Ryan, the T foils for a cat that you want to use as a “pitch” control I.E to stop (or dampen) the bows from diving AND rising dramatically, needs to be of a symmetrical profile (Luis’s post goes into a range of effects of different profile foils which could confuse the issue if you all you want from them is for the cat to be more stable fore and aft). Personally I feel that cats that could benefit the greatest of any of those cats “out there” at present are the Hobie 14 and 16.
As a pitch control, you want to ascertain the “ideal attitude” that you would want the hulls to be sailing at if you were sailing on the proverbial dead flat, smooth surface, and the wind was absolutely constant. (No such conditions could ever exist but we can dream), then project a line parallel to the “flat surface” from bow to stern of the hulls. Transpose that line onto the rudders in their fully down, “locked” position. That line then becomes the centreline of the foils, as that is the angle to the water surface that ideally you want the hulls to be sailing at all of the time, regardless of any “outside forces”.
You want symmetrical foils because you want lift to be generated both up or down depending on which way that the hulls are “tending” to pitch, so that the lift acts as a self regulating equal and opposite countering force to that “pitch”
As the bows of a Hobie 14 in particular, are so low in buoyancy, I personally think that I would set the T foils for them with a couple of degrees of positive lift rather than neutral I.E. set them up as described above then before fixing them into their final position angle them front down just a touch (2 or 3 degrees which is very small)

Wouter,

In the Trifoiler article there is only half a paragraph about "L" foils:

What Makes Them Work So Well By Greg Ketterman
Multihulls Magazine May-June 1994

"We have continued to pursue our L-shaped foils for many reasons. They are smooth and simple, and they have no moving parts below the water. Wing tips and "T" connections generally cause turbulence and drag. With our L-shaped foils, the aspect ratio of the horizontal section effectively adds to the aspect raio of the vertical section, and we have only one wing tip that creates losses. Sure, there is more stress in these fois, but modern materials can handle the stress without any problem."

There's another interesting (but unrelated) segment:

"In our research we discovered something that is surprising. The amount of wind required to get onto the foils is not sensitiveto the size of the foils...(supressed)... This is surprising because with airplanes, if you have a larger wing then you can take off at a lower speed and less power is required. Sailboats are unusual vehicles as they have the ability to increase their power as they accelerate. The power of most vehicles is fixed and therefore the thrust decreases as the vehicle accelerates; however, with a sailboat the thrust, or driving force, remains nearly steady as the boat accelerates. Since the thrust is steady and the speed is increasing, the power generated increases, magically. Once this is understood, then it makes sense to make the foils relatively small to yield better high-end performance. The top seed and efficiency of the boat are somewhat sensitive to the size of the foils. Our foils are surprisingly small..."

There is not much about L foils in Cogito's article as well:

The Aussie's Little America's Cup Reign is Sunk by Christian Féurier
Multihulls Magazine Jan-Feb 1996

"As the Australian early summer had been abnormally breezy, th Americans tried new rudders, L-shaped inside, in early January. Probably to try to stabilize hobby horsing in choppy seas, or reduce pitchpoling risks. They woked perfectly, but the cost was certainly an increased drag and they never used them."

PHOTO of Cogito flying a hull to show the L rudder with the following legend:

"The L-shaped rudders workd perfectly in choppy seas to stabilize the boat, but were not used in the races."

These were my sources. Summaries of real life experience. Your comments about L foils effective aspect ratio are very wellcome. Another view of my rudder's foils is attached.

All the best,

Agreed with everything, but you certainly mean "negative lift" (downwards).

2 or 3 degrees is not so small angle of attack. Symetrical Daggerbords work at about this angle of attack.

Rgrds,

Right Luiz when you are looking at the DIRECTION that the HULL transom is moving I.E transom down (negative DIRECTION) – bow up (positive DIRECTION), and visa versa, but - the usable productive lift generated from a symmetrical foil is always positive, (generated on the side that you want the "usable” lift) – it is generated through the inclination of the horizontal motion of the hull through the water and "operates in a direction", either, up or down (positive lift UPWARDS or positive lift DOWNWARDS
In the case of the Hobie 14', I feel that there should be a "static" amount of downward lift generated at the transom when sailing at the, so called, "level" state which will incline the bows slightly up simply to compensate for the lack of bouncy in the bows.
(Basically what is a little confusing is just a case of semantics)
When I said “a small amount” (2 to 3 degrees), it is because, with symmetrical T foils set at or near the bottom of the rudders, there is a certain amount of latitude for the actual angle(s) either side of the “level” which is quite “forgiving”. This is because within a few degrees either up or down of the actual “neutral” setting of the foils, the “real” lift/drag factor becomes progressively very small. (that is untill we look at very high velocities where the percentages are the same but their "effects" become "critical")
(We could make these descriptions REALLY confusing, by including, “pressure variables” into the mix, (I think that we would lose a great percentage of the readers if we did)

Luiz do you have rudders on the transom of the ama's as well as on the main hull, or do you only have a rudder on the central hull?

The theory of the curved “L” foils is very sound (and apparently also in practice as well); the primary problem with them is one of manufacture/construction. There are a LOT of side’s way leverage loads on the vertical part of the foil even under quite small sailing speeds and this alone creates difficulties in constructing a rigid/strong rudder, particularly one that has a high aspect ratio. The “T” foils on the other hand, have only minor sides way, leverage load differences acting on the vertical foil (they tend to balance each other out)
The main reason that I could see for using “L” foiled rudders would be where there is a problem with the “maximum overall beam” of a cat where the T foil would be wider than the allowable beam.
This in itself doesn’t have to be a problem if the T foils are “raked” aft to get the desired surface area and still not extend any wider than the outside of the hull.
By extending the foils back there is also a “hidden” advantage that, as the foils move up and down they also generate a small forward “thrust” to the boat.

The new light-wind weapon, jumping up and down on the bow.. Working the T-foil like a trampofoil. At last we can get to use some kinetics in cats also. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
http://ourworld.compuserve.com/homepages/roberthodgen/tramp.htm

With reference to angles of attack:-

On the Stealth, the rudders are held down by a pin through the top of the stock. Without the pin in the rudder moves up and down with changes in speed. As you sail gently in to/out from a beach you can watch the rudders rise and fall slowly. At quite fast speeds (IMO around 10 knots) the rudders are around threequarters down on my boat. So the loadings can not be particularly high. If you sit forward in this situation, as you'd expect, the rudders sink. When you move aft to put the pins in the rudders rise.

This is a useful trait when landing at a beach because all you need to do is unpin the rudders and then as you are about to land, move right aft and luff at the last moment. With these vertical stocks you retain steerage with even a small amount of rudder in the water at these lower speeds.

Interestingly, at Eastbourne a while back, the locking pin came out of one of my rudders on the downwind blast. This provided huge entertainment for Paul Warren right behind me as my port rudder launched itself out through the bottom of the stock! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
I, though, was not amused! <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />

Both "T" and "L" foils were tried on aircraft. Except for special applications "T" foils were dropped.

Every country that builds transport aircraft, that I know of, controls this information. Try looking up ITAR and Export Control.

Only one rudder on the central hull. It is designed to achieve max speed with the mainhull skiming the surface, so float rudders aren't necessary. It IS a cruising boat, with head, nav station, berths, etc.

Darryl,

Just to clarify:

The goat is to keep the boat from buriyng the bow, so the rudder's foils must generate downwards lift, to bury the stern (raising the bow).

In order to generate lift downwards the foil's leading edge must be positioned lower than its trailing edge.

That's about all one needs to know. The rest are nuances.

Is the situation the same for foils in the middle of the rudder, like mine? (lets call them "cross" foils).

On an aircraft "foils in the middle of the rudder" are called fences and used to control spanwise flow.

"T" foils on planes were not dropped, check out A320 family wingtips

Please forgive my ignorance, but what about the auto kick up feature? Is the a safety system on this set up? you just never know what you might encounter out there on the water.

Hey Luiz, NO boat is a cruising boat, if another one appears within 100 metres of it!! The race is on!!
In relation to the foils on a Hobie, I thought that was what I said - the front (leading edge) of the T foil lower (Pointing down) than the trailing edge, positive lift downwards at the transom??
Both T foils and L foils have their place on the wings of aircraft, If the foil is included as part of the wing in the design then a high aspect "curved" tip ending in an L foil is highly efficient (and desirable). When an existing "standard" straight wing ending in a "squared" off tip has tiplets added, T foils are more effective in “reducing “tip vortex than a single upward pointing foil with a "hard" angle at the transition between tiplet and wing. (Although just one “hard” angled tiplet is still more efficient than no tiplet at all). As I said earlier, the main detraction for an L foil on a cat is the manufacturing “difficulties” to compensate for it’s greater loads, not its undeniable efficiency as a continuation and integral part of the rudder.

I think there are plenty of T-foils on the tails of planes.

--Glenn

THey do not kick up ! You just stop PDQ !!!!!

"T foils on the tail of planes" are an entirely different subject for totally different reasons compared to T foils or L foils on the wings of aircraft.
Although as I think about it, maybe we should be relating the effects of T foils on rudders more to the “T” foils (elevators) on the tail of aircraft as their effects on the plane are much more closely related to the effects of T foils on a boat than any boat rudder foils are related to any foil(s) on the wings.
The elevators (T foils on the tail) control, primarily the forward angle of the aircraft, both nose up or nose down (and every angle in between) and apart from them being “articulated”, their “effect” works in exactly the same way that the T foils on the rudders of a cat work. The L or T foils on the wing tips of aircraft have only the one primary function of improving the efficiency of the wing mainly through reducing the tip vortex effect dramatically, apart from that it has no other “direct” effect on the stability, direction, or trim of the aircraft.
Has anyone seen L foils on the tail section of an aircraft??

Called a phugoid, test of aircraft stability in longitudinal axis, I have carried out this test on various aircraft. Similar to what you are seeing with T foils I guess.


http://en.wikipedia.org/wiki/Phugoid

When skipper-induced oscillation occurs on our boat it is of particular nuisance to my crew! <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />