11. D. Bellinga GAASTRA SAILS
Sailmast-cornbintion on saiLboards
This paper is made with the assistance of ir. J.A. Keuning of the ship-hydromechanics laboratory of the Technological University Delft.
Although boardsailing is a young sport the development goes very quickly.
The sailors skill improved a lot and the challange of cornpe-tition is a great stimulant for developing better material. Sailors could handle more wind and by the increased loading on mast and sail the sail became baggy and hard to handle.
The windloading on the sail increases the leechtension which causes the mast to bend more aft and the leech opens up. We get torsion in the sail or twist as it is called mostly. The windforces are balanced by hiking out of the sailor. These side forces cause the mast to bend. also sideways and also give twist in the sail.
With the older very flexible mast we often saw so much twist that the sail was only sheeted correct near the wishbone. The lower region of the sail was oversheeted and the top of the sail sometimes so undersheeted that it often got backwinded in the luff.
The centre of effort of the windforces moved for and aft in the sail and corrections were difficult.
In races where people had to use standard material you had to look for the stiffest mast.
The stiffness of the mast has major influence on the amount of twist that will occur in a specific sail.
ÀRCHIEE
Lab. y.
ScheepsbouwkuridTechnische 1-logeschool
As with the stiffness of the mast and the shape cut in the sail one can select the leechtension for the desired amount of
twist.
Sorne twist is needed as near the water the wind velocity is lower than at the top of the sail. By that the apparent wind at the lower part of the sail has a smaller angle with
the centre line of the board, than in the upper part of the sail. So the lower part is more closehauled and has a lower apparent wind velocity than the top.
To get the same section angle of attack over the whole luff some twist is needed.
As twist is governed to a large extend by the leechtension it is necessary to know the mast overall stiffness.
The mast data are measured in a 3 point bending test, as
described in appendix i and 2.
The mast is tested as a simple supported beam loaded at boom Position.
This gives a linear bending moment curve and is satisfactory for the relative small deflections needed for cutting the
sail.
The non dimensional deflection data at the measurement sheet are convenient to recognise the type of curve for the specific
mast. This is important for the margin one has for trimming
the sail and the dynamic behaviour of the rig.
By outhauling the clew, the mast bends and the sail flattens off as cloth from the luffcurve is pulled forward.
When the bending curve has a small radius at boom position and a big radius in the upper part the sail will flatten off more in the lower part and will stay full at the top.
The margin for trimming the sail will be rather small.
A fair bending curve is needed for well balanced trimming of the sail.
Apart from this mast deflection test we also gathered data by measuring on photo's.
For this purpose we made a lot of photo's with 2 synchronised cameras.
One camera is positioned behind the sail. The other
camera is beside the sail.
Both cameras take pictures at the same moment.
From these pictures a fair amount of data can be obtained about side and aft bend of the mast and amount of twist.
To study the foil shape photo's were taken from above the sail downwards.
These data are used in a computer program.
The description of this program is mainly Mr. Keuning's contribution.
These date are used to describe the three dimensional shape of the sail in combination with the seamposition between the panels.
The program computes the shape of the two dimensional panels which can realize the three dimensional shape, called sail.
The design proces for a surfsail is slightly different from the proces for a conventional sail.
For a main + foresail configuration there is a design
pro-cedure by which the maximum camber on both sails is determined in a way that a good interaction between both sails will be established.
The choice is influenced by boatcharacteristics and weather designed for.
Until now this design phase is to a high extend influenced by insight and experiences of the designer.
Computer calculations for optimizing the shape of 3 dimensional overlapping shapes are long, accuracy is a problem and they are expensive for daily use by the sailmaker.
For determing the 3 dimensional sail important factors are forestay sag, rnastbend and twist in both sails. These should be determined correctly and used as further input.
Note that by exchanging forestay sag by another mastbend, this could be used for tandernboards.
By the input some corrections for stretch deformation should be made in advance. Experience of designer with the program and the generated sails is important.
Comparing designed sail and the wind loaded sail, can lead to corrections in a sort iteration proces.
In designing a surfsail we face the same factors.
Mastbend and twist should be determined correctly and used as input.
The shape of the profile with amount and position of maximum camber should be determined in the design proces.
After the sail is defined this way, the position and direc-tion of the panels is determined.
These are input data for the program. The program computes the shape of the panels.
Output is in x-y coordinates or on a plotter.
There is an option in the program to have the panels shaped thus way that on a seam both panels are shaped or one panel shaped and the other straight.
The choice is mainly influenced by the way of the production after cutting.
This output is very favourable for numeral cutting.
Under development is a sort of maxi plotter with a focused laser beam as cutting device which will be coupled to the computer.
The result would be high accuracy panels which do not need a second lay for correction on luffcurve etc. after sewing the panels together.
Position of reinforcements and battenpockets can be drawn by a low density beam.
It improves accuracy in reproduction. Of each sail all
date can be stored in the computer. This is very efficient
for reproduction and evaluation. By comparing the sail during sailing with the designed sail an effective feedback is possible. A lot of experience can be gathered this way and evaluated for further use. The initial sail can be copied whenever wanted. This is a major step forward.
APPENDIX i
MEASUREMENT SURFMAST
As older masts were shorter we measure the deflection on a span of 4.300 m. Excess of length ot be divided equally on top and foot.
Deflection measured from baseline at half meter intervals from the top down.
The more recent measurements are also done on a 4.500 m span.
Be sure baseline is tight and secured to mast at support points so baseline does not go up by mast deflection.
Measure distance to baseline at each point unleaded and loaded with 5 or 10 kg at station 3 meter.
Subtract unloaded from loaded data to get deflection F in
mm. Divide by the load te get deflection in mm/kg.
By doing this it is not so important which load is applied. Some masts have been tested on linearity.
Mostly we found non linearity within 1 percent with loads up to 30 kg.
Maximum F is mostly at station 2 or 2.5.
We take this F max. inrnrn/kg as stiffness of the mast.
To get an impression on the type of curve the mast is bending
we make the data non dimensional by dividing them by F
max. So of all masts F max. becomes 1 and deflections at
other stations as ratio of F max.
Also note : Producer, type, length, weigth, diameter at various stations and possibly wall thickness.
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