Sail shape?

[Clyde_Wanderer]

I will soon be fitting a longer boom on my boat for various reasons, and will withen the next year or two, have to get a new main sail.
If I got the new sail made with a longer foot and shorter luff to keep close to the present sail size, how would this effect performance and balance etc?
C_W

 

[PuffTheMagicDragon]

The centre of effort would move aft and, in theory, would add some weather helm.

However, unless you are really going to add one hell of a lot to the foot, I don't think that it should matter much. At a guess I would say that anything up to 10% increase in the foot would have a negligible effect, easily corrected - if you really must - by adjusting the rake of the mast.

Easiest way to find out is to draw two triangles to scale that represent 'before' and 'after'. Draw them superimposed on the same base line and then find / mark their centres. (Don't bother about the roach)

 

[lw395]

Don't rush to shorten the luff, the area this affect is probably not going to change the balance much, it will look better and be more efficient with a taller main imho.
The lateral force of a sail is centred somewhere about 30% back, so you are not moving the combined centre of main and genoa very much.
Don't forget balance can also be controlled to some extent by relative fullness of main and jib, and also by sheeting.
Assuming you have reefing that works, more sail area is helpful in light air, and easily adjusted as the wind increases.
Make sure you can ease the kicker a reasonable distance before your boom hits the backstay in a gybe though!

 

[Clyde_Wanderer]

Dont want to make weather helm any worse than it is in a hard blow, and the only way to ease it now is to put 1 reef in the main, as letting the main sheet out doesent help, So I assume the weather helm is caused be the COE been too far aft or too close to the CLR, or am I totally wrong?
PS after I have added the extra 28" of boom, the aft end will still be aprox 12" from backstay, at normal sailing height, this works out at allowing me to raise boom aft end by 3-4ft

 

[PuffTheMagicDragon]

Too far aft of the CLR. Exactly how far aft is too far depends on the boat and the state of the sails...

In any case, the 'as drawn' position exists only on paper. In real life it's the effective position that matters, and that is constantly shifting each time you adjust the sheets, or your outhaul, or your kicker, or your trim, ....

 

[William_H]

Hello Clyde_Wanderer. In my opinion the excessive weather helm is more a as result of being over powered and heeling too much rather than caused by the balance theory of too much sail aft of the mast.

I wouldn't suggest increasing the size of the mainsail and boom unless you find you are sailing in light winds a lot.

In theory at least the lower aspect ratio sail will be less efficient than a higher aspect ration sail. Aspect ratio being height to boom length.
certainly for a given area short boom tall mast is better giving less turbulence across the top of the sail where air escapes from high pressure to low pressure over the top. But you may never notice any difference. good luck olewill

 

[LittleSister]

As I understand it a lower aspect ratio is not necessarily a bad thing for cruising. A high aspect ratio sail is like a highly tuned car engine, and a low aspect ratio a low tuned one. A high aspect ratio sail (and highly tuned car engine) will give more power when it's set up/adjusted just right, but is highly sensitive to being set up just right - a little bit off and you lose a lot of power. A low aspect ratio sail (& low tuned engine) doesn't have the same maximum power, but is much more tolerant of being a bit out of adjustment and will keep delivering usable power in a wider range of conditions.

 

[dt4134]

If you don't mind me asking, why are you going for a longer boom? Is it just to improve performance in light airs?

You can do a lot to improve light airs performance just by adjusting sail shape. Ease the outhaul & halyard a bit and put more twist into the sail. Also ease the backstay if it is adjustable (and the rig is fractional).

 

[Clyde_Wanderer]

Not lenghtening boom for any of the reasons you mention.
The aft end of boom is aprox 28" forward of the main track and car position leaving the main sheet at a very large angle of dangle, and putting a lot of unesseccary forward stress on the track and car, I cant move the track so hence longer boom, which will also alow me to fit a decent boom tent.
I will be needing a new main sail by end of next season, but will probably just have it made same as the present one, and not extend the length of the foot, or if I do extend it, it will only be 12" max, extra.
Thanks everyone for your advice, keep it comming as I am starting to peice together a good picture of the fundamentals of aerodynamics on a sailing boat under sail in different wind strenghts, which leads me to another question.
Someone above mentioned how another bit of area added to the foot wouldent make much difference in light airs, but would it help to give a slightly more positive feel through the tiller in light airs, just as it would increase weatherhelm in stronger blows?
C_W