Sorry about that and tank you for notice it.
The one has missed something is me that I forgot to retrace the CE, was going too fast.
I picked up an existing plan and just added a possible shape for a genoa as Alan was asking and forgetting totally the repositioning of the CE. I will go back and correcting the missing info.
A question remains nevertheless, the overlapping area count for 100% as contribution to the shift of the CE ?
Cheers
Claudio
Claudio I only have weekends to work and some week evenings, I’m sure I will be playing catch-up with you from here on in
Row …interesting perspective, I always thought that centre of effort, when viewed from the side is the balance for force that induces boat heeling, this is why the amount of overlap is never taken into consideration when measuring sail areas, only the triangular area of the sail in front of the mast and therefore the CE would always be the same position, regardless of the overlap %.
My thinking (perhaps wrongly) was that an overlapping of the headsail of the main does not mean that CE changes but the overlapping headsails was for greater wind efficiency, especially of the wind travelling over the main.
Whoa just seen you update of post # 19 showing difference of CE between Jib & Genoa is 42 mm ! the standard mast/keel box I use only has 35 mm adjustment in the mast box (including ram) and it not impossible to have the CE at aligned with front edge of the keel fin with Genoa.
I need a re-think the changes required for Genoa on this build :rolleyes: any ideas Claudio ?
I can’t even pretend to be an expert on naval architecture - my understanding of CE & CLR etc is purely from a practical perspective and the best part of 35 years and 80,000 logged nautical miles. Just kept looking at your original sailplan drawings and something just didn’t look quite right. Much head scratching ensued and eventually thought it best to put the question to you guys. Glad I did!!
Progress is looking good - are you going to be seeing anything of Mrs K1W1 Alan? You must have got her hooked on the AC120’s !!
Row,
you raised a good point, and actually I do not know where is the static sail plan CE , once the genoa is used, simply because on my models the genoa was never employed.
After your remark, I modified the drawing assuming you were right.
I went seaching on the net and in mine books and there I found controversial statements just enough to confuse me again ! I’m still searching !
Do not esitate , we are always on the learning curve !!
Looking at Claudio’s calculations with the Jib, CE is 47 mm behind the mast, the theory that increasing sail surface in lighter airs that CE moves forward on a Genoa, defies my logic.
Genoa’s are light-medium air headsails generally rated as #3 @ 100%, #2 @ 125%, #1 @ 150% overlap of the main, therefore as wind velocity decreases we go up to larger Genoas and “if” the CE does move at all with Genoas, it logically must go backward and not forward.
As wind velocity increases, don’t we “use smaller head sails to move CE forward”
Cheers Alan
P.S Hi Row, the missus hooked on model sail boats ? :lol: far from it, prevouisly I was living in Asia and a serious golfer, hence not home as often, as courses here in Germany are damn expensive total rubbish in compaision I took up model sailing here & guess she see’s it as the lesser of the two evils.
Like you, CE moving forward generates serious head scratching.
From your comment: ‘use smaller headsails to move CE forward’
While it may move the CE of the headsail fwd, smaller headsails are also usually associated with smaller mains (reefed or otherwise) so the overall effect is probably negligible. It’s also worth bearing in mind (from a big boat perspective) that when the wind really gets up and warrants the use of a storm jib, most sail makers recommend that it be flown on an inner forestay where possible to keep the resultant forces between jib & trisail (or heavily reefed main) as close to the designed vertical CE position, albeit lower, as possible. I know thats a slight over simplification, but I think it makes the point.
From earlier posts, from a design perspective we assume the CE is static for calculating the lead over CLR, but of course once a yacht is underway both positions are dynamic. Modern big boat designs & designers all come from/use the same sort of software and yet they all behave/handle differently and some are as near perfect as possible (very small group), so I guess us lot trying to get a handle on it is perhaps slightly out of reach…??
You are right Row, it is impossible to calculate the precise centres of the aero and hydrodynamic forces with any accuracy as they are constantly moving in different weather conditions, I understand geometric centres are only a useful tool for static reference that will help predict how the boat will behave but a large part of that depends on hull design too … not all hull shapes behave the same way :rolleyes:
Claudio leaves a trail of crumbs in his earlier statement
This question possible leads into how the design of this boat is different to the trend of current AC 120’s currently being used, if you look back at post # 18 you can see the water plan compared to the hull shape that won the cup this year.
After lots of pre-discussions, I came to understand that Claudio’s central thinking on hull design that dictates this project is: to fight against nose down and reduce the wet surface by keeping the fluidity of the hull.
In the mean time I’m leaving the genoa sail plan on another separate project as the questionable CE leaves yet unanswered question mark due to possible keel/mast position conflict on NZL-20.
If I substract the bulb weight, the appandages and the ballast, that leave only 996g for the rest. This represents +/- 25% less than Claudio’s earlier AC120 designs (SUI-100 at 1306g, NZL-92 at 1288g)! There is also a 7% reduction for the already tinny appandages.
I was ready to trash my first steps on my NZL-92 to embrace this NZl-20 who has, to my subjectively cosmetic taste, better lines. But after these calculations, it seems a project for seasoned veteran.
Is there some design subtilities or new building techniques that I don’t get or this diet is based on your building experience, great skill and desire to built the ultimate Ac120 racing machine?
Hi Sylvain,
the appendages are part of the total displacement since they produce a lift too, therefore cannot be substracted !
Now according to the AC budget developped years ago the following aplly :
Hull and Deck plus internal supports = 350g
Electronics servos + battery +Rx +cables = 200g
Rig for 7800cm² = 300g
Fin + Rudder = 170g
Total 1020g escluding bulb
Total with the bulb : 1020 + 2850 = 3870g
The NZL-20 above has a displacement of 4389g including a passive ballast of 115g that are used to reach the 4500g required by the Rules.
If we remove the 115g of ballast , we get 4274g that compared with the 3870g required for the construction, will give some 404 g to compensate for errors !!!
Today I’ve cut the building board & frame bases in prep for laminating using latex later. Then pinned couple frames to the base board to prevent the stringers from springing the frames off the board, while planking.
Next bevelled the frames and finally glued in the keel plank (two 3x7mm strips) and used plastic tie-downs to hold curved position of the keel plank to the frames while glue was drying.
Finally checked alignment using laser … next stage is planking the frames, looking at having this done by end of this week, all things going well.
Hi Alan,
I found a little problem with the deck line between shadow 2 and 0.
As a matter of facts the NZL-20 deck line does not have any funny shape is just strait or a little counter in the first half.
This is the way I will follow.
After what Alain told, here how to save also 10% of the wetted surface of the bulb.
This image show some exemples on how can be found the right bulb profile with the Bulb calculator !
According to studies, in term of drag, it was suggested that the best L/B ratio should be = 4.5. or 22.2%
Some modelers are using ratios of 5 and up to 6.
Thank you for pointing my confusion between the weight and the water displacement of the appendages. My mass balance diagram makes more sense now.
But in the end, my point remains as the 519g of “room for error” (404+115, as we won’t need the ballast if we are that way off) on NZL-20 is less than the 728g of NZL-92 and 740g of SUI-100. It proves my theory that you are more confident with your skills and/or experienced with these boats :)… unless I am again mistaken somewhere.
Still, there is some margin and I may be now interested to switch to this design… if you ever accept to PM the drawings to me :).
Your demonstration gives an interesting insight on your design process. Thank you.
Dear Alan,
Please keep in mind that this question comes from someone who has not been there yet! Why do you need the laser guided verification? Taken for granted that the base of the shadow paper profiles are glued parallel to a straight edge of the medium, how can you come up with a misalignment? Well, of course, the shadows could move laterally since they are only squeezed between two wood angles. Then I guess my question is, why not screw or glue the shadows on the wood angles, like we see on the Proto 50” OPEN70 of Kingplank, instead of just squeezing them?
I understand it more for Claudio since he cuts the base of his shadows and therefore cannot directly align the centerline of the shadows to the centerline of the base board.
Hi Sylvain,
in the past I was so confident that an AC120 could be made for 4000g.
Now if you look closely the NZL-20 Data, you will see that remowing the 115g of ballast, the boat will be out of the 4500g required by the Rules.
Why all that ?
Simply according to my “filling”, first the boat shall be as light as possible, since is already overdimensioned in weight, secondly by having 115g available and free, I could place them where I need.
Low wind = balast in the front to lift the transom and gain wet surface when running, Strong wind = ballast in the back to compensate potential nose down, Medium wind = ballast in the center …
It is not escluded that the construction being lighter, I have the option for an heawy ballast to reach the required 4500g.
Cheers
ClaudioD
with Genoa.
