction pic attached in exchance for your SUI 100 secrets :lol::lol:

I wish… I will be building Catamarans :wink:

As I suspected, it isn’t deliberate. There are no diagonals to help keep the mast in column.

Alan, the second jib cable is useful in case of heavy wind to increase the jib boom stability. However in Ravenna i noticed that the boat was faster without…remember? So now I don’t use it. I removed it from pictures because it was very bad looking.
Jim, a little lateral curve on the mast it is not a bad thing, you can increase, especially with low wind speed, the mainmast performances. Regard the jib sheet guide is my idea and copyright for this reason i use it, but onesty no one on the world on race use it and probably it is not an advantage. The tension of forsestay is important, especially in heavy wind. But as I told to Alan on the resturant in Ravenna, the final performances of a boat is the sum of many and many little good skill, no one does the difference. On my opinion hull and keel quality is the 15% of performance;40% id the rig and 45% is the skipper(set-up+sailing).
During this last training I used new sails made by me and Renato Chiesa, I noticed that respect Luca’s sails they are much more critical on setup. During the regatta 4 http://www.youtube.com/watch?v=UqmuQUZ77oQ I used a my normal mainsail set-up and the boat was very very slow, I had difficulty to overtacke Shosholoza RSA-83. After I changed the set-up and the boat starts over to go very fast. Another example is that: RSA-83 was slower than Luna Rossa, so I changed its set-up and after the boat was much faster than ITA-94…the set-up is the most important thing!!!
Alan, Jim, I hope to sail together soon…If you want to came a week end here in Roma we can sail together!


Hi Matt, I well recall our discussions and i think your 100% right :slight_smile:

Personally I’m not at a level “yet” where I can appreciate the loads of little things that can add upto a small advantage, that is why I started this thread “Rig & Sail Fine Tuning” to learn what is required to have a competitive package and help other interested 120 skippers interested into moving from a standard level up to a competitive level, if they wish.

The input of people in this forum is very valuable in achieving this and your skill & experience is exceptional & hope that you can contribute here too. :cool:

But be careful Matt, once I have that 45% boat package together, which I don’t have now, I will be wanting to live in Italy every weekend and sail together with you and other great skippers to fine tune that other 55% …with the burning desire to win every time :stuck_out_tongue:

Cheers mate :zbeer:

Ok after spending a week studying different rig set-ups I have boiled it down to what I will “try to summarise” into what I think are two systems, which have natural variations:

1) Flexi masts with two different rig set-ups for i) aluminium round or foil shape mast or ii) or carbon round mast shape, only because aluminium can be pre-bent and generally carbon masts are not.

2) Stiff masts which I see as being only a carbon foil shape.

Starting with flexi aluminium mast I understand two options A) “forward” pre-bent flexi mast where the tension applied to backstay pulls the pre-bend out. To set mast shape to match the sail luff curve you tighten the backstay for adjusting the mast/sail shape up and down the wind range the back stay tension either pushes fullness into the body of the mainsail mast straightens, or pulls fullness out of the mainsail when the mast bends more.

Pre-bent mast backstay adjustment directly transfers to jib stay tension, high back stay tension = high jib stay tension and conversely low backstay tension = low Jib stay tension. Back stay tension controls jib luff sag via pre-bent mast therefore you need to have very little jib luff allowance cut into your jib sail.

This seems to work fine in mid to upper wind ranges, The downside is that it is questionable if jib stay can be set-up “slack” in very light airs, otherwise the jib does would not develop sufficient draft. Jib stay tension can be changed by using topping lift according to the position of the jib pivot. If the jib pivot offset is closer to the jib luff, less tension feeds in to the topping lift, and vice versa as the pivot offset increases.

If the mast is positioned so that it cannot step fore & aft then the shroud base is fixed abreast the mast base which allows for minimal shroud tension adjustment then the sail plan centre of effort is balanced by raking the mast forward or aft and then fine tuned is done by adjusting jib stay length and the jib boom pivot point.

At the top of the wind range for this rig, the backstay can be tensioned further to increase twist at the head of the mainsail and to flatten the head of the mainsail. The head will flatten only if the spreaders and a “high” mast ram control the mid- and lower sections of the mast, otherwise increasing backstay tension will change the whole of the mast bend in ugly ways.

This rig set-up is seems to be the most difficult to fine tune and is used predominately by IOM class which limit class rules to round aluminium masts, mast ram, only one shroud and no jump strut…Then we have

B)]A straight flexi mast, here you can use any material or mast shape where you do not need to crank in as much backstay tension. This rig set-up has two shrouds (upper & lower), swept back “V” shaped spreaders and a jumper strut at the mast head, to keep the top of the mast straight, compared to a pre-bent mast.

Fine tuning of main shape is done by adjustment backstay tension along with upper shroud using spreader “V” together with the mast ram which pushes a little body into the mainsail, or takes some draft out. The mast ram continues to control the lower section of the mast and V spreaders mainly control mid section of the mast with shroud tension, which pushes the mast forward.

Shroud tension on this rig does a number of things, most obvious is that they keep the mast upright, and needs to be tightened as the wind picks up in order to hold the mast upright, but most important on this rig is that the swept back V spreaders become effective by also feeding tension into the jib stay.

The amount of tension that feeds in to the jib stay is partly related to the shroud base. The further aft attachment of the upper shroud from the mast step, the more tension is put into the jib stay. Conversely, if the upper shroud base is abeam the mast step, no shroud tension finds its way into the jib stay.

The amount of spreader “V” you need simply depends upon your shroud base offset. The further aft the shroud base, the greater the spreader “V” angle required. Lester Gilbert says that between 0% and 30% of the shroud tension is transferred into the jib stay depending as the shroud base can vary from 0 to 50 mm aft of the mast step on an IOM “A” rig.

The upper shroud aft base position (using V spreaders) means that backstay tension shares the jib stay tension with the upper shroud tension. Jib control stay tension depends much more upon adjustments to your shroud tension, depending on how far aft the upper shroud base is.

My assumption with this rig set-up is: you have more control over jib stay tension and mast bend and the up side is sailing in very light air, where you don’t want very much jib stay tension, you can better fine tune jib stay sag.

Remembering that Jib tension at the tip of the boom also increases when using topping lift tension, according to the position of the jib pivot. If the jib pivot offset is closer to the jib luff, less tension feeds in to the topping lift, and vice versa as the pivot offset increases.

The only difference between a “round” aluminium or carbon mast using this rig set-up would be the amount of stiffness vs weight of both options.

On my original rig I was using this rig set-up using aluminium foil shaped mast “without” 1) mast ram 2) jumper strut 3) aft swept spreaders …can you imagine how pretty my sail plan “was not” in upper wind range?

I’m now beginning to believe a straight aluminium foil mast with these “additional” fine tuning fittings will bring this rig set-up closer to a foil shaped carbon mast.

C – Straight stiff carbon foil mast “tapered” I emphasise a “tapered” carbon foil mast as “without the mast taper” I can only imagine would be very difficult to control any mast curve… it would be rock solid !

The characteristics of a “tapered” foil shaped carbon mast lend itself to minimal lower mast bend (hence low mast ram is possible) to control lower mast shape and as the mast tapers to the mid section of the mast, the curve here is directly tuned by backstay tension, while the jumper strut maintains straight upper mast head.

With this rig set-up you can easily fine tune COE by raking the mast forward or aft as needed (once final centre mast setting has been found) and the shroud tension does not affect jib stay tension therefore you can use straight spreaders and one single shroud base for the upper & lower shrouds.

With this rig set-up you use your backstay purely to manage fore-and-aft mast bend which means that backstay tension is more significant in affecting jib stay tension.

Jib control depends much more upon the jib stay by varying the tension at the hounds and length of your jib stay plus using topping lift tension according to the position of the jib pivot. If the jib pivot offset is closer to the jib luff, less tension feeds in to the topping lift (light air) and vice versa as the pivot offset increases jib stay tension (heavier airs)

Finally the other role spreaders play (straight or V shaped) is the control of the mid mast “side bend” which works off shroud tension. Sideways mast bend influences two things 1) it changes the slot, the gap between the jib and the main. If the mid-mast bends to leeward, the slot closes, and if it bends to windward, the slot opens. 2) the sideways mast bend also influences the effective twist of the mid-main. If the mast bends to leeward, the twist in the middle of the mainsail reduces, and vice versa if the mast bends to windward.

So there is my current mind set which is open to change if I’ve got it wrong (hope it was not too much in 1 hit) …comments :rolleyes:

Cheers Alan

Hi Alan,

Concerning the mast step, you should have a look to : http: //www.rcsailing.net/forum1/showthread.php?5990-Footy-XP ( if not already done.)

Could be interesting.


Hi Paulin, sorry for late reply I’ve have been travelling & just got back, thanks for the XP mast step link, conceptually it is very similar to what I’m doing …but for my new set-up a lot lighter.

New fin & mast box is made in fibreglass (keel mast stepped this time) comes in around 30 grams raw without supports etc.

Cheers Alan