Daggerboards, Centerboards, mini Keels on Catamarans

I've been away from boat designing world for a number of years as a result of a bankruptcy while building our Firefly tri project,.. Trimaran Design https://www.runningtideyachts.com/trimaran/

I wanted to see what new developments have come along with the leeway reducing elements on cruising catamarans,...not only their leeway reduction capabilities, but their deployment regimes, and their function when tacking. This tacking function was of particular interest.

I found this video that was well done, BUT it failed to address the question of operation during the head-to-wind tacking maneuver?,...and it failed to say anything about asymmetrical foils.

 

Btw, I long ago promoted the idea of retracting asymmetrical boards on the centerline of the vessel,..

Section 6 Galley, Dinette, Centerboard https://www.runningtideyachts.com/dynarig/Section_6_Galley_Dinette_Centerboard.php

It also provides for a central point about which the boat can pivot during tacking.

 

I put this posting into a LONG subject thread dealing with bridgedeck mounted boards,...lot of good yea & nay contributions,...

Bridgedeck centreboard why don't they work??? https://www.boatdesign.net/threads/bridgedeck-centreboard-why-dont-they-work.57051/page-4#post-795051

 

Here is an interesting photo,...

Bridgedeck centreboard why don't they work??? https://www.boatdesign.net/threads/bridgedeck-centreboard-why-dont-they-work.57051/page-4#post-795801
(there is a video of it sailing vet well in that posting submission)

Almost as though that central structure he has between his catamaran hulls was somewhat like the central nacelle I was speaking about,..his contains the board and the rudder

PS: Another good video of that cat with its twin sailing rig,...and its 'central nacelle'

 

I could get my central twin boards into a nacelle like this,...along with some nice stiff framing of the hull structure !

https://www.boatdesign.net/threads/...reboard-why-dont-they7051/page-11#post-797138

 

2 different CB arrangements

 

How about a curved ball in this discussion. There have been talks about 'fences' on boards that operate with a free surface with the water's surface.

Nature might have to be reserched,...
Whale Fins, nature's answer

Let me throw a curve ball at you fellows.

A long time ago there were some discussions of the shapes that mother nature provided us. One such discussion referred to the shapes of the leading edges of whale fins.


Here just one reference I found real quick. I'm sure there are more.
Bumpy whale fins set to spark a revolution in aerodynamics http://newatlas.com/bumpy-whale-fins-set-to-spark-a-revolution-in-aerodynamics/9020/
It seems the bumps have the effect of channeling air into smaller areas of the blade, resulting in a higher wind speed through the channels and a number of rotating airflows on top of the blade which increase lift. Furthermore, the bumps eliminate the tendency of air to run down the length of the blade's edge and fly off at the tip, causing noise, instability and a loss of efficiency
What if our centerboards, daggerboards had some sort of scalloped leading edge :?: (maybe not as radical as this image)

.....a few more references....
Whale-Inspired Wind Turbines https://www.technologyreview.com/s/409710/whale-inspired-wind-turbines/

https://www.scientificamerican.com/podcast/episode/005E4190-9DE4-32A2-015B43F0702189EA/

Tubercle effect - Wikipedia https://en.wikipedia.org/wiki/Tubercle_effect
The tubercle effect is a recently discovered phenomenon where tubercles or large 'bumps' on the leading edge of an airfoil can improve its aerodynamics. The effect was discovered and analyzed by Frank E. Fish in the late 2000s. The tubercle effect works by channeling flow over the airfoil into more narrow streams, creating higher velocities. Another side effect of these channels is the reduction of flow moving over the wing-tip and resulting in less parasitic drag due to wingtip vortices. Using computational modeling it was determined that the tubercles help to increase maximum lift and degrease drag. Dr. Fish first discovered this effect when looking at the fins of humpback whales. These whales are the only known organisms to take advantage of the tubercle effect. It is believed that this effect allows them to be much more maneuverable in the water, allowing for easier capture of prey. The tubercles on their fins allow them to do aquatic maneuvers to catch their prey.

The tiny hooklets on the fore edge of an owl's wing have a similar effect that contributes to its aerodynamic maneuverability and stealth

 

one of my new versions of those twin asymmetrical boards on the centerline nacelle,..

Bridgedeck centreboard why don't they work??? https://www.boatdesign.net/threads/bridgedeck-centreboard-why-dont-they-work.57051/page-20#post-798120

**************************
PS: Perhaps a change in that scalloped,...
Perhaps that multiple scalloped board I displayed in the first sketch I posted could be modified to a single big scallop at the leading edge waterline region,


.....with a mellow fence like the ones on this skeg



This might purposely mis-align the board's leading edge juncture at the waterline with the rest of the leading edge down deeper,...upsetting the direct path that ventilation might take down a leading edge.

 

Tacking posting,...hope this is true

 

Louisiane 37 Centerboards

I was the first importer to the USA of these cats. I liked their CB arrangement that hide the big trunks in the face of the big berths.

the Virtues of Shallow Draft

Rather than mini-keels, I might just have to go backe to the boards in the hulls idea if I found the central board idea unworkable. Why would you ask?? I love shallow draft capabilities,...I mean really shallow draft.

I imported the first Fountain/Pajot 37' catamaran to the US in 1986. It had two kick-up centerboards, one in either hull.

With the boards and rudders kicked up I drew 19 INCHES! of water. I once took that boat on a trip thru the backside of Cape Hatteras
brian eiland said: ↑
On another occassion I did the same with a Louisiane 37 catamaran that drew 19 inches of water with the CB's up and the rudders kicked up.

Come to think of it I took that same Louisiane 37 down along the backside of the outer islands of Cape Hatteras, NC (there were times we were pulling the boat along while walking the shallows....what a great trip that was). I'm sure there are not many boats of that size that have ever made that trip.


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Tacking a Malcome Tennant CS hull form.

I had written over on this subject thread, CS Hull Sailing Performance? https://www.boatdesign.net/threads/cs-hull-sailing-performance.35387/#post-465817


about adapting one of his CS hull designs into a sailing craft.

We often think the bows need to be of a form that allows then to slide sideways over the surface to more easily facilitate a tacking maneuver . I don't believe that necessarily so. We learned long ago that we really need to GENTLY turn the catamaran threw the tack,...certainly don't jam the rudders over as that just stalls us,..like putting on the brakes.
What this CS hull form does offer is less resistance to the sterns being turned over to a new side,...the vessel's sterns pivoting about our central nacelle mounted board(s).

Granted this might need a little nugging by a sort burst of motor power, but I am talking motor-sailer vessel here.

 

I had a Wharram and it was no problem tacking.

 

There are easier ways to get an asymmetric foil. You can have a tacking foil or a tab on the foil. Both have been done. Of course you need to compare the cost, complexity, and convenience to achieving the same drag reduction or performance gains though other means like a deeper fixed foils. Part of the benefits of dual boards is the lateral separation keeping the rudders in clean water and the ability to cant the foils to counter heel. Without lateral offsets and cants, I don't think dual boards are competitive with a tab on a single foil.

There is really not much advantage in the asymmetry itself. It just changes the zero lift angle, but has very little effect on a foil's lift - drag ratios. The drag bucket is somewhat different, but that isn't something that you can exploit on a normal boat.

 

I have never seen any aircraft utilizing symmetrical foil wings to develop their lift.

 

CB or DB Board in the Center vs Boards in the Hulls

Just a real basic feeling, its going to make tacking (pivoting) a catamaran easier when rotating about a central point under the CA of the sail rig,..than the confusion added when the lever arms are added to the forces of those boards by the separations encountered with the beam of the vessel.