Nacra dagger profile dimensions for 5.2/5.5 etc?
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Posted: Aug 14, 2017 - 10:41 PM
Working on a rebuild and the boards seem a little thin, or maybe they just don't fit the trunks as well as I'd expect. Is there a printable profile somewhere for the old boards or at least some basic profile dimensions for thickness? -
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Posted: Aug 15, 2017 - 07:32 AM
Suggestion
put carpet strips around upper and lower rim of dagger board wells -
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Posted: Aug 15, 2017 - 09:25 AM
For sure, I just wanted to make sure the boards had the right shape first before fitting the trunks to them. -
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Posted: Aug 15, 2017 - 11:13 AM
I have two old daggerboards, one that I recently repaired and another that is newer and much thicker in cross section. I was able to grind out a little bit of material in the trunk to accept the newer thicker board. I have only done this to one side and plan to leave it that way. I have the carpet on the topside of the trunk for both boards and they are now completely stable. I simply used the new board to figure out what needed to be modified.
Having done this modification for the newer thicker board and then also figured out how to repair the old thinner board I have mixed thoughts. The thinner board has got to be faster, the thicker one stronger. On the old thin board that failed it didn't actually break, it just delaminated a bunch of skin off one side. The repair was simply reglassing the board.
The trunks were always a very loose fit to the original boards. For sure the carpet helps hold the boards and with the side loading they just don't bang around anymore. But what I do miss is that with the old setup I could raise and lower the board with one hand. Now it takes two hands and a big pull to raise the boards and solo sailing that is an issue.
If it would help I can put these two different boards on the shop table and a give you the max cross section thicknesses.
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dg
NACRA 5.2 #400
This End Up
Original owner since 1975
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Posted: Aug 15, 2017 - 05:20 PM
Oh ok, well I can believe these are the thin ones. I think I remember reading that for rudders the profile is very specific to the speed the foil is meant for and that a thicker foil can carry a higher angle of attack before stalling than a thinner one. Not sure if any of that really applies to the daggers as much since apart from tacking, the dagger usually doesn't have much of an angle of attack to the flow and the thinner foil would just be a little less drag.
If you can just measure the thickness of the thin one I'd be curious if mine is at least similar. I sanded off the old gelcoat and repainted them. I was careful not to change the form but after seeing the fit in the trunk it made me worry that maybe I had taken down the thickness of the foil profile a bit. -
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Posted: Aug 15, 2017 - 05:40 PM
I just checked the old and new boards I have. The original board has a max cross section of .77" and the new board is .92". The other big difference between these boards is the weight that comes from the additional section thickness.
The board I repaired had a very hard core material (I had expected simple styrofoam) and surprisingly the original polyester glass laminate had not adhered very well to this core at all. When I repaired that board I coarsed up the surface of the core and used epoxy resin and I would expect that to adhere very well to this core material. Since this is now a spare board I just used the standard 12 to 1 bevel for an overlap of the new glass to the old. The bit of laminate that peeled away was about 18" long from the bottom of the board.
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dg
NACRA 5.2 #400
This End Up
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Posted: Aug 15, 2017 - 06:03 PM
Crazy, some of the stuff we read.
Over the years the factory was constantly evolving (i.e. stringers vs. foam core). Boards evolved also, thicker addressed the earlier boards breaking to lift loads, especially with the advancement of the 5.8/6.0 North American rig. It's that simple. In fact, about the time the Inter was developing, the technology was applied to a few sample legacy 5.8/6.0 boards. Carbon! I have some of the carbon boards in my stock and I have sailed the piss out of another carbon board on the 5.8NA.
I can remember many times when a board needed replacing by someone and the replacement was too thick for the opening. Caused all kinds of headaches.
Edited for spelling . . .
Edited by P.M. on Aug 16, 2017 - 07:53 AM.
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Philip
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Posted: Aug 15, 2017 - 10:12 PM
I think Philip has the answer, it was mostly due to strength with the bigger rigs...the N.American is a beast.
The idea that a thicker foil, more correctly defined as a lower aspect,(Reynolds number for airfoils), stalls at a higher angle is a very slippery slope. It is just not that simple, sort of like saying you're a little bit pregnant.
A thicker foil is more tolerant of small imperfections, in aircraft think frost on the wings, BUT, the speeds where compromise occurs is higher than we see on our beachcats.
Pure "thickness" is also heavily related to chord, the distance from leading to trailing edge, & the span,(length).
The least amount of induced drag is a function of aspect ratio. The most efficient foil is a very long thin one, (glider wings), but how do you do that & still keep it strong enough?
Beachcat rudders could easily be made more efficient, BUT, would the negatives be worth it?
Would anyone want a rudder 5' long, with all the grounding problems that would introduce.
And, as Philip alluded to, how do you make a long thin rudder strong enough to resist breaking when thrown full deflection at speed?
Aerodynamics & hydrodynamics are always a tradeoff. If you build to be exceptionally good at one thing, you become very poor at others. You can be mediocre at almost everything, but excel at nothing. In the end, profit margins dictate. Form has to follow function, & we build what people will buy. Not many people are America Cup sponsors, so we get what Joe six Pack will buy.
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Posted: Aug 15, 2017 - 10:44 PM
I think Ed has just well described the challenge/balance between engineering and marketing. I think for the most part the guys that engineered and then marketed our boats, or vice versa depending how you like to look at these things did a pretty good job in general. We still like the boats they made way, way back when.
Tom Roland was both the marketer and the engineer and I think the manufacturing guy for the early Nacra's. They got their foot in the big Hobie door with a boat that was lighter and faster and for sure handled bigger water on most points of sail and in my humble opinion was a lot more interesting than the Hobie's of the day. But these boats all (beachcats) evolved as they found and lost markets. We also know it takes a special person that wants to store and maintain these modestly expensive boats, as compared to the people that sailboard, or kite sail, either of which are easy to fit into a small garage or closet and are less expensive to buy and maintain.
It's all about smiles per mile and I think beachcats can deliver some fantastic smiles per mile.
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dg
NACRA 5.2 #400
This End Up
Original owner since 1975
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Posted: Aug 16, 2017 - 12:02 AM
Good info guys. I have an abnormally large amount of books on sailing and sail theory but nothing much on hydrodynamics related to cats our size. If anyone has some good texts let me know.
Even when you look to the top of a development class due to rules it can be hard to tell sometimes what is the best idea or just the best idea within their regulations.
With my square the whole idea was to have a boat I could play with little things like t foil rudders, making my own sails/rudders/boards/hulls etc if I wanted to and not be stepping on peoples toes with pushing class rules since I basically have none with this boat lol. I fiddle and build my own stuff for almost everything I own so with the H18s it was driving me nuts to not really be able to race with some of the updates I wanted to do.
I could see making a set of rudders and boards at least at a modern aspect ratio with square tips for the boat if I knew I was racing it in good water. I could even make (theoretically) foiling hulls for this beast and still carry the same rating... I'm like a kid in a candy store with this thing.
