I see that the Infusion and Capricorn have "wing masts", which appear to be modified teardrop sections. These appear to be quite different from what Bethwaite refered to as a "wing mast" in his 1993 book High Performance Sailing.

Bethwaite's wing mast is a laminar flow section with a square back just after it starts to narrow. This design neatly minimizes the inevitable separation bubbles behind the mast on each side of the sail, was developed in the wind tunnel, and improved the power of the main by 10+ percent compared to a conventional mast. It also leads to even better improvements in extremely light air (<3kts).

So, does anyone know if the Capricorn and Infusion "wing masts" are designed to perform similarly? Or are they simply more streamlined versions of the conventional teardrop? If not, why does no one use the Bethwaite design on a Cat?

Does Bethwaite use Bethwaite's design? I thought most of the Aussie 18 skiff masts were round? I'm quite sure they don't rotate either.

Good point Jake !

Another bethwaite design, the 49-er, doesn't have a rotatin squareback mast either !

with respect to the post made by Glenn





Not really, the wing masts are seriously different from a teardrop masts. It is not even a simple modification to make a teardrops shape into a wingmast shape.




I'm quite sure the "conventional" mast here is simply a round tube, nothing more. The difference between a wingmast a la Capricorn and the squareback will be a whole lot small is any difference can be measured.





Less then 3 knots is VERY little wind. And the squareback was optimisation for these conditions, I think Bethwaite even wrote that. For the high wind ranges the intended effects can be alot less significant.




I have a wingmast myself (Taipan F16) and more and more I believe its attractiveness is far more related to the way you can tune its bending. I'm sure some improved aerodynamics is present as well, but not as the dominant factor.


Wouter

I believe most skiffs can't have rotating masts due to their narrowness. In order to keep the mast "in the boat", they need to have spreaders on their lower shrouds which make mast rotation virtually impossible. Also, they have to carry considerably higher rig tensions than cats.

sm

Higher rig tension? Why?

All that sounds suspicious to me. I think they're skeered.

I purchased one of the first Tasars sold in the U.S. and raced it for many years (rig designed primarily by Frank Bethwaite) and it has a rotating mast, full battened mainsail, and roller furling jib. This made it easier to sail the similarly rigged Hobie 18 that I bought new in 1980. The main difference being that it was much larger and heavier and rigid. I cannot think of any reason that any dinghy cannot use the same type of rig proven very effective by the Tasar.

That may be a rules issue. IIRC the i14's (plus most others) are not allowed rotating masts which is most cases will preclude a wing mast.

Tiger Mike

Skiffs like I14s have non-rotating masts with high tension upper AND lower stays.

Skiffs like 49ers/29ers have non-rotating masts but achieve good airflow by sleeving the sail around the mast.

A 29er only sleeves the mast down low in the bottom 2' (or so) of the main along with part of the boom and the vang strut. the rest of it is a traditional luff groove hoist with an internal halyard arrangement.

The rig tension is basically a function of the force that the rig can generate and the geometry of the shrouds/forestay. The higher up the mast and the farther out the shrouds anchor, the lower the rig tension will be.

Consider an 18ft skiff- basically it generates more force from the rig and the shrouds anchor more inboard as compared to an 18ft beach cat. Suppose the shrouds anchor at 3 or 3-1/2ft out from the mast base- a cat anchors at 4 or 4.25ft. The cat as two guys with 8ft or righting moment. The 18ft skiff has 3 guys on a rack 9 or 10ft from center line, so more righting moment= more rig force so more rig tension.
I believe the skiff uses a smaller mast section and they keep it from deflecting to leeward by locking it with spreaders on the lower shrouds. This requires that the leeward shroud be tensioned as well as the windward shroud.
I'm pretty sure the 18s have hydraulic rig tensioning devices.

18s don't have restrictions against wing rotating masts. The main thing is no one has spent the money recently to try and get one to work, the current mast is efficient to an extent and capable of handling the huge loads generated. Providing no one else changes their rig design (currently CST Composites supplies the whole fleet) there is no need to change the design. It has been discussed in the bar quite often however.

They also do not have hydraulic rig tensioning devices. All the rig tension comes through a boat breaker, similar to a mainsheet set up on a cat (lots of purchases :P)

Cheerio
T_E

What a load of nonsense !

Several classes like the ORMA tris, open60 and even dinghies like the laser vortex and papertiger catamarans have solve the issue of lower spreader shrouds and rotating masts.

Also it is just nonsense to say that a 49-er is experiencing higher rig tensions then say a Inter 20 or tornado catamaran.

Those I-20's (F20's) overhere carry 1:10 mainsheet systems and 1:12 downhaul systems. Any idea what kind of loads that puts on the mast and mast foot.

Even Orange 2 has a rotating wingmast and its mainsheet (leech) tension is 22 tons !

I've sailed both 49-er and now F16's, these baots are very comparable in the way of sailarea and such. I can tell you from experience that my F16 experiences much high platform and rig loads then the 49-er. Beside the 49-er I sail only had 3:1 mainsheet.

Wouter

Bethwaite himself writes against sleeving sails around round mast tubes in his book. It doesn't do much as the flow is seperated before it can reach the sleeve. The round curvature in front is just too sudden. That is why the newer catamaran wingmasts all have an elliptical leading egde. Even Bethwaite write so himself

Wouter

The one we had on the 49-er was only 4:1; it could never put more then 200 to 300 kg of tension on the forestay.

My catamaran 7:1 mainsheet however can easily achieve 600 kg forestay tension as my boom is over 2 mtr long and my forestay bridles are only 1.35 mtr in front of the mast. These difference in leverage result in an additional 1.5 magnifying effect. Effectively resulting in a 10:1 boat breaker. I have to have at least 3 mm dyform or 4 mm 1x19 wire on my forestay or risk breaking it.

Try that on a 49-er. (or 4:1 boat breaker)

No really guys. I really don't believe the skiffs have such high rig tensions in comparison to high performance cats.

Wouter

If it can be made to work on the open 60 mono's with canting keels then it can certainly be made to work on a 18 ft skiff.


Beside on cat the biggest portion of the rig come from the mainsheet and leech tension. Think 2/3rd or more. The skiffs use ram-vangs and rather weak 3:1 mainsheets therefor don't have this component.

That makes a really big difference.

Wouter

Those F18 masts are copies of what classes like the C Class, A Class, Dolphin and later the Taipans have been using for a couple of decades. Nothing new here mate. I was racing with a rig like this on my first cat (Dolphin) in 1992.

As mentioned before, some monos do have wing masts like the Tasar, NS14, MG14

The skiffs don't use it because they have previously spent a lot of $$$$ developing their soft tip rigs and to scap everything and start again developing a wing rig which may not work for them and would cost mega $$$$$.... It is just not worth it.

They also need a lot more support at the top of the mast to carry mast head kites with can be up to 3 x the sail area as our kites. Put your cat kite up the top of an unsupported wing mast tip and see how long it takes before you are calling the insurance company.



Bit of repect there Wouter.

Firstly, compare boats of equal size to each other. A super Maxi will have more rig tension than a T. A 49er is 4.9m long ie 16 foot. An 18 footer is 2 foot shorter than a T or I20 however the rig tension is a lot higher.

I have sailed 12s, 14s, 16s and 18s as well as MANY different cats. Rig tension for a skiff is a lot higher. Just have a look at the compression to an 18s mast when they are cranking on the boat breaker to put the fore stay on. We often thought we were going to snap the mast whilst rigging.



Dont forget that a monos weight without crew is balanced over the keel line and with crew, its righting momentom is the weight of only its crew. In an 18s case, it is 3 crew 7.5 feet from the center line. 15 foot wing span I believe now.

A cats righing momentum is the weight of the platform and its rig from the windward side of the leeward board...... plus the crews weight, 8 foot (or just under 10 foot for a T) out from the pivot point.

Looking at this, the cat has a lot more righing momentum and uses the weight of the boat far more efficiently.

Below is a boat breaker about to be put into action to enable geting the forestay on.

I designed a 60lb lead bulb and built the aluminum keel fin for one that a friend owns. I can also tell you that the rigging loads, relative to even a 16' cat (don't know of a 15' cat to compare it to), are tiny on the skiff - it's got a 2:1 mainsheet! (and it's plenty of purchase)

This is not the boat with the bulb keel but shows the mainsail / luff groove arrangement.

The majority of tension on a Skiff rig is given whilst rigging. This is constant tension which is in another league to a cat. Nothing realy to brag about so don't get too upset guys.

Secondly, the boom vangs add a lot of downward tension to the boom, therefore large mainsheet loads are not required as it leach tension is already applied. Once again nothing to brag about or be disapointed about, so don't feel you must compete. Fact is fact.

My old Dolphin catamaran had a boom vang and it was amazing how much it took away from main sheet loads.