Mast Position ?

When building a new boat (I’m not talking about an existing boat) I’d like to understand more out how people go about finding the right “mast position”

I’m aware how aero (sail CE) the hydro (Hull/Fin CLR) dynamic forces work and how to find them using cardboard cut-outs of sail plan and BWL hull/Fin profile and pinning to a wall using pendulum effect to find their geometric centres, no calculations or science involved in this area thank goodness :slight_smile:

Then we couple these two centres by placing the rig CE in advance (% of LWL) to CLR so that boat will sail with slight weather helm and then move or rake the rig to fine tune the boats helm balance.

My question is how do people go about locating the right mast position and how is chosen for new boat construction ?

Cheers Alan

P.S Thanks Claudio for the illustrations

Hi Allan
I’ve been puzzling over this for ten years and the best I can figure is that designers use an “educated guess”. Claudio posted a graph some time back that gave a ball park estimate but it only got you to within a few percentage points of the actual position. On a one meter boat if you move you mast 1 or 2 percent you will change your balance a lot. This is not close enough to epoxy a mast tube to the bottom of a boat. You could use a multi-position mast tube. The only for sure method is to build three boats and sneek up on the position by testing. Just my opinion. I hope someone has a secret method they will reveal.

PS I think that on “real” boats they can re-cut the sails to achieve balance. In my case, IOM’s, that’s not an option. Ac120’s may be different.

Hi Alan,
interesting tread, allow me to bring my 2 cts,

here below I describe Step by Step the method I use my self in addition to what you have already presented to search for the static CLR and CE. Other methods can be found in the specialised litterature, but I’m happy with that since many years of modelling.

The KEY is to position the Center of Lateral Resistance (CLR) of the immersed side view with the Center of Effort (CE) of the sail side view by the amount called LEAD that is a percentage of the Lenght of Water Line (LWL) that produce the correct arm to create the boat movement.

These positions are assumed under static conditions and approximate. Only final Tuning will be producing the final Boat Balance under sailing conditions.

  • Step 1 : The first operation consist to suspend the Bulb under the
    Hull by some means, i.e. trought the fin box, until the LWL horizontal equilibrium
    is achieved. Note : on top of the deck shall be provisorily fixed representatives
    weights corresponding to the Rig , Electronics, Fin and Rudder at the right position they will have definitively.
    It may end up that the bulb CG may shift backward by some 5 to 10 mm from the basic position,
    also to compesate the dynamic forward tilt induced by the sail’s forces when starting sailing.

  • Step 2 : The second operation consist in the positioning of the Fin as
    such that the vertical line joining the LCB - Bulb, pass over the maximum
    Fin thickess extimated around 37-40% of the cord. This will also allow to insert
    the bulb retaining rod at the bottom of the Fin .

  • Step 3 : The third operation consist in the positioning of the Rig
    (mast and sail) as such that the vertical line from the CLR (found with
    separate methods) is crossing the sail’s CE by an amount called LEAD being a %
    of the LWL - This percentage is variable as function of the boat
    shape and generally can be considered between 6 and 8 % of LWL. This value is
    only a basic assumption since during final tuning the CE can be shifted
    forward or bakward by tilting the Mast or by moving the Mast Step on
    deck. Final tuning may also reached by shifting the foresail.

  • Step 4 : The fourth operation is simply derived by the third since is
    confirming the Mast Step position that pratically is at a distance that
    correspond to the sum of CLR - CE + CE - Mast (back face) when the mast is
    vertical at 90° with the LWL

This is an extract of one book describing in general terms this point .

That is all what is needed to get a good “static” balanced boat, no need for complicated calculations.

Obviously, on what reported above, the Hull Design and Sail design are not taken into account, but it is worth saying that part of the success is to get a well “balanced Hull”.

Eric Sponberg,, told me once that it is a good practice to let the CE falling close to the front edge of the Fin or at least in the first quarter of the Fin chord.


Hi Don,

I researched threats here before asking this question and saw you ask the same question in Feb 2004 but you never got an answer :slight_smile:

Hopefully some-one will have at least a basic explanation that help get my grey matter or what’s left of it around the question and we can figure out the secret :stuck_out_tongue:

I will have a stepped mast and I’d like to at least locate the mast in a zero position (center step) so I have room to play with, my last build I tried but found best balance my mast ended up further forward than I planned :confused: hence rather than just plonking the the mast on where the plan suggests, I would like to at least try and have little better understanding of what “I should be doing”

Cheers Alan

P.S I’m building three boats now & like to get all three right, not just one of em :lol:

Pray to god and ClaudioD appears :stuck_out_tongue: …amazing start thanks Claudio, I’ll start digesting your post now and will be back for further questions :smiley:

Cheers Alan

P.S Next request Claudio, how can I look 20 years younger :lol::lol::lol::lol:

Like me Alan, 96 - 20 = 76

i too am working on 3 boats. finished up the stock maritime rg65, working on an albacore rg65, and working on claudios America in an rg65 full keel (no fin) configuration.

on the stock maritime hull they have a sot for the keel, and I am guesstimating the front of the mast to be at the leading edge of the keel. and I will do the same thing with the Albacore hull. both will be conventional rigs. Eery fun keeled boat I have had the mast is right around the leading edge of the keel…

who knows maybe all these boats will sail like crap… At least with Claudios America. I have the plans which show CLR, so at least I can get in the ball park… But also all boat have deck stepped masts so the rigs can be tilted and easily moved…

so much for the educated approach on my end… throw it at the wall and see what sticks…:slight_smile:

Hi Marc, have you ever tried the push boat balance test in water to find CLR on your rg65’s ? it would be interesting to try it to find what type of lead measurement you have to the CE, it seems this lead % is the biggest variable.

I suspect with my last build the mistake I made was fixing the bulb 10 mm behind the LCB and then placing ballast at the ends which made my helm balance sensitive, especially in rougher waters. Now thinking additional ballast is better off being as close to the CLR/CE coupling as possible for better stability …but still thinking that one through :confused:

Cheers Alan

i would think hat having the ballast at the ends would create more “hobby horsing” of the boat in rough water… I have not done the “push” or pull methods. to be honest, not sure my bathtub is deep enough… the albacore I am going to try to have a very traditional, IE non high aspect ratio rig…but we’ll see at least with the rg I can massage the sail plan to accommodate the CLR.

well. off to measure my bathtub…:slight_smile:


If you are after an answer to where to place the mast/fin as a combination on a boat - there is no/very little information available. Dons “educated guess” is about the best you can find. It seems there is no definitive answer to this question - as I guess you are beginning to find out!

The most common method seems to work out the ballast position and then place the keel over/near the centre of that, and then mast position follows.

From my experiences in dinghy sailing, moving mast position aft helped upwind speed, and moving forward helped downwind. We used to change the position until we were “happy” with the inevitable trade-off. However, that was with planing type hulls, but it does seem to be transferable to displacement types.

The relative position of the mast compared to the keel is well written, but most do not mention the dramatic effect that the sail shapes can have. A fuller mainsail, particularly in the lower aft sections will produce a lot more weather helm than a flatter straight aft section mainsail. Identical boats with identical sail plans sometimes have quite different mast or keel positions to balance them.


I have found the CLR on a RG65 with the push test, then put the fin roughly centered to that. Then bulb is located to balance the finished boat. Mast was moved to get the lead needed for good balance. Worked pretty well.
I have found that you can put the fin further back than you would by this method and it still works well. The star 45 is done that way to get the lead needed. It does not go down wind well (wont track well and has to be steered), but goes to weather great.

I don’t know if I have well understood, but the “push” test shall be carried out with rudder , fin and bulb attached in the position that the design drawings are suggesting. Shifting the fin/bulb further can be done but the “push” test shall be repeated until is considered acceptable and feasable .
Care shall be considered since I believe that if the “couple” CLR - CE is too far backward from the LCB, the full boat may suffer unstable steering and further the bulb may be difficult to be maintained under the LCB ensuring longitudinal balance.

One possiblity is offered by the “reversed L” Fin, but this is another story. Some talk can be found in the Lester Gilbert site.

What could be a nice fixture is to have a dedicated servo capable to adjust the backstay / mast rake, as function of the sailing direction.
Unfortunately not all model’s Rules allow to add a specific servo for that purpose, unless a New modern Class is developed like the 100R !!!.


Hi Marc, I’m with you on the yawing effect having ballast at the ends, I will centralise the ballast in last hull, looks like little surgery is needed to move the Bulb C.G on the fin to bring hull back on her active LWL again though :rolleyes:

Claudio your four step guide to locating mast position is exactly what I was looking before I start construction ( I can see where I made mistakes before :rolleyes:) but now has me focussed on the path of confirming the CLR location on the hull, before I finalise the mast box position, without knowing this I can see how it can easily become a guessing exercise when balancing the helm on the water later.

Having studied over 20 of your plans Claudio, I have noted on your AC 120 designs that you started with fixed lead of 8%, then later plans increased up to 12% then your most recent plans the lead has come back into the area of 7%, or in other words the lead between CLR & CE coupling has ranged between 70-120mm (Δ 50 mm) on average 1000 mm LWL which seems huge variable in RC terms ! can you help me understand why by explaining what influences lead and roughly how the % is determined in hull design ?

Cheers Alan

Hi Jon

Bit of a contradiction there mate !

One cannot use the educated guess approach unless of course they have the education, that’s why I’m here asking questions and I find Claudio’s basic four step approach a logical guide to start with, having a heavily analytical approach to challenges I tend to look for cause and effect to problems in trying to understand and solve technical problems (tip: but never try it with women, it does not work) :stuck_out_tongue:

Example was learning the difference between for boat & helm/rig balance, for long time I assumed they were the same but with many bad experiences, and with shared experiences here, I’ve educated myself by understanding cause and effect of these two different dynamics e.g:

Boat Balance (hydro dynamic cause & effects)

  • Boat weight forward causes boat want to head up
  • Boat weight aft causes boat wants to bear off.

Rig/helm Balance (aero dynamic cause & effects)

  • Jib in will cause boat want to bear off
  • Jib out will cause boat want to head up
  • Main in will cause boat want to head up
  • Main out will cause boat want to bear off
  • Mast rake forward will cause boat want to bear off
  • Mast rake aft will cause boat want to head up
  • Windward heel (not applicable to RC boats) will cause boat want to bear off
  • Leeward heel will cause boat want to head up

When I’ve had bad boat balance and tried to tune it out with rig/helm balancing the over-all performance suffered badly, hence I’ve now learnt from sharing ideas here to get boat balance right first before trying to correct helm/rig balance, otherwise I end up chasing my tail all the time.

I’ve heard this saying so many times I honestly cannot believe, it seems more like an excuse than an answer, until some-one can explain how this is possible of course ?

Cheers Alan

Hi Alan;

IMO should be the contrary : jib out > bow cause boat to bear off.

About the Lead percentage, we did not considered so far that the hull ratio B/L is also playing a big role.
Generally, a large boat has tendency to head up much more easily then a narrow hull, this is why, some time, the tipical 6-8% can become 6-12% as reported on many yacht desing books. In this case it is necessary to move the Mast Step forward the bow compared to a relative narrower hull.

For instance given a LWL of 1000mm and assuming the 6-12% wide range this is equivalent to 6 -12cm shift range.
Is not drammatic since the tuning may requires even more and this depends on several factors as : hull shape, sail shape, sail chord % , CE vertical position, weights distributions, Fin shape, fin profile, Bulb profile, etc.
Often a mast step slide of few cm long, may help to cope with various sail form and CE positions.
Here below I prepared a simple drawing showing that, while may be interesting to shift backward, within certains limits, the “couple” CLR-CE , the fin shift is limited by the bulb CG position and attachment difficulties.
More the bulb CG will be off from the central fiber of the fin and more the bulb oscillations risks will increase expecially when changing boards or turning around the buoy ! As usual a compromise is the best approach !!!

At the end, the percentage has a relative importance if it is considered that the CE should fall in the first quarter of the fin chord. Personally I learn that at the begin, before any tuning, the CE is fixed “as rule of thumb” on the vertical line passing by the front edge of the fin.


Thanks for the correction Claudio … see I’m still learning :slight_smile:

was just a printing error , Alan !

Question of clarification Claudio: 6-12% of 1000 mm LWL is equivalent to 60-120 mm :confused: or is the 6-12% based on distant between shadows #5 & 6 which I’m guessing would be 100 mm in this case ?

Cheers Alan

you see, printing errors are always presents, sorry I will change mm in cm


I wouldn’t call myself an expert on this subject, at all.

But, correct me if I’m wrong, should the expected amount of heel not be a part of the equation? I have made an illustration to show what I mean. I have looked at the forces developed by the sail plan and the submerged hull and projected the forces in two directions forward, and sideways.

So from the sail there is a heeling force and a driving force which pushes the boat forward acting at the CE. The submerged hull incl appendages will also develop a heeling force and a drag force from water resistance, acting at the CLR.

When sailing without accelerating or slowing down, the driving force and the drag force will be equal.

I think the whole reason why we need the lead is to cancel out the torsion caused by the force couple F.Drive and F.drag. As you can see if the heel angle is increased, the distance between the force couples increases, thus the torsion increases. To cancel out this torsion, we need the heeling forces to generate a torsion in the opposite direction, which is done with the lead.

So in some way form-stability of the hull could also play a role. So if we consider a near cylindrical hull shape, it would need more lead compared to a more “boxy” one. Simply because it would have less resistance to the heel force, an thus move the two force couples further apart.

But don’t ask me how to calculate the exact % of lead. I guess it will also depend on the expected weather conditions.

But in Alan’s case with 3 different boats which is properly going to be compared in the same conditions, I would look into the “form-stability” to decide in which area the of Lead scale it should be…

I think you can calculate something called the Metacentric height which can be used to make a stability curve. But I would have to read up on that.

Hope it makes sense, and that I am not all wrong.

Again I’m not an expert; I just read a book at a pool in Egypt back I november.


P.S. How do you make pictures big when inserting them inline?