and interesting idea

i was at a friends place talking over a few beers… we both design IOMs…and we came up with a question…
most IOMs have their fins at 50% or 50 cm… the mast at 45…I can understand that and the rocker panel is normaly the deepest At 50%,
but i was wondering . with all the nose diving… what IF we put the leading edge of the fin at 60 and packed all the weight in the stern…we could then make the bow alittle sharper, ala the Iacc boats. what i was sort of getting at is most people put their wieght in the middle to help with turning… keeping the ends light.BUT what if we could sort of balance the boat so that 60 would be the middle… and turn there… keep the bow thin. so that it could cut the water . and we could lose some of the bouyancy upfront… has anybody tried that?
I know it cuts down the lever action of the rudder.But you have to trade something…

what do you think?

Hi Cougar,

I treated this subject for long on various models. I told that shall exist an “ideal” position of the CLR/CE couple along the LWL !
I raised the question with architects …!

So far, the problem is not coming from the CE/CLR couple but rather from the bulb position.

The Hull shall designed to have the LCB positioned afterward as well as such to keep the bulb CG under the LCB.
Not easy to design a recessed LCB.
The Bulb shall always stay vertically very close to the LCB in order to obtain the LWL being horizontal to the water plan.

If the hull is not modified then the problem will come up with the Fin/bulb interface and attachment.

The risk is to have a reversed “L” FIN-Bulb configuration where the bulb CG will sit outside the neutral fiber of the Fin.
Larger oscillations may be expected when changing direction.

Rudder may loose efficiency because of the shorter distance from the Fin, and exit turbulences from .
Probably a larger rudder is needed or positioning the rudder in front just behind the bow

A notorious architect working on AC boats and friend of mines, told me once about this question, …“you are lucky, because with little money you can find the answers to your query …”


PS : a couples of sketches just made for the discussion :

There still the dual fin option as on the NewZealand !

My IOM is some-what in the direction your talking Cougar

Front edge of Mast @ 50 cm & front edge of fin @ 51.5cm, with CB @ 56 cm and bulb CG @ 61 cm from the bow.

The major difference is having the bulb CG 5 cm behind CB has been offset with over 300 grams ballast in front of the mast base.

When I first saw it looked strange and the previous owner gave me another bulb which has CG under CB (centre of fin) this also meant had to move the ballast aft to compensate balance and needed to trail it to for right balance again.

Anyway, put it in the water as it was and sails in the groove perfectly (hands free close hauled) is very stable with bow-up without any yawing at top of A rig conditions.

Turns & accelerates very quickly, like its name it “carves” through the water beautifully. If any bulb oscillation occurs, it’s behind and not the in front of the Fin.

Cheers Alan

you sort of get the idea. what i was thinking. was taking your third pic… but keeping the rudder in aft of the keel…and using all the radio gear in the aft section…this way, you would not have to worry about the forward bouyancy… and you could lower the hull resistance. would it not improve the over all straigth line speed?

Hi Alan,
if may help, I copied your IOM and added the dimensional data you gave :

I’m only a little skeptic about the Bulb CG position !

ok … now why would this NOT work… I know the balance maybe off a bit… but i cant see why somebody has not tried this… IF you look at the 12 meters…( real americas cup boats) the keel was behind 50% so all the weight was behind 50%. and they seemed to turn well.
thanks claudio

Alan . whos boat is that? looks interesting

the boat is the fastest Italian design by gianluca nardo…the position of the cb depends on the position of the ballasts, (300 grams!!!)

sorry, you are talking about iom, but here two example of centering (by nardo) of my boats ac100…
ac100A by claudio diolaiti, very light wind condition boat
ac100Brutal by eupon (me), strong wind condition boat

My friends and I all drive IOMs… and we of of the opinion that there is something that we could be missing…every design that i have drawn. has had the fin at 50 and the mast between 45-47 … but what IF we could shorten the boat. keep the waterline at 1000 but put all the wieght aft of 50 .therefore the amount of forward bouyancy would just be required to keep the bow up and out of the water… not adding the job of keeping the wieght also… just thinking with the fin alittle futher back… could you also turn better. less hull in the water to offer resistance

remember this is an idea talked about over beer… is there any good in this?

the problem to solve is the bulb position as said above.
The CB-CG is a couple that cannot be separated to keep the horizontal alignment as well the CLR-CE as propulsion driving combination.
If you like to recess all stuff added to the hull, then you need to design a new hull with the Center of Buoyancy (LCB) recessed of the same amount you like.
Generally for better dynamic stability and efficiency, of the actual model as well the real ones, have the LCB somewhere between 3-3.5% behind the central shadow n°5 sitting at 50% of the LWL.
An LCB positioned outside the curve range vs Relative speed is already suggesting efficiency losses. See diagram below
Shifting the CB/LCB by 10cm is ‘enormous’ and not at all evident, of course the volume will be all sitting at the back and the bow could be as fine as you said, but …
In the third pics above, you may have noticed the underwater shape and the rocker shift toward the back.
At the end you will need to verify the CF in order to control the dynamic balance stability when sailing.
The rudder at the front is not simply meant to transfer some weight but rather to stay out of Fin turbulences.
Some new real boats do use also a second rudder at the front to compensate the reduction of fin surface when tilted to avoid drifting. see below

PS: added a Curve of Areas simulation
The red curve is rather typical of a good balanced hull, the blue curve I do not know what could offer ! see bulkhead positions and the large LCB percentage of - 12.5% against the normal -3%

Hi Cougar, This my IOM which was the 3rd prototype (of 4 boats I think?) in the development program of the new Italian IOM “Carving” designed by GianLuca Nardo (ITA 09) Hull construction was done by William Mazzotti and Luca built the sails & rig.

Luca has since sold the Carving design and is now commercially available here

Luca (ITA 09) is one of the top Italian IOM sailors and from his experience he has developed an extraordinary skill in how RC sail boats should be set-up. Every boat of his that I have had the pleasure of sailing is always perfectly balanced and very fast, he is also winning few major regatta’s in Itlay recently, but I should say, I don’t know how he has balanced out his own Carving.

Claudio I was like you being curious about the bulb position too, but I can only tell you that it works perfectly !! only wish I could have my IACC 120’s sailing so perfectly, few more photo’s

Cougar, if you have the time I can only suggest that you create what you’re imagining and explore !!! :smiley:

Cheers Alan

My 2 cents, just drawn this morning at glance.

The displacement is till too high since normally should sit around 3700cm3, this means that it can be made narrower or less drafting.

As can be seen, the LCB is at -8.5% and this is very much when compared with -3%.
I should check the variation of the CF.
Of course all what is above water can be modified as wanted to include eventually the fashion chine.

PS :
CF added and a slight shift of 0.6% backward is observed . Is not dramatic, but was preferable to be forward

Claudio’s point here was one of the most mis-understood for myself, that is not discussed very often :confused:

Having CF as close as possible to CB certainly reduces changing CLP of RC hull when sailing up and down wind, one of the other differences between real boats where you can move the “human ballast” when sailing different points of the wind, which we cannot do with RC sailing boats.

I had ballast in one my AC 120’s (400 gram) positioned half way between mast & stern and had good balance in light-to-medium air, but in heavy air it had bad weather helm (no bite with bow) but the boat never submarined going downwind.

To recitfy heavy air weather helm, I could not move mast any further forward without major surgery, so I moved keel/bulb back 20 mm and moved 200 gram of the ballast to the base of the mast, the boat now has very good helm balance throughout the wind range with increased the lead.

Now, sailing down wind she feels likes she wants to submarine in heavy air (slows down) as it looks like the bow wants to go under as stern comes up out of the water, then at certain angle, I think the combination bow volume (bounancy) with aft ballast, keel/bulb then appears to outweigh the CE and swings hull underneath the CE again (pulling bow back up) and she scoots over the surface of the water like a rocket !

I’m quite sure I really don’t understand all the forces & balances points of sailing vessels and how they interact with each other …but it bloody lot of fun learning by playing with them. :smiley:

Cheers Alan

Chine with a tumblehome ?

may be! just to please the fashion ! I’m not intending to go any further anyhow since IOM are not my cup of tea !

About CF, has nothing to do with the CB. The CF is a parameter that show the balance stability from upright and tilted boat.
Best balance is achieved when the CF will stay close to 1% variation preferably forward.

Kiwi’s IOM looks suspiciously like a Bantock Pikanto. If this is correct then the original fin has 2 degrees slope aft. This would put the bottom of the fin over the centre of the bulb. The current fin is a modification.

But I have been known to be wrong

OOPs just read page 2 forget everything I said

Funny you should say that, on another site people thought it was similar to another well known IOM design … The world of IOM design is more like car design … where everything today begins to look similar … but is somehow different !

The simple answer to your question, has a design with the fin moved aft of center worked, is yes. Take a look at the Lintle. The leading edge of my fin is 540mm back from the bow. I don’t have the fin in to measure the balance point ready to sail, but I can do that later if you want. all of the radio gear is located aft of the mast, so I did add 165grams corrector weight in front of the mast to get it to float on it’s lines.

It is also a fair amount wider than most designs, not just in the aft areas, but the whole boat, with the draft reduced as a consequence.