IOM's from ClaudioD

IOM’s are not my ‘Cup of Tea’, but it is too cold to use the garage/lab for 10R and Class M sanding, so I decided to run for an IOM design ! As a matter of fact they are two and here they are.

One is based on the “Narrow Deck” concept and the other calling for the America Cup monohull. Both are exploiting the ‘anti-drift’ surfaces of the ND and the deep “V” when heeled.


After months of absence, I’m back to recover the IOM designs. First, the SugarBox is abbandoned for reasons that will be explained. ND has been updated.

I like to start with the IOM Rules.

Every designer knows that the IOM rules are almost closed. The only parameter that a designer can change easily is the width.
The minimum weight required is 4000g with a maximum allocation for the Keel of 2500g and limiting the bulb weight to 2400g.
The Hull draft is limited to 60mm and the overall length to 1000mm
The max Keel depth is 420mm from Water line level
Mast is in aluminium or wood.
No carbon material allowed.
Sails are pre-dimensioned within small limits that can be freely chosen.

The parameter I’m most concern as with all models is the Hull Draft.

To make it short, every body knows that a deep hull is producing a deep wave when running.
A deep wave consumes a lot of power to be created and this power is taken out from the Sails.
Only part of the Sail’s Power is then used for the propulsion!

Every body also knows that another obstacle to the propulsion is the Wet Area.

IOM hull are rather short and the minimum displacement of 4000g is not easy to store without some drawing tricks that can combine minimum Wet Area and minimum Draft within 1000mm.

One of the first exercises is to establish what could be the immersed Main Bulkhead surface that shall be kept in mind as guideline for further developments.
Simple formula requires to fix main wanted parameters like LWL and PC :

Main BLK surface = (DSPL – Appendix volume / PC %) / LWL

Assuming a DSPL of 4050g
Assuming an Appendix Volume of 6.5 % of Sail Area (typical value)
Assuming a PC of 57% for medium wind conditions
Assuming a LWL of 98.8cm (1.2cm are used for the bumper)
The result will be :
4050cm3 – (6000x6.5%)_ = 3690cm3
3690/57% = 6473cm3/98.8cm = 65.5cm²

Thus the figure to keep in mind for the main Bulkhead immersed surface is : 65.5cm²

See example below :

Let’s take the drawing of the IOM CD1 .
Apparently is not far from the specified characteristics, the PC is 0.57
the DSPL is 4043cm3, the appendix volume is about 6.4% of SA (6000cm²)
the Draft is 54mm against the max allowable 60mm.
The LWL is 980mm.
The Water plan is 1172cm² - That’s means that for 1mm variation close to the LWL level, the volume of the hull will change of about +117.2cm3.

I would like to continue with this last point because in my opinion is rather important.
When the wind is blowing into the sails, a vertical component force is also exercised on the Hull.
Short calculations made by an expert, friend of mines, proved that at 10° of heel the wind force may push down the hull of about 1.5%, at 20° of heel by 5.3% and at 30° of hell up to 12.5%.
i.e. the 5.3% at 20° of heel is equal for an IOM class boat to 4000x5.3% = 212g pushing down force and 500g at 30° of heel.

This condition will alter the water plan form and hull volumes and the IOM LWL will not change too much since already close to the max length of 1000mm.

Assume the same conditions for a hull with overhangs. The sinking will produce an important variation also of the LWL length. It will turn out that in spite of the increased volume and wet area, there is a compensation derived by the LWL increase that will favours the increase of the Critical Speed. It is obvious that if the boat is heeling at 30° the wind velocity should be elevated.
Such a thing like Critical Velocity increase will not occur with an IOM type hull that exploits the maximum LWL length.

I wonder how many models designers take this aspect into consideration.

This is my first thinking !

Please pardon my ignorance on the subject matter.

I am certainly not educated in engineering, but I think that you are saying that an IOM should probably not be allowed to be overpowered and consequently heeled to 20 or 30 degrees because of increased drag without any increase in the critical speed of the hull. Reasonable to assume that smaller sail area to keep hull upright would be advantageous as the wind increases.

Another assumption is that the effective downward force could be varied to a certain degree by sail trim. i.e. allowing more rolloff (under powering) of sails to keep hull more upright. Would that tend to increase speed over the other condition of more drag? If so, do sailors tend to adjust the trim a lot differently on an IOM vs. traditional overhang type hull vessel?

I should mention that your posts, Claudio, are interesting an inspiring! Please continue.

Hi Clark, I do not suggest anything, the IOM are all the same and racing together, no mixing with other hulls. My reasoning was a sort of comparison with other type of hull with overhangs. Most of the IOM have Design Displacement slightly above 4000g.
Assume an IOM with a displacement of 4400g.
Assume also that the same IOM will weight 4010g in accordance with the Rules.
Of course this IOM would appear out of water a little more father but once in the water will exhibit a shorter LWL.
I do not know if I well explained my concept.
Under low wind conditions the hell angle will be also low and the push down force will be much smaller. The hull will lift up and the LWL length will be shorter.
1st observation: with shorter LWL and hull lift, the Wet Area will be reduced with consequent advantage with low wind conditions.
2nd observation: just the opposite, the wind will raise, the heel will increase, the hull will sink more and the LWL will get longer helping to recover the IOM critical velocity. The Push force will be compensated since already taken into account at design level.
I shall make a sketch about!
Actually on most of IOM hull the LWL variation remain very close at low and high wind conditions since the LWL is similar to the total Hull length.
The Hull will simply sink.
Now there are IOM with LWL of 980mm, 2 cm shorter then total hull length.
Shorter LWL will imply either deeper draft or wider hull.
My idea, that may be wrong, is to develop an IOM hull with LWL of 960mm and to get close to a draft of 50mm.
This of course will force to have a greater initial displacement and the draft will be automatically adapted to the Wind force. Why shallower hull? Most of hull drag is due to wave making resistance that produce a hollow wave at middle length. The hollow wave consumes a lot of sail power.
Reducing this phenomenon, the boat speed should increase.
Hope was sufficiently understandable.
Practically I should design a hull with a larger displacement instead to be close to the minimum Rules of 4000g and this in order to anticipate the hull sinking when the wind and heel are increasing.
Next time

The basic principle I follow.
At model level a deeper hull produces hollower wave

Hi Claudio,

One thing to note in the modern narrow IOMs is that the boats are full forward (the volume has to go somewhere - so it is moved to the bow). Also, the water line at the stern is right at the transom, but the waterline forward is back from the bow as much as 20mm. This gives some reserve buoyancy forward to help resist the pressure of the tall rig, especially on a run. So you may want to redistribute the volume to give the boat a ‘bow up’ attitude.


Hi John,
You may be right and on my side I have already taken some precautions.
First, the volume that count is the one above water and you can observe the nose shape from the front Shadow.
Second, I worked on the hull lines in order to recess the LCB. noting that the LCB is moving further backward when the boat start heeling.
Third, The LCB recession allow to move backward the Fin and Bulb in order to increase the distance between the CE vertical line and bow. The Arm rotating point of the full boat is about 150mm below the hull bottom.
Fourth, arrange the water plan lines in order to fix the Center of Flotation when the boat is heeling to get the best Balance

Hope to make some drawings !


The prototype IOM-ND Hull construction is just started with the Polystyrene foam method.
Here below the pictures taken:

Pic 1 Bloc cutout following the Deck outline and then sliced at interval of 96mm. Very important to trace the Center Line where the shadows will aligned.
Pic 2 All shadows are printed on Bristol paper A4 format - Color chosen to contrast Polystyrene color. Note the Center Line and Shadow alignment.
Pic 3 Bloc marked with shadow’s outline… This mark in Red will be the ‘alert’ to indicate during sanding that the shadow contour is very close and soon the carton color will appears.
Pic 4 Shadow paper inner contour cut away
Pic 5 Shadow paper bonded on polystyrene bloc with White glue
Pic 6 Shadow paper under gluing

Continuing Pictures taking :

Pic 7, 7b, 8, 9 showing various assembly steps for bloc’s bonding
Pic 10 All blocs bonded with Epoxy distributed in the inner shadow space cutout as per Pic 4

Other Pictures showing construction process.

From pic 11, 12, 13, 14 shaping with manual cutter
Pic 15 showing paper sanding work with grade 60, 80, 120
Pic 16 side view - notice support blocs to facilitate sanding work. Later will be also important to use the supports when starting glass-epoxy lamination.

Hi Claudio - for photo #2 - how were the blocks cut? By hand or on bandsaw or ?

Thanks, Dick

Hi Dick,
Self-made Hot Wire Cutter as in the picture

Work in stand by due to big back pain !!! sh…

Is the back better yet?
Construction on RG65 3b is still moving but painfully slow :frowning:

Hi everybody ! The back pain was simply due to a hernia at the lumbar vertebrae L2-L3. Chirurgical intervention the 2nd of July. Long and painful recovery at home for almost 2 months. Actually I can walk with the help of a walking stick and started Kine-therapeutic treatment. I can spend some time in front of the PC elaborating my new design concept based on the increased Hull Volume while keeping the weight close to the nominal one. In other terms a hull of 4400 cm3 shall be weighting a little more then a hull of 4050cm3, but offering a wider range of flotation and variable LWL. The picture below shows the parameter I have taken into consideration. My friend Pierre suggest that the vertical force contribution on a 4.5kg model is about : 68g /1.5% at 10° of heeling 151g /3.3% at 15° 263g / 5.8% at 20° 563g / 12.5% at 30° Assuming a Flotation plan of 1200cm², the 563g force at 30° will correspond to : 563/1200 = 0.469cm sinking at water level. In my opinion is a bad condition that can modify drastically the waterlines inducing more drags.

Hi, Claudio,
good to have you back on board!

The vertical force contribution you mentioned in your post can be calculated easily from the torque of the keel bulb (righting moment) and the torque of the wind pressure. Both values have to be in equilibrium for the different heeling angles. I did this caclulation for a typical RG65 design a while ago and came up with nearly the same (relative) values. They correspond to Pierre’s values within 10% deviation.

Thanks for confirming my approach!

Hi Haegar thanks a lot.
Just in attachment a simple explanation for my approach presented at the begin for my new Hull concept, ex.3 . Numbers not to scale, but the principle is there.
A larger hulls is only few grams heavier.
I shall calculate the surfaces to have more precise values.

According to my point of view, when the wind is low, the Hull will exhibit a short LWL, a smaller Wet area, etc. The down pressure will be very low.
As the wind force will increase, the down force will also increase and the hull will be pushed down. The LWL will increase to, authorizing a higher speed.

The point is find a compromise between an acceptable increased Hull Volume and the LWL lenght that shall be kept shorter then : 1000mm - bumper, when the wind is stronger.