Jump to content

ETNZ sucks big hole


Guest

Recommended Posts

That makes a lot of sense Wheels

 

Do the foils need to be shaped like a wing or is the lift generated from the angle of attack enough?

 

Is there a need for the perfect keel shape (maintaining flow across both sides) or can the foils be flatter in profile?

Link to post
Share on other sites

Island Time, you forget a few things.

 

The old ferries could lift free but did not due to the restrictions imposed by the technology they had which did not allow them to put propellors in the foils.

 

All propulsion /forward momentum came from props that exited the hulls. Therefore they stayed quite low aft to keep the props in.

 

The foil's also had to resist side motion equivilant to or in excess of what a sailing craft. This was because of the higher loadings due to the gross weight they were lifting/moving plus the instances of large side wave motion. ( something the AC72 will never sail in)

 

The side loadings on ETNZ are not as much as you think due to the forward speed.

 

If you did a loading /force vector diagram you would see that the sideways load is less due to the high forward speed.

 

These boats are like planes. All the loadings are on takeoff. The foil design/camber is designed to lift and maintain lift in the speed window they are designed for.

 

The stall angle, like a plane, is very narrow as they cannot change the shape of the foil to allow for velocity increase ( like a plane can). Thus they will stall and prior to this will create unbalanced pressure which in turn creates a Pressure wave. ( maybe the hole?)

 

Also because the foil is angled as it exits the hull it also gives lift when pushed sideways and thus the side/leeway loadings are less compared to a vertical dagger board arrangement.

 

The difference in lift are not that much different. But the weight and construction materials are leap years apart. Which make the comparison look different.

Link to post
Share on other sites
Do the foils need to be shaped like a wing or is the lift generated from the angle of attack enough?

Yes shaped like a "wing" which is the term "foil" not "flat board". The reason is due to fluid flow. (air acts like a fluid also). A flat board has all sorts of issues. A large part of the lift created is due the length of path that the air has to take over the surface. A flat surface has less path length. But the main issue a foil seeks to solve is with a fluid "breaking away" from its surface, caused by turbulence which is created on edges, especially the leading edge. A foil has a leading edge shaped so that the fluid moves around with the least turbulence.

Interestingly, a flat plate as a rudder will suffer from fluid break away at angles as low as 5deg. Normally a 7deg angle is about max before the rudder starts to stall. A foil shape allows angles of 12deg all the way up to angles of 20deg and maybe higher, (Dependent on foil type and fluid flow speeds).

Is there a need for the perfect keel shape (maintaining flow across both sides) or can the foils be flatter in profile?
Depends on purpose. A rudder has the Foil shape both sides due to it needing to work both directions. An aircraft wing has more shape one side due to only needing lift in one direction.

 

The old ferries could lift free but did not due to the restrictions imposed by the technology they had which did not allow them to put propellors in the foils.

Sorry, don't understand. Or if I do understand, the last place you would want a propeller is in a foil. Aircraft have a propeller on the wing due to usually having too, rather than wanting to, due to the turbulence that is created. Note that many designs of aircraft have been made with the propeller in places away from wings, so as to overcome the turbulence and make for more efficient aircraft. The reason why not all aircraft have gone that way is due to other factors of design. So with Jet engines today, the Jet is hung down low under the wing to get the engine air clear of the wing.

With the Ferries, the main idea behind them was to create a better ride. The idea was to lift the hull clear of the chop and thus maintain a better speed. It takes power to lift a Hull. Lifting on a Foil should not be considered more efficient. As I said earlier, 7tonne of hull going up is because 7tonne of water, plus a fair bit more for losses and down force and so on, is being forced down. 7Tonne is 7tonne. There is no sudden change in the laws of physics that allows a mass to be lifted suddenly a lot easier. The only restriction being overcome with the AC boats is friction with the Hull in the water. The Hull is really only there to keep the entire mass afloat when off the Foil.

All the loadings are on takeoff.
Not really. The faster a fluid flows over the wing,the greater the force. Takeoff (and landing) is the point where the wing is passing through Stall and the pressures are a lot less. As already said by someone, waterskis at 30Mph is not as hard as waterskis at 35Mph. The faster you go,the greater all forces become.
Also because the foil is angled as it exits the hull it also gives lift when pushed sideways and thus the side/leeway loadings are less compared to a vertical dagger board arrangement.
Not really. It's more about the materials and stress. A straight down dagger with a foil at right angles would be the most efficient. But think of then trying to corner. The danger of tipping would be ten fold. A rounded corner on those blades is more about taking the stresses with a greater degree of strength. Dissipating stress is the "new engineering buzz word" rather than biulding structure strong enough to resist stress. It is why we see Buildings reach ever higher upward today, because designers have learned how to dissipate stress in a structure. A car is lighter and yet stronger today, because impact stress is sent around the driver/passenger area. With the AC Foils, the Curve is about strength and obviously designed into that is purpose.
Link to post
Share on other sites

without doing lots of quote things ( which i've never tried to do ) here's a couple of silly points.

 

Most foil sections have a working range or sweet spot of 6 to 10 degrees. so for example a typical rudder will happily go 5 degrees off centreline without stalling.

A point to note for those that will say I can turn my rudder to 45 degrees no problem, is that your boat will most likely all ready be turning in an axis about the keel/board so the true rudder deflection is around 5 degrees. or the rudder is stalled but working as a barn door.

 

if you look at an asymmetric foil section, what they have done is bent the working range of the foil. this means it will work better in one direction than the other off centreline.

A typical wing is trying to provide lift in the up direction, so the working range of the foil is bent to work better for it's purpose.

 

the early hydrofoil ferry's use helix of arc foils. they still worked as a foil ( although crudely )

 

David kieper and williwaw was a good example of this helix foil

Link to post
Share on other sites
Also because the foil is angled as it exits the hull it also gives lift when pushed sideways and thus the side/leeway loadings are less compared to a vertical dagger board arrangement.
Not really. It's more about the materials and stress. A straight down dagger with a foil at right angles would be the most efficient. But think of then trying to corner. The danger of tipping would be ten fold. A rounded corner on those blades is more about taking the stresses with a greater degree of strength. Dissipating stress is the "new engineering buzz word" rather than biulding structure strong enough to resist stress. It is why we see Buildings reach ever higher upward today, because designers have learned how to dissipate stress in a structure. A car is lighter and yet stronger today, because impact stress is sent around the driver/passenger area. With the AC Foils, the Curve is about strength and obviously designed into that is purpose.

 

did it work ?

 

they always talk in resultant force, which is the maximum centre of pressure. So think of all those pictures with three arrows showing the loads ( should actually be a minimum of 6 arrows )

As in all cases this changes rapidly between speed and angle of deflection. the engineers have to predict the worst case, then add a safety margin.

usa 17 showed us what happens when you get that wrong on one of its first sails

 

the dissipation of stress has really come about from engineers understanding flexural stresses and being able to measure them

carbon is a difficult one as it has no stretch or compression as you see in metals, so it has to be engineered within it's flexural failure.

unlike metals that you can build in fatigue life

 

the bear away most likely sees the highest loads. and is less likely to stall than you think

Link to post
Share on other sites

Good responses Wheels and FNG. Considered and accurate answers....

I decided not to answer Topsail, I did not follow part of his post, and I don't want to give an engineering lesson on here. Anyway, its a long time since I did my engineering training, and my understanding of foils is pretty fundamental.....

I still believe that what ETNZ has done with foils here is pretty revolutionary!

Link to post
Share on other sites

Island Time,

 

Thanks for not enlightening me with you engineering degree. :D

 

Unfortunately my post did read a bit funny, when I read it back, and when I went to edit it. The edit button vanished and I thought... to much hard work.

 

Hydrodynamics is a specialist field and we should leave it to the experts.

 

But I still think ETNZ and all the other teams foils are not that great.

 

What the foiling moth does is very good though. :thumbup: :thumbup:

 

ETNZ is just a development of that thinking but with two hulls/foils and a stable platform.

Link to post
Share on other sites

No problem Topsail! :lol:

 

Here is a really simplified comparison between the Moth, which you say is cool (and it is) and an AC72 - which is cooler.

 

The Moth has 2 simple T foils, one on the rudder, one on the centreplate. The AC 72 has a very complex, curved asymmetric primary foil, and a small asymmetric (ETNZ's preferred one) on the rudders .

 

The Moth has elevators (Flaps) on the primary foil to adjust height realtime, constantly while sailing- done mechanically without human input. The AC72 does not.

 

The Moth has an adjustable rudder angle while sailing, to prevent pitch-poling. The AC 72 cannot adjust this while sailing.

 

The fact that the AC72 foils in a stable manner, primarily on ONE foil, AND without these controls is a feat not yet duplicated on any other vessel, and is a credit to the engineering competence of ETNZ :clap:

 

But hey Topsail, everyone is entitled to their own opinions!

Link to post
Share on other sites
The fact that the AC72 foils in a stable manner, primarily on ONE foil, AND without these controls is a feat not yet duplicated on any other vessel, and is a credit to the engineering competence of ETNZ :clap:

Amen to that. Another way to look at it, the Moth Foil size in relation to what it is lifting and what the AC72's foil size is and what it is lifting. Quite a feat of engineering when you think about it.

Link to post
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...