Hi all, after my initiation into model boat building two years ago with NZL 92 build, that happened to be my first ever scratch build, since I’ve built few more on my own and now have the privilege to work together with Claudio on this project
Looking forward to those interested in joining us on building NZL-20 also. All you have to do is PM Claudio and he’ll send complete plans… hope you have good service provider Claudio
It started few weeks ago chatting about this and that and all of a sudden Claudio had done 12 revisions on a previous design, funny how time fly’s when you’re talking boats, then we looked at twin keels and all sorts of hull forms until the arrival with this design, amazingly Claudio was drawing up complete plans in within hours sometimes two sets a day, I think this is version 16 or thereabouts.
Anyway, I only just got this version off the printer late this afternoon and sitting here cutting out shadows ready to start framing over the weekend when Claudio send me the e-mail saying [i]“I’ve Started a thread !! and you are called up !!! you cannot withdraw anymore !!”[/I] so Claudio lets rock & roll on this winter project NZL-20 and hopefully have in them water next Spring…and I thought it was going to be a quite winter this year sheesh !
Hey Gio, Kiwis don’t cheat mate, they just read the rules little differently to others
A little back ground info courtesy of Patrick Carpenter:
What is a tandem keel?
A normal keel sees the keel bulb supported by a single fixed fin in its centre that attaches to one point centrally in the hull. A ‘tandem’ keel uses two fins, one at each end of a long slim bulb that is connected to the hull at two points. Each keel blade is able to move independently on its own axis, allowing its attacking angle to the water to be adjusted. With this arrangement the yacht sails with no rudder, instead totally relying on the movement of the keel fins to steer.
Sails 20, 29, 31, 59, 78, 87?
The idea of a tandem keel is not new. In fact, in every AC event since 1992 tandem keel designs have actually made it into the water but not all competed in regattas. Oracle’s designer, Farr Yacht Design tasted some success with 1992 tandem keel design NZL20. The yacht was fast but difficult to sail, and only performed well in the hands of those who had time to truly understand her. Since that time other yachts trialed these concepts without success and most later changed back to classical arrangements. The +39 challenger, (SUI)ITA59 started her life as a tandem keel yacht, only to be later purchased and converted by Alinghi to single keel.
A tandem keel can make a boat faster in a straight line, especially upwind. In essence, being able to adjust the angle of both fins independently reduces drag and increases efficiency. Both ‘lift’ and ‘wave’ drag are reduced meaning twin keels can do the job better than a more traditional single keel design. Other benefits include moving the yachts centre of gravity forward, which increases down force on the bow. Wave drag is reduced with the boat actually creating less wake (disturbance) in the water. It pays to note, the super long bulbs amongst the most successful of recent conventional yachts, are not that different to those used on tandem keel designs of the past. Past Tandem bulb trials may have already influenced modern bulb designs.
]Historically, the main downfall tandem keel designs have been maneuverability issues. This is due to the removal of the rudder, and rather relying on two central fins with limited movement to direct the hull through the water. Once engaged in tactical dueling situations, these designs can be slow to regain their momentum after several maneuvers in a row. This doesn’t mix well with AC Match racing, as an opponent could attack this as a weakness.
Tandem keels also have a larger wetted surface, meaning more resistance pushing through the water. Also eroding efficiency gain is ‘junction’ drag, which is where the actual fins attach to the hull and keel bulb. These moving parts create areas of inefficiency in their design. Overall, the design gains in a tandem keel have always proved small, and the challenges faced both in engineering and actual sailing have made it a risky choice.
Another interesting concept that the Kiwi’s trailed was the hula…“but thats another story”
All started with the confirmed success of the AREVA FRA-93 at the AC120 Italian Championship 2011.
A deeper analysis on the hull, suggested that perhaps there is still some room to improve the hull performances among sails and thumbs.
This model proved to be very fast in all points of sail expecially during running.
The major effort being dedicated to try first further reduction of the wet area and the hull balance, with the “return” to more round form like Desafio in 2007 and on general setting.
Initially the idea was to introduce two news, but old , AC120 models, the Defi-Areva of 2002 'called ’ the Yellow one and the NZL-20 the Red one.
This approach was confirming the will for a change compared to the actual existing AC120 design that are almost very similar ‘squared’ except the Il Moro.
Still keeping “inside” the AC character design , I proposed to Alan to “approach” the hull caracteristics of the real NZL-20 and letting the Defi-Areva in the corner for now in spite of the effort dedicated to improve the parameters as mentioned.
With the NZL-20, the elements not considered for the time being, are :
bowsprit that is not needed and may be a danger for others boats during racing, may be experimented later !!!
the tandem fin for the reasons above, unless a solid construction is found and a classic rudder applied. To note tha the appendage shall be removables to chek for the weight limited by the Rules to 3000g. Reduction of wet area may be achieved by splitting in 2 halves the actual single fin.
the max beam is reduced to a reasonable size of 225mm against the 255mm of the original, being an RC model . A model too large compared with the lenght is generally more luffing-up and difficult to stear as a model.
More details will be come up during the discussion.
I found the tandem keel concept intriguing (and fits within AC 120 rules) reading various other articles they say it is a real upwind speed machine that is achieved by adjusting the angle of both fins independently, reduces drag and increases efficiency of both ‘lift’ and ‘wave’ drag, meaning tandem keels (they say) can do the job better than a more traditional single keel design.
Then I was completely surprised that “six” AC boat designs were considered for have tandem keels in 1992 and said to Claudio, six designers must have seen some merit in the concept so he pursued the design further and finally came up with too many questions marks e.g how to work out the relation between the CE,CB and CLR.?
For a RC version I also saw the remote adjustment of the tandem keels being impossible to get right when looking at the boat from a distance, plus the AC 120 class rules don’t allow for movement of the keel fin. And then the weakness of lack of manoeuvrability as a trade-off, with the rudder being so far forward, so we ended up with too many uncertainties and parked the idea.
The 100 year old dream of Nathanael Greene Herreshoff, looks like remaining a dream but I continue to think about it :-)))
Cheers for that Alan, I guess some big boat aspects just don’t scale down sufficiently well, especially when it comes to ‘control’ surfaces. In terms of the movable fin not being allowed in the AC120 rules, surely if so many challenger/defender designs utilised this aspect, may it be time for the rules to be looked at? I’m sure it would present additional difficulties in construction - keeping down to a competitive overall weight would be one, but even if the rules couldn’t be changed, it would certainly present a fascinating technical/engineering challenge.
Going to show some extreme ignorance now, but what was Herreshoff’s dream?
In the meantime, good luck with setting out the frames etc & looking forward to seeing your progress.
Edit: Like your idea for including a genoa - would you use a sheeting arrangement like the Sprinta’s? Don’t think the Eurgle winches would be up to the job though…!!
I need to do some calculations, but 125% would be max and should be good to go, managed to cut frames and mount to board today, love the lines on this baby unfortunely balsa won’t be here until Tuesday.
Tommorrow I will cut the building boat as I willl be doing latex lamination
Could speak to the powers to be about rule revision, but figure that would be easy once one has the cup in their possesion, then you can change rules as you like :rolleyes: right ?
The turning of the entire fins would not be too hard, but not being on the boat can imagine it would be hard to gauge how far you need to adjust them in various conditions from the bank, they say the orginal was different to sail so RC would be much harder I’m guessing.
Herreshoff’s dream was the link in post #11 above, quite interesting ideas been around for long time, it’s worthwhile short read. Yes I grafting the genoa set-up across from the Sprinta onto AC 120 and thinking should be on this boat, but weight maybe limiting factoring depending on winch being used which will be RMG or Graupner Regatta if it is upto it for speed at 7 volt, I would not trust an Eurgle on Genoa :rolleyes:
Not sure if I’ve missed something here but surely if a 100% foretriangle jib is replaced by one of 125%, then the CE of the whole sailplan must move aft. Your method of calculating CE has a beautiful simplicity about it (assuming you cut out a profile, hang it & then mark the verticals, the intersection being CE). However, as soon as an overlapping foretriangle is introduced the hanging profile doesn’t work?
Obviously the mainsail CE doesn’t change, but the 125% jib/ genoa must generate a CE slightly further aft than a 100% Jib - if the adjustment were to be ignored then the resultant relationship between CE & CLR (the lead) would be less than anticipated which would then impact on helm balance. I recall from my ‘big boat’ sailing days when as the wind picked up if the genoa was reduced in size then unless the main was also reefed the helm was effected quite dramatically. On that particular boat it was perfectly possible to steer by sheet adjustment and no need to touch the tiller (which didn’t even need lashing midships or anywhere else), in other words, an absolute joy to sail. Beating in 20 - 25 knts of breeze with everything up meant two fingers on the tiller instead of one!!
As I said, I may have missed something and welcome your thoughts.