In my uninformed opinion which may well be wrong:

Go for cams (unless it's an Ezzy with heaps of shape already built in)

The idea of a sail is to change the direction of the wind. F = ma and all that, you want to accelerate the wind behind you, giving you a bit of a push forwards. The best way to do this is to gradually turn the wind using a foil, rather than a flat plate.

Cams make your battens stay the same full shape regardless of how much wind is pushing on them. Uncammed sails belly out more the more wind they have (which is pretty much the opposite of what you actually want them to do).

However, cams will mean that the sail is heavier, the bigger luff pocket will make it harder to fly the sail when waterstarting or uphauling, and it's harder to rig.

But I like my 6 cams on my biggest sail, and zero cams on my smallest

quote:

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Originally posted by nebbian

In my uninformed opinion which may well be wrong:

Go for cams (unless it's an Ezzy with heaps of shape already built in)

The idea of a sail is to change the direction of the wind. F = ma and all that, you want to accelerate the wind behind you, giving you a bit of a push forwards. The best way to do this is to gradually turn the wind using a foil, rather than a flat plate.

Cams make your battens stay the same full shape regardless of how much wind is pushing on them. Uncammed sails belly out more the more wind they have (which is pretty much the opposite of what you actually want them to do).

However, cams will mean that the sail is heavier, the bigger luff pocket will make it harder to fly the sail when waterstarting or uphauling, and it's harder to rig.

But I like my 6 cams on my biggest sail, and zero cams on my smallest

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Sounds like some slightly dodgy aerodynamic theory there Nebs! Bernouli's theorem is that if you make air move faster, it's pressure will decrease. That's why our sails have that curved foil shape - the air travelling over the bulged side of the sail has to go further than the air on the inside, which means that it has to go faster to meet up again after the sail has passed through..

So the aim of the exercise is actually to create an area of lower pressure on the side of the sail that is away from you, so that the sail is effectively sucked into the lower pressure area due to being stuck between the fast moving air, and the slow moving air on the other side of the sail.

Of course, that only works if there is some air-flow happening to start with, and that's where pumping, and apparent wind comes into it.

You can induce an airflow by moving the sail through stationary (or slow moving) air. You can also give yourself a bit of momentum by scooping air, and chucking it out behind you. The back pull part of the pump chucks air (not much aerodynamic theory there!), and the forward scooping action gets the air flowing across the sail.

If you're standing on your board feeling 8 knots of wind, and then you pump a bit, you start moving forward, and the apparent wind will increase - say to 11 knots where you might be able to get planing.

But you knew all that already!!

Camber inducers effectively lock the draft (that's the deepest part of the curve) of the sail into one place. This means that the shape of the sail stays pretty constant in the cammed area regardless of the airflow, or load on the sail. Usually in a free-ride oriented sail, the cams are low down, in the engine-room of the sail, and the higher up parts are still free to twist and flex. (not true of Neb's 6 cammed sail - I'm guessing it was designed as a race sail??)

Cams can make flipping the sail more difficult.
No cams means that the sail generally needs some wind to make it the right shape, and the draft (depending on the sail design, and cut, of course) is more free to shift forward and back.

I have sailed a cammed Hot Sails Maui 7m sail, and a newer North cam-less 7m sail on the same board in the same conditions, and found that I prefered the cammed sail. It seemed to have greater range, and the cam-less sail kept shifting it's centre of effort, so I had to constantly adjust (wrestle) as I sailed.

Cam-less can be easier to flip, and easier to rig. It seems that the modern trend for free-ride sails is towards cam-less. Maybe this is a cost thing, or maybe convenience to the end users I'm not sure.

The other point to note is that you can't "turn off" a cammed sail - as long as there's air moving past it, it'll be pulling. Cam-less sails tend to pull less when feathered (i.e. if you hold it by the mast and nothing else). Wave sails are cam-less for this reason.

I like cams on the big sails..

Think I agree with mkseven, although I haven't had a huge experience with cambed sails. They have full shape at 0 wind, (well in theory, if they're good sails, more wind shouldn't be able to increase their shape)
As before said non cambed sails need some wind to turn them on.
The trouble here is how much wind????
Some sails fully rotate and shape up with about 5kns others seem to need more like 15-20kn to get full shape. If you have the later I'm sure it won't be much good in light winds.
This is probably another area where mast compatibility plays a roll

A bit off topic.... no.... yes......no ?

Mentioning pumping: remember that half (or more) of the effect is due to loading the fin briefly to create more lift and help the board lift up. Also (on a wve / freestyle board) rocking it back on its rocker to stop it from pushing so much water up front.
Not all of it is about the sail...

Anyway back on topic.... cammed.

plenty of dodgy aerodynamics going on in this thread. Air does not meet back up at the trailing edge, smoke tests of foils prove this. nebbian pretty much has it right I reakon. people argue newton (f=ma) or bernoulli, or even more ridiculously percentages of both to explain lift. Both are ways of describing the same thing, one by conservation of momentum one by conservation of energy. By some mathematical trickery they can be shown to be equivalent.

In my opinion cams are desirable on big sails where you want all the lift you can get but less desirable in smaller sails as you have stacks of lift and what you want is instant control by powering and depowering the sail, cammed sails are less easy to depower.

Thought for the day:

Would a reflexed sail provide less turning moment, so therefore allow the use of a smaller fin, which in turn would allow you to go faster?

So, cams are best for low end, light wind power?

Would a Severne Overdrive be a good choice?

mkseven said...

As always I disagree with Gestalt. Big sails with cams built for light wind have far more draft than a non-cammed sail. If the non-cammed sail was to attempt to achieve the same it would be baggy as a bag of wotzits, which is just wrong.

Where non cammed sails are good is with pumping.

Traditionally the most powerful bottom end sails built for getting heavier people and boards on the plane have been 2-3 cam sails.

yeah i know mk, you always dissagree . so how many 9.5m plus race sails without cams have you used? i've used a couple myself form different manufacturers.

draft is not just about cams. cams help lock the draft in giving stability after planning occurs, correctly tapered battens with no cams can also do the same.

draft is built into the sail by the sail cut. a nocam sail can have just as much draft as a cam sail.

"typically" soft sails are far better in non planning conditions including pumping.
the weight the cams add is also a big issue. so a compromise is made with regards pumping/stability/weight so some sail manufacturers use minimal cams and less battens making the sail more user friendly.

leach is also another area that is critical. a tight leach tends to work better in lighter winds (ie. race boards). Typically cammed sails have loose leaches which is great when you're planning but not as good in the really light stuff. however the foot tension strap does help get around this when it is cranked tight as it reduces the sail twist on the cammed sails.

most sails marketed for light wind recreational cruising don't have cams like the sailworks retro, KA kontender, or aerotech zenith. some have 2-3. like the aerotech dagger and severne glide which have more stability at speed for the racers.

check out this post and see how a ond design soft sail faired against the gig cammed race sails.
www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=31813

Gestalt said...
check out this post and see how a ond design soft sail faired against the gig cammed race sails.
www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=31813

That's a stretch isn't it???!

How about comparing apples with apples.. the same weight sailor, and the same board. Then we might be able to make some reasonable judgement about the impact of the differing sails on the outcome.

If I put a 2hp outboard motor on a kayak, it'll probably go faster than a 200hp diesel engine on a barge.. no reflection at all on the engine though..

If I put a 2hp outboard motor on a kayak, it'll probably go faster than a 200hp diesel engine on a barge.. no reflection at all on the engine though..

555 i agree with you're point.

i have also compared apples with apples. i always found the no cam sails felt faster off the mark, better to pump and more powerfull than cammed. but once the wind picked up a bit i just wanted the fully cammed race sail with better top end and the locked in feeling. i have also seen no cam race sails beat cammed sails around a formula course in light winds.

probably cause nocam sails tend to perform better than cammed sails when under downhauled. could also be because of the weight saving?

the question was "The long and short of it is that I want a large sail for light conditions. It doesn't need to go fast, or be particularly stable. All I want is to get planing in minimal wind. Cams or no cams?"

for me that is a no cam sail, if DL had said he needed it to be fast and stable i would have said camed. also i haven't tried the aerotech or severne and they sound like they are cammed sails built specifically for light conditions and will hold their shape too.

so maybe they are the go.

Avoiding all technical bizzo, I've got a 7.5 (3cam) Gaastra GTX, love it!

Dunno about the guys at the shop's intentions, selling it to me as my first sail, but on light-medium days, it's great.

The rigging is supposed to be more difficult, but...(here I go, I said no tech. crap......) the cams 'create' the sail's shape, leaving the guesswork out of rigging, although the outhaul tension must be right, otherwise you look like a 'dill' trying to 'pop' the sail after a turn. The only downside I find (mentioned earlier), is when waterstarting in gusty conditions it's touchy, but that could be just my lack of ability showing!

Its simple - may I be so bold as to say "if you are planing, cambers are faster in pretty much all conditions", assuming everything else being equal.

If they weren't they why are they in the biggest race sails and in the smallest speed sails?!

from what i have found most modern cambered sails have very open leeches which gives them great top end but the down side is that low end power is reduced and the ability to pump the sails in marginal condition is also reduced. Cambers on most sails exert pressure onto the mast which 'forces' shape into the sail which is why the draft position is held in place better than 'raf' sails. So if you are looking for a sail with better bottom end look for something that has a tighter leech .

Does anyone have information on the weight of the Severne Overdrives?

It doesn't seem to be published on the Severne website.