Fin/Mast location

Hi All
I wonder if someone(or everyone) who designs thier own boats(IOM in particular) could run through thier method of determining fin /mast location? I’m not new at this and mostly understand CLR and CE, how to determine them and thier effect. I’ve just never managed to get it right. My boats always lean towards the weather helm. On all save one, after a few test sails, I have had to move the mast tube ahead(not an easy task) or move the fin back. All the stuff I have read puts the lead at 1 to 12% of the WL but most IOM’s seem to have the CE almost directly above the CLR(assuming that the hulls CLR and the fins CLR are in line). Could anyone who has had success with this please help!

Hi Don,
this is my method I use for my projects.
The matter of facts is that I toke a graph published by an Italian architect related to real boats.
I justt extrapoled the graph by adding the curves for small boats like ours.
So far I’m very satisfied.
The interesting thing is that the graph includes the ratio BWL / LWL.

The curves are presenting the nominal value and the tolerances varying from one design to another

As an example take a boat with an LWL of 1000mm and a beam at water level of 165mm :
The ratio will be : 165/1000 = 0.165

On the base of the graph you select the 0.165 an going up you’ll cross the nominal curve that project on the left the amount of LEAD in % of LWL, in this case is ~8.7 %

This value is the LEAD that separate the CLR from the CE. The Mast step position is fonction of the sail forms that determine the CE.

This method is valid for all type of boats


my method to search the CLR, simply using a carton and nails

Hope is OK for you

Thank you Claudio
I noticed in you example that you used the lower limit(I didn’t know I could read Italian). Do you always use the lower limit?

Another question. Do you always put you fin on the CB(Center of Buoyancy?)

sorry Don is a graphic mistake I repaired the problem !!
Take a look !
Tank for mention it

No, myself I use to place the fin as such to cross the max thickness with the vertical line passing by the CB.
The Fin profiles I use are generally exibiting the maximum thickness around 35/40 % of the cord.
Actually on my last AC100 I will put further back the fin as to cross the vertical to the CB at around 10% of the cord, see sketch below :

Obviously I’m not sure yet that will work on all course conditions, but the driwing idea is to recess as much as possible the CE and all the RIG in order to better contrast the “nose down”.
The limit of course is the bulb position, that shall remain under the CB vertical +/- 5mm and still “attachable” to the fin.

Hi Claudio
I tried your chart on a boat that I had modified so it would balance. I determined the CLR using the " pushing the boat sideways while it was floating method". It turned out very close to what the chart suggested. This is good. I noticed that you include the rudder when determineing the CLR. Everything that I have read says to not include the rudder or to include half the rudder. To me common sense says that the rudder should be included. Do you have a reason that might be more substantial than my common sense?

I have also tried moving the fin/mast aft to provide more buoyancy forward. It seemed to work. The boat has a very fine bow but does not nosedive any worse than the average IOM. The forward prismatic coefficient is .422 and the aft PC is .644. The LCB(CB) is 569mm back from the bow. I mounted the fin at the LCB. Your chart also works with this boat. The chart says I should have the CE about a cm. further forward. Unfortunatly I have the mast right up against the raised deck and the boat still has some(not much) weather helm. I could move the fin back but that takes the bulb with it and makes the bow pitch up. I think the moral here is to not design a boat with the buoyancy so far aft.

Thanks for the chart. I will use it on the next boat.
Does anyone else have a method we could compare with?

Hi Don
Many boats, many methods !!
Is another valued method to push laterally in calm water like a pool, the same should be done with the tilted boat, but difficult to do it without adding extra weight at the top of the mast.

Reasonably, according to my sense as you said, the most actives surfaces in opposition to the boat drift are the fin surface as well the rudder surface just because are flat compared to the china form of the immerse hull and the bulb. Since my design use an hull rather “squared”, once tilted, will offer a ‘V’ shape volume under water acting better against the drift when close hauled. The hull draft is well reduced when running.

To be strictly in line with the method one should take the projected surfaces with the tilted boat at 25°-30°. The fin and rudder surfaces will be shorter, but the only parameter affected will be the height of the CLR. The bulb, because of his roundess is not very effective against lateral drift therefore is ignored, similarly for a round hull I will consider only 50% of the immersed surface.

If you look to my last drawing you will see that the CB is at -2% behind the shadow 5 and further the bow volume is relatively more buoyant, not only, but the form of the shadows above the water level are “V” shaped to get more volume as soon the nose go down.

The boat is not yet constructed therefore I’m not sure that it will sail better than others , wait and see !!


Bugger you Don & Claudio :rolleyes: (say that with a smile on my dial) I’m just about to start construction of a new hull, fully confident I had everything planned out nicely and then this very interesting threat pops up and starts me thinking & doubting myself again :scared:

It was once said (by who, I don’t know) there are three most important centre’s in sailing…Centre of Gravity (C.G) vs Centre of Lateral Resistance (CLR) vs Centre of effort (CE), nail these suckers and you’ve got a well helm balanced boat, which until now satisfied my KISS (keep it simple sailor) thinking.

Honestly, I really didn’t think about the importance of Central Bouncy (CB) until reading this threat & I began thinking more about this area & did a little research and found in technical terms, the CB, also known as the centre of immersion, is the centre of gravity (CG) of the volume of water displaced by a floating object. In other words, it’s the centre of the total force that is pushing upward on the hull. Together with the boat’s CG, this design element determines how stable the boat will be.

Therefore I’ve now come to the conclusion that defining the “static CB position” is of paramount importance when building a new boat, as all the other static balances are be based on the CB being in the right place with the vertical line falling from the CB in order to fix the Bulb position to obtain a longitudinal equilibrium of the finished boat. Get this right to start with and other balance points will be easier to find.

With this in mind I’ve done an exercise to check the CB for my next new IACC 120 build and I would like to share what I’ve done and ask for your comments & thoughts on how valid it is.

Step 1: After making a new hull shell, I used original plans and cut another set of hull frames to use as construction cradle & this time cut the frames so that the marked LWL is on the “outside of all frames” so they’re same distance from the base board (level LWL) in the cradle.

Step 2: The “inner frames” (cut from above cradle frames) can then be used to sandwich the hull in the cradle and mark the LWL position on the inner hull surface, to check hull shell shape to plans (it is a very good fit)

Step 3: Thinking in reverse engineering terms about hull displacement, I poured water into the hull shell until the total weight of the hull & water together equalled minimum racing weight (4500 grams.)

Step 4: With the hull LWL level on external cradle makings on frames being level, then checked internal LWL levels and once again perfect.

Step 5: With the water in the hull, then placed a round stick roughly in the centre underneath the hull so it lifted the hull off the base of the cradle base by approx 10 mm & then moved the round stick until I found the hull was evenly balanced on the stick and marked the position on the side of the hull.

Step 6: Compared this static measurement with design plan measure and found that the static CB position to be 10 mm more forward than what is indicated on the design plans.

Is this the correct way to check “static CB position” of a hull shell or is there another why of checking this ?

Cheers Alan

P.S I have few further questions I would to clarify but need to be sure what I’ve done so far is correct first and have delayed this hull construction to use the shell for further testing & checking for this threat topic.

Hi Alan,
I’m not sure to have well understood, but probably the variation is due to the fact that you raised also the hull with his own weight.
In any case the CB you find in my drawings is coming from the Curve of Areas. The form is then cut into a small bristol sheet and successively balanced with a nail. Tracing the lines , the cross point indicate che CB or CC for the italians. The small grid is composed of ten parts, each one representing 1% of the LWL. Knowing the LWL lenght is easy to calculate where is the CB from the stern or simply from the Shadow 5.
In the example you will see that the CB is 1.8% of LWL behind the shadow 5. Because it is behind the shadow 5, being also the midle of the LWL, the sign - is put in the front of the percentage therefore reading : -1.8%

The method I use is rather precise.
Difficult to detect of the weights distribution of the hull and of what is put inside, this is the reason why the bulb position has a tollerance of 0-10mm around the CB axis in order to balance the boat if necessary. The starting point of course is the the Bulb CG that shall be aligned with the CB/CG of the hull.


PS : normally the Cb should be around -3% of LWL, therefore in this example is suggested that the volume in the front is more buoyant.

Is it possible that the rod you are balancing it on is deforming the bottom of the hull and throwing off your results? I don’t know how stiff your hull is. I tried this once and I could not get the water to stand still. Every time I moved the rod tha water would slosh to one end or the other. You must have very steady hands or the patience of Job.

Hi Claudio, Now I understand what that CB % value means, thank you. I’ve now calculated that measurement from LWL and measured from frame 5 and it is exactly where my static CB measurement mark is, which validates the static method I used as physical check, I must have measured the plan wrongly it’s the only reason I can think how I had a 10 mm difference.:confused:

Hi Don, the hull is very stiff with 2 x 200 gsm carbon and was very little deflection, could feel it in my fingers but could not see it …you make me laugh about water sloshing trying to balance shell full of water, I tried the same thing and was only successful in spilling water all over the kitchen floor, he who one must obey, smartly kicked me into the backyard.

If you look at the pic, I finally did the balancing of the hull while the shell was inside the cradle, this helped considerable, but took a little patience to wait for the water to settle after each minor adjustment was made. The 4-5 mm gap between the bottom of the hull and the cradle helped limit the hull tipping.

Ok now for the next evaluation to see how the LWL hull canoe shape looks at 30 degree heeling angle compared normal level LWL, this was more to satisfy my own curiosity as to what it does look like rather than imagine it.

Really don’t know what to think looking at it …any comments ?

Finally I wanted to see how much the LWL changes when hull tilted 2 mm fore & aft, resulting numbers tell me when the bow is down 2 mm the LWL shortens 6 mm and when the bow is up 2 mm the LWL lengthens 14 mm.

How much do these numbers affect the boats sailing characteristics ?

Cheers Alan

It looks like you gain a bunch of waterline when it’s heeled, that’s good. I’m thinking about the gain and loss when you pitch the boat forward and back. I wonder if the design(deep chin and long sloping exit) are that way for use with a spinnaker. The spinakker would have a tendancy to lift the bow which would lengthen the waterline. This could be a bad thing as without a spinaker as the boat will go bow down which will shorten the waterline. Just a wild-assed guess but it sounds feasible:confused:

Wow Don, what you smokin :icon_smok there buddy :lol:…Spinnaker not allowed in class rules, love to have one just for fun playing around with, but far to complicated on RC model, first thought that comes to mind is the extra weight required in the bow would offset the lift it could possibly create.

The key to keep it going bow down (my opinion) is to have enough volume in forward part of the hull displacement together with having balanced weight distribution of fittings inside the hull, this is why I was looking closely at CB in construction as “the” major reference point by building to maintain this static CB position in balance, the final deciding factor would be adding the bulb 5-10 mm behind the CB line to have an artificial LWL with 2mm bow up.

Then under active sailing conditions the bow would go down 2mm and then be sailing on her designedl LWL, well that"s how I imagine it anyway, then easier fine tuning comes into play (having the correct fixed CB) with CE adjustment, maybe Claudio can correct me if I’m wrong here.

Cheers Alan

Note: btw I “fine tune” CLR by changing size of the rudder and found that by having varying weighted rudders can also help “fine-tune” the artificial LWL, maybe an over-kill by some people but I find it works very well for me.

Hi Alan,

You are right , the bulb position shall compensate the weight of various masses to keep theoretically and statically the LWL horizontal.

Aside the static adjustment with some bow lift, during normal close hauled sailing , the wind on the sails exerce a force to push the boat down in the water probably more then 2mm, perhaps also 5mm.
Considering also that the CE is slightly advanced compared to the CB, it is necessary to compensate the nose down, by increasing the foward volume.
A friend of mine, calculated the extra volume needed to be around 11% at 30° heel and 5.5% at 20°.

For the foward volume, I intend the volume of the hull above the water and not only the volume under water.
This because during the nose down, the constrast shall come, in my opinion, from the volume that is available above the water line.
To be efficient, this volume, should increase very ‘rapidily’ and the shape that offer this characteristic is the V shape. By comparaison a U shape do not offer ‘rapid’ volume variation.
That’s mean that the contruction should develops an increased volume compared to drawings . The fact that the lamination is done above the wood master, the thickness of the lamination will help by adding some small buoyant volume.


Sorry I wasn’t clear. I know that spinnakers aren’t allowed, but the models rocker follows the general shape of a real AC boat. They are/were designed to fly a spinnaker.

Hi Don, understand what you were saying, no worries :wink:

Claudio, I have a what may be a stupid question to you, but I really don’t know the answer & would like to know.

If we reduce size of boat rudder (let’s say by 30%) and move the keel fin aft (lets say 30 mm) and keep the bulb C.G is orginal position (make it longer) in other words we have larger lead.

What would the resulting behaviour would you anticipate such a boat would have with such change ?

Cheers Alan


Claudio, I have a what may be a stupid question to you, but I really don’t know the answer & would like to know.
If we reduce size of boat rudder (let’s say by 30%) and move the keel fin aft (lets say 30 mm) and keep the bulb C.G is orginal position (make it longer) in other words we have larger lead.
What would the resulting behaviour would you anticipate such a boat would have with such change ?

Hi Alan,
The Lead has nothing to do “directly” with the the size od the rudder or position of the fin, but more with the CLR position, created of course by the appendages.
Seams a strange game of words but what it is important for the lead is to have a starting point that is the CLR.
The Lead according to my graph is depending , among others, by the ratio Bwl/LWL.
Now, if you reduce the rudder surface , the fin surface become more predominant and the CLR will move forward and the Lead will advance toward the bow.
So what you suggested , will produce the contrary effect you were looking for !
Further, by recessing too much the fin, the boat will become difficult to handle during running course because of the shorter distance beteween fin and rudder. The turbulences created by the fin wil also disturb the rudder.
I guess the boat will be rather unstable !

Longer the bulb, higher the horizontal oscillations at any change of course ! Loss of speed waranteed !!

I may be wrong !!! you try and let me knows !!


would you all not say that each designer has his/her personal quirks???
for me. I put my mast around 45-47 % of the hull length… so lets say a hull is 1000 mm… I would put the mast in at 460… but also that would leave room for the fin leading edge to be at 500…all this depends on the winds that you sail in…I tend to like to have the wind pull me in the direction… now as far as the fin… i look at the CB and the CLR. most of the boats I draw have the bouyancy around 500. so that is where the hull can support the weight. that is where the fin goes… i know it is a simple way of thinking. I have drawn over 30 IOMs and built them all to see IF i know what i am doing…
all i can say is that I am in the ball park. you need to read some book talk to people… but i will tell you 1 thing…
I built my 3 boat with a real screwed up rig… i have a mast mount with 10 holes so that I could move the mast back and forth starting at 450 -550 … same with all the rig attachments… doing it this way you can learn WHY the mast goes in a certian way…every wonder why a boat with a mast too far forward behaves a different way??
there are 2 books i use almost like a bible…'s+elements+of+yacht+design

both these books have been invaluable.

good luck

Hi, Claudio
where are all your drawings gone? I was just looking for the diagram showing the lead in dependence from the LWL/BWL ratio …