TACK Downwind.....
Your boat should be fastest (speed and VMG to the next 'mark') downwind at about 135 deg. instead of 180 deg. At 135 deg. the "apparent" wind will be much higher and the sails will be 'dynamically flying' (as when close/beam reaching or beating). Just gybe over a few times so that you stay somewhat in the middle of the course. Tacking downwind will cover a lot more distance traveled but the **speed advantage** on bearing off ~45° off of 'true' DDW will be dramatic ... as the boat will be 'making' its own apparent wind due to the 'angles'. At DDW there is no dynamic flow over the sails, just DRAG of the wind on the sails to move you along. ( FYI ..... On some of the FAST planing hulled boats ... or iceboats .... this tactic will sometimes have the sails quite in close (as in for close reaching) due to the apparent wind coming from 'ahead' when going downwind). If you're flying a FULL set of telltales .... @ luff, midcord and leech, simply set/shape the sails so that all the tales are 'flying' or streaming back while youre going down at ~135° (or 225°). Set the boat to hold the course then set the sails to match the course .... and then keep adjusting the sails in/out as the wind pressure/speed changes .... while holding that course. The above is probably optimum for 12-15 kts. true wind. The angles will be less than 45° off deaddownwind at higher wind speeds; and coversely more at less than 10kts. FORGET bouat -- double headsails, wing on wing, etc. as they are toooooooo slow in comparison to 'tacking downwind'. If you have a GPS set a waypoiint at the downwind mark ... then shift the GPS function to VMG and then sail the course angle that gives you the HIGHEST VMG .... this is only valid at the beginning of a 'tack' and VERY unreliable at the end of a tack (due to the intricacies of 'trionometry' --- at the beginning of a tack follow highest VMG; but, forget VMG when at the end of a (long) tack.