Sail Shape Control - Have I got this right?

[Hamish Norton]

Hi Guys, I sail an Javelin Skiff (javelins.org)

Fingers crossed this renders okay / is readable.
Could you please let me know how well I've done with the following and perhaps some feed back. You'll see I've got a few questions in there too that I'm not 100% on.

The Javelin is a two handed skiff (crew on the trapeze), square top main. And in my instance the main sheets off a bridal at the back of the boat and we use boom sheeting.
I've put this little grid to together of the main sail and it's controls, and then filled out the effects each has.

Controls and there effect on the mast and sail

Mainsail...	Mast	Boom	Sail	Purpose
Cunningham / Downhaul	Pulled On: Bends the mast forward in the middle. It will bend the mast the most where it is weakest (tip/tip) and the least where it is stiffest (Perhaps between the base and goose neck).	Pulled On: It will reduce the effectiveness of the Vang tension, and some load will transfer to the mainsheet	Pulled On: Primary effect - the mid draft is reduced, making the sail not as deep in the middle. Secondary effect - load is reduced in the leech allowing it to twist. Sheer - is to the aft compared with the middle of the sail, which increases tension between the aft square top corner and the middle luff of the sail	Primarily to control the power of the mainsail (flatten it for stronger wind, or for very light winds). Can be played even for gusts. Q. By preloading it more does it improve gust response of the mast (in the tip)?
Vang / Kicker	Pulled On: It bends the mast forward at the middle and pulls the tip of the mast aft. Vang vs GNAV... The GNAV puts the bend further up the mast compared to the more traditional vang.	Pulled On: It places a bending moment on the boom to the outboard end by the Gooseneck and Vang load	Pulled On: Some tension... It makes the leech stand up, as the line of tension between the head and Clew pulls the roach to windward. Lots of tension... it flattens the sail in the middle and allows the leech to flatten off. Sheer - as this tension pulls the mast tip aft it creates a sheer to the aft compared with the middle of the sail. Q. What does this mean for the sail?	To control the twist of the main for the conditions. More wind more tension to hold the leech twist required. Choppy condition less tension to allow more twist widening the boats groove of power allowing you to steer more yet maintain power at these ever changing angles.
Outhaul	Pulled On: Could add some tension to the head of the mast bending the tip back.	Pulled On: Places compression on the boom	Pulled On: Primarily pulls the bottom of the sail tight taking out draft, also pulls tension from the head to the Clew flattening it out along that line, and all of the rest of the sail between the head, tack and Clew. Q. Am I over using this and causing the stress lines I see in the upper half of the sail between the head and the Clew?	To control the depth of lower 1/3 of the mainsail
Mainsheet	Very little, unless the vang tension is minimal and the mainsheet is sheeted on hard. Also depends on the amount of purchase setup.	Allows you to adjust the sail as a whole in and out to match the angle of the wind	Minimal, unless the vang tension is also minimal. This may be the case in very light wind where it's used to control the leech tension (sail twist)	Adjust the sail as a whole in and out to match the angle of the wind
Baby Stays / Lowers (they attach to the up point of the GNAV)	Pulled On: Shifts the mast bend caused by the Cunningham and Vang to be moved further up the mast, and limits the bending effect of both controls		Tends to flatten the upper sail (perhaps at the 1/3 point instead of the 1/2 point) Vang and Cunningham tension affect the leech tension, and the flatness of the Square top.	Pulled On: Allows you to keep the main powered up and adjust the leech and square top to the right amount of twist for the conditions (flat, choppy)

Jib...	Forestay	Mast	Sail	Purpose
Cunningham / Downhaul	Negligible, perhaps a very small increase	Nil	Pulled On: increases luff tension, pulling the draft further forward increasing the angle of attack and thus causing lower mode.	Control the sails draft for the conditions.
Inhaul (Jib car distance from centre line)	Nil	Nil	Angle to the wind	Angle to the wind and mast flow Q. How do you know when you have got this right?
Jib Clew Plate	Nil	Nil	If set low, it pulls the foot tight. If set high, it pulls the leech tight.	Adjusted to set the twist for the conditions
Jib Sheet	Pulled On: Depending on Jib cut it can add tension to the forestay and bend it aft	Can bend the mast further in the direction it's bending if the forestay tension increases	Pulled On: pulls the sail as a whole flatter, but typically setup to control the leech tension allowing the leech to twist off in a strong wind or choppy seas.	Control leech tension, thus controlling twist.

Specific Questions...

1. Jib - Inhaul: How do you know when you have got this right?
   [*]Main - Outhaul: Am I over using this and causing the stress lines I see in the upper half of the sail between the head and the Clew?
   [*]Main - Vang/Kicker: Sheer - as this tension pulls the mast tip aft it creates a sheer to the aft compared with the middle of the sail. What does this mean for the sail?
   [*]Main - Cunningham/Downhaul - By preloading it more does it improve gust response of the mast (in the tip)?

Thanks for reading.
Hamish

 

[lpdsn]

Just got as far as the first box. A Cunningham won't bend the mast. It tightens up the luff drawing the max draught for'd and is usually quicker to adjust than the halyard. Comes at a cost of increased twist as it doesn't tighten the leech.

The North Sails book "Trim" is a good resource.

 

[Chris 249]

Cunningham bends the mast on many rigs, including cat rigs and fully-battened dinghy sails. The tension on the cunningham pulls on the head of the sail and that compresses the mast into a curve.

Re question 1 - inhaul position is normally a judgement call depending on you want to go lower and faster or higher and slower, or whether you are depowering.

Question 2 - sounds like overbend wrinkles caused by the mast bend being greater than the luff curve of the main.

By the way, I'm going to be pedantic and say that the Javelin is not a skiff, and it should be proud that it's not. Skiffs as we know them today come from a largely separate line of development to the Jav. Spencer never wrote about the Jav being a skiff in anything I can find. The NZ/Australia line of lightweight development class dinghies that includes the Javelin was often a lot more progressive and modern than the Skiff classes were. Around the time that the Jav was designed there were ultra-light self-draining highly efficient and very fast dinghies like the Cherub, R Class and Gwen 12 - the skiffs of the time tended to be big fat heavy things that banned trapezes and buoyancy and were slower than the dinghies.

To call the Jav a skiff may be trendy, but in some ways it's an insult to a line of dinghies that led the world in some ways. It's like calling a Lotus sportscar a V8 just because V8 racing has a bigger advertising budget. Those of us who have sailed in the NZ/Australian lightweight development dinghy titles need to highlight the contribution those classes made, not take the title of another type just because they're cool. Reclaim the "dinghy" name!

 

[Hamish Norton]

Hi Chris and Isdpn,

Thanks for the feedback.

Chris - I see your perspective on a Javelin being a development dinghy class, which is obviously is. Perhaps the term "Skiff" these days by many as seen as anything that sails more in a planing mode than a displacement mode, which I can say my Javelin does well.

Re Q1 - Inhaul: I'm adjusting it for the conditions, but probably need to build up a better set of guidelines than I currently have. Typically if it choppy I'll bring it in but run an eased sheet to create twist and open groove. When under powered I'll typically move it out a touch and have a slightly tighter leech (less pull on foot via clew plate adjustment as well). And in overpowered conditions I'll open it up, look for move speed which tends to lift our VMG.
However the big question for me so far is how do I know if I've got the inhaul in the right spot compared to the mainsail? At the moment my rule of thumb is I'll use the combination of jib sheet and inhaul to bring it in so the top leech wool starts to wrap around the back about 20% of the time. It doesn't really have an effect on the shape of either sail. Although if you bring the jib right in does reduce the leeward separation bubble behind the mast / at the luff of the main? Or does this choke the up wash effect of the main?

Isdpn - Here is a photo of the Cunningham cracked on fairly firmly, you can see with the carbon mast it bends quite a bit (with no load the mast is straight).

I've got the Norths book "The Art and Science of Sails" which from memory talks about the aerodynamics of interaction of the jib and main. I shall review it again, and perhaps search for the "Trim" book too.


Does anyone have any thoughts of Q3 & Q4?
cheers,
Hamish

 

[Javelin]

Rule of thumb is the top third of the sails does 2 thirds of the work.
Given that, the outhaul is frankly, a low priority control so I tended to pull out to the measured mark on the boom and then forget it as round the course you have more important things to worry about.
The Vang on a trapeze boat is your power control especially in marginal trapezing conditions.
I would play the vang upwind more than the mainsheet to keep the crew flat out on the wire which in turn keeps the rig fully powered up, also to get the top batten leech tell tail to stream around 40 to 50% of the time.
Downwind the vang again is the power control, though with a huge asymmetric its more important to keep the boat flat.

Also don't forget the board from your matrix, playing the board makes a huge difference and can often promote planning upwind increasing vmg big time.

Expensive but will win you races as you can play the vang and sheet at the same time.