bendy masts on ice boats

[b_rodwell]

I have noticed photos of racing ice boats showing what seems to be extreme mast bend compared to water boats.

Does anyone know why it works for them and whether the same principle is applicable to other boats?

[National3434]

Originally Posted by b_rodwell

I have noticed photos of racing ice boats showing what seems to be extreme mast bend compared to water boats.

Does anyone know why it works for them and whether the same principle is applicable to other boats?

First, I am not an Ice sailor, and, living in the south of UK, am unlikely to even see it, let alone do it!
It is a question I asked when I first saw photos of DN iceboats with the masts bending laterally the WRONG way ie bending to leeward in the middle, rather than at the tip as in a 'normal boat'. I think I was reading an artticle in 'wooden boat' about the DNs and the explanation was that with the fairly modest sail area, low aspect ratio and very high stability, this is how the power of a gust is absorbed (rather than dumped). I have not seen this on pics of the rigs on the bigger yachts (skeeters) which use a wing mast and much higher aspect rigs. I think of the bend as being a bit like the downbeat of a birds wing, the max lift condition.

I guess this principle could be used on water craft. I have been nosing around the various speedsailing sites recently and most have cottoned on to balancing the forces vectors through a single point. Having decoupled the thrust from overturning moment, then maybe this power absorbtion in a gust is just what they want?

[Overdraft]

AFAIK, because of their high speed the apparent wind once they get going is forward... WAY forward... one of the strategies of the bendy masts is that as wind speed increases, boat speed increases and the apparent moves forward and and the bendy mast sucks up the extra fabric making a progressively flatter draft to better match the finer and finer apparent wind angle....

[tspeer]

Landyacht, and I presume iceboat sails, are constructed dead flat - no built-in draft at all. They get all the draft they need from the stretching of the fabric.

I believe the bendy DN masts have more to do with controlling twist than they do with controlling draft. The extreme bending down low moves the tip of the mast to weather. If you look at DN photos you'll see this allows the leech to stand up. The whole sail is like it's made on a curved surface like a horizontal cylinder instead of a flat plane. But all the sections are lined up without a lot of twist.



When you trim the sheet on landyachts (which I've sailed) and iceboats (which I haven't, much), you're really working the twist at the head more than sheeting the boom in and out. The boom mostly moves vertically, and the yachts rarely have travelers. If the boom is ever sheeted out horizontally, it's because your going very slowly - like at the start. Once the yacht gets "hooked up" it's sheeted in tight all around the course. You'll ease a bit rounding the windward mark, then back in to accelerate, etc.

So I suspect the DN masts work with the leech tension to enhance the twist control, either at the head or in the belly of the sail.



The other reason for it may be to get a downward force component from the lower part of the rig. In high winds, we loosen the stays on landyachts to let the rig tilt to leeward by 15 degrees or more. This imparts a downward component that helps keep the yacht from hiking and improves traction. The DN mast is doing this automatically - the harder the rig is loaded, the more inclined the lower portion is. Offwind, the apparent wind is less, and the rig would stand up, removing the tilt when you don't need it.

With an inclined rig, the farther the rig is located to windward, the more effective the inclination is, because the downforce has a longer lever arm. It's not practical to move the mast step to windward (although it's been tried on boats), but by putting the inclination down low, the DN mast does keep it to windward as much as possible. The upper part of the rig, which would not contribute anything to reducing the heeling moment, since it's right above the leeward runner, stands straight up for best aerodynamic efficiency:

[National3434]

Tom,
Thanks for finding the fascinating pictures (rather wierd to a soft water sailor!) and a clear explanation. I guess the key difficulty in applying this to speed sailing on water is the low power a flat sail would generate at low speeds and the inevitable associated high drag (versus wheels or ice skates)? I guess that the highly curved kites used by kite surfers are using the same effect to control the foil shape?

Cheers

[tspeer]

Just because the sail is constructed flat doesn't mean there isn't camber when it's operating. Remember, these sails see apparent winds of 30 - 60 kt all the time.
You can see the draft in the sails in the second photo.

If you draw a straight line from foot to head, you can see what the curve in the leech is. This is the "leverage" leach tension has in reducing the twist. Now imagine that the mast were straight. The luff would be pulled to windward and the twist would be much greater, especially in the belly of the sail. So you might think of it as having a leech curve that is established by the elasticity of the sail material and the available leech tension, and then letting the luff sag to leeward to finish controllng the twist.

BTW, it may not be evident from the photos, but the masts are short-chord rotating wingmasts.