Referring to the bend curve method here (excerpt at bottom of this post):

www.peterman.dk/windsurf-mast-article-gb01.htm

If a manufacturer gives you only the 3/4 measurement in %, is it possible to estimate the full method explained by Peterman ? (ie: is there a mathematical relationship between the 3/4 point and the measurement described by peterman ?)

Cheers



"Bend Curve.
If you measure how much the mast deflects at the 1/4 point of the mast, at the midpoint, and at the 3/4 point you get some figures telling of the way the mast are bending under load (called bend curve or bend characteristics). If we for instance say that a mast deflects 13 cm at the 1/4 point, 20 cm at the midpoint, and 15 cm at the 3/4 point, you find your calculator and do as follows:
First you calculate how much the mast deflects at the 1/4 point in % from the deflection at the midpoint. In our example it's (13 x 100 / 20) = 65 %.
Next you do the same calculation for the 3/4 point - that is the deflection of the mast at the 3/4 point in % from the deflection at the midpoint. The calculation is (15 x 100 / 20) = 75 %.
Now all you have to do, is to subtract the two figures - that's 75 % minus 65 % = 10. To be correct, the denomination is not 10 % but 10 %-points. Anyway, you say that the mast has the bend curve of 10 *).
To make it easier to understand the bend curve figure you can translate the figures to these IMCS terms:

0 - 6: Hard top.
7 - 9: Hard top - constant curve.
10 - 12: Constant curve.
13 - 15: Constant curve - flex top.
16 - 18: Flex top.
19 - 21: Flex top - super flex top
22 - : Super flex top."

Math not my strong point but I think no.

The manufacturer gives you

length
IMCS (which is mid point deflection essentially)

Petermans thing gives you the difference between top and bottom deflection - so its representative of tip flex, constant or hard top.

If you only have 3/4 measurement, and IMCS, it does not tell you what the 25% measurement is.

But you could infer roughly tip flex or hard top from the 3/4 measurement I guess...

MCS (mast check system)is the stiffness of the mast as an Index.

IMCS (indexed mask check system) scales that index according to the mast length.

If you have the % difference between top and bottom and the stiffness index number (e.g: 460-25/12%) you should be able to work the equation backwards to find that actual deflection at each point. In theory of course. Actually trying to do it would make my head hurt!

www.sailworks.com/pdfs/MCS_test.pdf

does anyone understand what constant curve means? I would've thought if the curve was constant the 25% and 75% deflection would be the same and therefore the bend curve would be 0?

Constant Curve is a term used as a marketing tool and can mean whatever they want it to mean!

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bodia1 said...
does anyone understand what constant curve means? I would've thought if the curve was constant the 25% and 75% deflection would be the same and therefore the bend curve would be 0?

All windsurfing masts bend more at the top section, the hard top less so and the flex ones more so, and the constant curve are in between. That's marketing for you

Thanks all. Sailquik, that's food for thought.

Below from: www.unifiber.net/windsurf-gear/masts

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jn1 said..
Referring to the bend curve method here (excerpt at bottom of this post):

www.peterman.dk/windsurf-mast-article-gb01.htm

If a manufacturer gives you only the 3/4 measurement in %, is it possible to estimate the full method explained by Peterman ? (ie: is there a mathematical relationship between the 3/4 point and the measurement described by peterman ?)

Cheers

Curious to know jn1, have you tried to contact the mast manufacturer to find out the actual bend curve of the particular mast?

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sailquik said..


If you have the % difference between top and bottom and the stiffness index number (e.g: 460-25/12%) you should be able to work the equation backwards to find that actual deflection at each point. In theory of course. Actually trying to do it would make my head hurt!

www.sailworks.com/pdfs/MCS_test.pdf

I thought he was only talking abotu the 3/4 measurement
Then he does say %

jn1 which one? are you saying you have the IMCS / length and % difference? Or IMCS / Length and 3/4 deflection measurement?

Gazman: I did that, I get an approximate answer. BTW, thanks for posting the graphic.

Mark: % points. I want the constant curve figure to be the subject of the equation.

What I'm getting at: if you're a sail maker and you want to order masts to a specification, what (functional) spec does the mast manufacturer ask the sail maker to provide ?

- Approximate ? (hardtop, CC, flextop, "a bit of this and a bit of that" ?)

- Specific IMCS, bend curve figure like the above Peterman link ?

- A CAD drawing, bill of materials etc ?

That's why sail manufacturers recommend that you buy their own masts, because they already have the equations all figured out to a science for their own equipment.

Most of us windsurfers don't have the money or refuse to buy a name brand mast to match the same name brand sail they want. That being said, we end up mixing and matching up sails and masts the best we can.

To prevent your self from getting a headache on this subject, just call the mast manufacturer to see what sails they recommend for your mast, because if you miss something on your math equation, your gonna be screwed. That would suck!! But perhaps you just want the challenge, I get that.

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jn1 said..

What I'm getting at: if you're a sail maker and you want to order masts to a specification, what (functional) spec does the mast manufacturer ask the sail maker to provide ?

I think you'll find that there will be a slight difference in masts anyway if you take 3 masts of the same length and brand they will not be the same some might even be longer as well.

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choco said..
I think you'll find that there will be a slight difference in masts anyway if you take 3 masts of the same length and brand they will not be the same some might even be longer as well.

"a bit of this and a bit of that" :)

Good point Magic Ride. I have asked my mast manufacturer their recommendation. I reckon that is a good way to go. Regarding you're first point, I'm a technician, it's in me to understand how things work :)

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GazMan said..

Below from: www.unifiber.net/windsurf-gear/masts

What I want to know is if you did this test with 40kg or 50kg on both SDM & RDM would the results match?

I am betting they won't.

Testing both at only 30kg does not prove similar performance between RDM & SDM.

I wonder how much tension/force is applied when down hauling ?
A wave sail must be over 30kg, race even more.

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col5555 said..
I wonder how much tension/force is applied when down hauling ?
A wave sail must be over 30kg, race even more.

The two forces are not equivalent. The downhaul force acts in line with the mast, while the 30kg load acts perpendicular

^^ more than that for sure but that's not the same as hanging a 30 kg weight in the centre of the mast.
The 30kg was chosen because it produces a deflection that is commonly found in rigged masts.

It's a matter of vectors. If your mast was perfectly straight and your downhaul was concentric with the mast it would be impossible for mere men to bend the thing at all. Putting a little bit of bend in the mast by pulling the luff taught and having the downhaul slightly eccentric from the mast foot gives you a manageable resistance to deal with but it's about 20+kgf after 3x reduction through pulleys on a wave sail (guess).