Sport Today

I'm thinking of something that uses two sawhorses, some weight, and a tape
measure, but I need some idea of how to calibrate the system.

Thanks,
Jeff

   I remember a while back someone posted instructions on how to calculate the
   stiffness of a mast using common household tools.  I'm not looking for extreme
   accuracy here, I would just like an idea what the stiffness is of my old
   epoxy masts so I can compare to one I might purchase.

   I'm thinking of something that uses two sawhorses, some weight, and a tape
   measure, but I need some idea of how to calibrate the system.

here it is:

Article: 15701 of rec.windsurfing
Newsgroups: rec.windsurfing

Subject: Re: Cutting a Carbon Mast?
Organization: University of Hawaii
Date: Mon, 30 May 1994 02:46:42 GMT

: I have a 5.00 m. one-piece carbon mast (Gaastra) that I would
: like to cut down to 4.65.

: Anybody out there who can give me any advice on this?
: Can I do it - or is it too much? Will that mast work as well
: as a an intially shorter mast would (stiffness etc.)?
: Should I cut it from the bottom or top? Or a little from both
: end? What tools are best to use - and can anything go wrong
: if I don't watch out? Do I risk it breaking easier if I cut
: it from the bottom? Is there an enforced section on the mast
: for the boom - and is this section long enough to serve its
: purpose if I cut 35 cm from the bottom?

: -Harald Skinnemoen

You can measure the flex your 500 cm mast would have if it was cut,
before you actually cut it.  If you know of a 465 cm mast that works
with your sails you can see how closely your cut 500 cm mast would match
it by supporting both ends 5 and 460 cm from the base of the mast if you
are going to cut off the tip.  Hang 30 kg from the middle of the
supported area of each mast and see how the flex curves of both masts
match.

As others have mentioned you don't want to cut the mast so that you
won't have reinforcement in the base or boom area.  The manufacturer
should be able to tell you how far the reinforcement extends.  You
should also make sure the inner diameter at the base will fit your
extensions and perhaps buoyancy plugs may have to be moved.  You can
however tell what the flex will be before you cut the mast.  If the base
diameter and reinforced areas permit, you can cut some from the base.  
Put the flex testing supports 5 cm in from where you might cut the mast
to see what the flex will be.

If you want the MCS numbers, here is the method for measuring mast flex
I had earlier adapted from the March 94 Windsurfing Magazine.

Support each end of the mast 5 cm from the ends  (or future ends if to
be cut),  use a ladder  or saw horse or hang non-stretch rope or wire
from garage roof.

Mark mid point of mast and then 1/2 way between support points and the
mid point. For a 465 cm mast the marks would be as follows from the base
end...

5 cm       for base support
118.75 cm  for bottom flex point (B)
232.5 cm   for mid point         (M)
346.25 cm  for top flex point    (T)
460 cm     for top support

__________________________________________________________
 |          |             |               |             |
 |          B             30 Kg           T             |
 |                        M                             |

Hang 30 kilograms from the mid point.  If you don't have an even floor
to measure from, you can stretch a string above the mast to measure
from.

Measure the deflection at points B, M, and T with weight hung on the
middle point and after removing the weight.  Make sure mast doesn't fall
off supports when weight is added.

MCS flex = flexed mast length/deflection at mid point (M), ie. for a 465
cm mast that deflected 17 cm

MCS = 465/17  = 27.4

The bottom flex is deflection at B/ deflection at M
The top flex is deflection at T / deflection at M

If the deflection at B was 11cm, M was 17cm, and T was 13cm then..

Bottom flex = 11/17 = 65%
Top flex =    13/17 = 76%

As an example of what will happen to the flex,  I cut 30 cm off the tip
a too-soft 465cm Kilwell Light Wave MCS 17-61%/85% mast.  After cutting
the mast was MCS 23-60%/83%.  

If you know where the reinforcing is and what the flex will be, you
shouldn't have any surprises when cutting the mast, but it might be
easier to buy Stig Johansen's masts.

Good Luck,

Steve Spielman

: Norway

---

William L (Bill) Kleb                          195/6.3 (44kg/190cm)

mosaic: http://ab00.larc.nasa.gov/~kleb/       8.4 (260cm) finally-done!
mail stop 366, nasa langley research center    Gastra, UP, & no-name UP
hampton, va, usa 23681-0001                    3.3     4.0   5.0,6.0,7.0
ph: (804) 864.4364 day / 838.7853 other        White 77 Jeep Wagoneer

--
=============================================================================

NASA Langley Research Center          Mosaic: http://ab00.larc.nasa.gov/~kleb
Hampton, VA  23681-0001  USA          Phone: 804 864 4364   Fax: 804 864 8670

Wilton

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This is in response to a question someone had on mast stiffness.

In the summer 1991 edition of "Performance Windsurf Report" (a free magazine I
picked up at a local shop) there was an article on how MCS stiffness
measurements are made.  Here is a summary of the test:

1.  Support the mast horizontally, with the supports 5cm from the base
        and tip.  Measure the distance between supports.
2.  Measure the vertical deflection of the mast at the quarter points,
        due to the mast weight only.
3.  Hang a 30kg mass from the mast halfway between supports, and measure
        deflections at the quarter points again.
4.  Subtract deflection w/out weight from deflection with weight to give
        net deflection.
5.  The overall MCS stiffness is given by dividing the distance between
        supports by the midpoint deflection.
6.  The stiffness percentages are given by dividing the deflections at the
        1/4 and 3/4 points by the midpoint deflection.  The shape of the
        mast curve determines these numbers (i.e. whether a mast is
        constant curve or flex top).

Example:

Total mast length = 460cm
Deflection length = 460 - 2(5) = 450cm
Point spacing = 450 / 4 = 112.5cm

                             1/4     1/2     3/4 point
                            ----    ----    ----
Deflection w/out weight      0.6     1.1     0.9
Deflection w/ weight        12.2    19.1    14.7
                            ----    ----    ----
Net deflection              11.6    18.0    13.8

Overall stiffness = 450 / 18.0 = 25

The article also says that the DIN test involved*** a weight 420cm
from the mast base and measuring tip deflection.

As far as someone saying they thought an MCS number of 3 was a mistake,
that it should have been 30, I've got a North two-piece aluminum mast
that says 7.8 DIN, 3-4 MCS on it.  Maybe that's an "old" MCS number.

It's getting too late in the year to sail here anyway, the lakes are
starting to freeze over.

John Brunner

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