BYYB Forums

Hi all,
I'm (finally) working on sails done in Tarpaulin as shown in the guide and in the DVD and I have some doubts / questions about.

1. Sails trailing edges. shuld they remain a "crude" cut or should they be reinforced in some way?
2. Clews attachements. I've attached the stress-line tapes at clews corners and the extra grommets. I've seen in the video when a reinforcing lines are knotted through, but not how they finish and how they are joined to boom and jibsheet, because that part of the video doen't show the home-built, but a dacron sail. Can someone show me a clear view of these particulars?
3. Reefing. I've read somewhere that 1st refing should reduce the sail of about 30%, the second at about 40% of the original size. Can someone show me a sketch of the best lines path? How many reef points shoud I put on a same line? What is the smart way to attach chords/lines in intemediate points?

My considerations:
a. Tarpaulings available in Italy (but made in Vietman) are blue on a side and green on the other. With the silver duct tape sails look really disgusting!
b. Sails plans are wrong! I put main and jib dimensions and angles in a cad drawing and the true trailing edge resulted longer more than one inch in the main and more than 5 inches in the jib. Can someone confirm this?


Gianluigi

Greetings Gianluigi,

Well, I think that saying that the plans are wrong is probably a little harsh.

It would be more accurate to say that the plans are not very precise.

If you look at page 36, diagram 25, you will see that the main sail is really made up of two triangles.  The lower triangle has sides of 118 inches along the luff, 127 inches along the foot, and 157.5 inches from the throat diagonally down to the clew.  The diagram also shows the angle at the tack to be 80 degrees.

Well, it doesn't take a CAD program to figure out that the figures are not exactly precise.  The law of cosines says that a^2 = b^2 + c^2 - 2*b*c*cos(a).  Rearranging the terms we have cos(a) = ( b^2 + c^2 -a^2 ) / (2*b*c).  Substituting the diagonal for a, the luff for b, and the foot for c, we have cos( tack angle ) = ( luff^2 + foot^2 -diagonal^2 ) / ( 2 * luff * foot ), which works out to be cos( tack angle ) = ( 118*118 + 127*127 - 157.5*157.5 ) / ( 2*127*118) = .1751  The tack angle is then the ArcCos( .1751) = 1.3948 radians or 79.92 degrees.

I dare you to measure the difference between 79.92 and 80 degrees using a protractor on a floppy piece of fabric, even one as stiff as polytarp.  But if you put 80 degrees into your CAD program, you will be making the tack angle larger, the diagonal longer, and the top of the throat angle more vertical, all of which will make the leech of the sail longer.

It turns out the throat angle is not quite 142 degrees either, but a little less.

The rest of the angles work out to be ...

[table]
[tr][td] Length of Side ... [/td][td] Opposite Angle ... [/td][td] Name of corner [/td][/tr]
[tr][td] 118" [/td][td] 47.53 [/td][td] Lower Clew [/td][/tr]
[tr][td] 68" [/td][td] 23.57 [/td][td] Upper Clew [/td][td]71.1[/td][td] Clew [/td][/tr]
[tr][td] 157.5" [/td][td] 67.85 [/td][td] Peak [/td][/tr]
[tr][td] 170" [/td][td] 88.58 [/td][td] Upper Throat [/td][/tr]
[tr][td] 127" [/td][td] 52.55 [/td][td] Lower Throat [/td][td] 141.13 [/td][td] Throat [/td][/tr]
[tr][td] 157.5" [/td][td] 79.92 [/td][td] Tack [/td][/tr]
[/table]

The plans were written with the sail measurements given to the nearest half inch, and the angles rounded up to the nearest whole degree.  

But none of that matters.  As long as the foot is about 127 inches, the luff is about 118 inches, and the head is around 68 inches, the sail will fit onto the spars and the sail will propel the boat.  A five inch difference in the length of the leech will not be noticed in the performance of the boat.

Cheers,
Tom

Tom you are right, plans are not wrong, but there are simlpy some dimensions incorrect.
I'm used to check with CAD in order to verify if I have well understood parts shape and than I scale all to covert inches in cm. In this way I introduced main sail and jib into my tarpaulin CAD shape and then I marked line references on fabric edges.
All this seems to be long, but is pretty fast. Doing this by hand calculator is longer, you cannot realize eventual input errors, any modification imply a new chain of calulations. Spreadsheets are better, but you lose the visual impact that a very simple CAD sketch gives.

Gianluigi

What about reefing system adivices?

Gianluigi

I put about 5 reef points into my sail about 60cm up from, and parallel to, the bottom of the sail.  For them I put a cross of tape in the spot and then a grommet in the middle of the cross.  A short length of line (about 30cm) with a knot on each side was put in.

I used it on my first (and so far only) launch when the wind was blowing at 20 knots and they did the trick other than the fact that I had really poor sail shape.