An older friend of mine has started a debate with me about the physics of masts.

He thinks that if you had a mast that was perfectly stiff the boat would be faster than a boat with a semi flexible mast.

I started to debate back but I had nothing, all I could tihnk of was the ability to spill aire in big wind and not over power the boat.

Is there any other advantage to a flexible mast over a very stiff one?

With a perfectly stiff mast you can build a sail to match exactly one condition. With a flexible mast you can control the amount of draft and draft position yourself and be faster over a wider range of wind (and seastate) by bending the mast.
With a correctly tuned flexible mast the mast will automatically help you alter sailshape as gusts hit, making you faster. Perfectly stiff masts will not be fast on a small boat, as you are not sailing in a windtunnel where all conditions are set and well known. You have to change your sails shape to match the conditions, and you do so by bending the mast.

Additionally, in one of the books I was reading it says all masts bend, and I've started noticing it when I look at other boats. With the mast bending, it allows you to get a much flatter sail when sailing to wind, since it allows you to get your blocks much closer etc.

On DN iceboats, the bendy mast acts like a shock absorber, soaking up the initial energy of a gust, then releasing it as the boat accelerates.

Jake,
There are times when I think what a bugger it is that I was born in such a warm Climate.

That looks just so cool!

I would really enjoy that.

Regards,
Phill

Pretty much to echo what was said above. If the conditions that we sail in were completly static, then yes, a perfectly rigid mast with a perfectly rigid rig would be fastest. i.e. perfectly steady wind on perfectly flat water sailing only on one point of sail.

However, since we sail in a constantly changing environment, the flexible mast (along with all the other controls) allows us to vary the foil shape to suit the conditions. Additionally, a dynamic rig which self-adjusts can be made to dampen the ride. When a gust hits, the top of the mast flexes allowing the top of the sail to open automatically. The boat stays flat and keeps going fast.

Of course, this is great in theory. It all depends on the designer to make it work. If the mast doesn't match the boat and sail, then you're out of luck.

sm

Do you really Phill? I needed an hour to thaw my toes after last wednesday night race..

In addition to acting as a "shock absorber", dont the bendy mast also give better grip and stability when you decompose the forces developed by the rig?

Rolf,
Some of the pics you have sent me of your homeland appear to be really spectacular.
Having never experienced that type of cold I can only imagine in a fuzzy kind of way and admire the beauty.

Just like imagining the type of speed that I could get out of an Ice boat in a good breeze.

You know what they say- "No sense, no feeling"

'Go flying in an Ice boat."

One of the 10 things That I must do.

Regards,
Phill

The biggest thing most modern catamaran masts do is allow you to flatten the sail to control the amount of power you have as the breeze increases. Most sails are built with some luff curve, and most mast a built straight. So you add diamond wires and add some prebend into the mast, but its still straighter than the sail (if the sailmaker has done his job) This causes the sail to be full and the sail in this shape produces lots of lift (power)

So, without any downhaul, you have a very full sail shape, which is good up until the boat begins to heel to much in puffs. Adding more downhaul bends the mast back and gets the mast closer to the prebuilt luff curve of the sail, and flattens the sail. This allows you to go faster in higher winds. So if you are sailing in higher winds and the boat feels sticky or overpowered in gusts, add more downhaul. Modern wing masts like the Acat masts are really good at this. You can actually play the downhaul instead of the main on the HT, which has a modern wing mast. This is really fast by the way.

A really good example is an airplane wing. When you are taking off, and landing you need full power with lower wind, so the pilot has uses lots of flaps (think full sail shape) When the plane is crusing along the flaps are retracted and the wing is less full, (think flat sail shape)

Fore and aft bends helps with flattening the sail and works with downhaul. You also want some sideways bend, which needs to bend differiently than the fore and aft. Most masts (modern wing masts) are more stiff fore and aft than sideways. This sideways bend helps with the mast twisting off in a gust. Diamond wires keep the mast in column depending on your weight, so the top can bend off, but the middle stays straighter side to side.

The Acat masts a built with two bend numbers in mind. The first is the sail makers luff curve and sail charactoristics and the second is the weight of the sailor - which influences the side bend of the mast.

The net is that a mast needs to bend, but in a very specific, very calculated way and bend differiently fore and aft vs side to side. Rotating wing mast on catamarans are actually some of the most advanced masts and sail combinations on the water, not counting a CClass type rigid wing sail. Cat rigs are way more efficient than monohull masts/sails and its one of the reasons modern cats are so fast.

So now you can argue that mast have to be really, really bending to be fast.

My Acat mast wiggles like a noodle when I carry it around without any diamond wires.

Thanks guys this was the kind of info I was looking for. I knew it needed to be flexible but I couldn't put it into words\

Would you ever bend the mast past the sail shape or is this not usually possible - doesn't sound like a good plan... do you A sailors know where that point is on your own boats if it's possible...?