Can one explain me in which conditions a staystail will give extra speed when the asym is up? Or is it just because of the very long bowsprit making the distance between fore- and mainsail too big?
I think youíre right Robelz, at least considering asymmetric spinnakers and code sails.
Because the relatively small staysail can only add efficient sail area and give extra power if thereís enough distance between the luff of the mainsail and the spinnaker.
This only works with a (long) bowsprit. Without this, the bigger spinnaker and mainsail are too close to each other. The airflow between them is very much slowed down, because the upwash from the mainsail is fighting the downwash from the spinnaker.
The staysail between them would then not only be inefficient (little airflow) but also disturbing this slower an therefore more delicate laminar flow, which is essential for the power of the mainsail.
Miniís also carry swinging bowsprits that can bring the tack of the asymmetric upwind. Not only to catch the upwash of the main earlier (= stronger and under a better angle), but also to produce a more leeward and thus weaker downwash from the spinnaker.
In this case less is more, because the delicate laminar airflow between spinnaker and main is less slowed down, making a staysail between them even more efficient.
With a symmetric spinnaker things are of course very different. Especially dead downwind when these sails are much more efficient on boats that donít benefit from gybing downwind for an optimal VMG. But then thereís no more laminar flow, so almost no interaction between the sails and therefore also no added value for staysails.