Hey Brad, Ozzie colours on Kiwi boat !.. don’t tell your mates ? they will rag you forever buddy
Claudio, I’m about to start fixing the bulb to the keel fins, I planning on having a 5 mm adjustment of the bulb to fine the right position when I finally get into the water.
My question is: As a starting point, what correct position of the bulb C.G in relation to the hull C.B ? currently have guessed bulb C.G to be 15 mm behind hull C.B line … or is there a formula or something else I can use as a reference point ?
Hi Alan,
in teory is OK,
From one boat to another there is always some variances.
This is the only criteria I hnows, the bulb, among others, shall be fixed in order to keep the fully equiped boat in horizontal position on the still water, along the water line.
You remember the " Floating decimeter" presented under the AC33 discussion ? This simple instrument I used to ensure the horizontality. The height of the bow and stern are known and easy to check.
Once obtained the horizontality, is a good practice to slide the bulb a couple of mm backward in order to lower the stern. This asset will become normal again once the wind will start blowing into the sails.
From the plan you can also fix a scotch ribon on the bow and at the stern where the water line is supposed to pass and mark the positions for the time needed to fix the bulb.
Cheers
Claudio
I got a long exchange with Luca, the winner of IACC 120 Cup, about various methods used in order to find out where to put the CV and consequently the Mast step.
Obviously everything is linked to the position for the CV that need to be found first.
The two methods :
The Luca one, is based upon the search of two CAD 1 & 2 Not supported by any theory but establishing at priori a 10% of LWL as advance, trimmed after with an empirical methods. As depicted in the attached drawing.
The mine is based on the CLR position found with the famous carton, use very often.
In principle, I should give a weighting factor to the hull and bulb since their counteracting efficiency is not very high, while I can assume 100% efficiency for Fin and Rudder.
The basic experience show that an advance (CLR - CV) of 8.0% to 9.5% will be sufficient to trim the boat.
My method adopt a percentage that can vary in theory as reported on books, from 8% to 12 % of the LWL, actually averaged for the AC120 to 9.5%, but appears to be too high, better if around 8.5%.
Obviously many others factors may induce to get finally some adjustments.
Claudio & Luca, thanks for the update very interesting but when you use the term CAD you are reffering Computing Aided Drawing calculations ? … just wish to be sure that this not an abbreviation for something else.
I recall the " Floating decimeter" you presented us in the AC33 discussion, I shall follow this process, thank you.
Today I had final testing of water migration through the FBG and we are now 100% waterproof !
Needed two filler applications on the hull, 1st application showed water migration still happening and only after 2nd application was it ok, photo’s are using super marco function on my camera as I could not see the holes with my eyes … in need of glasses I think
Planning to close hull up this coming weekend. Cheers Alan
CAD = Centro Anti Deriva just the same as CLR in english.
After a long talk with a professional expert, I revised my text above.
Personally I will continue to stick to the “Carton Method”, until some one will come out with a proved and mastered theory.
I know is not perfect, but is working reasonably well.
Too many variables and unknons.
One exemple, take a hull, charge her with some weight a such to tilt by some degrees and then give a push. The boat will start turning.
Are all the boats turning of the same amount ?
Certanly not, but this parameter should participate in the establishment of the “ideal” position of the CV or not ? etc. etc…
Back to your nice work. Very effective, but how many gramms have you added ?
What sort of filler you used ?
GREAT example of what some of our builders should be aware of if building using fabric cloth. Appreciate your taking the time to post, and am requesting your written permission to use them as examples throughout the sailing hobby.
I’m with you I will stay with orginal CLR position for the time being
I used an off the shelf car fibreglass filler from local handy mans store, I’m on business travel at the moment & can’t recall the brand. It was diffulcult to fill the pin holes but sanded very easliy. I will also check additional weight when I return but I’m sure it would not have even been 1 gram as pin holes were micro-scopic.
Hi Dick … no problems you can use photo’s where ever you like, if you wish I can send you orginals without my K1 imprint, just let me know?
Never mind Alan, this stuff is all the time the same “polyester” filler.
If you are interested, the AC120 Plans collection will be filled up soon with two new projects : AREVA and DESAFIO
Cheers
Claudio
It’s really great to see u guys so active in the RC sailing scene … may i suggest u guys to open up a thread just for plans ? It’ll be easier to locate them whenever needed without searching through the building thread
I’ve downloaded the ETNZ and some other plans from here but not too sure whether that is the updated one.
You can download them from here : http://iacc120cup.altervista.org/Main0E.html
The only updating concern the Lead % between CLR and CE.
The data are updated taking into acount the test results obtained during racing. Many models are under construction but few are already in the water.
Actually this Lead is located bewteen 8.2% and 9.5%. This variation is due to the size and shape of the sails from 80dm² of rig1 to 60dm²of rig2-
From the same site you can also get the Class Rules.
I was looking closely at the schematic of the CLR in post # 501 (above) and noticed that you have the Keel bulb on tilt of what looks like approx 2 degree’s knowing your excellent drafting skills Claudio, this is not a mistake ! can you explain why this particular hull has bulb on an angle like this ?
Sorry to hijack the thread (again) but my displaymodel is allmost complete, the only thing that is left is the rig. But my next model will be a very fast sailing one, my only regret is that I don’t have the plans for Victory Callenge, since I’m Swedish. ( Claudio, would you be so kind;))
Hi Z,
nice looking hull, but I got the impression that the fin is closer to the bow or is just an optical effect ?
Sorry The Victory is not yet drawn also beacause I have very little informations about her.
Sincerely, having translated the original appearances, since I have no plans, into a AC120 class, most of the peculiarities of each boat disappears. The Rules do not provide margins of interpretation also because the scale 1:20 is not observed at hull level.
Just recalling the dimensions of the real AC Cup LOA = 24mt, Displ. 24000kg , ballast 20000kg, Mast 35mt
Reducing all that to the 1:20 scale, one should get : LOA 1.2mt, Displ. 3kg, ballast 2.5kg, Mast 1.75mt
Under these conditions the model could be a real leaving room copy but it could not be capable to navigate . Simply beacause the difference between 3.5kg and 2.5kg will not permit, with the 0.5kg left, to build the complete model of 1.20mt lenght.
So the “escamotage” was to increase the displacement to 4.5kg and limiting the ballast to 3.0kg.
Obviously we got a rather “fat hull” . Difficult to create differences.
If you take the AREVA, it appears to be close enough to the shape of Victory and then you could use the Swedish decorations. Some had already used Alinghi model with ETNZ deco.
So long
Claudio
Claudio, thanks for your exellent info (as always). I will look into the drawings of Areva and see what I can do. The fin is where it should be, it must be the angle from where I took the picture that makes the optical effect.
Lester Gilberts document is very interesting reading to me for two reasons:
1 Bulb Shape: he states for IOM’s is usually a simple torpedo shape and creates much more drag relative to its very small contribution to lift. As you know I made 1 bulb myself using torpedo shape. I also brought 1 bulb from TD Models (Italy AC120 parts maker) which has a wider flatter shape.
This surprised me as I wondered why a wider flat shape, but did not think any more about it at the time, as most Italian AC120’s are using this TD bulb. but I will come back to this later.
2 Bulb Tilting: Lesters conclusions that tilting the bulb (and he refers to IOM class only) is that it does improve the boat’s handling and performance in the stronger winds, but will also hurt it in the lighter winds, therefore the amount of bulb tilting is dependent upon the kinds of winds you are sailing in.
In winds above 6 knots the suggestion is 3-4° tilt and in lighter airs reducing the bulb tilt to 3-2° reading this article convinced me that if works for IOM’s why not AC120 so I will incorporate a adjustable tilting bulb on one of my AC 120’s to see how effective it is.
The question is how to make the bulb adjustable ?, any ideas from your side ? I am thinking of placing two small screws in the bulb, 1 each side of the Fin bolt going through the bulb to adjust the bulb tilt against the base of the Keel Fin.
Finally coming back to bulb shape and putting it together with the concept of a tilting bulb. If in fact, by tilting the bulb the leeway angle will minimise the bulb drag. Would not having a flatter wider bulb increase drag compared to a torpedo shape ?
I ask this question as I have not drilled the TD bulb yet and thinking that maybe I should turn the TD bulb sideways so that the bulb is not mounted in wide flat position on the Keel Fin but in Tall narrow position
First are you sure that the TD Bulb is keeping the symmetry on both sides ?
Second , is cheaper to make a new fin with a tilted bulb. You could swap them during the testing.
Cheers
Claudio
A wider flatter shape will give a better end plate to the fin, improving its efficiency; think about the little winglets that have appeared on the ends of airliner wings. For a fixed draft a wider, flatter shape will also lower the CG of the ballast providing a marginal improvement to the righting moment of the boat. The downside is that it will have a higher surface area than a ‘rorpedo’ so produce margianlly more drag. Like everything else, if you take a gain somewhere you’ll pay a price somewhere else.
Hi Claudio, both bulbs are bilaterally symmetrical, the difference is the radial symmetries as you can see in attached photo are different.
I will change Fin/Bulb combinations between my two boats when in testing.
Ray, I fully agree with what you are saying, excepting you are assumming the bulb is always in maximum efficiency position when sailing, Lesters point is just that … it is not always in the most efficient position when the hull is heeling the boats balance moves toward the bow resulting in the boats LWL & bulb having 2° down tilt, Given this sitution, a wider flatter bulb would create more increasing drag as the boat is driving forward
My question to help reduce drag (with or without bulb tilt) was would it not be better to run flatter surface vertical and not horizontal together with 2° up tilt to maximise the bulbs efficiency?
I was about to drill the TD bulb when this topic came up and was seeking opinions before deciding what to do with my TD wider flatter bulb :rolleyes:
Yes, Lester’s points, as always are interesting, and I understand what he is saying; I wasn’t making any assumption regarding these points, merely explaining the possible benefits of a wider, flatter ballast shape. You might also consider that a wider flatter bulb on a heeled boat will act like a secondary fin and produce some lift that might counteract leeway. At the end of the day the variations we’re talking about are going to be small and swamped by the ability(or not) of the skipper.
knowing your excellent drafting skills Claudio, this is not a mistake ! can you explain why this particular hull has bulb on an angle like this ?