Daggerboards, Centerboards, mini Keels on Catamarans

This is true for most aircraft, but it has nearly nothing to do with lift/drag performance. Aircraft can and do exploit the drag bucket, which boats simply can't, but that's not the main reason for asymmetry. Camber shifts the center of pressure rearward. A stalled symmetric foil has a pressure center in the middle of the cord. An attached symmetric foil has a pressure center about 25% of the cord. Pitch control becomes very difficult when taking off and landing. A great deal of aircraft design is controlled by take-off and landing requirements. Camber shifts the center of pressure of an attached foil closer to the center of the cord so that pitch control becomes less of an issue. This is advantageous for weight as well - the wing girder system has less twist to deal with.

These are aircraft-peculiar issues. Boats have their own issues, but none of them drive a designer of normal boats towards asymmetric foils. If a foil stalls, the bow might tend to fall off. That usually isn't a problem. For cats, there is the problem of getting them to tack without stalling the boards. But the boards are tiny compared to aircraft and the shift of the pressure center is negligible. It's the loss of lift that matters.

So again, there is nothing to be gained by using asymmetric foils on normal boats even if you have decided to use twin boards. Maybe you can reduce the toe on the cases a bit. From a production standpoint, you don't have to make a left and a right.

 

Wood Board Experiments


If you look back at my dwg for the twin asymmetric bridgedeck CB's you will find that it relatively easy to remove and replace either of the boards, ...and the 'center bearing' of the board itself can be as simple as 'precisely cut hole' that fits around the UHMWP 'bearing rod'.

That makes it relatively easy to carry out experiments on what particular board shape might best meet our needs.

We could set up a simple small computer milling machine to cut us a variety of asymmetrical board shapes out of wood blanks (maybe just decent laminate plywood for the testing), and go out testing with a good GPS on board measuring our leeway for different shapes. Then maybe a few waterproof go-pro cameras looking at the flow around the variety of shapes.

For those concerned about 'identical conditions of testing', we could have a 'pit crew' that can make quick changes during the day of testing. And we might even carry two (2) different asymmetric shapes at one time for comparison on two different tacks.

Just read back thru this other rather long subject thread, and discovered this posting of mine at the end. I rather like this idea, wonder if it could be carried out on one of those older Stiletto 27's that had problems,...or maybe some other cat like that Thai one where the were the central board and rudder was built into the nacelle structure mounted between the hulls,...LINK
Daggerboards, Centerboards, mini Keels on Catamarans https://www.boatdesign.net/threads/daggerboards-centerboards-mini-keels-on-catamarans.70660/#post-981805

Or an older Stiletto 23 like that fellow who used 2 of them when he was developing his twin mast for Saphira, Saphira 70 cat, twin masted, well conceived https://www.boatdesign.net/threads/saphira-70-cat-twin-masted-well-conceived.70600/

 

Another forum posting in reference to 'Happy Feet'

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Very interesting.
two questions.
What mechanism slides the center nacelle left and right.
How does crew move from one hull to the other on a tack.

 

Hello Skip,
Actually when I first viewed this vessel I did not realized that the center nacelle was fashioned to move across the beam of the boat.

I was not attracted to that aspect, ....nor the foiling capabilities of the design.

I was interested in the application of streamlined nacelle application to the center of a catamaran,..and the operation of a free surface daggerboard. (with respect to application to a cruising cat design)

 

Having spent most of my life around shoal waters, foils are also problematic for me ;-)
My only comment about free surface daggerboards is I know that canting a free surface rudder forward a few degrees does help minimize or eliminate ventilation.
Doesn't do squat for sea grass though.

 

Might have to do more surfing the internet to try and find best foil shapes:

https://forums.sailinganarchy.com/t...rboards-are-they-more-efficient.220133/page-2

https://groups.io/g/HarryProa/topic/asymmetric_daggerboard_foils/101734140