Mainsail / Jib ratios ?

In AC 120 class we have quite wide range of sail dimensions to work within the rules unlike other classes & I’ve being trying square head and elliptical mains to experience the differences.

Now I’m looking at the rules where we have sail area with main / jib ratios limitations starting with main of min 45% & jib up to max 55% to main of max 65% & Jib min 35% ratios.

Max sail area for rig one is 80 dm2 and for rig two 60 dm2

Question is, What is ideal main / jib sail area % ratios for which conditions ?

Cheers Alan

Hi Alan,

What an excellent question!!

I reckon this will be one of those questions like “how long is a piece of string”…

Not being familiar with the AC120 class, I am assuming you refer to a Bermuda rig, maybe fractional, maybe not. The image posted shows a fractional Bermuda rig.

The first thing that springs to mind for me would be the effect on weather/lee helm. Leaving the boat unchanged otherwise, a larger main and smaller jib would increase weather helm as you move the COE (centre of effort) back in relation to the CLR (centre of lateral resistance). If you have a mast that you can move, you could use this in concert with the rig sizes to maintain reasonable helm balance (eg larger jib, move mast back)

The idea would be to move the CE lower I would think, this would give your improved performance by having less heel for a given wind speed & direction.

Other consideration might be how the sail area is actually measured - you could perhaps add a lot of curve to make the jib into a gennaker if area is measured by the length of the sides… better downwind performance…

Looking forward to other comments on this one…

Hi Pen,

I’m glad some-one finds this question as interesting as much as I do, I’m been researching the question on the net and am surprised at lack of information available considering sails are the “engine” of a sail boat.

Btw: I’m completely ignoring hull design /speed in this exercise & only focussing here in how to quantify the most efficient sail shape and area.

I should clarify that boat helm balance is accepted as the norm by mast stepping the rig with micro adjustment using mast tilting & the settings of boom angles.

Good point you make about being fractionally or masthead-rigged ? but the question still remains the same.

The difference being: a mast head rig fixes jib head to the top of the mast which greatly determines the CE height that you can only change with the foot length of the Jib whereas, the fractional rig provides the flexibility of lowering the CE height by having variable fixing point of the Jib luff (below 80% max of the main luff length) as well as a variable foot length and here we begin see the fundamental differences between the two rig types.

Further, a high aspect or taller sail plan results in a taller mast with increased weight of the mast, rigging and spreaders that contribute further to the heeling force as wind speed increases, therefore it’s great for light air but becomes more detrimental as wind speed increases.

The compromise catch with high aspect ratio - heeling moment goes up - puts a practical limit to the tallness of the rig and low aspect ratio performs better off wind, conclusion: Higher aspect with light air and low aspect with higher winds speeds ?

Then there is the influence square & pin head mains on RC boats and my limited experience so far, I’m beginning to drift toward having high aspect square head for light medium air and pin head for heavy air, which based on the amount of control you can have on main leech and twist that declines rapidly in higher winds speeds using square head main, unless you are prepared to support the head in less conventional ways, than just having traditional head crane and vang adjustment for sail settings.

With RC sail racing, when the sail suit (and the wind) causes too great a heel (slowing-up of the speed) it is time to put on the second suit, continuing to the third, if the second proves too large. There doesn’t seem to be any hard and fast rule as to how much smaller the second and third suit should be unless you test and trail different sail suits, that can be expensive. Experienced eyes use rule of thumb saying if you heeled more than 30 degree’s it’s time to rig down, but I ask myself again how much should I be rigging down 10-15-20% in sail area, to have best the engine (sail) efficiency for the conditions.

Working toward trying to define ratios between jib and main sail areas, I’m looking at the aspect and foot length and throwing into consideration sail shape with the pro’s and cons of square head vs pin head mains for a RC yacht, that provides maximum engine power in three winds conditions of:

  1. Light air 1-5 knts
  2. Medium air 5-10 knts
  3. Heavy air 10-15 knts

My wish is to gather information into illustrations and measurements and finally percentages that could be helpful to different RC classes, not just AC 120.

Finally a PM tells me, “Generally speaking the fore sail is increased with low winds force to get more power. It is important to know that a jib equivalent to 30% of the total sail area, is providing 50% of the total power” Now that’s a very interesting number to think about !?

Looking forward to reading more comments on this interesting topic.

Cheers Alan

Hi Alan,
your question is very valid.
The ratio between Main and Jib is not an easy matter to find an answer. Generally, books consider for 2/3 for Main and 1/3 for the Jib.
What can be said, is that with low wind there is a large Genoa to get some speed. By analogy I would consider the max Jib as possible.
For what I’m told, the increase of speed is nevertheless counteracted with less up wind.
Each boat has is own set of combinations and this is part of the game and rarely you will get an answer. Just try step by step.
Certainly one of paramount parameter is the CE that shall be as low as possible, but on the other end an high aspect ratio is offering a better efficiency and speed…
The Rig 1 is good for 0 to 5 kt with stable wind and up to 7/8kt with gusts
The Rig 2 with 10% lower surface can be used up to 8/12 kt with stable wind and 14/15 with gust
The Rig 3 with 20% lower surface can be used above 14kt

If you have only 2 Rigs, then is better to have 15% difference in surface.

The Rule of Thumb that consist to change Rig is as you quoted , if the boat is heeling, with stable wind, for more then 10 seconds over 30°, then is better to change rig. This is because the Hull Drag over 30° of heel is getting too high and the speed will drop proportionally !
Boat can be, nevertheless designed to be efficients up to 35° of heel, see rounded hulls.

The other condition, to get the best sailing efficiency, due to the Lift/Drag, is to use an elliptical shape like a 1/4 of ellipse.

Large Main Head is favorings the air flux exchanges from one face to the other of the sail creating more Drag, this phenomenon is most of the times ignored.

With fractional Rig, the jib can go up to 7/8 of the Main luff.
The important is to have as much as possible parallel sides of the Main/Jib window and the Jib Leach as close as possible to the Mast and deck. The ideal would be like on real boats, where the jib base is in contact with the deck, this avoid to air flux exchanges as said above.
Finally a short Mast presents a valid advantage with stronger winds, by lowering the CE.

Only one way, try and decide !!


Unfortunately this is NOT possible under IACC 120 class rules :rolleyes:

Hi Alan,
clearly is a lack of the AC120 Rules !
Generally one should expect as minimum 10% or even 15% to be consistent with the change. Below 10% the effects are minimal.
As already discussed, the Rules should be modified applying -15% to the 80dm² for the second rig as per 68dm².
24% variation from the first to the second rig is definitively too much
If then is not possible, you knows what you can do !!!

The answer to that question is what I’m looking into now Claudio … it’s winter & no sailing or construction time leaves plenty of time for learning which is one of the things I enjoy with sailing.

Thinking inside the box with the limited Sail & Jib area at first glance, it makes no sense what so ever to have two different sail suits to cover RIG 1 wind range, it is just too expensive when you throw in cost of two: masts, main & jib booms and sail suits to cover 9% sail area limitation.

But thinking outside the box, it “maybe” practical to have just one Rig with two only different Jibs e.g 80 dm2 sail suit with 55% Jib and change Jib to 35% to have min 73 dm2 ?

Added to the sail area limitations is Main luff max of 1750 mm (no minimum) Jib hoist is max 80% of the main luff and once again no minimum and main head width of 60-200 mm, so we are now talking more about aspect and Jib to Main sail area ratio’s.

Here my search began with firstly to have understanding of loads on sails and there is virtually nothing on RC boats in this area, until I found this little gem from Rod Carr about Wind Velocity Gradient (converted to metric) and laid it beside chart using Martins formula for Wind Pressure on RC sail area.

I’m working on the principle that heeling force acts at the height of the center of effort and taking the CE as percentage of the rig height, so my next step is to draw up some sail plans to locate highest CE for 80 dm2 and lowest CE for 73 dm3 playing with sail aspect & changing only the Jib sail plan, using data from these charts.

Any experience, ideas or comments will be greatly appreciated & I will update my progress including 2 boat testing this sail combination next year, when soonest spring sailing weather permits. In the mean time waiting on few books by C.A Marchaj’s to arrive & read.

Cheers Alan

reducing the Jib area, will not bring you very far in the overall sail area reduction.
Because this is the matter, when the wind will increase is most of the time wise to reduce the sail surface.
Reducing the Jib area will not produce an effective reduction overall, but it will induce a recession of the CE that in turn will force to shift the full rig forward and jeopardizing the initial intent to avoid rig change.
On one hand you like the idea to change easily the sail area by substituting the Jib only, but on the other side you need to move the full rig anyhow to reestablish the centering conditions.
I do not see any advantage to work only on the Jib.

Instead one could decide to keep the same sail area and work with stronger winds , an example in the picture below.
The CE is almost at the same place and can be made exactly the same, therefore no change to centering/tuning.

May be this will help to work around the rational solution for low and high winds conditions on the same day !


Hi Claudio,

Your point is having “one surface with two Rigs” whereas I’m looking at different perspective of having “one rig with two surfaces”.

Two Rigs, One Surface: Emphasis is having two sail suits with different CE position 1) high and 2) low with C.E Δ of 74mm where the ratio of Jib/Main area ratio only varies by 1% by example of your illustrations is 29 cm2.

Makes perfectly good sense until we look at the wind pressure on your two rig 79 dm2 sail suits is the same and the we more across to see the difference the wind gradient makes on 1650 & 1485 mm luff on 79 dm2 (C.E Δ of 74mm) is absolute minimal at RC scale, unless I have missed something ?

My conclusion from this option is that energy & expense for two rigs is not going to result in any real significant performance gain through in the wind range, I could be wrong and always open to learning why ?

One Rig, Two Surfaces: Emphasis is on Main/Jib area ratio meaning having one main with two Jibs where the area is reduced as % of main. Example is main of 5160 cm2 with 55% Jib (2840 cm2) = 80 dm2 sail plan and 41% Jib (2140 cm2) = 73 dm2 sail plan. The difference in Jib areas being 14% or 2140 cm2. Granted it is only overall 9% reduction of total sail area, but they are the parameters I have work within the rules.

The CE position would be roughly be the mid-point between the 74mm CE difference of the two sail suits shown above, but as already understood by the wind gradient at RC sail scale, this is not significant.

Now, I wish to learn if there is going to be any significant performance difference by changing Main/Jib sail area % ratios for different wind conditions ?

It’s little like the difference between front and rear wheel drive cars, where front wheel drive pulls and the rear wheel pushes the vehicle mass is the same with Jib & Main ratios where I’m thinking max Jib area in light air provides lot of power (jib equivalent to 30% of the total sail area, is providing 50% of the total power) and main is controlling the direction & torque of heeling forces and in heavy air “maybe” it is worthwhile to change the power/torque ratios for the higher winds speeds.

I hope made myself clear as to what working toward understanding more about with this exercise, as I cannot find the answer anywhere so far & all good advice tells me the only way to find out is to … try it !

Cheers Alan

Hi Alan,
you miss a small detail, the sail area remain 7900cm² from Rig 1 to Rig 2 also when used with strong winds against the 7300cm as you obtain with a smaller Jib.
The two rigs are in compliance with the Rules !

Hi Claudio,

I fully understood your sail areas being the same with difference between the two being in main luff length, which are both fully compliant as is my example of 80 & 73 dm2 has same main luff length but different Jib areas. The point you’re making is that “low aspect 79 dm2 plan” has larger “total sail area” than 73 dm2 sail plan, question is the 79 dm2 really as efficient as 73 dm2 in high end of the wind range ?

Cheers Alan

Hi Alan,
in principle yes since the power obtainable should be proportional to the surface. Of course nothing is certain until tested !

Yes it’s the principles that I’m trying to clearly understand, ideally I would like to validate the theory before putting test into practice… I don’t want to be tugging my dinghy … if you get my drift !!

Taking one more step forward, I did plot on the wind pressure graph using of 30 degree’s heeling around 25 km/h (13.5 knts) which is considered to be top of the wind range for 80 dm2 sail area. Then plotted min Rig 1 73 dm sail area @ same wind speed and result says we have approx 400 grams less pressure on the C.E with 700 cm2 less sail area.

but I have no idea what heeling angle we would have with 73 dm2, I’m guessing it would be less than 30 degree’s, meaning possibly the 73 dm2 could go little further up the wind range before max 30 degree heeling is reached.

Am I on the right track here ?

Hi Alan,
the difference in wind pressure to the sail CE is not very different at 13.5kt alias 6.94m/s or V² = 48.19

Taking the classical formula from Bernoulli we can get the force developed by the two sail sets : F(Newtons) = 1/2 x rho x S (m²) x V² (m/s) x C

With 0.80m² at 25kt we have : 0.5 x 1.29 x 0.8 x 48.19 x 1.25 = 31.08 Newtons / 9.81 = 3.16kgf

With 0.73m² at 25kt we have : 0.5 x 1.29 x 0.73 x 48.19x 1.25 = 28.36 Newtons / 9.81 = 2.89kgf

So, if I’m not wrong, the difference is 270g only !

That substantially means that changing from 0.8m² down to 0.73m², the heeling force variation is minor.
Introducing a tilt angle may change a little.

You can also use another formula applicable to Kites :

F(kgf) = 0.13 x S(m²) x V²(m/s) x sin alpha (~45°)


Thanks for that Claudio, I take that to mean when take with the two different sail area arms vs same bulb arm applied to the two sail areas, 73 dm2 has better righting moment for same wind speed. (bulb arm has bigger influence)

Ok, now I will dive into the theories behind sail efficiency …why am I feeling this is going to hurt my peanut.

The peanut is coping with deep dive but had been going around in circles with different theories until I decided to go back and start with the basics of how a sail work …then came across Mr Arvel Gentry review of modern how sails generate lift, and how two sails interact with each other are discussed in light of modern aerodynamic research…great read

Page 15 Figure 37 touches how Jib can generate x 2 drive of main sail however, it is only in reference to over-lapping sails, I now suspect drive from RC sailing yacht Jib maybe considerably less.

Much of the old sail theory in the sailing references I have been reading he proves to be wrong. The origins of these old ideas are discussed and the new and correct explanations are presented in laymans speak, in the attached PDF document.

Following this reading at least 3 of my books found their way quickly into rubbish bin.

There is only one C.E ? and it can only be in one place ? That’s what I thought anyway, but my peanut is well and truly stumped on this one !

In drawing up sail plan for two different Jibs to go with the one Main sail, I’m at the stage of calculating the geometric centre of the two sail plans to position the stepped mast box in the best position in the hull, out of curiosity I started reading my reference books on the subject of C.E and found differing opinions on how the combined sail C.E is calculated :confused: then when on the net and found more :reyes:

I understand that it is impossible to calculate the precise C.E with any degree of accuracy because they are constantly changing as the boat moves through changing wind and water conditions, therefore, using the geometric centres is a useful tool that gets the job accomplished the starting point…I have done 6 different methods of equations and I get 5 different results with variations or 84 mm of the combined C.E from main luff edge, which is 8% of my DWL and that’s huge !

For simplistic explanation purposes, I prepared standard triangles sail shapes ignoring roach and leech hollow etc…to demonstrate the six (and there are more) different methods I’ve found

All of these methods come from reputable designers and authors on the subject of sailing …but I cannot understand how they can be so vastly different in their results …can anyone offer an explanation ?

Cheers Alan


Let me begin by saying that I am no expert on this matter. However, the following is offered for your consideration…

Your comments re the CE constantly changing are correct IMHO as the apparent wind and sail shape move around and the boat heels.

Using simplistic flat triangles to represent the sails is reasonable for the exercise.
Also assuming that wind effort is acting directly at right angles to the surface is reasonable.
Considering anything else is the path to (mathematical) madness IMHO

Given these limitations, one can the calculate the CE for each of the sails by simple triangulation.
Draw a line from each point to centre of opposite side, where the three lines intersect is the CE.
If the three lines do not all intersect at one point you are not working with a triangle (if you include the roach, this will be the case).

The final step is to determine the combined CE of the two sails.
This point will lie on straight line between the CEs you have determined for each sail.
It will NOT be in the centre of that line (unless main and jib are the same area), but will be offset toward the sail with the larger area.
The offset amount will be determined by the relative area of the two sails.

Worked example:
Assume a mainsail twice the area of the jib.
Assume the length between the CEs is 300mm
The combined CE would be closer to the mainsail CE than the jib CE, 100 mm from the mainsail CE.

This is how I understand it. Maybe others can add to or correct what I have offered.

Hope this helps…

Hi Alan,
Out of the 6 methods, personally I used :
The A and in my opinion is OK
The D , I drawn the plan and used the pendulum and found 69mm instead of 71mm. 1mm variation while tracing the lines justify the variation.
The F would be OK - your calculation is wrong, personally I would split the CE connection line as: 381/8000 x 2842 = 135.35mm and 381/8000 x 5162 = 245.84mm. The junction point (blue) is at 69mm from the mast.

The 3 models mentioned are giving the same results geometrically speaking of course !
That means also that the pendulum method is very sound .

Hi Pen,

Apologies, I had left out the basics of finding centre of triangle (single sail) which is exactly as you explain, you can see where I have marked this intersections on each sail, this is the only one point that everyones seems to agree on, the fun begins when we wish to find the combined CE of both sails.

Claudio, thanks for adding another formula which validates 4 of equations (A, D, E & F) examples in the area of the geometric C.E being within 2-3 mm of each other with avaerage of 68 mm from main luff, which is acceptable.

The other examples B & C are within 3 mm of each other and average out to be 46.5 mm from main luff, difference of 21.5 mm, so we have two seperate opinions.

What is really interesting is that the B/C group are from big boat sources and the the other four are from RC sailing sources !?

Cheers Alan