Rig & Sail Fine Tuning

After learning construction skills building two IACC 120’s under the expert guidance of ClaudioD, I have been happily sailing my creations alone for roughly 6 months now. I was happy with myself that I had a good set-up and confident in participating in my first competitive RC regatta.

Over the two day competition, good ole mother nature threw everything at the fleet from high winds from 20+ knots and white caps to go, down to virtually glassy mirror conditions and everything in between.

It was exciting changing mast rack, twist, sail draft, tightening & loosening up, down and out hauls, topping lifts, shrouds changing from flat to round sails to have the right rig settings and boat balance for the given conditions…that’s what sailing is all about.

Then I put my boat in the water and sail next to other boats to see how I compare on speed and pointing ability and compare the look of other rigs and sail shapes to decide to tweak it or not which I was doing often…hmm my boats pointing was lower and speed not as fast as other boats…I could not figure out why ?!

Ignorance is Bliss when you have confidence in your own knowledge which is inversely proportional to how much you know about a topic… I talk about myself here, after this regatta I quickly came to the conclusion, what I know about fine tuning, you can write on the back of a postage stamp, hence the reason for starting this thread, I would like to learn more and hope that newcomers (like me) to the sport will also find hints here when setting up their boats.

Most are ‘au fait’ with the skills for “rough set-up” in finding helm balance by changing Center of Effort (C.E) mast position and boom angles etc. (good reference is Lester Gilberts IOM rough set-up http://onemetre.net/Race/Roughset/Roughset.htm)

What I wish to learn more about is “Rig & Sail Fine Tuning”.

The first topic I like to ask about is “mast bend” I am in the middle of constructing my next hull and changing the mast box structure as per attached drawing.

When looking at other boats at this regatta they could set a flatter main than I could, the major difference between my boat and others was I had “deck mast step” while everyone else had “keel mast step” and were using a “mast ram”.

Asking for opinions here about the pro’s & cons of changing before I go any further ?

Cheers Alan

Hi Alan, yes as you found out the hard way, fine tuning is the difference between being at the front or somewhere back in the bunch.
It is an acquired skill which can be learnt by anyone although some will always be better at it, just like life really.

In a yacht the size of an IACC 120 a keel stepped mast with a ram is a huge advantage.
The ram controls the lower half of the mast fore & aft curve, which in turn controls the draft position and depth of the lower portion of the main.
There is an alternative if you wish to retain the deck step and that is to fit a deck mounted ram.
Either way, some method of controlling that section of mast is essential.

The next question comes with the control of the top section of the mast.
Should one use jumpers for this or are they simply adding windage and weight where it is not needed.
I personally am a jumpers believer. They allow you the luxury of making the choice with the draft position and depth again.

Next we come to mast material and the shroud set-up.
I have just been away to check that any material may be used for the mast construction.
That pretty well tells me that carbon fibre is the only way to go if you are serious about having the fastest 120 on the planet.
How big a diameter, wall-thickness, section, taper, straight, groovy, round, aerofoil, luff wire, tie-on, etc, etc, these are some of the options a builder has to choose before the question of rigging is considered.

Perhaps I should stop here and allow you to answer some of the questions I have posed here and to ask some more from the many thoughts that you must have.
Others will also have much good advise on this most excellent forum.
Best wishes from middle earth.

Here is a link that you may find helpful, where the EC12 class have published a complete manual on the building and rigging of their class.
Whilst the IACC 120 may be more advanced in their use of materials, the tried and true methods of controlling sail shape for larger area sails should not be discounted.
You do not have to reinvent the wheel just because you want to move up to “mags”


Hi Ian, One advantage at being on the tail end of fleet is that it can only get better from there :lol: …and your right, it is all in the fine tuning.

Currently I have upper and lower shrouds with one straight spreader just below hound position @ 60% mast height (no jumper strut at top section of the mast) I thought at the time I can use the shrouds and fore & back stay tension to control mast shape. Top shroud base is abreast the mast and lower shroud aft of the mast, but feel this is not right.

Without a mast ram I now think the lower shroud should be more at 40-45% of mast height to better help mast shape control.

On my largest sail plan (78dm2) I have mast height from the deck of 1.7 meters, and mast shape is clearly critical on such a high mast, hence my search for mast shape control was first on my list, next size down sail plan is 74 dm2 with 1.6 meter mast height, these are my first sails and I know can change sail plan to have shorter mast height, and albeit a shorter mast would help mast shape control I’m more interested is what the effect of a lower sail plan C.E would be like for better heeling stability.

The wind maybe free, but sails aren’t so I’ll come back to this later on when I have better understanding on what I have to work with for now.

Reality check on mast bend: The backstay is tensioned so that the mast bend matches the mainsail luff curve. Fore and aft mast bend changes the shape of the mainsail in two ways. 1) the mast either pushes fullness into the body of the mainsail when it straightens, or pulls fullness out of the mainsail when it bends more 2) the head of the mainsail either twists off as the mast bends more, or twists off less as the mast is straightened.

The mast ram is then necessary on a high mast for controlling lower section of the main, I’ve come across no negatives so far? I’m more convinced now why a mast ram is the way to go ! btw could not find anything on deck mounted ram you touched on, if anyone has any details, would be really appreciate to look at this arrangement.

I sometimes think rigs on 120’s are designed to represent the real boats rigs rather than more suitable for best performance as a model. In particular the headboard does not hold the sail well and the main twists reducing power significantly maybe this brings us to your next point about of top mast control, using jumper struts.

As said earlier, I have no jumpers, and only because no-one else has (sheep syndrome … I know) are you saying the problem with a fractional rig where the forestay pull at the hounds creates a curve in the top of the mast with the pull from the backstay via head crane ?

Further, often thought that a long head cranes create too much leverage (read mid mast bend) anyway, I remember looking closely at what I think are the two best sailing/rig tuning in Italy (Luca RSA 09 last years Cup winner & Gabriel USA 71 this years Cup winner) on what was different on their boats compared to others … they both had jumpers, whereas others did not…never quite registered then, but now it is beginning to make some sense.

Gabriel had traditional jumper strut each side of the mast using turnbuckles for tension adjustment, it looked mast top heavy and Luca has single jumper strut which is mounted on “front of the mast” (facing forward) with one threaded adjustment screw through the strut for adjustment, looked smart and much lighter with minimum windage…thinking about it, both were always top 3 fleet racing finishers, so that discounts the argument about jumper weight and windage for me …it’s baloney and I’m now becoming a believer too.

On jumper adjustment I’m I right in thinking:

  1. Jumpers too tight - mainsail too full in heavy air and can’t be depowered with luff and foot tension ?

  2. Jumpers too loose - mainsail overly flat most of the time and you may get an “overbend wrinkle” mainsail crease that usually runs from the clew, diagonally up and toward the luff near the middle of the mast, especially when sailing up-wind and sheeted-in tight ?

Btw: don’t know if you noted in the drawing posted, but I’m also changing to a telescopic Fairlead post, to keep fairlead sheeting point as close as possible to the main boom. I found when the gap was too large (mast forward tilt in heavy wind) I could not sail tight-hauled due to the widening gap between post and boom.

Next you mentioned mast material oh la la … until now I have only been using aluminium groovy mast (foil shape) and I was thinking this was the reason for no stability to help control mast shape, I think I was wrong, without a mast ram, would have the same problem no matter what mast material we’re using, albeit the stiffer carbon mast would better than aluminium (without a mast ram)

I have purchased a carbon groovy (airfoil shape) when I was in Italy and also a round carbon mast to try all possibilities, just out of interest.

Ian we covered a lot here, thank you and really interested in what good advise other members have to say on this interesting topic also

Next question, what is the best position to have your shroud base (chain plate) in front, beside or behind the mast and their relation to spreaders? (Straight or V shaped) particularly for supporting fore stay tension.?

Cheers Alan

P.S thanks for extra reading from ec12 site and middle earth is a good place to be …weather forecast here saying first snow on mountains here this weekend :indiffere

“if” the mast rotated, (like on a cat) it is an auto-function of mast bend to side, that when rotated will pull sail flatter. Keep in mind, we multihull guys use a forestay, side shrouds and if a tall mast, a single diamond wire. There is NO NEED for a backstay, jumpers, etc. - even when using an asymetrical spinnaker.
Let the mast rotate, bend out of column depending on wind strength and diamond wire tightness, and instant (relatively) a flatter sail. Want a fuller sail? - reduce mast rotation.

Ahhhh - this would be some of the “trickle-down” technology on loan to monohull sailors from us multihull guys. :stuck_out_tongue:

Didn’t mean to hijack the thread - just introducing an option that we use to eliminate a lot of standing rigging. Is a rotating mast legal in your class?

Cheers, Dick

BTW - a rumor - some of us may show up with an IACC shape hull in the RG-65 Class next spring. :cool:

Hi Alan,

Here are some examples of a deck mount mast ram.

The ram does one additional service than shaping the lower portion of the main. It also resists the forward thrust of the goose neck. Try this. With your boat rigged and set to close hauled, lift the end of the main boom (to simulate a gust hitting the main). Watch the mast bend forward at the goose neck, and see that the upper main leach opens up.

IOMs (similar fractional rig) try to get the mast ram as close to ‘in line’ with the goose neck as possible. Now, the boom cannot force the mast forward, and so the boom cannot lift, so the leach remains in control.

This is one main reason that modern IOMs use a raised foredeck and a ‘skiff’ after deck. This allows the goose neck/vang to be lowered, and the ram to be raised.

If you have a skiff style aft deck that extends to the mast, you could lower the goose neck/vang a bit and install a foredeck level mast ram, and get a bit closer than in your diagram.

You would also get a stiffer mast if you went with the half inch high tensile round alloy mast than the same sized groovey. CF tubing has various degrees of stiffness, stiffer the better.

Yes, get the adjustable sheeting post (that’s what they are called) and set is as high as possible yet still to clear the boom. Raise and lower it as necessary with changing mast bend/vang tension. The sheeting post allows you to bring the main in closer to centre line without generating a downward pull (that closes the upper leach).

It is also important to control mid and upper mast bend. Most IOMs use forward pre-bend, and then tighten the back stay to bring the mast straight. Even though you have hounds higher than an IOM, this should still work. A jumper strut will also help the top portion. Also don’t be afraid to bend the spreaders to pull (or push) the mid section of the mast.

With upper and lower shrouds, the normal set up would be to set the lower chain plate aft of the uppers. That way the lowers can limit and tendency to forward bend in mid mast. So the uppers would be in line or just aft of the mast step (20 to 30 mm), and the lowers a bit further aft again. Don’t go too far aft as that will restrict the boom from going full out down wind.


Hi Dick …Yes rotating mast is within the rules, can you fit a mast ram to it ? The idea of eliminating the backstay sounds appealing, can you post illustration or photo of how it looks?

Trickle down from multi’s uh…about time something comes back our way :p:p

Hey like to see IACC shape RG 65 hull, when the rumours surface into reality…cool

John thanks for the link with examples of deck mast ram. Your point on a ram also resisting the forward thrust of the goose neck rang a bell with me, I have had goose necks jam on me a couple times & once the bottom joint on one on my 120’s was forced off its rotating ball joint & could not figure out why!? also remembered one of the Italian boats goose neck blew off in last years regatta and the skipper tied it back with plastic tie down (photo attached) and you can see albeit he has mast through the deck, no mast ram. …very very interesting explanation on latest IOM developments in this area.

Adjustable sheeting post on order … thanks John great stuff !!!

Cheers Alan:zbeer:

Will see if I can find/get some photos of a beach cat (5.5 meter or thereabouts as it is easier to see. In the meantime, I will try a sketch to perhaps explain the concept. We still use outhaul, inhaul and downhaul controls plus sheet tension at boom end (not middle) and a wide traveller for offwind broad reaching.

I have a break coming up soon here at work, so maybe can get it posted before tonite.


Here is an example of a deck stepped mast ram as we use them on EC12`s in NZL.
We call this a “Stubby” and it remains on the boat when you change rigs.
If you would like to call me on Skype (captainbit) cos I can talk far better than I can type.
So much to say it just takes me so long to type. Cheers.

“DO NOT” I repeat, Do Not, consider eliminating the backstay.
IMHO it is a necessity on a rig such as yours.
There is no way you will keep a tight forestay without a backstay.
Loose forestay = can not point.
No backstay = cannot set mast curve for varying wind strengths.

Hi Ian …wow! now that looks like a nice idea, will skype Sunday evening your time to talk a little about this “stubbie” could be good solution to implement on my NZL 92 which was built with deck mast step, I considered it was too much surgery to put in keel mast step…but the stubbie looks like the answer, look forward to talking with you about it.

Don’t panic :lol: I could not imagine running a 1.7 metre mast “without” a backstay … but interested in what dick was talking about, it maybe useful for smaller boats.

Cheers Alan

Having trouble finding my photos - so may have to draw and start with some agreed upon assumptions.

My 18 Square Meter cat - 5.5 meters in length by just under 4 meters in beam - carried a 10 meter mast with 18 square meters of main sail area as a uni-rig. It had a side shroud on each side to hull tang, a 1/2 meter “pigtail” forestay with a single long forestay connected to the pig tail from the bow of each hull, and a diamond wire with single spreader. “THAT” was my standing riggging!

I think I will put together my thoughts and description in a WORD doc and post as a PDF. I can then incorporate drawings, descriptions and any related photos.

Basically, the mast bends sideways limited by diamond wire tension. Because it has a long chord fore/aft, it does not bend very much in that direction. If mast rotates towards apparent wind off port side, the middle of the mast will bend forward as far as the diamond wire allows. As this bend takes place, it pulls the center and lower portion of the main forward (and flattening it). You can still use downhaul on main and outhaul on main to flatten sail more. Likewise, tighten diamons, less downhaul, less outhaul and restrict rotation and you have maximum sail camber. Add an in-haul and I had enough camber between mast and boom that I could stand between them. This will not be to any advantage (or work) on a round tube mast or groovy mast. Widely used in the 10R class here in U.S. - and perhaps elsewhere.

The caveates - mast must be foil shaped, freely rotate toward apparent wind and boom must be sheeted at/or near the end of the boom and above traveller. Some form of rotation control is also needed - either as alever from front of mast base - or from mast to boom with adjustable line.Will work on drawing this weekend if time permits.

Hi Alan,

I think the kind of mast twist you are looking for is demonstrated here…!

The spreaders do that on both tacks!!

Hi Guys,

Mixed feelings on rotating mast idea, I have same effect with mast deck step now (with no control) as on Jim’s mast cam pic. Great pic Jim! I’m seeing changing shroud tensions on the spreaders as the mast rotates against the fixed shroud base …hmmm not quite sure if this is a good thing or not, need to run this movie through my minds eye a few times.

I have is foil shaped groovey mast using bead/tube attachement luff to mast, with a keel mast step I can’t see how rotating mast could work …ideas ? (mind you Ian’s stubbie may be able to do something here)

The other side of the coin and reason for buying round mast was to have the same effect “without” the extra complications of a rotating mast by using simply mast loops to attach luff onto the round mast …am I right in thinking the effect is the same as a rotating mast ??

Cheers Alan

P.S Just thinking about foil shaped mast for a minute (not wing mast) …always thought the point of the foil shape was to reduce the angle of incidence when beating (angle of wind into main) when the mast is fixed square onto center line, having this foil shaped mast rotate into the wind, would surely defeat this purpose!? … :rolleyes: little confused on rotating foil shaped mast point.

Hi Alan,
There is a 6th attachment method; a jack line that is tied to the round mast.

The problem with the luff tie is that theory and practice are different. In theory, the sail edge slides around the mast, in line with the wind, reducing turbulence behind the mast. ie the sail is at right angle to the circumference. But in practice, the sail slides around the mast, tangential to the circumference. The force that slides the sail forward is the low pressure sucking the sail. This allow the sail to flatten, and so you loose control of the draft that you carefully built into the sail.

The jack line tied to the mast is intended to overcome this effect. The sail is able to move around the mast a little to align with the wind, yet the luff is now stiff enough to be held in place. The amount of ‘float’ is controlled by adjusting the tension on the jack line.

See Lester’s site




EXCELLENT explanation John !

The rotating mast - whether foil shaped or larger wing chord is designed to have the leading edge of the mast align with apparent wind, and the wind on the leeward side of mast flow onto the leeward side of the sail with much less turbulence.

:scared: Now - to jump over to the “other” even darker side of sailing … the ice boats allow their masts to bend to a ridiculous extreme in an effort to get their sails really flat when sailing. Keeping in mind an iceboat develops tremendous appraent wind speeds, travels as tremendous speeds relative to monohulls or even multihulls, and has little need for power (full sails) to “punch through” waves — the guys driving these want sails as flat as possible (like an airplane wing) to speed airflow for acceleration and top-end speed. It must be noted that these things (DN Class in photo) have very little friction or resistance other sideways on the three runners - unlike multihulls with surface drag and waves or monohull with the same plus weight/lead keel.

Apologies to those who have seen this photo. Never found out if it eventually broke or not - but mast bend is actually a “black art” in it’s own right in ice boating. Regardless - this is one flat sail.

Go figure ! :stuck_out_tongue:

Hi Guys,

John thanks for 6th attachment method with wire on luff for round mast, that’s the version I will try with my round mast :slight_smile: makes good sense what you say…great stuff !!

Lesters always good for his research on a wide range of topics but I need to keep reminding myself that his reference is mostly directed toward IOM and sometimes not suitable for other classes which have more choices such as IACC 100/120 (e.g tear drop profiled groovey carbon masts) the applications & theories for rigging & sail tuning I’m finding are considerably different…but a great starting point for learning.

Just summarisng my thoughts and ideas gathered so far and will post for comment before moving onto next topic.

Cheers Alan.

I’ve learnt that boats helm balance is the most important indicator of correct sail setting, but even with if the helm is neutral (I prefer slight weather helm) it is no guarantee that the sails are set correctly, as I have found out :help:

My original set-up (without Jumper struts & mast ram) Rig fine trimming is at best a BIG PROBLEM especially in high wind ranges with limited rig control, which was evident with my original rig set-up :scared: (attached pic)

Light was not so much of a problem but had lot of trouble with leech being too far open

I’m now beginning to understand that fine tuning a rig can be improved with the addition of jumper strut & mast ram controls to have more control over sail shape with new set rig set-up. (attached pic)

Have posted below 3 illustrations of the cause and effect of new rig settings through various wind ranges when close hauled… open to comment to check if I’m looking at the right basic settings :confused:

Cheers Alan

Hi Alan, the first question is “are you starting with a mast that has forward pre-bend or a straight mast”?

For pre-bent mast, apply enough back stay to bring the mast straight on light air. for medium air, apply more back stay, and apply some ram to flatten the lower third of the mast (note - applying ram will affect the twist of the leach and so the vang will need to be re-adjusted). For top of range, move spar back, or lengthen head stay to move COE aft, apply more back stay to flatten main and ease vang to increase twist, apply ram to get desired shape in lower spar and main. Use jib boom topping lift to adjust jib leach twist to match main twist through out wind range.

For a straight spar, to get sufficient head stay tension, you need to apply back stay. As soon as you do that the spar begins to bend. So add ram to straighten the lower spar. With increasing wind, you will need to apply ever more back stay, resulting on more mast bend. Dial in more mast ram to control the lower third, and more jumper to control the upper third. Move COE aft as before.

In light air, I use just enough ram to counter the forward thrust of the goose neck. I have not found the need to use mast ram to increase fullness of the lower main.


Here is an item I found on the internet. Although titled Hard Wind tacks, it talks about sail trim and mast position. Note the section about using twist rather than moving the mast forward in higher winds.