Rick,

Thank you so much for all the time you spent doing this, much appreciated! Very impressive what you can achieve with simulation.

There was a lot of info in these to digest.

I must say that I liked the speed numbers much better on the first set of charts :-)  Seriously I understand these are dynamic analysis and not VPP.

I had a mail writen up when I got your last post ... had to delete my answer and had quite a lot of reading since. I understood before that the higher aspect ratio have a better Cl but I also understand now that the higher Cl/Cd move to a lower angle of attack which I guess is the reason for better pointing ability of the 40 sqm sail.

The better pointing ability is the result of higher efficiency of the rig.  The angle of attack in both values of sail area was 10 to 11 degrees to give the highest Cl for the chosen sail camber (draft) of 10%.  The rig is adjusted to maintain that AoA irrespective of the apparent wind angle.  With an Aerorig the slot stays the same irrespective of the angle of the boat to the true wind so only needs one sheet adjustment, whereas the slot changes with the schooner, requiring both sheets to be adjusted.  I did not get much variation in the Cl by changing the slot.  With the high aspect rig the aft boom would likely need to be pulled windward of the hull to get the best pointing although I did not do the full geometry. 

 

Since the chart do not account for the ww hull and the speed is lower than I would like, I am considering now a higher aspect ratio 50-60 sqm sails to compensate. 

I think now a little more bow volume would be good … but as you have shown, bigger sails is not always better !

It is likely the two hulls will be slower but not guaranteed.  I would not assume that two hulls sharing the displacement would be higher drag than one hull taking it all without analysing.  

As for going close to dead downwind, as you pointed out, it would be uncomfortable (wallowing) and best VMG is reaching. 

I read that high AR sail are not good when going off wind but what is not clear to me is how far can a proa use sail lift going downwind or at what true wind angle does it only uses sail drag (since there is no shroud the sail can be set at a much higher angle to the boat and still give higher forward forces than using drag only) ? 

It maybe could be sailing best VMG using lift all the time ? Then high AR is good all the time :-) No more questions about big downwind sails.

If the boat can reach boatspeed near windspeed or better then it is going to be faster reaching downwind unless there is ability to massively increase sail area when going downwind.  

I was thinking that such a light boat, a 15m hull and a 6 ton-m moment, I could be sailing very close to wind speed … maybe the schooner rig max Cl is the limiting factor.

A factor to take into account with the schooner rig is that there are two sails of identical size.  AR is determined as Span^2/Area.  So you need a bigger span with sails of the same area than the span for a fractional rig to get the same AR.    The aspect ratio of 144/50 = 2.88 is quite low.  An A-class cat has an area of 14sq.m and typically 9m mast so AR is 5.8.  The 2D lift coefficient for the schooner rig will be around 1.7 so there are drive benefits of going taller without increasing area as well as the efficiency gain.    

Agreed that this hull is overpowered at 10 m/s and 50 sqm ... the bow do not come up much! Very good to see also the 10 m/s 40 sqm graph.

 

The other thing this tells me is that in good wind but not much waves, the limit will be the overturning moment, not the pitching. Did more research about water ballast and found the Gougeon 32 Cat does this ... definitely will be added to the design. It could help very much when sailing a much lighter boat since the chart are at full displacement.

Eliminating waves for the 6m/s windspeed case the boat speed on the wind increases by almost 10%. 

 

I installed Autodesk CFD so will try to get a Cd out of it, may take some time to make this work though. Would be very interesting to have a good VPP but these plot is more than I could ask for at this point :-) Again many thanks!

I made a guess at windage.  Once you get some better values I will include those.

Luc

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