Gennaker for Swan 38

Hi all,

I would like to know good measures for a Gennaker for my Swan 38 Kidogo III.

Any informations about this is appreciated.

Thanks,

Enzo

Dear Enzo,
I think that Gennaker dimensions can be determined in two different ways.

1 - Having an IRC or ORC Rule certificate, make it as big as the largest Spinnaker you are already "paying for".
2 - Disregarding a specific target area, measure the boat and ask your sailmaker what are his recommendations. I'd like to suggest the following guide-lines.

Compared to a generic simmetrical Spinnaker, a good Asymmetrical can be used in more wind because
a) is built stronger along the Luff, where most vertical load is running,
b) has a larger Head and Tack reinforcements,
c) Tack and Clew remain always the same, they don't swap when jibing like they do in a simmetrical Spinnaker
d) having an truly asymmetrical shape, entire Head portion is twisted, effectively driving more forward (where the boat is going) and consequently heeling less (side loads component are reduced).
More driving force combined to less heeling force = better efficiency and more speed.

If speed is not an important factor, Gennaker can be reduced in area for easier handling.
A Gennaker has four principal dimensions. Luff, Leech, Foot and Mid-Girth (distance between mid-Luff and mid-Leech, measured across the sail). Using these four dimensions, different Rule foumulae are computing the rated area, with very similar results. Let's see now how to determine the variuos dimensions, starting from our boat.

Luff length is related to the distance available between Halyard full hoisted and Tack desired position above Deck.
Without a bowsprit, I think the safe Tack position is just above the pulpit, saving some room below, to pull down the Tack when the wind pressure is reduced.
Leech length is depending by the sheet lead position in the back of the boat, combined to Foot length.
Foot Length is a major area producer. The longer the Foot, the lower the Clew (i.e. longer Leech) is required, in ordre to maintain the correct sheeting line. This is very important to share the sheet load, being well balanced between Leech and Foot.

Having set the three external boundaries, the Mid-Girth can be handled separately because it depends by sail fullness, Luff projection forward and degree of Leech Twist (sideways vertical curvature). Compared to a simmetrical Spinnaker, designing a Gennaker the last two items can be controlled independently. This can make a real difference, because they are genereting the amount of power, while setting the direction where is aiming.

Racing boats with only Asimmetrical kites may have a family of different sails available, often with similar boundaries, but very different mid-girths to match smaller or greater wind pressures. Cloth weight is also an important factor, related to area and fullness. When one only gennaker is desired on board, a good compromise can be found, but I don't see why it should be that much smaller than a traditional simmetrical Spinnaker, partucularly if most handling related issues are nicely managed with an appropriate hoisting and dousing sock device.