From a performance perspective boat windage is most detrimental going to windward.  At best VMG apparent wind will be around 20 degrees.  Take a few sections at that angle through the boat to see how it presents to the wind - remember bluff trailing edges are usually worse than bluff leading edges in terms of adding drag.

From a motoring perspective frontal windage becomes dominant.  With the Melbourne proa half of the power goes into air doing 5kts into a 15kt wind.  In no wind and clean hull it can do 8+kts sucking about 9kW from each of the two batteries.  It can hold position in wind between 40 and 45kts depending on wave conditions but will not go directly into the wind at that strength.  The thrusters reach their rated current each pulling 6.6kW under static pull conditions so they are overpropped in terms of delivery maximum bollard pull.  Once of the units can be pushed well above its rated current for about 10 minutes but breaker trips if sustained.

When manoeuvring in tight places the side windage dominates.  The Melbourne proa gets to about 2kts sideways in a 12-15kt breeze.  That reduces the time to do anything.  The owner has resorted to a side thruster at the opposite end to the forward pushing thrusters.  It is the same basic design as the forward thrusters so good for about 1300N thrust.  That makes getting into its berth possible in winds to about 30kts.  There is an open buoy mooring or other sheltered spots in the marina that can be used in heavier conditions.  Transverse sections will give a good indication of the aerodynamic profile laterally.  Again bluff faces are likely worse on the lee side than the windward side in terms of creating drag and remember when mooring, the boat may be the wrong way around in terms of its windward facing side.

It is noted here that high aspect dagger boards off very little lateral restraint when not moving forward.

From my perspective windage dominates the powering requirement.  It takes very little power to move the 18m proa at 6kts in light conditions.  It is only a 4.7T boat with very long waterlines.  However it has the windage of a 100ft or even bigger mono hull and is no where near as planted in the water.  

I have discussed windage many times with the owner and it is what it is.  With hindsight there are quite a few things that could be altered to reduce windage.  There is a lot of the cabin that does not require full head room.  In fact it has 2m headroom throughout the cabin and in the ends of the ww hull.  About 3m over the galley area.  In my view only the designated walkways need full head room.   Much of the cabin roof is above seating or over consoles where it could be lower without impinging much on usable headroom.  

Reducing windage should be an absolute priority with any HP design.  That requires taking sections at various angles to observe what is being presented to the wind under various conditions.  From those it is possible to estimate the air drag. I have a reasonably well sorted VPP for the HPs that I can use for determining windward performance using the boat forward and lateral Cds as two of the many inputs.  The powering requirement really gets down to what conditions you think you may need to motor in.  In my experience being able to hold in 60kts would provide a safe margin in most circumstances.  Weather worse than this is usually predicted well in advance and there is somewhere to hide or gain sea room.  Or you stay at home of course.

One other costly lesson regarding windage is that you have to remember you are anchoring something closer to a 50T sailing vessel than a 4.7T vessel in terms of anchor holding power , chain size and chain length.   If you have wing masts then you can at least double the anchoring loads based on Ozone's experience.

Rick