Bow forms for a- & c-class cats

[patrik111]

Hello,
I'm wondering why nothing like the flyer or Nikita bows (a-cats) on the C-class cats? Is it shear size? If so would anyone have a ballpark guess at what kind of lengths that would be?

Cheers
Patrik

Below a link to show the nikita shape.
http://home.planet.nl/~dwars000/id40.htm

[grob]

I have to say I think its just fashion, there does not seem to be a marked difference in speed between these different bow shapes, you need something to differentiate your product from the next boat.

all the best

Gareth
www.fourhulls.com

[SailDesign]

Gonzo.
I'm not sure about the fashion thing. Most hulls with bows like that have butts that are actually foil sections, designed that way to lift the bow out of the waves (i.e. to stop nose-diving) and to create little wave action when they do immerse.
Steve

[CT249]

It's not fashion; I sail against the guys who have won the 2003, 2001 and 1 other recent A Class worlds. The Flyer-type bow does not pitch as much in chop because of the volume, and the narrower hull sides reduce wave impact drag and reduce nosediving. They also put holes in the opposition's hulls at a more vulnerable point (closer to the waterline....).

Moths tried the same thing but encountered handling difficulties and abandoned the idea.

I think the C Class Cogito came out before the Flyer. The Aussie C Class uses old hull moulds because they felt they would get more performance from spending their resources on rig development.

Does anyone have more detailed information on the design theory behind these "reverse bows"? And the hulls that go with them?
Any published sources or links on state of the art high speed multihull design particularly under 30'?

[grob]

I'm sticking to my fashion theory, until I see some hard data, my opinion is that the quicker boats are more to do with other things, rig, weight and crew ability. I cannot see a good reason why these bow shapes will have any effect in anything other than small chop. ( by bow shapes I am talking about the shape as viewed from the side as in this picture.



Its interesting how very similar these hull shapes are (other than the bow of course).

I think the most important feature of a "wave piercer" is having a rounded section on top of the hull so once it has pierced the wave the drag remains low.

Although going one stage further I don't see why a sharp bow will create smaller waves or less drag than a fuller bow (viewed from above). All the aerodynamics and fluid dynamics I have seen indicates that blunt shapes are better. And I have never seen any hard data on wave generation by bow forms.

Like Lorsail I would like to see some theory or data behind these shapes.

But the Flyer and Nils definately look the coolest and most radical.

Gareth
www.fourhulls.com

[patrik111]

Originally Posted by grob

. I cannot see a good reason why these bow shapes will have any effect in anything other than small chop. ( by bow shapes I am talking about the shape as viewed from the side as in this picture.


Its interesting how very similar these hull shapes are (other than the bow of course).

I think the most important feature of a "wave piercer" is having a rounded section on top of the hull so once it has pierced the wave the drag remains low.


But the Flyer and Nils definately look the coolest and most radical.

Gareth

I believe that it has to do with oume distribution. Less volume above the DWL(Waterat & A2 both by M&M) click link for piture

http://www.geocities.com/patrik_elfving111

This would, as I understand it, prevent the bows from
"Stopping" and
"popping up" when entering a wave.

I think the idea is to not pierce large waves, but to ride the large ones while swallowing the small ones.
http://www.flyer-acat.de/pages/german/konstruktion.htm

It would indeed be very interesting to get some hard data from this. Maybe by towing a Waterat and an A2 in small chop with sails down. ( the underwater shape is virtually identical, and could be compensated for by a calm water test.)

Couldn't agree more about the coolness...

Still scratching my head.

[tspeer]

Originally Posted by grob

...Its interesting how very similar these hull shapes are (other than the bow of course)....

Do you have cross sectional area distributions for these boats?

[CT249]

Re "I'm sticking to my fashion theory, until I see some hard data, my opinion is that the quicker boats are more to do with other things, rig, weight and crew ability."

So why did the world champions sell their old boats and pay out the cash for new ones? Why does the guy who made the Auscat admit that they new boats are faster - now he has to pay license fees for Flyers.

The profile sketches don't show how much narrower the topsides are in the boats with reverse stems. They are also U-sectioned with flat spots along the keel. Boats like the Auscat 5 have more Vee in section. Basic geometry shows the Vee has more wetted surface for volume, and less dynamic lift.

The trend to putting more volume under the waterline and finer shape above the water is a long trend. Look at something like I-14 design from Avenger on and you'll see it.

Having more volume above the (calm water) waterline does nothing to add speed. All it does is add drag when waves strike the fiuller sections above the water. Every development class from Moths to IACC boats, IMS boats and 60' tris has followed the same route to fine topsides.

They also have the advantage that when they are at a negative angle of attack, stuffed down a wave, there is less dynamic downforce and a vastly better chance of recovery.

But I'll tell Gashby & Stalky that they were wrong to have gone for the Flyer, as soon as they get back from Gashby's 3rd or 4th worlds win; Stevie only has 1 world's win.

CT 249

[patrik111]

Originally Posted by CT249

But I'll tell Gashby & Stalky that they were wrong to have gone for the Flyer, as soon as they get back from Gashby's 3rd or 4th worlds win; Stevie only has 1 world's win.

CT 249

Would you please insted simply ask them to explain as elaboratly as possible, how this works, in what sort of chop, percieved downsides a.s.o.

I think you can see the small volume and the tumble-home on the pictures on the below link
http://www.geocities.com/patrik_elfving111/

The scan is however not really anything but very rough. If anyone has sections and or area distribution of these beasts it would be realy appriciated!

Cheers

[grob]

Tom, Patrik,

I should be able to make a crude CAD model with the views of the A2 that Patrik posted, if anyone has any other views of the flyer, Bim or Nils, I'll see what else I can do. Give me a couple of days.

Gareth
www.fourhulls.com

[grob]

Originally Posted by CT249

This term "wave Piercing bows" leaves me with a fuzzy understanding also. All bows pierce waves. Some bows/stems are vertical; some lean back at the bottom and some lean back at the top. That doesn't make any difference. What does make a difference is the height of the bow relative to the other vertical measurements of the hull because that sets forward hull volume and that has a large effect on pitchpole characteristics. Remember the hull modifications to Playstation. They were all about increased pitchpole margin.
The other thing that affects the drag of a bow as the front of the boat goes through waves is the shape of the foredeck. If the foredeck is flat or very nearly flat with slightly rounded corners, that is draggy as the bow punches through waves. That hull/foredeck shape does not shed the water easily that is slamming down on it. On the other hand a foredeck shape that is round or elliptical with no corners at the deck to side intersection, no shearline, will shed the water much more easily and is less draggy punching through waves. It is the foredeck shape immediately behind the bow that affects how draggy the hull is as it pierces the waves.

Patrik, also posted this question on the catsailor.com forum and this is the reply he got from Bill Roberts, designer of the ARC and SC cats and also a pretty good sailor.