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Old 27-09-2008, 15:44   #16
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Originally Posted by Nordic cat View Post
That is what they are for. The idea is that they will not bury, but rather cut through a wave, and by being narrow and sharp at deck level, they can pull up and out of the wave without creating a braking effect that can lead to a pitchpole.

The traditional way for non-racing boats is to ensure a large increase in bouyancy above the waterline, but if you ever bury the whole bow into a wave, it will offer a lot more resistance, slow the boat down, and maybe slew the boat round, or end in a pitch-pole.


I think the modern trend started with the A-cats initially. Another advantage is the lower windage this gives. Good for racers, not for cruisers who should be sailing more conservatively.

Alan
The same reason that trampoline netting should be more open....
Also the wave piercing design means the boat will not tend to pitch in response to realtively small waves, which means the sails keep driving better.
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Old 27-09-2008, 18:54   #17
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Heres a newer reverse bow boat

New Launch: Blohm & Voss 118-meter "SIGMA" - YachtForums.Com



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Old 27-09-2008, 20:38   #18
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Another pic of US 87

This boat is a no holds barred, no expense spared all out racing machine

With Russell Coutts as Skipper, Franck Cammas as technical multihull advisor, and a who's who team of the most experienced AC Cup, and French Maxi Tri vets

Designed by VPLP, it gives new meaning to "State Of the Art" with fibre optic cables imbedded in the hull laminate, which are now connected to load sensors to monitor the enormous strains that 13,000 sq. ft. of sail and 40+ knots of hull speed produces

The'yre taking it easy in the preliminary sea trials - ramping up slowly as they learn to deal with this phenomenally powerful boat

But they've already hit 30+ knots, and they're trying to keep it throttled down

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Old 27-09-2008, 21:24   #19
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Gorgeous!! I want one!!
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Old 28-09-2008, 21:35   #20
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We saw the boat in Catmando's photo in Sardinia two weeks ago.

In my opinion it is truly ugly in the "flesh".
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Old 29-09-2008, 01:09   #21
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I hate to say it……. but I think soon, we will have the first Americas Cup sponsored fatality in a yacht race.
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Old 29-09-2008, 01:33   #22
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We saw the boat in Catmando's photo in Sardinia two weeks ago.

In my opinion it is truly ugly in the "flesh".
Sardinia??

In the flesh??

Us 87 has never been in Sardinia

???????????
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Old 29-09-2008, 02:20   #23
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When I stated Gorgeous I was talking more about the actual speed or being able to harness 2.5 times the knots of wind.. I guess I should've used Awesome!! I know that it is purely a skeleton and nothing too sexy or livable.
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Old 29-09-2008, 02:44   #24
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holy crapolas

That is truly a thing of beauty. You monohullers may be content with sailing Conestoga wagons. A boat like this really stokes the fires.
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Old 29-09-2008, 13:27   #25
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Us 87 has never been in Sardinia


Merlin, perhaps you should read before you write , strange but true.

The boat I was commenting on was the one in Cat man do's photo which precedes you second last post and you will find that it has been to Sardinia and in fact is probably still there!!

As to US 87 that is truly beautiful and I agree with your comments.

daniel
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Old 29-09-2008, 13:36   #26
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I hate to say it……. but I think soon, we will have the first Americas Cup sponsored fatality in a yacht race.
I agree...this is getting to be dangerous. But then there are others sports that are even more dangerous. Not that the Americas Cup should be hazardous to the competitiors.

I just noticed the outside hulls have a hard chine. These are supposed to plane?...wow
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Old 29-09-2008, 13:44   #27
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See the banana shaped foils in the amas, that also create lift to ensure a "nose up" attitude and prevent bow bury.

I doubt that the amas will ever plane, as you can see on the picture above, the port ama is definately in displacement mode. As these boats are so fast, the relative wind will nearly always be forward of the beam, so one side will always be loaded up, and primarily in displacement mode.

Alan
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Old 29-09-2008, 13:47   #28
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Originally Posted by Nordic cat View Post
See the banana shaped foils in the amas, that also create lift to ensure a "nose up" attitude and prevent bow bury.

I doubt that the amas will ever plane, as you can see on the picture above, the port ama is definately in displacement mode. As these boats are so fast, the relative wind will nearly always be forward of the beam, so one side will always be loaded up, and primarily in displacement mode.

Alan
Why would they want to increase the wetted surface area per amount of volume with a hard chine as opposed to a parabolic cross section?
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Old 29-09-2008, 14:28   #29
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Why would they want to increase the wetted surface area per amount of volume with a hard chine as opposed to a parabolic cross section?
The major factors in hydrodynamic resistance are:

1.) Friction due to surface area
2.) Wavemaking resistance

To keep this simple I will attempt to explain my understanding of what happens in basic terms.

Friction as a function of speed grows roughly with the square of velocity.
Once we get into the higher speed range (or Froude number) the wavemaking resistance comes into play. (this is akin to the "hull speed" of a mono, but we can't use the usual 1.34 x SQRT Lwl here)
Wavemaking resistance has 2 main components, but simply put, the wavemaking resistance grows roughly with the cube of the speed.

So the amas are optimised to keep wavemaking resistance down, and this is best achieved with this kind of shape. Long, slim and with an immersed transom that "cheats" the water into thinking the hull is longer.
The use of the hard chine to help bring down wavemaking and lower drag is seen more often these days, but I haven't seen published data on how efficient this really is, and the guys who know aren't going to tell us,as they have paid alot of money in tank tests and advanced CFD (computational Fluid dynamics) tests.
The theory is that they create a seperation zone to the highly turbulent flow, (that creates drag) by helping air get mixed with the water, and thereby lowering the viscosity. This happens best when the hull has vertical movement, like in a chop.

Some of the big French tris use a flared chine to push the waves away from the hull sides to bring down the secondary drag this creates as well. See the picture showing the hull shape on Idec, the current singlehanded round the world record holder with Francois Joyon at the helm.

Alan
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Old 29-09-2008, 18:07   #30
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Us 87 has never been in Sardinia


Merlin, perhaps you should read before you write , strange but true.

The boat I was commenting on was the one in Cat man do's photo which precedes you second last post and you will find that it has been to Sardinia and in fact is probably still there!!

As to US 87 that is truly beautiful and I agree with your comments.

daniel
Sorry Daniel, My bad

I agree with both of your observations
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