Sail design - Furling Headsail

[seanfoster]

After numerous responses to a previous post regarding size of sail required for a furling 130% genoa (Many thanks by the way) I have decided to make a new set of sails. I'm in the process of gathering information and so far have found a mass of extremely useful information of the web.
I've downloaded and used Sailcut (free sail design software) which is pretty damn good in my opinion!

A few questions I have, should you have a zero luff round if you are using a furling system? I assume this to be the case as you have a straight spar that encloses the forestay?

Sailcut gives a default twist angle of 18 degrees, what are the effects of changing the angle?

Any advice from anyone who has made their own sails would be extremely welcome!

Just for info, the sails are for a 19ft colvic yacht.

Cheers

 

[neil_s]

I think you have to expect your luff spar to bend, mine certainly does. The luff spar is pretty floppy on the ground so I would guess that its not much different from a simple wire forestay.

Best of luck with your sailmaking!

Neil

 

[boguing]

Whilst you can, and should, set some positive (forward) bend into your mast, your forestay can only have negative bend.

You counteract that with backstay tension, but it's impossible to eliminate it. You need to go for a sail with your present foresail to estimate how much sag you have. Even though you'll probably have less with a new sail.

As for the 18 degree twist, I have raced at a high level, but not so high as getting that sort of data. The amount of twist built into a sail is a function of it's working height vs boundary layer. On the assumption that windspeed is zero at water level, and increases in a linear fashion with height, the amount of twist to design in is a result of the change of height in the sail.

To test the software, try changing the luff length and see if the twist varies. If it doesn't, then I'd be wary of it.

 

[santeana]

I don't know your boat at all nor the design software. Just a few thoughts that may be of help.
* check your E dimension (foot of main) and compare with J before finalising size of genoa. You need a balanced rig. You don't want the boat head driving to lee. Is the Colvic design main or headsail dependent in terms of where does the primary sail drive come from?

* Check out the design software to see if the amount of twist changes with height of clew. Generally with a higher clew the top of the sail will twist off less (angle of sheet with clew and leech tension - changes and adjusted as sail is reefed).

* What height of clew do you need? Clearance of coachroof/guardrails - lower better upwind; higher better reaching - the higher, the longer the track needs to be. So sail size, height of clew, track length need to be considered and compromise reached.

* Does the software permit you to build in a degree of aerofoil shaping? A RR Genoa from a professional sailmaker would permit reefing to about 25% of original LP (e.g. 130%J), maybe a little more, before rolled shape becomes inefficient. You are likely to struggle to meet this standard. So what wind range will you mostly sail? Low or high - when does the iron horse take over? May help you decide on genoa range needed.

 

[William_H]

The difficulty with puting luff curve on the jib is that it should match the inevitable curve of the forestay (and foil) Stronger winds mean more curve.
The curve inward of the luff tends to put more cloth into the body of the jib hence more fullness (camber) under stronger wind conditions just when you want less fulleness (camber) So this becomes another design compromise for a furling headsail.
With individual jibs of course you have a straight luff for genoa and increasing amounts of concave curve of the luff o the smaller strong wind jibs. olewill (foundation and only member of the anti furling jib society)