Ben
No the best VMG is to sail fast on the wind. Probably around 70 to 80% of windspeed gets best VMG. In 5kts of wind that is going to be 3.5 to 4kts. The problem it that there is no margin for error. If the wind shifts, windspeed dips or helmsman looses concentration resulting in boatspeed below 3kts the boat is bound to round up when sailing with the lw hull is on the lee. The reason being that the rudders cannot create enough moment to overcome the drag from the ww hull that is rounding it up.
Any time the boat speed gets below 3kts with the lw hull on the lee side the helm has limited response. Trailing rudder is stalled first and becomes a brake. It cannot generate enough side force to overcome the turning couple between the sail thrust and the ww hull drag.
There may be a slight improvement when the rudders are decoupled because then both rudders can be close to stall at the same time as Arttu commented earlier.
There may also be further improvement when hulls are clean such that the drag will be lower at a given speed. However current performance with lw to lee side is not far off my polar. Highest angle possible in 5kts of wind is shown to be around 60 degrees true. I believe we have already done better than this by sailing the ww hull on the lee side. I have not done the polars for this mode of sailing to see if there is a fundamental difference. The practical difference is that it is easy to recover if speed drops off. Simply bear away and pick up speed.
What is not immediately obvious with the polars is that there is no recovery from the situation when the lw hull is on the lee side. Boat is still moving at reasonable speed for the light conditions but is slowly rounding up and slowing down. More helm increases deceleration. The rig does not allow the CoE to be moved forward as can be done with a normal sloop rig. There is no means of easily depowering the main without depowering the jib.
My experience with boats having lateral symmetry, where their sail drive is aligned to drag in light air, is they still respond at very low speed. As long as they are moving, the helm responds.
Rick
On 07/01/2012, at 1:09 AM, bjarthur123 wrote:
rick,
i think what you're trying to say is that the polar for light winds might be such that the best VMG to weather is made for less than 2.5 kts boat speed, and if you try to go that slow with the rig to lee then you'll go into irons.
it seems to me, however, that if the rudders are ineffective at slow speeds, they will be ineffective no matter which way the wind blows over the boat. if the rig is to lee you'll go into irons; if it's to weather you'll bear away. either way, you won't be able to maintain the best upwind VMG speed or angle. no?
sure, you'll be able to temporarily pinch with the rig to weather. but it's only faster because you reduce the likelihood that minutes will be added to your finish time while idling in irons. not because you're able to sail a better VMG.
ben