Two hulls one rudder

[Trident]

Can anyone explain to me why catamarans have two rudders other than why would you not fit one to each hull?
I recently met a man in Spain who, after a fire, had rebuilt his Prout cat, leaving the stock two rudders in place but also adding centrally a third rudder which could connect to a wind vane. He then found that using just this centrally mounted rudder (which was originally to be emergency steering when the wind vane was not attached) was much lighter and more responsive than the original hydraulic twin rudder set up. It was also positioned behind the central drive leg on the Prout meaning that he had prop wash over the rudder too.

So he had simply locked off the original rudders never to be turned again and used the central one all the time. Obviously this is just anecdotal evidence but it made me wonder if there was a reason architecturally why cats always have one rudder per hull ?

 

[jwilson]

Can anyone explain to me why catamarans have two rudders other than why would you not fit one to each hull?
I recently met a man in Spain who, after a fire, had rebuilt his Prout cat, leaving the stock two rudders in place but also adding centrally a third rudder which could connect to a wind vane. He then found that using just this centrally mounted rudder (which was originally to be emergency steering when the wind vane was not attached) was much lighter and more responsive than the original hydraulic twin rudder set up. It was also positioned behind the central drive leg on the Prout meaning that he had prop wash over the rudder too.

So he had simply locked off the original rudders never to be turned again and used the central one all the time. Obviously this is just anecdotal evidence but it made me wonder if there was a reason architecturally why cats always have one rudder per hull ?

One rudder per hull still works if you are flying a hull - OK not many cruising cats do that but I know of one French cruiser-racer cat owner that did regularly for long periods, though not this one - http://static.wixstatic.com/media/2....jpg_srz_626_293_75_22_0.50_1.20_0.00_jpg_srz

After it finished several days ahead of us on a race that took us (42 ft monohull) 6 days, I consoled myself with the thought that they must have had a lonely wait for the rest of the fleet to arrive to party.

On more ordinary boats, the two hulls also offer a nice strong point to attach pintles....

 

[lw395]

I don't know about rudders, but centreboards on cats are said to be very inefficeint if they are surface piercing instead of being in the hulls.
I'm sure central rudders have been tried and rejected in racing cats, but racing cats will fly a hull, they really operate as a monohull most of the time.

 

[William_H]

No I am sure it really is about structural mounting of the rudders. As said. A rudder can produce a lot of sideways or angular force best taken by pintles well spaced ie down to the keel of the hull. A central rudder would work just as well but a nightmare to try to support sideways as low as possible.
Now the additional rudder of the OP's anecdote probably seemed to work better and have lighter steering loads because of rudder ballance. On a rudder on a canoe stern or deep transom it is difficlt to get rudder ballance. ie area of the rudder ahead of the pintle line, force on which tends to aid any turn commanded on the tiller. So if you had one third or the rudder area ahead of the pintle line this will ballance 1/3 of the area aft of the pintle line leaving rudder turning loads only 1/3 of the loads of the total area if it were all aft of the pintle line. Further rudder forces can be a lot lighter on a deep high aspect ratio rudder. ie deep but short front to back.
So I presume his new centre rudder had a high aspect ratio and ballance so making it seem to work better. The trap being that the rudder will still generate as much side force in turning (or counteracting weather helm) as the total of the 2 original rudders. The new rudder would have to be cantilevered from the stern cross beam down to water level then deep into the water so very hard to design. (Or likely to snap off sideways at the balde itself or the mounting.)
I am sure that ordinary cat cruiser owners will never fly a hull so lifting of a centre rudder out of the water is not a concern. good luck olewill

 

[markhomer]

Yo lw395 right on the button , aim of game to have only one hull in the water when going in any direction ,

 

[lw395]

Chatted about this last night with a chap who has sailed cats a lot.
Couple of interesting points.
1) Cat rudders are more vulnerable to damage from flotsam, so having two is nice.
2) A central rudder would be in the bow wave of the hulls, and in chop it's in a different part of the wave from the hull. This creates a lot of opportunities for the rudder and hulls to be working against each other, creating a lot of drag sailing in a straight line.
3) In light airs, with a conventional set up, you can often raise one rudder for less drag.

A single rudder might work very well some of the time, but you'd need to know it was as good as conventional all of the time.
Fast cats and dinghies though, only need a small rudder when up to speed, but can struggle for control at low speeds.
The original system described by the OP sounds like a compromise, if the orignal rudders can be locked off while the new rudder steers the boat, the original rudders must have been small or not effective?
Maybe their design was a compromise for beaching or something? Or the boat just needed fin area at the stern to balance the rig?

 

[Seajet]

It strikes me that having the original rudders locked off might make them something for the boat to trip over in big beam on seas & winds, a normal pivoting rudder would offer little resistance ?

 

[srm]

My last boat was a Prout and I considered a centre rudder for wind vane self steering. (They are heavy and relatively slow cruising multihulls so wind vane steering works). Decided against the third rudder as it would be very vulnerable to damage from flotsam and difficult to securely mount against side thrust. Prout rudders are not balances and are supported by substantial skegs. They also provide part of the lateral stability of the boat.
The 'heavy steering' is a product of the hydraulic connection to the wheel. When using the wind vane steering conected to a tiller I would disconect the hydraulic ram, giving light and responsive steering from the linked rudders.
On my other cat the rudders would lift, but having sheered both off at the waterline while surfing in storm conditions I discovered the hard way that they were also an essential part of the boat's lateral / directional stability and had to take very high side loads.