Rudder T-foils

Peter- I’m very interested in hearing some details of your rudder T foils as well as any additional comments you might have regarding their installation and use.
Do your rudder foils have servo variable angle of incidence? Or a flap? What angle are they set at relative to the design waterline? Do you use an asymetrical or symetrical foil? How thick a foil for what chord length? (t/c ratio) Approximately what area do you find most usefull?
Is the foil deep enough to still be effective if the main hull flies? If the foil pops loose does a pitchpole result? Do the foils stay in place in lightair?
Thanks; I’d really appeciate as detailed a response as possible; many multihullers could benefit from this info.

Doug Lord
–High Technology Sailing/Racing

We add t-foil rudders when the conditions require it. At this stage number one rigs and t-foil rudders are used by us when the wind gets to around 12 knots. We change down to number two rigs at around 18

The t-foil rudder is a fixed unit that is a complete new rudder that is added. I use a rudder that is about 10"(25cm) long. The t-foil rudder is slightly longer (about 11")28 cm), both rudder are about 2 1/2" (6 cm) deep and about 1/4"(6mm) wide. The rudders are made symetrical but the “t” setion is asymetrical. It is shaped then glassed into place and therefore is permanent. The T-foil rudder is fitted into the boat so that the T section is parrallel to the waterline. Therefore when the boat start to excellerate, the bow tries to bury and the T counteracts this.

The loss of boat speed is minimal, but the stability is improved. T-foil rudders don’t restrict the course racing performance of the boat. I have my rudder under the boat. It is about 5" (120mm) from the transom.

I have experimented with different sized T-foil rudders, and found that the one I use suits our conditions best.

Unlike most Multi’s I have seen, our members don’t have our platforms flat(all three hulls in the water) there is about 6 degree up angle on the beams. This means that in light air the windward hull doesn’t contact the water. It also allows for that Multihull “flop” to occur. The other point is that the boats tack more like the speed of monohulls, but because they accelerate out of a tack quicker, monohull’s can’t match them.

So many multihull problem are removed by doing this including
Going into irons.
Slow tacks.

These boats can now, in 5 knots of breeze go from port tack to starboard tack and back again one after the other 5 consecutive times before you need to power them up again. This ability makes for really good tight racing.


Thanks, Peter. In your normal racing do your trimarans fly the main hull? Does that happen often? Do you find that it is sustainable for more than a few seconds at a time?

Doug Lord
–High Technology Sailing/Racing

Main hulls are flown my all the “new” designs. This happens every time we race. The boats start to fly main hulls in about 6 knots and quite often the only time, on a windward leg, that the main hull is in the water is during tacks. The boats are often sailing on leeward hulls only for 10-15 seconds.


Peter, I’m not clear on the t-foil dimensions you are using: are the foils 2.5" X .25" or were those dimensions the rudder blade itself?

Doug Lord
–High Technology Sailing/Racing

Rudder without T -foil is 10"x2.5"x.25"

Rudder with T-Foil is 11"x2.5"x.25"

T-Foil dimension is 4.25"x2.5"x.25"


Thanks again, Peter. Your t-foil area works out to 131sq. in. SA per of foil area. On the F3 it is 98sq. in.SA per sq. in. foil area. And on the X3 I’m going to try 164 sq. in Sa per sq. in. foil area. Interestingly ,the area of the t-foil on the full size Rave is 112 sq.ft. SA per sq.ft. foil area…An almost direct parallel to the smaller F3 foiler.

Doug Lord
–High Technology Sailing/Racing

Peter, I may have misunderstood what you said regarding the t-foil angle of incidence below: it seemed to me that you said the foil was mounted parallel to the waterline which ,if thats true in both directions(viewed from sideways and viewed from fore and aft), would mean a zero degree angle of incidence. But in another post you said:" rudder foils are set at 7 degrees off the waterline of the boat".
Could you clear up for me and others interested in rudder t-foils which is the correct (the one you find works best)figure? Thanks in advance…

Doug Lord
–High Technology Sailing/Racing

just wondering!

Doug why are you asking so many infos about a multi, that YOU said was "#$%&’# ???
still wondering…


if it isn’t broken, don’t fix it!

  1. I never said this multi or any other was “@%$%^**((*&”. I had/have reservations as to some multie’s that have been described on this site as being the “fastest in the world”.
  2. I’m interested in building a database for use by all multihullers of accurate and pertinent information regarding various set ups and their raison d’etre.

Doug Lord
–High Technology Sailing/Racing

Has anyone ever had a T foil work too well? I was getting a big rooster tail behind the lee hull at speed the last 2 times I’ve sailed my cat with the new rigs and T rudder. First yesterday was in a Force 3-4 wind flat water and today in a Force 5 with 6inch waves It was even dipping the aft beam at times. Still goes bloody quick even then!

Luff 'em & leave 'em.

The fact of the matter is this. You are definitely not interested in what we are doing over here. The development in multihull sailing over here is always improving, which is more than can be said for what you are doing to help establish the class in the US, yet you critise posts by me concerning our boats. I have timed them sailing courses, and the averages that are coming out seem very quick. We have designs that sail with/against us from other parts of the world, that are rated as “quick” boats, yet they are consistantly following the fleet home.

Your opinions of multihull sailing worldwide have no foundations what so ever, and therefore any readers of your posts relating to r/c multihull sailing should take your opinions with a grain of salt.

As for what we use specifically over here. If I post the correct information that is for me to know. Especially when the question is asked by you, I will reply with whatever sounds good at the time.

As was suggested by someone to me some time ago, maybe you are asking so many questions because you just don’t know where to start when it comes to building a fast rating boat.

BTW it wasn’t Dick or Wis


Oh Peter, I am interested ,I am interesed! I consider it a great privledge to get even inaccurate information from the the home of the “fastest rc multihulls on the planet”.
But maybe it would help others a little better if you could try to contribute accurate information- don’t worry: I’ve already posted my information on t-foils under Foiler Design below.
Try to think of the masses who could benefit from your enlightented multihull design; the individuals working in their own shops who don’t have a clue. You could help all of them Peter, instead of this other thing you are doing…

Doug Lord
–High Technology Sailing/Racing

thread locked by moderator

hi im going to be building the nightmare tri in the new year from plans i was given for free from the designer ernst zemann and wondered if you could show me some pics of your winged rudder set up?.also any pics of your boats also as i like to see whats out there.i hope you can help me out here and thanks in advance mate.

Peter -

defintely make contact with Andy Wright there in the UK - as he will be of immense help before/during your build. I will PM you his email address. Let him know I referenced you to him.

Here are two photos of my T-Foil rudder taken during construction of my self-designed 1 Meter trimaran. The foil is set at a negative 3 degrees. In light air there isn’t much drag, but as wind picks up the slight angle does keep the stern down - although the bow has lots of built-in reserve buoyancy. I have not experienced the “rooster tail” effect noted by Matthew.

hi thanks for that, i allready have andys email ive been talking to him a bit about the wing for the rudder and also sails .thanks again for the help.

For what it is worth - and for anyone considering an F-48/Mini40 - here is a starting point for rig sizes based on GHOST TRAIN plans. I suggest starting with one of the two SMALLER rig sizes until you get some experience sailing a boat without training (lead keel) wheels.

It will be underpowered, but keep you from too many tip-overs ! :wink:

thanks for the tip dick id allready decided to do that great minds think alike lol.i do have the details for 4 sets of sails but im sure others could use them too.

Here are a couple of photo`s of the winged rudder on my tri.
I have since sold it but have a few photos if they are of any use.