Arne Kverneland wrote:

«Le mieux est l'ennemi du bien.»

You are certainly right with this quote.

The question is I am not yet looking for the best but still trying to know and understand the fundamentals (if any) of aJR.

You suggested one of the two sheeting systems described in your "Junk for beginners" note for Ar Vag Melen.

Both are 3+3 systems. The (very few) french documents about junk rigs all present a 3*2 system. It then seemed to me that having 3 sheets controlling each 2 battens would give a better control of sails twist.

What is more, I imagined that having a triple pulley finally receiving these three sheets linked to a "master" sheet would be convenient : using the master sheet to control the whole sail, using any of the others to adjust the twist.

See 3X2 SHEETING

I am wrong somewhere ?

Concerning this last "master pulley", should it be linked to a traveller track, to a fixed point on boat center line or to a"pantoire".

"Pantoire" is a french term I did not find a translation for, possibly "bridle", a system where attaching point of the sail sheet is more or less movable along a fixed rope from port to starboard.

My Tilapia (Ar Vag melen) is originally equipped with a "pantoire", the more recent ones are equipped with a traveller track.

I had better avoid a "pantoire' as they never proved efficient for me (unless for very small sized sails). Buying a second hand aluminium traveller track should not be a problem.

Thanks for, once again, any comments or help.

Claude

Arne Kverneland wrote:

I see two ways to move the CE aft:

·         Move the JR mast top up about 50cm. This will let you pull the sail further aft on the mast, if needed.

·         I would also widen the sail a bit by adding about 15cm to the boom and four lower battens.

What I aim for on such a boat is a setup which gives a bit lee helm in light winds, fully close-hauled. I am sure the weather helm will soon take over as the wind picks up or if you bear away onto a reach.

Hope some of this makes sense.

Arne

As already said, Ar Vag melen having rather flat bottoms, shallow draft, a rather high  Bwl/Lfl ratio and then a V horizontal shape forward of her gravity center, she heels quite rapidly and her buoyancy lines distort and diverge with an increasing angle from original centerline (see Waterlines and heel) .

Rolling of its present sail moves sail CE forward and somewhere compensates for this weather helm tendancy as wind grows and picks up so I do not expect any real trouble with this question of CE position even if junk CE ends little forward or JR CE.

You suggest to keep the mast top upper than on my sail designs which would help setting the sail further aft on the mast if needed. I do agree and intended not to shorten the mast until sail trials had been made.

I finally changed my mind to your preferred H5 sail design and followed your idea of widening the sail by adding some length to the boom and four lower battens.

Sail design is now the following H8 . Widening it increased the sail + battens area by about 1 m2 and certainly does make it more poweful under light winds.

So I am now less worried by the importance of the "shutting door" effect with light winds due to mast aft rake we discussed before  and ready to take the bet of keeping the mast as it is.

However, I have not fully conceived yet how this aft raked sail will mechanically jibe and how it will have to be sheeted for this purpose.

Comments are still welcomed.

Claude

Arne Kverneland wrote:

Claude, if you are keen on a bit experimenting and even dare to fail, I would say go ahead. I think the chances that it will work is over 50%.

Annie’s worry about the weight making the sail falling aft to the centreline must be taken seriously.

PS: I hope you plan to make the sail with camber in its panel.

Here in Britanny we often make fun of our Normandy neighbours who always answer, whatever the question is, "May be yes, May be not" !

Do you have norman origins, Arne ?

Though, you wrote "chances ...are over 50 % " which makes a lot of difference for me, having read all your documents on the site and then being sure you certainly know what you are talking about.

I do take Annie's worry seriously. There's certainly a "shutting door" effect of an aft rake and I fully admit the sail may tend to fall to the centerline of the boat. Considering the mast as an inclined hinge axis, I tried to identify forces applying (see Aft rake forces) .

Once this done I considered a weight of 35 kg (sail, battens, yard, boom, pulleys ) to evaluate the "shutting door" moment of this force E and compare it to the moment of wind pressure SF. These calculations led me to the conclusion that Annie is right about the question of light winds as, depending of sailing angle (and consequent center line/sail angle), the "shutting door" effect may be a problem up to 4 or 5 knots wind.

Of course this is a simple static calculation.

As said elsewhere, Ar Vag melen heels very quickly so this "shutting door" force due to aft mast rake should be offset quite rapidly upwind and the increased power of junk rig downwind (if compared to an unfolded Jlrig 20% smaller in area) should somewhere compensate.

I shall then take the bet of trying a junk rig with the mast "as it is".

Concerning sail drawings offered to comments in my last post, H1 and H3 did not satisfy the goal of a shorter mast while elliptic H6 and H7 where more designed to be transformed ultimately in a split rig.

I then agree with your preference for H4 and H5.

As the AR ratio of H5 infers an higher drag coefficient I shall then hold the H4 design and I subsequently need more help from JRA experts.

1/ positionning sail center of H4 sail

Much is said on this site about the matter but I am just unable to know what is, if not right, at less appropriate or convenient : the old marconi sail center (in my case the old Jlr sail center), a sail center aft of x% LWL of the marconi sail center, a sail center forward of y% of CLR (rudders being considered or not).

I know there's no divine law, yet any help to decide where the X position of the H4 sail should be fixed  would be appreciated (CLR of Ar Vag melen is 3.09 m without rudders and 2.72 m with rudders; LWL is 6.28 m without rudders and 6.63 with rudders.Center of gravity with 2 persons aboard should be at 2.88 m)

2/ modifying H4 sail for a multi-purpose use (in fact not a multi-purpose use but another useful use ie with a vertical mast)

In case experiment of this H4 rig with aft rake mast "as it is" may fail, I shall have to consider setting a vertical mast somewhere aft of present partners (30 cm suggested by Arne) and I would prefer H4 sail to be reused.

I know one should not expect to "keep his cake and eat it too" , nevertheless investing in a new sail  without major works on the boat itself before deciding to invest further makes sense to me.

Thanks for any help.

Claude

 Ps :

As usually suggested, H4 sail should be 8% cambered at 35% of chord with a Nacra 0016 profile modified with straight line aft 60% chord length.

Here under are links to different sail designs for Ar Vag melen conversion to junk rig.

They were designed with the mast "as it is" shortened of 2 meters. Some may be kept with a vertical mast if compulsory.

They all increase the sail area  from 21 to 24 m2 so as to obtain a sail ratio to displacement (SA^1/2 / D^1/3) of 0.45 instead of 0.41, 0.45 being the usual minimum ratio for a cruiser.

(X of CLR of Ar Vag melen is 3.09 m without rudders and 2.72 m with rudders; LWL is 6.28 m without rudders and 6.63 with rudders.Center of gravity with 2 persons aboard should be at 2.88 m)

Comments of pro's and con's of each would be appreciated.

H1

H2

H3

H4

H5

H6

H7

Claude L'Honnen wrote:

Regarding the aft rake, where's the problem ?

When the mast rakes aft, the sail falls in to the centreline of the boat.

When the mast rakes forward, the sail swings out - this helps tremendously in light, following airs with a disturbed sea state.