I have mentioned that the wetted surface, would this not be a good start?
I am concerned that the playing with the fin length & bulb weight and compensating with the sail area is going too far.
There are so many variations to test which is expensive and two boat testing is a real advantage in this instance.
If you scrapped the above and stuck with the original max draft or pushed the draft to a fixed 450mm then I would have no issue with the proposal as it Simplifies the whole rule and you go racing!
I am curious, is a rating, or a rule being proposed? A rating, in my mind, would be similar to the full size Performance Handicap Racing Fleet (PHRF) or the Portsmouth Yardstick used for dinghies, both used for races with dissimilar boats and racers can win on corrected time. A rule, is like the IACC, the International rule for the “metre” boats, the Universal rule for J, Q, R etc boats, Skerry Cruiser ‘Square Meter’ rule, or rules for dinghy development classes lsuch as the International 14 or the Merlin Rocket in the UK, and all of these classes have ‘first to finish’ type racing.
Maybe there is room for both types of racing within the IACC120, if there is an interest in building the older designs. Or, like the racing within the 6, 8 and 12 metre classes, a classic or vintage division and a modern division are raced, without any type of rating or handicap system
I do see problems with adjusting too may variables prior to a race…full size sailboats don’t usually change the keels before heading out of the harbor…
Hi Tood,
It is a proposed guide for “Rules” indicating the extreme parameters and the “Rating” resulting from the choice made.
The rating systems you mentioned cannot be applied to medels, unless you have a precise idea on how can be done.
The proposed “rating figure” pertains to that boat model only and for ever and used for all races.
If the owner decide to modify the essential parameters, the boat shall be cerified again.
This principle, is applicable by all Rules and not limited to the rating formula.
Jim,
probably you are not aware that the AC120 is rather under/over dimensioned, either by the SA or by the Bulb. It is sufficient to apply the static stability moments to find out.
Proposing a longher fin or an heavier bulb or limiting the minimum and maximum SA, will help to improve the performances stability. Once the limit parameters are fixed by the owner, his boat will certified with the corresonding rating.
IMO the Wet area is not measurable with sufficient presision and I do not know what rating should be retained and how can be applied. In the proposal there are figure expressed as rating 31<>34, if you have a different good idea, we will be pleased to discuss it.
As Claudio says AC-120 hulls do not come from the same mould a wide range of “design” and “construction variations” are present within the class rule dimensions. You are right Jim the only differences in this proposed “rating” system is wetted surface coming from hull shape & trimmed racing weight.
Only two boat testing can approx prove this, but my opinion is that RC yachts are very sensitive to wetted surface area differences.
Once a boat is built within beam dimension rules (16-25 cm) the approx designed wetted surface is determined and the only variable that remains is the final trim weight.
These two basic variables are finalised & builder can then simply calculate trimmed yacht “rating” to find max SA that can be used for the regular two sets of sails (light & heavy winds) are then fixed, planned around the rating…and we go racing !!!
Racing on corrected time, makes things more complicated for RC yacht racing where we can have upto 10-15 races per day, my two cents is for first across the line.
The question raised about having a fix Fin/bulb length is a good one, as Claudio has highlighted AC-120 being under-dimensioned having same 42 cm length as IOM, original thinking was to improve point with increase to max 45 (variable length option) to give skippers more options based on weather conditions of the day without affecting the rating.
Which one does everyone think is better, fixed or variable fin length ? albeit variable would be limited by 2-4 cms (Keel box limitations) for single fin only. Rules don’t allow to change fins during the day.
I think that the discussion could be interesting… very interesting for developing different projects but not to simplify the problem for skippers, organizers and so on…
The exactly same formula were used in Turin and also in AC120 rule for few years a lot of years ago… than there was an evolution to the actual one that is more simple to be used, measured and understood from skipper and modeler.
Anyway the starting point is not so correct:
The max total draught (lengh of fin + bulb+ hull immersion) of a IOM is 42 cm: OK
The max draught of AC120 for the italian rule is 42cm (fin + bulb) + 4-6 cm of hull immersion that, finally, give 46-48 cm of total depth and quite a similar ratio between heeling/righting moment of the IOM (approx only a 15% less for the AC120 depending from different sail plans)
Considering that the AC squared mainsails could loose more lift in the higher part due to twist, when wind come up the balancing is not so different and we see a lot of time this situation in our Regata field in Aosta where IOM and AC120 sails together at the limit of Rig 1. When IOM need to change sails to use Rig 2 also the AC120s need to change rig… not before.
Near to the “usefull” limit of IOM rig 1, for example, my first AC120 Black Urca seems cope the situation better that IOM.
I suppose due to the quite triangular main sail used, as it is possible to see in attached images
The Italian AC-120 rule is simple YES but Fair … NO !
It loads the “playing field” in favour of narrow beam hulls that have lower wetted surface compared early wider beamed AC-120 designs, as touched on eralier with Renato’s ITA-25 with wetted area or 1,680 dm2 compared to USA-98 wetted area of 1,620 cm2 and both hull designs able to use same max SA area of 80 dm2, but don’t you agree that the 60 cm2 lower wet surface area of USA-98 is going to advantageous ?
The proposal here is to open discussion to “even up the field” where the likes of ITA-25 can use max SA of 80 dm2 while USA-98 using rating system is limited to max 77 dm2.
Thanks for the heads up on IOM “total” draft being 42 cm from LWL, I’ve seen the 6 cm hull draft check and admit I assumed it was “added” to total draft limit of 42 cm bulb, not “deducted” as we do in AC-120…maybe there is no need to increase Fin/bulb to 45 cm, we just leave it 42 cm + hull draft of course.
Just to adds my 2cts, the comparison of 3 models IOM, Class M and ac120 in terms of bulb dimensioning, I check
3 differents set of formulas, including simple approximations for static righting and heeling moments.
Only one conclusion : the ac120 bulb is too light by about 800g when used with 80dm² and 5m/s wind speed.
claudioD
Hi Jim,
a fast check out of 4 models, I found an average of 1234 cm² for the Water plans surface. This means that the minimum displacement increase with 1mm sink, will correspond to 123.4g.
It is not necessary to recover the max 800g missing since the use of 80dm² is only for low wind conditions.
Personally I will be happy to increase the actual bulb of 2.85kg to 3.2kg with a delta increase of 350g.
Now taking the 350g / 123.4 = 2.83mm hull sinking which is a worst case. The Water Line Lenght will increase from 985mm to about 1000mm as fonction of the overhangs entrance angles.
The ac120 rules “last revision” accept up to 1030mm.
Longer fin could still keep the heavier bulb if so desired, within the rating, noting btw that longer fin will increase the hull displacement and compensate the LWL increase.
Ciao
Claudio
PS : for those would like to play with numbers see the attachment below. The first formula is very simple and can be played modifiyng step by step until the results are similar. Interesting to note that with two different formulations the bulb weight for the ac120 will be 3.65kg for one formula and 3.74 kg for the other !! and I repeat, with the improbable use of 80dm& Sail area with 10kn wind ! authorising therefore a smaller bulb .
Why couldn’t you allow the removal of the equivalent corrector weight in the hull to compensate for the new bulb weight? Reducing the mass a bit… Would help keep the acceleration.
I apologize if I haven’t read all post of this discussion, but my opinion is extremly simple: I don’t like to introduce rating in IACC120 Class.
The reason is very simple: are you sure at 100% (and say 100%…) that the formula used is correct also for scale 1:20? Why transform a simple things in a complex things?
Only becouse in your opinion a model-boat with 290mm of beam is slower then a model-boat with 200mm or less? For me rating could create only problems and controversy when you lost a regatta… becouse is too simple to say: “the rating formula is wrong!!!”
More of this, I remember to all that 99% of IACC120’s project has a beam less then 200mm and the Moro is the only one exception. If you think that Moro could be slower then other project, I have a very simple alternative solution instead of Rating: DON’T BUILD MORO… simple But Il Moro has a wonderful design and, maybe, is also very fast!
And even if Il Moro could be slower then other project on the paper… in my opinion the truth is not on the paper, but in the water… in the wind, so… let’s put all IACC120 toghether behind the same start line and… WIN THE BEST
Just as it happens, I have a blank for a new bulb (10.7g/cm3)
It’s been sat under one of my tool boxes for a while. Maybe I should start shaping a 3.2 kg bulb, remove some of the correctors and see how she goes…?
if the Victory is still the one I have drawn, the water plan was of 1152cm² and the Lwl was of 960mm and a beam of 177mm and the bulb of 2850g out of 4472g without additional ballast to reach 4500g minimum.
Increasing the bulb weight will increase the lateral stability as obvious and reduce the heeling and increasing the sail performances.
Under above conditions with 3200g the hull will sink by 3200-2850 /115.2 = 3.0mm and the LWL will increases to 989mm
The hull draft will pass from 51 to 54mm and this is not very good but still acceptable.
Good to try and see what happen. Certainly a hull design around 3.2 or better 3.3kg would be better if the maximum sail area is desired.
Personally I beliewe that 78dm² is already too much for the actual bulb size and probably 76dm² would be better.
Reducing the heel of 78dm² from 30° to 10° , because of a larger bulb, the exposed sail surface will increase
Cheers
ClaudioD
Hi Alan,
with my past message I’d like to correct only the wrong interpretation of the IOM and AC120 rule. If is true that IOM are a little bit “on the paper” more balanced, performing regatta on the same water field and together shows immediately that it isn’t a problem: they sail together till to change rig 1 to 2 and AC120 is always faster. I’ve observed this directly with my AC120 and my IOM together with others: in piemonte in the past there was a couple of “club” competition with all the boat =/> than one meter together
I understand well your intention and it is an interesting proposal but, from my point of view, it is very difficult that a “large” model sail boat (especially with ACC shapes) could reach well all weather performance.
The first IACC was larger than the last ones not because formula was changed: for hull measurements, quite nothing changes from rule first version to last one.
The optimization of design and mass reduction allow to use more weight in the bulb so the research study performed demonstrated that the “ faster” IACC isn’t a “large boat”. The shape stability doesn’t win against weight stability.
In scaled models is quite the same, but with more advantages in making boats with short beam because the possibility of heeling stabilization in the right optimized angle for a large boat is more difficult, quite impossible.
A large boat, when go over the good angle, loose completely the equilibrium and the performance become worst due not only by wet area increased but also for the worst shape in water and for the angle of attach of appendages that change a lot making a lot of drag.
So, I think, the question is: if you want to make a model class similar to true IACC, why you want to try giving advantages to boat that the evolution of true ones made obsolescent and withdrawn?
Is this the “spirit of IACC rule” that you want to serve?
From my point of view the answer is: NO
The actual AC120 rule is quite a simplified IOM rule that allow more “free space” to modeller and avoid any hi level techniques for materials and cost. It is sure not perfect but is a good compromise that allow to play regatta with beautiful model that seems IACC.
The true IACC rule allows development and if you want to make true improvements, the formula don’t need levelling performance in low level direction to allow a potentially slower boat to sail together with a potentially faster boat.
If you like to do this could be useful introduce a Static evaluation of boats that give “points” and make competition ranking with static evaluation points + regatta points like in model airplanes F4C categories… but it is again another rule very difficult to be implemented.
An other way is to use a scaled true IACC rule like in the past in France try to do with AC10 (in this case 1:20) in this way is sure that the commonality with the real ones is completed but I think that only very few modeller could design and manufacture a similar boat.
Bye
Hi Claudio,
SWE 96 is my own interpretation, she is very like her big sister.
The rocker is around 45mm @ 4500g my aim is to stay at 4500, but increase righting.
However, there is no way I will sacrifice any sail area… As I have said before, the performance already is great upwind, however you will loose out downwind…
… NO !
But Il Moro has a wonderful design and, maybe, is also very fast!