Hi,

I’ve been fiddling around a bit with Ilvy’s current sail design (which right now is Arne Kvernelands design, originally made for Boudicca). My aim was to cut off 1-2 of the top panels and add the cut-off area as parallelogram panels at the bottom.

Doing this, the CE of the new sail shifts (only slightly, see the following drawings). In contrast to the usual design approach, this time the mast position is fixed (already built). Thus, the horizontal distance between CE and mast (plumb) has to stay the same – but the mast balance changes. One thing to keep in mind: the original sailplan of Ilvy was drawn with about 24%. In reality, I shifted the sail more forward to about 27% mast balance.

Original sailplan of Ilvy:

First panel cut off and area appended as one further panel below.

First and second panel cut off and area appended as two further panels below.

Several observations:

• ·         The CE, though shifting slightly, almost stays at the same position!
• ·         Halyard angle/drift is bad at the one-panel-off version, and undoable bad at the two-panel-off version – IF the halyard attachment point stays at 55% chord, and all battens/yard chord lengths are the same.
• ·         Not a serious one:The original 60° yard is simply most pleasing my eyes. But then, I’m biased from getting used to that shape by looking at it day after day.

Overall, it would look like a quite straight-forward surgery, if it wasn’t for that halyard angle!

Would a sail set well with the halyard attachment point more forward, let’s say 40% or even 30% chord?

Slieve, was it due to this issue, that you reduced the yard length in your high mast balance, low yard angle sails?

I have a lot of fun looking into this, and hope for some good discussion!

Paul

Regarding best yard angle, Amiina sailed in the Round the Island Race with the Mark 1 rig on three consecrate years. With the ‘too big’ full rig first, with the bottom panel permanently reef for the second one and with the top panel removed for the third. So for the second and third the ‘yard angle’ was 30deg and 5deg and it is not possible to tell which was the most efficient. In both cases the performance was above average for cruising boats of similar size. I built the Mark 2 rig simply to get a rig of the desired area with what was thought to be the best aspect ratio and yard angle, (and as I wanted to try some new building techniques). I do believe the Mk2 rig was better, but it is impossible to prove anything unless direct side by side tests are performed.

Slieve, a little bit further below in this thread I remember you put up the question, to what amount the low yard angle of the SJR contributes to its superior performance and if the low yard angle might be even the dominating positive effect of the SJR.

The tests you describe with MK 1 and 2 are - though highly interesting - only comparing 5° to 30°. Both rather low angles (would you describe a 30° yard as a fanned top?). 

Plus, the camber in the top panels of your Mk1, reefed by one panel, and your Mk, with the first panel cut off, was probably not the same, was it?

Cheers 

Paul

Hi Colin,

You say, “Tell tales, in my opinion, can be a double edged sword. While they indeed show airflow over a sail, they do not exemplify how efficient a sail is. One may have a baggy, stretched out sail with all the tell tails flying, but the effective power created by the sail is much less than what it should be. Tell tails can help ensure a sail is working to its own maximum effectiveness, but beyond that they cannot represent exactly how well a sail is working overall.”

Here I believe we disagree. Telltales are not a design tool, but a method of helping the skipper achieve the best performance from the sails in use.

If you have ever been in an aeroplane when it stalled you would know just how dramatic it is when the airflow over the forward section of the upper (leeward) surface detaches. With the sudden loss of suction the aeroplane initially literally falls out of the sky. In a spin it is more dramatic. With the loss of lift on one side the aeroplane immediately turns upside down in a fraction of a second and then points at a rapidly spinning earth. With the sailing rig, the loss of lift force when the flow breaks away can be about 60%, and the loss of up-wash will also kill the windward ability.

 The problem with a boat is that it will not stop dead when the drive is lost, as an aeroplane effectively does, but will continue at a slower rate. It is amazing that so many cruising sailors accept the inferior performance. Telltales, particularly near the luff can clearly show when the flow detaches from the lee side of the luff area.

Regarding best yard angle, Amiina sailed in the Round the Island Race with the Mark 1 rig on three consecrate years. With the ‘too big’ full rig first, with the bottom panel permanently reef for the second one and with the top panel removed for the third. So for the second and third the ‘yard angle’ was 30deg and 5deg and it is not possible to tell which was the most efficient. In both cases the performance was above average for cruising boats of similar size. I built the Mark 2 rig simply to get a rig of the desired area with what was thought to be the best aspect ratio and yard angle, (and as I wanted to try some new building techniques). I do believe the Mk2 rig was better, but it is impossible to prove anything unless direct side by side tests are performed.

Cheers, Slieve.

Slieve 

The big question is what would be the best angle for the yard for best overall performance. No doubt the ideal yard angle will vary with wind strength therefore the best result will be a compromise. In my case I have chosen 30 degrees as it looks right and mathematically it's easy to calculate as sine 30 = ½, but I have not experimented to find the best result.

I have received quite a few letters from people who have built rigs with splits in them, mostly complementary. When I have chased up the ones who have not been satisfied with the performance I have generally found them all to have disregarded the shape of the head of the sail. This encourages me to believe that it is not just the split and the jib that are the strength of the rig but that the overall shape is also very important. The low angled yard also has structure advantages making a low stress rig.

To examine the actual influence of the sail's top shape, wouldn't it be quite interesting to cut off the upper 1-2 panels of a well working, cambered junk rig, and add that area to the bottom of the sail with 1-2 parallel panels?

With every other variable of the sail left untouched, any performance differences must result from the decrease of yard angle. (Edit: almost true, for this work would also change the camber in the then new topmost panel)

Ilvy would be a good candidate for this surgery, with her very well working and proven rig made to Arne's method, and her curious skipper, who enjoys experimenting more than looking at what's already working well on his boat.

Of course that would need some redesign/ recheck of the sail, regarding i.e. CE, mast balance, drift/halyard angle.

Just a thought...

Cheers

Paul 

Graham, I don't use any rocket science when I am to make a suitable camber in in the top panels.
My rule of thumb is to start with finding the needed round in the 4 lower parallel panels (same camber in all of them). Then I divide the round I found there with 4-6 (depending on boat type  -  flatter top section on lighter craft) to find the round in panel 1. Panel 2 is given twice the round as panel 1. Panel 3 may be given as 3 times that of panel 1 (More details on p.7 in Chapter 4 of TCPJR).

Example:

Say we found to need 25cm round in the lower panels.
Then the round in Panel 1 could be 25:5 = 5cm
Panel 2 would be given 10cm round, and
Panel 3 could be given 15cm on both sides, or 10cm along the upper edge and 20 along the lower edge. This ‘asymmetric’ round in Panel 3 is to make it easier to match the sides with the edges of the adjacent panels.

In the two top panels, I let the max round sit at the middle. On panel 3 I may jazz a little and have the max round at the middle at the upper edge. At the lower edge, I place the max round 35-40% aft of the luff.

I don’t bother too much what the actual camber/chord is up in the top panels. I am even unsure on how to calculate it. I can only say that the upper and lower section appear to match each other during practical sailing on real water.

As said, this has been described in TCPJR, Chapter 4. I warn against overthinking this subject, as it does not appear to be critical.

Cheers,
Arne