The state of the art

What is the state of the art in the design of RC sailboats?

Naturally depends on the means that someone have. Several have access to University resources like tank test, sophisticated softwares for VPP (Velocity Prediction Program) and CFD (computational fluid mechanics), wind tunnel, etc, but the most of the peoples have only personal computer and free softwares, like me.

So, for  peoples like me, the great majority, what will be the state of the art?

What we can obtain free for do a good RC sailboat design?

For me, to answer the first question for IOM boats, is :

A – Establish the principals design parameters

1 – Prismatic coefficient – Cp

Cp is associated with wave resistance, the book Skene’s Elements of Yacht Design by Francis Kinney suggest:

V/√L            Cp     (V in knot – L in feet)

1.0           0.52
1.1           0.54
1.2           0.58
1.3           0.6

We have some options, which choose?  Some says that is better to have high Cp at low speeds than have low Cp in high velocities. Some says that 0.55 is a good average Cp.

I discovered the IOM RC sailboat in July 2009 and my first design,  Xique-Xique, the boat in the header, I do without any study. In that moment I was very anxious to sail an RC and where I live do not have none , I choose Cp = 0.55. My second IOM design, the Jegue D’água (Water Jack) has 0.574 for Cp. I am still learning which would better. What you say?

2 – Wet area

For me a good number is about 0.145 m² (1.561 ft²) but I do not get this in my second design, the minimum that I get was o.151 m², the first design has 0.158 m², a little high I think. What you think?

3 – Breadth in design waterline

As displacement, length, depth of the hull from waterline and draught, are fixed or have maximum values permitted, remains free the breadth.

Most of IOM have design waterline breadth between 0.165 m to 0.195 m. The breadth has influence in the shape of the sections and in the stability. More stability, lesser heell angle and by consequence less deformation in heeled waterlines and more sail efficiency.

On the other hand less breadth less deformation in heeled waterlines. My IOM designs have less breadth than normal. Xique-Xique has 0.16 m and Jegue has 0.15 m. Xique-Xique has good stability, I think, it has 0.125 m righting arm in 40 degrees heeled and Jegue 0.116 m.

How to compare all these values?

This is the problem and this Blog came to solve this. With time we will do a Data Base with all these values and much more others. You can not help?

4 – Sections shape

We have several shapes to choose, circular, rectangular, elliptic, etc. What is the better? Associated with waterline shapes, parabolic,elliptic, etc, we have a great number of choices.

My intention when I started the Jegue D’Àgua design is to do a sailboat with Cp = 0.580 with circular sections under design waterline to reduce wetted area, but with the time, to agree with Turner’s theory, Cp, etc, the final form was circular forward and rectangular after and the Cp is 0.574.

I do not know any proven formula on how to draw sections shape.

Another interesting question is stern form. When I do Xique-Xique, what I see in You tube, my unique font to do my first IOM design, was a  very large and flat stern.  I do not understood why that. For me, wide, flat sections is to achieve planning and I do not thought that a IOM was a planning sailboat and with the little race course and with a minimum leg downwind will be not a good idea have flat and large stern. Flat large stern was a good denotation that the LCB goes astern when heeled, denoting a big tendency to put the bow down , then when I do Xique-Xique design I had two conflicting ideas :

Large and flat stern X moderate stern

and the result we see in the photo:

At all I decided do the sides aft at right angles to minimize the bow down. And thanks to this, I think, the Xique-Xique do not has any tendency to put the bow down when heeled.

Xique-Xique 02 heeled

Xique-Xique 01 more heeled with painted DWL

For my second design I decided do the stern lesser. When I was testing the design for Turner’s Moments the stern was more lesser that I imagined but I decided leave as it was. I do not have any commercial or myself competitive intentions, my intention is learn how to do a competitive IOM and share.

Jegue D'Água


Jegue D'Água plug

Let’s see what happens when Jegue D’Água is done.

5 – LCB – LCF

The longitudinal center of buoyancy – LCB – reflect the distribution of volume in respect to mid length, it is the volume center of gravity and is the resultant position of the pressure forces actuating in the hull. If we have more volume astern the LCB goes aft.

The longitudinal center of flotation – LCF – is the waterplane center of gravity.  By LCF pass the hull rotation axis when doing trim.

The maximum usual position of LCB is 4% LWL aft mid length.

By the formulas do by Delft Systematic Yacht Hull Series we can establish an optimum LCB position for each velocity. This formula also take in account the relative LCB/LCF position, this relative position is seen as a measure for hull distortion fore and aft.

Delft Series formulas can not be applied to IOM. They are good in the range of the lengths used to establish them. What I do is scale my IOM design to 10 m, but we need see the others limitants factors as L/B, etc.

If we can express the Resistance of the hull by this formula, so, we can achieve the bests position for LCB and LCF for each velocity.

Xique-Xique has LCB 3.9 % aft and LCF 0.43 m from astern and Jegue D’Água has 1.3% aft with LCF 0.484 m from astern, so the LCB and LCF are more close to mid length and more close between them.

But something is very important: when heeled, LCB need stay in same position or go forward, never astern. LCB astern signifies bow down.

6 – Curve of sectionals areas

The traditional way for do the curve of sectionals areas is do a curve with the fore part with senoidal form and the after part with trochoidal form.

As I use Delftship software I do not mind more in sectionals areas curve, I change stations form and see the resultant volume, the Cp, wetted area, LCB, LCF, resistance, etc. The process is very quickly.

B – Check the result

1 – Resistance

Have done the hull drawing wee need check it for resistance to compare with the design that we need hurtle.

Happily we have excellent free drawing softwares. I use Delftship or Freeship.

But for resistance  we  have a big problem. The softwares do for predict hull resistance, like Delft Series, Michlet, etc, are not do for boats with one meter length and we can have a distortion in the results here. I solved this problem changing my hull drawing to 10 m length. Is the inverse process that we do for big boats. For big boats we do models, for models I do a big boat and now I can use the softwares more confident. Naturally we need transpose the results for our models. But we can see also the  results for the bigs boats and compare them as a first step.

About resistance we have some others problems.

1 – We do not have data in frictional and residuary resistances for others models.

2 – The softwares availables for  free do not calculate heeled resistance.

3 – Michlet do not have prevision for boats with overhang and do not calculate heeled resistance.

For 1 I hope that this blog came to improve. I am going to put all my boats data here and in future others boats that I have drawings or table of offsets.

Let’s  see the resistance by Michlet for Xique-Xique and Jegue D’Água:

Xique-Xique LWL = 10 m

Jegue D'Água LWL = 10 m

Blue line is Jegue D’Água hull form before changes to agree with Turner’s theory.

Green line is Jegue D’Água hull form after changes to agree with Turner’s theory.

Red line is Xique-Xique resistance.

2 – Stability

Stability is the boat aptitude to return to initial position after the disturbing cause cease to heel or to trim the boat.

A way that we use the stability study is calculate the righting arm. The righting arm value is a good indicator of the boat capacity to resist the transverse component of the wind force that heel and trim the boat.

When the hull is inclined some problems occur:

a – The sail lose effectiveness

b – The waterlines deform causing increase in resistance

c – The boat driveability can change to a very bad situation.

We have some data for righting arms from some IOM boats at 40 degrees heeled:

Topiko/Pikanto – 0.133 m

Widget – 0.13 m

Cockatoo – 0.14 m

Isis – 0.13 m

Gadget – 0.135 m

Oscar – 0.125 m

TS2 – 0.158 m

Xique-Xique – 0.125 m

Jegue D’Água – 0.116 m

3 – Heeled  Driveability

The Turner’s theory for heeled driveability is controversial but we have several testimony that boats done to comply with Turner’s theory have excellent sea behavior. Jegue D’Água was done to comply with and what I see was that after change design hull forms to comply, the resistance is very diminished. In the comparison graph above, the blue line is Jegue D’Água without comply with Turner’s theory an the green line is Jegue D’Água after change to comply with it.

Some says that Turner’s theory do double ended boats and Jegue D’Água is a double ended boat after changes to agree with the theory. But Earl Boebert in his paper:

THE 18th CHESAPEAKE SAILING YACHT SYMPOSIUM – “That Peculiar Property:” Model Yachting and the Analysis of Balance in Sailing Hulls

says that is possible do a boat to agree with the theory without be an double ended and show an example.

But IOM is not a real boat and therefore I found interesting see how a double ended RC boat behaves.

Turner’s theory is very hard-working but I do a AutoLisp program to use in Autocad  that after do a file with the table of offsets and more a few data the program do the graph for any angle. See below the result:

Xique-Xique 40⁰

Xique-Xique 40⁰

Jegue D'Água 40⁰

Jegue D'Água 40⁰

By Turner, Xique-Xique has a very bad heeled driveability. Will be? I do not have problems with Xique-Xique heeled driveability, but I do not have any experience in RC sailboats, I am faraway from another IOM, some thousands kilometers.

Jegue D’Água has a good heeled driveability, certainly, I do hull lines to comply with, and in future we can say what happened.

For the time being what I can say is that when I adapted the hull lines to agree with Turner’s theory the resistance diminished substantially.

4 – Inclined Waterlines

The sailboat pass more time heeled than upright so we need study very well the heeled waterlines. We do not have any systematic study about. The sailboats design books have some indication about but is more or less obvious that waterlines form that disturb the boundary layer is not desirable. A great asymmetry in port and starboard lines is not good also.

One problem to us is the extra work to do inclined waterlines drawing, we need do the drawing of several waterlines in several heeled angles. To facilitate this job I do another program in Autolisp that do this without much work. Below we have the inclined design waterline  for several heeled angles:



Jegue D'Água

Jegue D'Água

We can see on the drawings that we can have when heeled a situation that the inclined waterline cut the station in the same side two times, changing the form abruptly like what occur  in Jegue D’Água station 0. Some abrupt changes in stations form introduces also distortions in waterlines, what occur in station 7 and thanks to inclined waterline drawing we can see these problems and adjust boat form.

What we can add here? Contribute.

For the second question: What we can obtain free for do a good RC sailboat design?

The softwares that I use is:



My autolisp programs for AutoCad: Taylor, inclined waterlines

If you are interested in my autolisp programs, say me that i send to you.


About Fred Schmidt

Engenheiro Naval interessado em projeto de veleiros radio-controlados. Naval Architect interested in RC sailboats design
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6 Responses to The state of the art

  1. Don Case says:

    Hi Fred
    I’m trying to insert my statics in here. I can make a screen shot from Hullform but I can’t paste it here. Any suggestions other than typeing it all in.

    • frediom says:

      Hi Don


      You can send me the archives by e-mail:

      If you have much material I can do a page only for you like Future in RC Sail page or Construction page, however the design data was good to have in Data Base page.

      What you think?

      The blog is totally open for you.


  2. Don Case says:

    Great idea Fred, I just hope you get the input you want. I have just designed one boat(in Hullform) and am sailing the plug. I am quite pleased and it seems fast. I have very little experience with boat design so a lot of the terminology will requier explanation. I am just leaving the house for a camping trip but will start sharing the details(and asking questions) when I return.

    • frediom says:


      Thanks very much, it is all we need, share knowing.

      I will be very honored in work with you.



  3. frediom says:

    Hi Donovan
    Thanks for the words.
    I use Delftship and Freeship. Freeship is better because it has trim calculation and I have the same problem in pass a drawing between them.
    In the Forum: some peoples made some threads about this ṕroblem. Import the table of offsets is a solution but you need do according Freeship standards to import it.

  4. Donovan says:

    Great blog Fred!
    Thanks for the english version…
    Do you use FreeShip? I have a design in Solid Edge but am battling to import it into FreeShip to do Hydrostatic calculations….any ideas?

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