Since drag is proportional to lift, the rudders would be prone to kicking up at speed... more so than conventional rudders.

Is the foil slapping the water's surface going to interfere with the Dotan stock operation?

Wear on the line/rudder entering the channel at an angle is an obvious concern. A steel rod bearing would help.

Wouter rightly pointed out geometry problems with the line/channel placement. These could be addressed simply by making the line ride a cam around the top axel, with the cam radius matching the bottom axel-line separation.

The "channel" through the rudder need not be parallel to rudder trailing edge.

You need some adjustment somewhere to get the line length just right. You probably need a little stretch in line line to so you can get the rudders down with tension on the line.

The hinge can only add drag, but kick-up eliminates the problem of cassettes scratching the foil, so you could possibly use a laminar flow foil section for less drag and make up for it.

I think the foiling Moths have a setup like that, where they can adjust the anlge of the rudder foil by tweeking something on the tiller.

They used to twist the tiller to adjust the attitude of the rear foil - however, I believe they've all gone away from that and have found that a fixed rear foil works just fine and is less hassle.

This system has obvious merit if a kick up system is required. The cassette system also works just fine. Any conceived concerns regarding a cassette system do not seem to be any problem in actual practice. The idea that hitting the rudder(s) on any solid object when they are in the cassettes is greatly over rated. We have used them now for some two years and sailed in some fairly suspect water depths with many solid underwater obstructions and although contact has been made with the rudders none have been broken whilst sailing. At the same time, during the same events we have seen more than one set of “kick up” rudders destroyed because the kick up release mechanism(s) have been set so tight to stop them releasing prematurely if and when they were to contact weed, muddy bottom etc, and as such, to all effects they become a permanent (whilst sailing) lock down system
The drag induced by T foils on the rudders has a negligible effect on the cat’s performance and any minor drag that may occur is greatly out weighted by all the positive benefits, (particularly pitch control), gained from the foils.
To understand the way in which T foils operate one has to view their effects as a global formula and not as a two dimensional concept.

The plain old conventional Hobie 14 castings used on the hydrofoiler rudders in my thumbnail pic worked pretty well and never broke. Neither did the rudders. They are on my boat, the only hydrofoiling Hobie 16 in the world. 8^)

The current generation top end moths can still adjust the rudder T-foil angle of attack with the twist grip extension, but the whole rudder box now rotates using a screw drive type arrangement. Usually it is set for upwind or downwind and not played with otherwise. Essentially they're fixed most of the time.

Darryl, do they also make it a bit more difficult to lift a hull in marginal conditions because of the down force they generate on the windward hull?

There are scaling problems with the following anecdote but...I have built and raced model trimarans (mini40) for awhile and have found that T-foils haven't been faster in most conditions. Upwind, the extra drag from them and extra drag from not being able fly the hulls was slow. They made it harder to sail downwind, which was when they were supposed to work, because the boat wouldn't accelerate as fast in a gust, which would load up the rig making it want to nose-dive.

As I said, there are scaling problems with the anecdote but the main problem I see with T-foils on multihulls in general is making sure you can still lift a hull, probably only a problem in marginal conditions.

Gero,
Interesting that you should mention lifting hulls.
Two things to note:
I believe John P has mentioned on the forum that the T foils dampened the affect of gusts lifting hulls upwind.
So in gusty conditions they made the boat more stable giving greater acceleration from the gusts as well as the additional affects of stabilizing the pitching motion allowing the sails to generate more power.

Someone using Darryl's T foils was telling me at a regatta that he was lifting the windward hull downwind and then moving a little further aft and resting the windward hull back down on the windward foil with the hull remaining clear of the water.
This allowed a slight bias in the downward loading of the hull making the boat more stable while still flying the windward hull. So he could keep the hull up in marginal conditions reducing the wetted surface drag as well as being able to drive the boat better with the added stability.

I think we still need to do a bit more experimenting, even so I expect T foils will find a permanent place in all progressive catamaran classes in time.

Now the next step could be independent adjustment of foils to help lift the windward hull in light conditions.
Interesting thought that would be fun to play with but unlikely it could be made simple enough to not cost more time through poor adjustment than gained.

But you never know until you give it a go.

Regards,
Phill

Lifting a hull to windward is exactly the same with or without T foils. The only times that T foils produce ANY lift either up or down is when the hull pitches forward or back, therefore due to the forward motion of the hull, the lift generated by the foils is mostly directionally forward, and not simply up or down therefore there is virtually no extra loads on the transom, if the hull is level with the water surface then the foils are lift neutral and lifting the rudder and the hull offers only minute (not even noticeable) upward resistance
We have found that the great amount of pitch dampening by the foils lets the sails work almost to their optimum all the time instead of darting forward and then back at which time almost stalling in light choppy conditions half of the time. Since we put T foils on the Alpha F14 the increase in overall performance in all conditions (and particularly in the light) has been “exhilarating”. We sailed a point to point race in the Southern ocean last weekend in very light conditions against 20 odd cats all of which were larger than the F14 (F18’s, F16’s, 5.8 NACRA’s etc) and for a large part during the middle of the race two Alpha F14’s were leading all of them (actual). At the finish the first cat across the line was a Hobie tiger followed closely by an Alpha F14. On yardstick Alpha F14’s took 1st and 2nd by some minutes. Before the fitting of the foils that just would not have happened. Since fitting the T foils to two cats neither has ever “pitch poled” regardless of the conditions. Before fitting both had regularly pitch poled in heavy downwind legs under spinnaker.

QUOTE; The plain old conventional Hobie 14 castings used on the hydrofoiler rudders in my thumbnail pic worked pretty well and never broke. Neither did the rudders. They are on my boat, the only hydrofoiling Hobie 16 in the world. 8^)

VERY INTERESTING,how much wind does it take to lift the cat above the water surface, and does it have wand type variable foil adjustment to it's lifting foils (like the moth or to a lesser degree the Tri foiler)??

Agreed. And I guess cats would use L foils instead of T foils. They are more efficient. But, as you said, we will only know if we try.
Cheers,

Gero

We felt in testing that in gusty conditions the T-foil equipped boat was less 'flighty', in other words it was less prone to violent hull lifting in gusts, this is dificult to measure and was more a feeling, we put it down to the following effect.

When hit by a gust the non t-foil boat would accelorate at the same time as the hull lifted, this extra drive produces a small dip as the bows go down under acceleration.

On the T-foil equipped boat the bows don't dip because the T-foils hold the boat steady, but of course to do this they apply downforce , also as the rudder lifts it is no longer travelling horizontally but is sloping up, these 2 effects produce downforce on the rudder which on the windward one equates to an extra force resisting hull lift.

The net result of this is that it is just as easy to fly a hull and the same wind strength is required to do it either way, it is just that it dampens the effect of gusts, and lulls(because the reverse happens then).

Darryl, I keep waiting to hear you are starting to get strong sales..
What would you charge for a set of the rudders and stocks?

The rudders normally come up after and go down before the boards so they are of course more susceptible but with the kick up function that is not an issue. For example I pull the boards up and surf in until I’m in about knee deep water. If I misjudge the tide then once in a blue moon I hit a rock and a rudder kicks up. No big deal. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" /> Maybe better to build a cassette with a simple kick-up function (not unlike top cat) as insurance against ripping the stocks off on the reef? <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />

Bob,

Darryl tested his T-foil rudders as plain kick-up rudders. Meaning you use the exact same stock as you are using now but not with rudderboards with the little foils attached to the their ends.

He reported that there is no issue hitting the bottom, the T-foil rudders will kick up just the same as the normal rudders do. The T-foils won't break as they need to be strong enough to withstand the normal operating forces anyway. and maybe apart from some extra drag that slows the boat down nothing else happens. This may all not be a bad thing when landing.

The only issue left is leaving the beach with the rudders kicked up. I don't fancy working my way through the surf like that, but experimentation might proof otherwise. Afterall the T-foil rudders aren't that large in area. Also if you don't have to launch through the surf but can walk the boat in to sufficiently deep water and put the rudders down then there is no issue left at all. Then you can just have T-foils added to your current rudderboard design and use all the old kick-up hardware.

Again Darryl sailed the F14's like that for a long time now and everything is still fine. I think they even didn't modify the rudder pintle setup from what the quick snap on system they had on there. Apparently the forces generated on these part are relatively small and it is the large leverage they have there in the back of the boat that does the major part of the work.

I have no experimentation money left otherwise I would order a test rudderboard like that and just fit is to my current ruddersystem and find out what really is a problem and what isn't. The Taipan is one of the hull designs that could really benefit from the T-foils.

I was very impressed by them when I sailed the Stealth a few times.

Ohh, and there is another principle that is of interest here. John Alani expressed a while back how he approaches a landing site with the cassette rudderstocks.

When he approaches shallow water he pulls out the retaining pin, now the rudderboard can slide up and down through the cassette depending on the vertical forces produced by the T-foils. In effect he can lower the rudderboards by moving his weight forward and putting the hull in a small bow down angle. And he can raise his rudderboards by moving his weight back and putting the hull in a bow up angle. He uses this phenomenon to lower and raise his boards when leaving the beach or landing. Just before the rudders are about to hit bottom he apparently moves his weight back and thus have them lift themselfs clear. This way he covers the last stretch of water and then steps off.

This phenomenon may even be an advantage of the cassette tape rudderstocks, because you can always get as much rudderboard in the water as you need/can without having the large tiller loads or having to use your hands or go to the back of the boat ! Your hands are now free to hold the sheet and tiller. Once through the surf you can head up into the wind and fit the retaining pins and go sailing.

I think there is some excellent development to be had with the T-foil rudders and I truly believe that we will see them fitted to beach cats more and more. Especially in the range where hull length are relatively short. Personally I'm planning on using them on the F12 design and I would love to have them on my F16.

Wouter

There is the old adage for any design, K.I.S. (keep it simple) and there is a lot to be said for the absolute simplicity of the cassette system. No cams, no pivoting points, virtually no wear points, never any necessity for fore and aft adjustment for tuning, etc, and their sheer simplicity overcomes a lot of potential (and real) problems that are inherent in all other rudder systems. I was initially doubtful about using the cassette system, considering it “old fashioned” but after extensive testing and use now, I actually prefer them to a “kick up” system

Ditto! There's something to be said for set and forget, and the KISS principle.