Upwash

An article from AEZ

When sailing a fractionally rigged sloop upwind the boom may be sheeted on centreline, but the upper part of the mainsail will be twisted off and the luff will have camber in it. All this compounds such that the luff tape may be leading out of the mast at an angle of 45 degrees to the yacht centreline. If the sail is not backwinding in the top part it suggests that the apparent wind angle in the head of the sail is at some 45 degrees. Yet, the instrumentation is saying the Apparent wind angle is only 25 degrees. WHY IS THIS? Well the main answer is UPWASH. The wind curves down in advance of the sail. In general we can say that the more righting moment a yacht has and the more powerful the sails are then the more upwash it will create. By tacking back and forth and using other inputs it is possible to isolate upwash and remove it from the angle that the wind instruments measure. Clever systems are designed to do all this for you automatically as much as possible, so the average sailor does not have to think about it. Interesting observations about this are that if the upwash is caused by the sail shape and it is of the same order as the AWA itself, then anything such as a change in mainsail shape may in theory change the upwash and require recalibration of the whole system. Not only that but clearly there is a strong reason why it is that grand prix yachts use vertical wands for their wind gear: It is to get as far away from the influence of the mainsail as practically possible! Secondly, if the "local" apparent wind angle at the masthead is up to 45 degrees then the brands of vertical wand which are aerofoiled probably stall and cause lots of drag, and in fact the round circular masthead wands offer much less drag. This is the reason why AES designs round masthead wand tubes and we feel that the preference many racing sailors have for long narrow aerofoil wands is just plain wrong. Even with minimal upwash these aerofoils are likely to stall at angles of 25 degrees or greater. Very flat mainsails with little roach will create low lift, less upwash and these do have less luff angle. Whereas as a deeper high lift mainsail with lots of roach can create huge upwash (such as America's Cup yachts). So without wanting to bore you with an advertisement, THAT is upwash and that is why wands should be round.