Aftmast rigs???

[Pericles]

Yeah, I say unto you thrice.

Traction kite.

Traction kite.

Traction kite.

Pericles

 

[masalai]

Pericles, does that mean you prefer "Traction Kites"?

Something similar to the attached image is what I mean. I think that is slightly different & suited to smaller applications... It will point higher into the wind...

 

[Pericles]

Masalai,

Traction kites make great sense on downwind legs and as an emergency sail in the event of being dismasted. Brian Eiland has been promoting the idea for some while. You posted there.

http://boatdesign.net/forums/showthread.php?t=20319&highlight=brian+eiland

http://www.skysails.info/index.php?L=1

http://www.youtube.com/watch?v=q4A0B_-aQK4

http://www.kiteship.com/

http://www.kit-cats.com/kite/kite_sail.htm

http://www.dcss.org/speedsl/index.html

http://www.voilecerfvolant.com/Test/Gb/home.asp

Regards,

Pericles

 

[masalai]

Do you want to log in to my head as the mental de-fibrilator - can't think of another word to get my memory to find what I am thinking I would like to recall... - Thanks

 

[brian eiland]

Idealized Sail Plan

Having lots of smaller sails with sharp leading edges is good for versatility, and each sail should perform well in it's own right....but when reaching or running, small sails lose more wind around the edges than a big sail. Also, the best foil shape is achieved when the rig consists of only one sail (no gaps between sails).

So...in theory, the most ideal rig would be just one big self-supporting sail with no mast or stays. The sail would magically stretch to become tall and narrow when beating to windward, and round when sailing downwind.

I try to keep this image in mind when I am looking at various rig possibilities.

.....humm...no mast, no stays....sounds like the kite sail

(kite-assisted powercat)​

.....or a single sail that would stretch to be tall and narrow, and round for downwind...sounds like the modern Dynarig
Interestingly the overall profile of this sail plan almost perfectly matches that of the idealized semi-ellipital/parabolic planform shape. The lift/drag factors for this optimized shape are so much superior to those for the triangular sail-shapes of the Bermuda rig.

 

[masalai]

Aaaah He stole my device, a skyhook attached to the boat

 

[Fanie]

I was considering the mast setup for ie a 10m cat... if one uses two masts side by side, each close to a hull, could one end up with lighter masts, but more sailing area yet more controllable ?

Ok, how about if the one mast is more foreward and the other more aftward ?

 

[masalai]

Not a real good thought there Fanie, two masts do not make for "redundancy" as one would pull the other over.... Better to stick to a single stick unless one is looking at 100m? (lost a "0" somewhere?)... Remember, bigger is better and 12 is such a nice rounded number - with overhangs makes 13 - a bakers dozen

 

[Fanie]

Hi Masalai, have a look at this.

http://www.youtube.com/watch?v=A7vD7wALP5M

Ok, it's a hydrofoil, but look at the sails. What would a single sail look like to replace those two ?

 

[Fanie]

Here's another one
http://www.youtube.com/watch?v=0y91vkBUvHw&feature=related

I was lying in the soap box thinking (my most productive place ) two masts that have the same sailing area as the one big sail, ok, but your combined forestay would be longer thus better speed to windward, same speed when running, smaller and lighter masts, aft mast sail lift ie on a cat should be better distributed - less heeling ?, much less down force on the beams (no beam centre down force), lighter construction, easier to handle.

One thing no one seems to mention.
On the bermuda rig the mast is the device pushing the boat foreward, the boom supported aft on the boat
On the aft mast setup the forestay is pulling the boat foreward. Ok on the mast too, but better balance.

As I have it,
a boat being pushed is more difficult to steer
a boat being pulled is easier to steer

 

[masalai]

It is a matter of "horses for courses", do you put a thoroughbred horse in front of a plow? when you really should be using a reliable draft horse? or do you put a jet turbine on the top of an outboard leg? and expect fuel economy and performance reliability as opposed to 1000mph speed? if you get my drift?
See post 21 and imagine a slightly shorter mast set a bit further back so the stays near midships went forward a bit so a furler could be used... http://www.boatdesign.net/forums/showthread.php?t=21274&page=2 There would be reduced downforce to push the leeward hull deeper, easier to control, and no boom, (or attendant sheets & wenches etc), to decapitate or kneecap (ouch), you whilst reeling in those fish

Remember those earlier thoughts of yours to keep it simple, several smaller sails for ease of handling... That rig on the foil is strictly for performance and demands constant attention... You will note that "Crash Helmets" were worn!!! So, make 2 boats, one 12m as "mother-ship" and small toys for the speed thing, carried on the aft bridgedeck roof, as there will be no "main" there to cause other problems...

Re post 70, "as I have it," - sometimes...

Look at the angle from the central mast and the forestays/furled sails and think of the reduction of bad down-forces... It helps to make a simple model to see where it is at... Look carefully and be pleased... not necessarily a "sailing model" but a convenient post, mark the boat with whitewash (tennis court marker from your tennis court) or even driveway and tent pegs to mark the bows then a bit of bedsheet (whilst wife is away shopping) and see how it sits...

 

[Richard Atkin]

Brian Eiland
The Dynarig is very interesting. The curve of the sail is symmetrical, unlike an optimised foil shape. Does this affect performance much?

 

[Richard Atkin]

Fanie, regarding pushing and pulling... I've heard that cats don't like the centre of sail area to be too far forward. This can cause steering problems. This is why cats generally have their mast further aft than monohulls. But I'm talking about bermudian rigs. I have no idea if this principle applies to the aftmast rig, which should have more upward lift

 

[ironmatar]

a question iv not seen posted here why not leave the mast<s> vertical and instead maby use a forward rigged arm or gaff in order to move the top point<S> forward the desired amount of rake?

 

[Fanie]

Good idea, Ironmatar. It would be a way to decrease lift and increase foreward pull.