DN Bendy mast

[rapscallion]

Is there any decent explanation out there for why the DN bendy mast is so much faster? And why don't we see bendy masts on A or C class catamarans?

[Sistrunk]

Do your research in the area of windsurfing rigs.

[Corley]

This description is from the Jeff Kent the leading dn rig builder and championship winner is from Seahorse.

The benefits of bendy rig are initially their ability to depower and have gust response which needs to be very fast as the boats arent very able to spill wind without crashing. I'll just put an extract of the article up verbatim.

His description about their performance in steady state winds follows:

So far we've talked mainly about depowering and gust response. While this is an important feature of the bendy rig there are also reasons why the same rig is faster in steady-state conditions.

A) The luff is more parallel to the leech. It's been said that for maximum efficiency the leading and trailing edges of a foil should be parallel - think of a bird or aeroplane wing. Since you can't pull the arc out of the leech another way to keep them parallel is to bend the front - the mast - to match the curve of the leech.

B) It 'increases' the apparent wind angle on the sail. As the leading edge moves to leeward the sail sees a wider angle to the wind, thus increasing power. This is akin to slackening the forestay in light air on a J-24 or Etchells to improve pointing. It also redirects the driving force of the sail more sideways. In a soft-water boat too much of this would lead to excess heel and possibly stalling the flow around the keel but in an iceboat that isn't a factor... The extra power and pointing are more than worth the extra bit of side force.

C) Drag reduction. This is paramount in iceboats as drag increases as the square of the wind speed. As the mast bends with increasing apparent wind speed the sail becomes very flat, decreasing drag. Masts are also set up to under-rotate in breeze, presenting a smaller profile to the oncoming wind and decreasing drag still further.

[rapscallion]

Are A class and C class catamarans too slow in comparison to a DN to benefit from a DN bendy mast?

[Corley]

I think the reason that they dont have a bendy mast on A's and C's is that better sail and wing control are available the dn has quite a few class restrictions which mean solutions like bendy masts are more attractive. Mind you fibrefoam masts on A's are becoming quite popular they have quite a bit of flexibility fore and aft and significantly less from side to side.

The really fast ice and saltpan record breakers I've seen have all used fairly standard if radical looking rigid wing rigs. The dn's are a sort of midfield contender in the speed stakes. The 130mph greenbird holds the landspeed record. The ice record sounds like its a matter of contention it's held by a soft sailed boat but consensus among modern sailors is that the craft which is suggested to have gone 143mph could not have achieved that speed.

You might like to check out the site below for more info.

http://www.windjet.co.uk/index.php?o...=68&Itemid=111

[Doug Lord]

Best explanation I've found so far: http://www.wb-sails.fi/news/98_2_SideBend/SideBend.htm

Excerpt:

Killing twist with negative bend

While sail twist is the friend of the salt water sailor, it is the enemy of the ice boater. With very little resistance between the skates and a good, slippery ice, all drag in the air is detrimental, and twist does no good to the lift/drag ratio of a sail. With the wind pressure at apparent wind speeds around 40+ knots, there is no way to keep the leech from twisting off.

So, the witty ice boaters use negative mast bend to eliminate twist from their sail. The mast follows the curve of the sail leech, providing essentially zero twist. Seagulls, as birds in general, do the same - as the feathers at the trailing edge of the wing twist out under pressure, the seagull bends down its wing to eliminate the harmful twist.
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Raps, I've seen a lot of bend in some A Class masts:

[gonzo]

It acts like a shock absorber. Depowers in a gust and springs back, recovering some of the energy when the wind speed goes down. In other words, the average power produced by the rig is higher.

[Sistrunk]

Like I said, study windsurf rigs and ask all the same question within that group. You'll get your answers and then some. They are the kings of bent rig technology and all the folks mentioned above are sort-kinda playing with the idea.

[Doug Lord]

Originally Posted by Sistrunk

Like I said, study windsurf rigs and ask all the same question within that group. You'll get your answers and then some. They are the kings of bent rig technology and all the folks mentioned above are sort-kinda playing with the idea.

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It seems to me that the windsurfer mast bends the exact opposite direction relative to the apparent wind than does the DN mast-probably not much in common technically, in my opinion. Also, the windsurfer rigs are designed to twist where the DN rig is not:

[Sistrunk]

So you say, but I don't agree. You are comparing stayed rig bending characteristics to unstayed rig issues. The luff prebend issues are nearly the same.

Again... talk to the windsurf guys and get a handle on the issues from them. They know volumes about mast structure, sail cutting, power generation, the whole works. Here's a good place to start:
http://www.hansensails.com/AboutUs.htm

Bill is one of the gurus for sail design and bending rigs and I would bet that he doesn't have to stretch his knowledge too much to give you some feedback. He'll certainly point you in the right direction and that looks to be worth a few million posts by folks who are not hooked in very well.

[gonzo]

Windsurfer rigs are very different. They also produce a lot of lift because they are tilted to windward instead of leeward. When masts bend, the distance between the mast top and the clew shortens, which loosens the leech and normally depowers it.