[PCW] asymmetric boards

[Malcolm Tennant]

Bob Kupps.

Dear Bob,

If you go to Abbott & Von Doenhoff "Theory Of Wing Sections" you will find
out more about wing sections [plan forms are another matter] than you 
probably
ever wanted to know. You will find it is obvious as you look at the data for
the airfoils that the asymmetric sections have a better lift to drag  ratio
ie: for foils of a given area, aspect ratio and percentage thickness the
asymmetrical section may develop as much as 50% more lift for the same
amount of drag. It may also do this over a greater range of angles of
attack. So why aren't asymmetric section foils more widely used? Firstly
foil sections, are usually used on keels which can't be easily raised and
lowered which obviously means they are useless for a boat of this type. 
Secondly, they usually have
to be able to be raised and lowered except on proas which often change ends
to "shunt" [see Disco Volante]. This in fact is the crux of the whole issue-
the raising and lowering of the foils. Early C-Class cats used asymmetrical
foils and raised one and lowered the other when they tacked. They no longer
do this, the gains were just not worth it. I have used asymmetrical foils on
a number of trimaran designs  [sylph, stinger,wild thing, flying circus]
because on these boats the windward foils mostly come clear of the 
water,reducing the loads making them much easier to retract,
when the boat tacks because they are located in the floats.

They have occassionally been used on keel boats as dagger boards. In fact 
they are de rigeur on the various round the world race boats but these are 
racing boats that will often spend thousands of nautical miles on one tack 
and are prepared to expend the effort required to raise and lower them. And 
effort there is, lots of it. We had tandem dagger boards on the "Cordova". 
These were hydraulically operated and "chinned" themselves up and down in 
their cases. It was found that such were the loads when underway that there 
was not enough battery power to drive the electro-hydraulic unit that raised 
and lowered them. If you wanted to change the position of the dagger boards 
it was necessary to turn on at least one motor. So the owner very wisely 
decided to remove the daggerboards. In fact, we found that with the CS hull 
form, the dagger boards were not really necessary. The CS hull is very close 
winded by virtue of its shape. However if you will be operating in similar 
conditions to the Volvo boats, and your hull form requires the use of foils, 
it might well be worth considering them and installing systems that make 
them easily retractable.

I was glad to hear that the 20% foil was a typo. It seemed more like a 
lifting body!  I have used a number of different foil sections over 40 odd 
years and kept coming back to the good old traditional NACA0012 [I actually 
use a slightly thicker 13% section] for the rudders and a more laminar flow 
section, the 65-013 for the symmetrical dagger boards.Asymmetrical foils are 
usually 64A413 sections, or as close as I can get them without CNC 5 axis 
milling.

Abbot & Von Doenhoff is one of the most heavily thumbed volumes in my 
reference library.

I hope this helps with your decision making.

Regards,

Malcolm Tennant.

MALCOLM TENNANT MULTIHULL DESIGN LTD
PO Box 60513 Titirangi, Waitakere 0642.
New Zealand.
ph +64 9 817 1988
e-mail: malcolm@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com