Rudders & plates along centreline between hulls on Cats

[Sailor Sam]

Can anyone explain why on catamarans, rudders and plates are not fitted along the center line between the hulls, but on each hull.

thanks

[catsketcher]

Hello Sailor Sam

Some of the earliest cats had centrally mounted board and rudders - a few still do. One of the problems is that if you fly a hull then the foils will be out of the water or reduced in waterplane area.

Surface piercing foils are not as efficient as underwater foils. This is why many cats and tris have underhung rudders even when a transom hung rudder is much easier to make. The underhung rudders work very well in rough conditions.

On the other hand there are a few cats that have the usual twin rudder set up with a pivoting centreboard hung from the bridgedeck. Not a bad idea for cruising cats as you shouldn't fly a hull anyway and you can have a really big board with few space limitations.

cheers

Phil Thompson

[Sailor Sam]

Hi Phil
Thank you very much for yoiur speedy reply. Your info. is very interesting paticularly your last para. Can you provide the names of the cats with C/L arrangements.

cheers

Colin

[Bruce Woods]

Sailor Sam,

To name a few with this arrangement:
14' arrow- neil fowler design.
11' arafura- neil fowler design
16' quick cat- charles cunningham design
16' tathra- similar to quick cat with jib.
the cruising cats I can think of off the top of my head with all foils on the centreline are predominantly kelsal designed i think.

Regards

[Sailor Sam]

Hello Bruce
Thankyou for your input.
I will look at these cats.

regards.

[doug kay]

I am rebuilding my ground effect after extensive damage due to a twister and it struck me that a centre board in the centre of the bridge deck would be a good idea. The case can go down to the W/L but it's location with respect to the mast could be a problem. Because i'm 78 years old i reduced the aspect ratio of the rig quite considerably so flying a hull would be doubtful.

[Sailor Sam]

Hello Doug
Some low altitude hull flying may not be a problem because as the windward hull lifts the leeward hull sinks deeper so compensating to some extent by lowering the whole boat in the water. A simple sketch shows this clearly. Taking the centre casing as low as possible is a god idea as this better distributes the side forces from the plate. I see centre line plates as a labour saving device when tacking so you might find that aspect usefull.

[Petros]

It can also be done on large high performance cats, check this one out:

http://www.wsl2009.com/canais.asp?id_canal=66

It has not been in the water yet, but I would assume the designers know something about cat design.

I think on smaller cats it is assumed you would fly a hull at times, and you would not want to go sailing with little or no center board or rudder in the water.

I had also considered this and if you are not designing for sailing with a hull flying, you would not need more than one center board or rudder. The bottom of the hull also provides end-plate effect and make them more efficient, but I suspect that one larger center board and rudder would have less drag than two smaller ones, unless the end plate effect is more important (add a surface plane on a center mounted keel and rudder?). The fewer the parts the lighter and less complex it would be.

[grob]

You should be able to arrange it so that when the catamaran is sailing level with both hulls in the water the boat is balanced. i.e. center of effort CofE and Center of Lateral resistance CLR are in line.

When the hull lifts and the center rudder is lifted out of the water the CLR moves forward, and so the boat luffs up, the boat de-powers and the hull drops back into the water.

Gareth

[brian eiland]

Have a look here:

http://www.boatdesign.net/forums/showpost.php?p=36348&postcount=36

http://www.boatdesign.net/forums/showthread.php?p=36348#post36348

..under ASYMMETRIC CENTERBOARDS
http://www.boatdesign.net/forums/showthread.php?t=13389&page=2

DynaRig MotorSailer
http://www.runningtideyachts.com/dynarig/

[doug kay]

you have some excellent ideas Brian, I shall have a good look at the double centreboard idea over the winter. I tried to use a foil instead of the normal sail rig but the mast needs a deep bury and on a cat. that is difficult but I did look at the idea of counterbalancing the rig with a plate and the mast able to spill wind as a mono does you may be able to dispense with the shrouds also if not a frame of aluminium would do the trick.