[MHml] DynaRig, Mast-Aft, Bi-Plane sailing rigs

[brian eiland]

I recently wrote this letter in response to an inquiry I had on a few sailing
rigs I've promoted.
_____________________________________________________ 

SUBJECT: DynaRig, 40-ish Catamaran motorsailer 

Hi Brian, 

I am of the definite opinion that all these rigs you talk about are 
inefficient and heavy, not what one wants on a catamaran. It has been 
proved time and again, what makes an efficient rig for windward work, a 
wingmast and a sail of high aspect ratio, and some means of minimizing tip 
vortices. 

Why make it more difficult? 

I have decided to go for a bi-plane rig on my 45 ft. cat, using unstayed 
masts, and the masts will have a wingshape. The masts and booms can turn 
360 degrees. Can easily be released and reefed at any angle to the wind. 

XXXX 

_______________________________________________________________ 
Brian replied: 

Actually XXXX I am not trying to "make it more difficult". I am just following
aerodynamic principles that end up leading me to the rigs I chosen to promote
and expand upon. I try to consider these rigs in light of cruising sailboat
designs rather than racing designs, or specialty applications. 

I'm having trouble understanding your choice of a 'bi-plane rig'? I understand
the desire to get a lower COE of the total sail area, and I do know these rigs
have distinguished themselves in a few specialty applications (smaller
vessels, and a few speed record seekers), but at what cost to all-around
performance on a cruising vessel?? (I believe this is the type of vessel you
are seeking?) 

The bi-plane rig can certainly be made to perform to windward. And the use of
higher aspect ratios and wingmasts will assist in this effort. But now you
have tall narrow triangular sails that are the worst form, that produce
considerable induced drag from vortices off of their tips, and you have two of
them. Your induced drag on this bi-plane rig is going to be considerable
higher than a single free-standing uni-rig. 

My real concern with the bi-plane rig is with its performance in reaching and
running, two of the primary and preferable sailing angles for cruising
vessels. I just do not see any way to make the leeward twin rig effective at
all in a beam-reaching situation. And remember EFFECTIVE downwind sailing is
not just about projected sail area to the push of the wind, but rather FLOW
over the sails, that will not easily be attainable with the bi-plane rig. Here
are a few excerpts from a forum discussion on Sail Aerodynamics I made; 
Aspect Ratio (AR) of rigs on cruising vessels
<http://boatdesign.net/forums/showpost.php?p=93500&postcount=168>   

  "In general we have three basic sailing directions we need to consider,
upwind, reach, downwind. And for the cruising sailor the upwind 1/3 of the
total is not even an equal partner (as many cruisers often chose not to fight
upwind work). As Marchaj and many others have reported, high aspect ratio is
principle beneficial for upwind work." 
  

  "When we talk of speed, are we talking of 'around the race course' speed, or
straight-line, head-to-head speed? From a purely aerodynamic point of view, I
would prefer to compare the vessels with different sailing rigs on a
head-to-head basis in each of the 3 primary sailing directions. So hopefully
you can see where I have some problems with your statements such as "the most
efficient boats of all use cat rigs", and "fractional rigs generally go faster
than masthead rigs". If you are talking 'around the race course' your
statements could well be considered correct in many cases, but on a
head-to-head basis they might not hold up so well." 


Are you proposing fully battened sails on these freestanding wingmast? Are
they to be reefably?...on each mast?, or just one up, one down? I would not
want to be the autopilot unit trying to cope with the turning moments
generated by COE shifting from one side to another. 

I'm just not impressed with the idea of a bi-plane rig on a cruising vessel. 

Here is another interesting collection of discussions on existing bi-plane
rigs:
<http://www.boatdesign.net/forums/showthread.php?t=10197&page=2>

I don't know how you conclude that my mast-aft design is "heavy and
inefficient"? It shouldn't weight much more than the ketch rig it is intended
to replace. As concerns efficiency I will just say I've written quite a lot
about this in the past and won't bother to rehash it here. Suffice it to say
it will perform very well on all 3 major points of sail. 

  

The email I sent out on the Dynarig was meant to show how the Falcon's Dynarig
could be brought down in complexity, price, and weight. Certainly it is still
a 'square rigger', a very modern version at that, and will never compete with
a free-standing unirig in expense nor weight aloft. But it has shown its
capabilities in performace, and on all 3 points of sail.