best rudder foil section for a catamaran

Well, race boats have higher-AR rudders with a certain amount of area where the flow will be quasi-2D. That's where a foil shape do have an influence on the overall rudder characteristics.

The choice of the foils and the planform shape will depend on the rudder type, boat data and intended usage (pure racer or mixed racer/cruiser), requirements for rudder performance, materials, money at disposal and some other info.

Rapscallion, your best bet imho is to pick a designer and work closely with him in the definition of these goals and during the design stage. I personally wouldn't do it by gathering info through various forums because you'll end up with 10 different and often conflicting opinions and advices. You either know your stuff or you don't. In the latter case, let a designer do it for you, or together with you.

Cheers

 

Side by side - trials

Gooday 'Par' & to you to 'Wisconsin-guy'. Our side by side trials were on a 15 sq mile salt water bay, average 30' depth with good wind & flat land 95% around. Every weekend for 8 years - we sailed many types of monohulled yachts up to 40' & multihulls from 12' up to 30'. Very tight class racing was the order of the day. Refinements were easy to judge due to the closeness of the various fleets. At that time I was building, repairing & modifying yachts for a living & of course sailing everyday after work & the 'chicken-races' on Wed nights & Saturdays & Sundays as well. I was able to be involved in modifying several 1/4 ton, 1/2 ton, 3/4 ton & 1 ton yachts & many catamarans - doing extensive mods to both keels & rudders. What a fun - hot - time that was & a very steep learning curve. Had the great luck to 'play' with some of the 'big-boys' in the marine design field - boy oh boy - lucky me.

Lessons learned - size is not the governing factor to efficiency. General foil selection needs to be regarded as very important (& doesn't seem to be) - so does section & profile at both the top & bottom of the foils - both c/b'c, keels & rudder/s.
It takes almost as much time & space to explain it all (& to get it exactly correct) as it does to do the actual doing - - however the rewards are rather astounding & instantly noticeable in both handling & performance.

At one stage - we re-designed & built our 2nd 'A' class cat & - came-in at 5% under minimium class weight rules - thought we'd done well. Then some 4 months later we redesigned c/b's & rudders (reducing the surface area by 55%) - result - were much farther in front & eventually went on to win a world champs - with a score of 7 wins out of 7 races & all the 5 invitation races as well. The skipper was one of the very best at that time - however some credit was due to the advanced design of the whole boat concept. Sure sounds like a load of 'crock-****' when I reread it - but it did happen & is a matter of official record. Except for the likes of - Steve ('C' class world champ) Doug Lord & Gary (K1W1) - I've not come across anyone who fully appreciates the importance of - getting the whole package correct & paying a lot of attention to foils - all 3 - - sails, c/b's or keels & rudders. Damn - there I go again. Sorry guys.

Hay there 'Wiscon-guy' lots more info on your boat - please - & then let's see if we can make a positive contribution to your end goals?? Can I see some pics on the net - about the design etc etc.

All of this information is - of course IMHO - & truely do not want to enter into an arguement with anyone - as has happened many times before.

Y'all over there - have a great day & enjoy - sun - sailing - smiling & s*^ - cause life is all to short. Go follow your dream - do it now. Ciao, james

 

The best answer to the topic question is it depends on the design of the boat and the use to which it will be put.
The builder/sailor seeking advice can be given many opinions, some even conflicting.
The best thing he can do is read the excellent information found in the book "Rudder design for sailing craft" (AYRS NO:79).

 

Importance of Rudders

Gooday Chris. With all due correctness to 'Tim B' - - I totally disagree. If & I quote "I bought the boat to race it" - then the very highest end refinements are of great importance. The wetted surface area of the rudders & c/b's is a large % of the total wetted surface area of the entire racing catamaran.

I'll do everything that I can to assist you - to get to the - 'front of the fleet' & stay there. Whilst - Tim B's comment has some importance - however even he should realise that everything about the yacht as a whole plays a significant part & all of them are linked - at the hip - so to speak.

I've already asked several - very quick knowledgeable experienced sailor persons to assist you to come up with the 'advantage' if there is such a thing (& of course there is).

I'll try to keep this discussion - - open - - & here in forums - - if all the nay-sayers - just button-up for a bit - - - even they might learn something - - well - - I will continue to live in hpoe.

How did your sailing go these last 2 weekends??????????????

Ciao, james