Following on from the thread recently that touched on the slot effect; here's a couple of related links to the boatdesign.net sailing forum...

http://boatdesign.net/forums/attachment.php?s=&postid=9891
http://boatdesign.net/forums/showthread.php?s=&threadid=457

Regards,
Colin
www.sailwave.com

Hi Pete,



That's my experience too and was surprised by the inference; I used to race a Dart18 single handed under main-only against double handed Dart18s with main and jib. I would always point higher up-wind and (usually) go faster up-wind, and then (usually) go slower down-wind.

Interesting stuff nonetheless...

Regards,
Colin
www.sailwave.com

In general, a boat without a jib can point higher than a boat with a jib. It has always been my understanding -- from others and from just plain common sense -- that the jib bends the wind so that the mainsail sees the wind coming from a lower angle. And that is even with a well-sailed and tuned sloop rig. The worst, and most common, problem with sloop rigs is the jib backwinding the main, causing you to sail low and slow. With the fully-battened mains we have on cats, it is much more difficult to notice a backwinding situation than with the soft sails that most monohulls have.

I'm skeptical of that theory because I don't see any of the sloops furling their jibs when going upwind. There has to be some marginal payoff to having a jib when going upwind. I do agree that you'd be better off having that jib area in your main instead, though.

I've also heard many sloop skippers claim that they can't point well unless the jib is sheeted in hard. Maybe that's because they steer to the jib telltales, while it'd be more profitable to let the jib out and let it luff a bit. I guess I'll have to experiment with that.

Hi Mary,



My understanding too; I post this stuff because it seems to contradict our common(-sense) understanding about how sails work and is written by reputable folk... In this case a (big-)cat designer as far as I can tell... I'll ask him what he meant by "Put another way, the mainsail provides an upwash for the jib that makes the jib both more efficient and able to point higher...".

Regards,
Colin
www.sailwave.com

Maybe they are talking about monohulls with great big, overlapping jibs. On a lot of those boats the majority of the sail area is in the jib, so maybe in those cases the mainsail is there to help the jib, rather than the other way around. I don't think that would be true of beach cats. But maybe in light air when you are able to carry a Hooter upwind....? All theories are interesting to think about.

I think the document refered to have some grammar problems and miswrittings that are unintentional

Their base models seem to be okay but some phrases are open to multiple interpretations

I think the updraft comment intends to say that a sloop rig can point higher than expected when adding the optimal angle of both individually. Which it does.

I don't see uni rig pointing 15 to 20 degrees higher when a normal battened sail needs about 20 to 25 degrees to operate fully powerup.

The difference between unis and sloops is more in the 5 to 10 degrees.

I think no monohuller would seriously claim that a sloop points higher than a uni-rig.

The document however does stated that the first improvement would be adding a jib. It does however ot say that the gains will come from pointing higher. It leaves this point open. I think the intent here is to say that adding a jib would improve power enough to attain a better VMG by going faster despite the fact that you will point lower.

There is definately more material on the subject that the main makes the jib more powerfull than its size would suggest. In stead of teh jib making the main more powerful by a slot effect.Both Bethwaite and gentry are exponents of this line of thought. Net result is of course the same.

Colin, did you had a better VMG than the sloop when doublehanding the uni-rigged Dart 18 ?

Wouter

If we are talking about beach cats, I think there is a benefit going upwind with a jib especially in choppy conditions and relatively light air, because it helps to power through the waves and keep the boat on course and also to tack. I think a unirig might be at a disadvantage in such conditions.

Hi All -
What they may mean is that because a jib is set on a wire and has a clean leading edge it has the ability to point higher without stalling ,--The mast -typical section --addes a disrupt factor to the leading edge of about 10 to 15 percent dependant on wind velocity ,--according to what I,ve read ,-
The advantage of adding a jib other than more sail area is the attached flow aspect ,-particularly in seas when the rig is being swung around making it more difficult to keep attached airflow over the sailplan ,--We have all seen out tell tails in light air and choppy seas bounceing all over .

The lead of the jib deflects the wind approaching it ,-so by the time it reaches the main it is deflected at a slightly higher angle which requires you to sail a little lower to keep the same apparent wind angle ---Thats why a main only rig sails higher ,but has a more difficult time keeping attached flow in seas and tacking .

-Think of the apparent wind with the spin up and how much that effects mainsail airflow ,--its generally centered on a cat downwind ,--The jib does the same ,just proportionately to the size , and in this discussion the upwind mode.
If you can look up wind tunnel tests of rigs that use colored smoke ,--it makes this very visual --
Hope this perspective and explaination is helpfull ,
Always enjoy design related discussion posts ,-thanks
Carl

Hi Wouter,



Never did that; always single-handed so weight was involved...

However in the Dart and double-handed and overpowered (I had a light crew) we found it very fast (VMG-wise) to dump the jib into a stable S-shape up-wind rather than dump the traveller or the main; I read about it in the the Dart manual and it worked a treat. It's tricky to sail like that because you can't sail to the jib, but when you get the knack, you seemed to point higher and go faster... My son used to call it "s-club seven-ing the jib"... We would go as fast as the 'heavy weights' up wind and point higher and then take off down wind because of our weight... Apart from the occasional pitchpole it was great...

Wouldn't work with high aspect ratio jibs I would guess...

Regards,
Colin
www.sailwave.com

Okay, then for going upwind why don't we just have a really big jib, with a really clean entry, and drop the main? Seems like the main would just be kind of fighting the jib -- jib wants to point high, main is getting a different wind angle and wants to go lower. Is the main just there to keep the helm balanced?

Hi Mar

There are some cruising cats that do have that type of sailplan --large jib small main ,-it may be what the designer had in mind --jib efficiency ,-that along with moving the mast aft which reduces the pitchpole tendancy --in effect more bow length forward of the mast and center of effort on the sails . -Balance of sailplan C E to hull and underater profile center of lateral resistence CLR are always key in any sailcraft design , --balancing the rig location and rake to helm .--Generally the C E leads the CLR by 10 to 15 % dependant on sail shape and hull type as other factors also .
To answer the question of aft mast and jib only ,-With an aft placed mast only there is still the drag of a 30 ft plus mast ,-placing a main on it to add sailarea and make use of it makes design sence , better than it there doing nothing but holding the forstay and large jib .
A very areodynamic light A framed rig configuration with both main and jib set on a wire in betwen the two masts is an intriging design concept ,
The efficiency of the rig would be similar to existing wing type sails as on C Class cats but potentially much lighter and less complex or prone to damage ,-the key would be designing the A frame sections to be pivoting and efficent and lightweight enough to overcome the drag ,-maybe the C F materials and great sections as we see on the A-Class cats now make this a possibility ,-

One rig configuration I really like for its simplicity is the ancient Dhow type rig ,--basically what is used on the sunfish sailboat today , a short mast with longer angled one with the sail attached for raising and lowering ,-been around a long time .--The angle helps create lift ,-we generally see these rigs set well forward ,-the dhows sometimes add an extra smaller same sail set aft .
This would be a great rig for a cat ,--the catyak used one .
Some fun ideas for the other armchair cat designers out there ,-
Carl

Hi Carl,

Have you seen this...

http://schachtdesign.com/proafile

Regards,
Colin
www.sailwave.com

Thanks Colin

liked the Zen of proas part ---agree

The only one I,ve seen locally is SLINGSHOT
the only pic found was one on this site to scoll down to
http://www.geocities.com/CptinRn/strange.html
A proa design speed trials attempt in the 70s by the Gougeon Brothers ,-West System Epoxy guys,-here in MI .
The current attempt at the speed sailing record is also in a proa ,--much more refined .

Recall an earlier post with Grob in your area ,
he has been working on an interesting 4 hulled fold up sailing craft which initially had a two boardsail type sailplan for it ,-suggested the A frame type rig used on the 12 ft Catyak --{no longer made} ,-but was a good simple rig like the proas .
Will be interesting to see how that progresses .
The basic platform was fasinating and has all types of interesting potential applications ,-paddle boat -fishing platform ,etc brilliantly designed to fold up and fit on the cartop.

thanks again
Carl

His reply is at the bottom of this thread:-

http://boatdesign.net/forums/showthread.php?s=&threadid=457

CJ

Taking Mary's Idea a bit further why not hang the sails from the shrouds, then we get two jibs that meet at the top and the mast does not get in the way.

I have noticed that alot of the boats currently trying to break the world speed record have biplane rigs and there is even an A frame type rig. See http://www.windjet.co.uk/water/competitors.php

Carl, thanks for the plug, should finish the build on my strange boat this weekend, and am hoping to get it in the water next month, (althought I've been saying that since xmas).

Gareth
www.fourhulls.com

O.K., here's my take on it. It's all about required power in conjunction with pointing ability.

Monohull: Weight is high, righting moment is low. This boat needs GOBS of power down low to drag it through the water since it is so dramatically heavier. Aerodynamic efficiency of the sailplan is a little less crucial since the speeds are lower. More power down low relates to less healing moment and better grip in the water if you can keep the keel closer to vertical. Therefore, if you get your power down lower, i.e. with a sloop arrangement, you can put more boogy in her dance and still point higher. Though they can sail less high with them, Genoas are used in light air simply because a blade jib just doesn't pull hard enough. The tradeoff in pointing ability for power is worth it because of the lack of wind there would be extra righting moment and they need the additional oomf. They need to use all the righting moment they can at all times. If you put a uni-rig on a monohull and attempt to carry the same amount of sail area, while it's efficiency can be higher and the power about the same, it's healing moment will be much higher and it will loose it's grip in the water.

Enter the catamaran: Weight is low. Righting moment is high. Keep in mind that most the uni-rigs (A-class, 18HT, etc.) are working with a box rule sail area. They take what sail area would be in the jib and add it to the main. Now the main is taller and with more sail area than a main sail on a sloop rigged cat. It's also designed with a very high finesse ratio (more efficiency through the air) and since we have better control over our righting moment we can manage the taller main. A jib is draggy and much less efficient because it presents a lot of windage that a main alone doesn't. However, just rolling up your jib doesn't make you point higher because you just lost 20 to 30% of your sail area, power, and speed to present lift to the boards. If I took my Nacra 6.0NA, built a new taller mast and added the jib sail area to a new bigger mainsail, it would point like no other because it still has the same power but a lot more efficiency. (although it probably wouldn't be very controllable!).

Downwind, I don't think anyone disagrees that a jib is an advantage because of the slot effect.