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Re: See the F20c in person [Re: pitchpoledave] #222239
10/21/10 02:14 AM
10/21/10 02:14 AM
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Australia
macca Offline
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Originally Posted by pitchpoledave
Macca can you talk a bit about the development process? On boats that were built prior with curved foils, what didn't work and how are these foils a step forward from prior attempts?


I will put a story on the development together in the next day or so. I am on my way to Australia today and i'm sure to have time to write it up on the trip...


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Re: See the F20c in person [Re: mikekrantz] #222240
10/21/10 02:20 AM
10/21/10 02:20 AM
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Australia
macca Offline
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Australia
Originally Posted by mikekrantz
I don't have enough time on the boat in heavy air to comment just yet. However, Macca should chime in with some real time experience in those conditions and comparisons.


At the moment we have a lot of heavier crews sailing the boat, but Fergie and I are just on 150kg combined and have been sailing a lot on the boat in all sorts of weather. Recently we sailed in Hyeres in full mistal conditions (30-40kts) and despite the terrifying ride we were really happy with the control we had over the boat.

As for competitive weight ranges I think in time we will see the weights come down to just above the ideal F18 weight. The hulls can for sure carry more volume than an F18 and the rig is massive when you compare them, but the 3.2m beam is a lot of RM to play with and as such the lighter teams can keep the boat moving even in the big wind.


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Re: See the F20c in person [Re: macca] #222256
10/21/10 07:23 AM
10/21/10 07:23 AM
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Portland, Maine
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ThunderMuffin Offline
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macca,

how about 200kg crews?

Re: See the F20c in person [Re: ThunderMuffin] #222257
10/21/10 07:33 AM
10/21/10 07:33 AM
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Australia
macca Offline
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Australia
For sure it will handle 200kg crew better than an F18, or I20. Simply due to the larger volume hulls, lifting foils and bigger rig.

As to the best crew weight once we have all worked out how to sail these things at full pace all the time.... We have to wait and see.

Last edited by macca; 10/21/10 07:33 AM.

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Re: See the F20c in person [Re: ThunderMuffin] #222310
10/21/10 03:04 PM
10/21/10 03:04 PM
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Asuncion, Paraguay
Luiz Offline
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Originally Posted by Undecided
Originally Posted by scooby_simon
Originally Posted by Undecided

The extra width is also needed for the curved boards since the lift forces on the curved boards decrease righting moment via the leward board.


Eh? The leeward board is pushing the hull up, thus increasing RM!!!


Yeah but since the board is also curved - some of that force directed upwards at the tip end of the board is going to result in a "torquing" of the hull, meaning that its effectively going to rotate the leward hull outboards.. increasing the amount of capsize moment.

You might be right though that the effect of the board lifting overall would equalize the effect of the boat spinning the leward hull outwards.


AFAIK, the issue here is the balance between transversal and longitudinal stability.

Suppose we start with a traditional 2/1 L/B ratio catamaran, that reaches both stability limits simultaneously at a given wind speed.

We can increase its righting moment by making it wider and then it will be able to carry more sail area. However, since its longitudinal stability remained the same, the extra sail area will make it pitchpole earlier then before, so it will not reach the full speed potential of the enlarged sailplan.

Lifting foils placed forward of the CG increase the longitudinal stability at speed, solving the problem - granted, at the expense of a fraction of the extra righting moment.

Conclusion: keeping hull shapes unchanged, curved foils allow a cat to be wider and to carry more sail area, so it becomes faster then before.

Check: the new AC cats have curved foils and their L/B is 1.5:1

Check: the tris with even smaller L/B (1:1) are where curved foils were developed, for they need them badly. Without those foils they reach their longitudinal stability limit a lot earlier than their capsize limit. You certainly remember how the first 1:1 tris were pitchpole prone.

Cheers,


Luiz
Re: See the F20c in person [Re: macca] #222317
10/21/10 04:12 PM
10/21/10 04:12 PM
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Originally Posted by macca
Originally Posted by scooby_simon
Originally Posted by Undecided

The extra width is also needed for the curved boards since the lift forces on the curved boards decrease righting moment via the leward board.






Eh? The leeward board is pushing the hull up, thus increasing RM!!!


reduced RM... look where the COE of the board is in relation to the hull centerline.


OK: can understand that once the foils are starting to lift the boat will pivot about (or roughly about) the COE of the board; but is this still true in "non flying modes" as the boat is then pivoting about the leeward hull.

I'm no boat builder, just keen to understand this bit that I do not appear to....

Last edited by scooby_simon; 10/21/10 05:50 PM. Reason: getting comments in the right place.

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Re: See the F20c in person [Re: scooby_simon] #222340
10/22/10 08:02 AM
10/22/10 08:02 AM
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Daytona Beach Florida
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orphan Offline
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Macca,
You said Nacra tested with straight and curved. Did they ever test with curved being moved foward of the center of effort? The idea to provide lift without the balance issues.

Re: See the F20c in person [Re: orphan] #222345
10/22/10 09:31 AM
10/22/10 09:31 AM
Joined: Jun 2001
Posts: 1,021
Australia
macca Offline
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Australia
There are no "balance issues" you simply have to sail the boat and not just stand anywhere you like. Its the same as any other cat with the exception that when you get it wrong on the F20 its clearly wrong.

The Board position was decided from the testing on the original test platforms and also calculating the effect of the wider beam from what was tested.

As for Simon's question: as soon as there is load on the foil then the COE is part of the RM equation. Load is on the foil very early, even on straight foils.


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Re: See the F20c in person [Re: scooby_simon] #222348
10/22/10 10:22 AM
10/22/10 10:22 AM
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Asuncion, Paraguay
Luiz Offline
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Originally Posted by scooby_simon


OK: can understand that once the foils are starting to lift the boat will pivot about (or roughly about) the COE of the board; but is this still true in "non flying modes" as the boat is then pivoting about the leeward hull.

I'm no boat builder, just keen to understand this bit that I do not appear to....


Erase all this.
Lift from the foil replaces part of hull flotation at speed, so righting moment drops with speed simply because the foil's center of lift is more inboard (closer to the mast) than the float.

Cheers,
Luiz


Luiz
Re: See the F20c in person [Re: Luiz] #222773
10/27/10 08:12 AM
10/27/10 08:12 AM
Joined: Jul 2001
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42.904444 N; 88.008586 W
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Macca, (Kevin)

What's the latest?

YOu still 'down under'?

Speaking for myself, It would be cool if you posted your latest excursion, events, etc. on a regular basis. It would be nice if some of the other 'Rock Stars' would as well.

For me, being in WI now, winter is beginning to set in. I did get to do some excellent one design racing in Milwaukee harbor last Sunday. It was nice, even though it was on a Pearson Ensign!

http://www.ensignclass.com/images/stories/pdf/EnsignBrochure.pdf



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Re: See the F20c in person [Re: Luiz] #223304
11/04/10 10:55 AM
11/04/10 10:55 AM
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Reno NV
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Originally Posted by Luiz


Erase all this.
Lift from the foil replaces part of hull flotation at speed, so righting moment drops with speed simply because the foil's center of lift is more inboard (closer to the mast) than the float.

Cheers,
Luiz


This may be a dumb question, but why not have the boards bend the other way (i.e. the tip extending outboard when down)? In other classes there's a max width, but that shouldn't be a problem here.

Re: See the F20c in person [Re: Rhino1302] #223456
11/06/10 03:49 PM
11/06/10 03:49 PM
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Solomon's Island, MD
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samc99us Offline
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Having not sailed the F20c or any other curved foiled boat, I won't speak to why they placed the boards under the boat from a performance aspect.

I will speak of one good maintenance reason however. If you have a 10.5 ft wide boat sailing along at 25 kts in 20 kts of breeze up the Florida keys, hugging the coast as that is minimum distance, you likely can't see the water very clearly under the leeward hull. Now the keys have lots of these things called shoals, usually made of rock hard coral that will take your boat from 25 kts to 0 kts in about 3 seconds if you hit one. Imagine your curved foil hits one. Now imagine your curved foil sticking outside the boat hitting one. You now have a situation where:

a) you can't even see your foil moving in the water b/c its 11 feet away from you
b) your foil hits first as usual, but instead of a blunt impact, which is generally survivable, the entire boat is torquing around a 3 foot moment arm...snap is usually what happens when that kind of load is applied to a thin airfoiled surface.

With the foils under the boat, moments like that are a non-issue, its far far easier to know your full boat width so you can avoid hitting shoals, and more importantly your not worried about touching some F18's $1500 carbon daggerboard with your $1800 curved carbon daggerboard while on the start line. That would just get confusing and dangerous.

Last edited by samc99us; 11/06/10 03:51 PM.

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