Tuesday, 5 October 2010

Tweaking and tuning your DNA

The DNA is very sensitive on the position of the daggerboards, on mastrake and bodyposition.
The daggerboards can be adjusted in the horizoantal axis by the glider in the hull and in the vertical axis  by adding or removing filler or tape.
Horizontal adjustment can be done on the water. If the screws are not too tight you can move the slider by pulling a board up and then by using the board as a lever to move the slider forward for less lift or backwards for more lift.
If the wind is above 18 knots the boat is faster and more controllable if you use less lift (= less rake in the daggerboards). In medium conditions you use maximum lift (standad configuration). In light winds we are not sure yet, though you have to pull up the boards while going flat downwind.
In stronger winds you are faster downwind (about half a knot)  if you pull up the luff board.
The amount of toe-in (vertical axis) differs between 3 mm and 5 mm (both boards combined).  3mm for minimum drag and 5 mm for some extra lift and height. The rule of the thumb here is that lighter guys use more toe-in. In really strong winds extra toe-in gives better control downwind , but there is a penalty upwind.  More than 5 mm toe-in feels draggy.
If the wind picks up more mastrake seems better. You keep he bows out and the boat is both upwind and downwind significantly faster. The optimal amount of mastrake depends on the type of sail and/or the battens you use.
The flatter the top of the sail the more mastrake you can use. Chris Field has used this principle to his advantage and has designed his sail so he can use maximum mastrake if the wind picks up. He uses a twisty sail with a relative flat top and a full bottom. Many other sails get very full if you ease the sheet a bit.The fuller head moves the center of forces too much backwards which leads to too much pressure on the rudders.
The amount of mastrake can be measured by holding a trapezewire on the deck at the forestay and then you look were it touches the deck at the back.
In light winds the best position is just behind the hatch, in medium conditions on the transom and in strong winds 10 cm down the transom if the top of the sail is not too full and the battens are not too soft.
If you use a lot of mastrake steering may improve if you put the rudders somewhat less under the boat. (standard 15 mm)
This weekend PJ sailed in Hellevoetsluis in force 5-6 in such a set-up (increased mastrake, striff battens) and he could easily beat a Nacra 20 carbon and the F18's  upwind  by saling both higher and faster (downwind the Nacra was a rocket). Downwind he struggled a bit due to very nasty waves. In such waves you easily loose the rhythm, where the spinnaker boats just glide on.
A few weeks ago Chris Field could match the speed of a M20 in medium to strongwinds on flat water after increasing the mastrake on his DNA.

Body position on the boat is critical as well. If you stand too far behind the transom sinks and you will be slow. The boat will saill at 9-10 knots upwind. The fastest way is to play with your weight a little bit; you step forward if the boat is picking up up speed and once it is up to speed you step a bit backwards and get the boards working for you. (speed 12-13 knots) If you step back to early the boat is too draggy and you cannot get the boat over the  threshold of 10 knots.
Downwind the same principles apply. It is easy to get the nose out of the water, but the fastest way is to get the nose just touching the water. Troubles (steep waves) are circumvented by increasing the heel of the boat, though generally the boat is sailed flatter downwind than the previous A-cat designs.
The boat asks for a more active (fysical) style. You really have to work the mainsheet to get the best out the boat and you have the move your body as well. You can see that style used by Chris in the pics taken at the Dutch Nationals. Sailing the boat in medium conditions is quite easy, but in strong winds you really need to master the boat to get te best out of it. The reward is huge though because Chris reached amazing endspeeds and downwind he sailed away from the others both in Garda and in Hellevoetsluis.

It will be interesting to see which sails Landy, Glenn and Stevie will use in the Australian Nationals and how they will sail the boat to get the most out of it. Three sailmakes which differ both in bodyweight and in rig philosophy.

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