I think you are right: asymetric foils seem to be more draggy and less efficient in light air.

The Open 60 tris' solution is to use asymetric foils in heavy air and replace by the symetric daggerboard in the central hull when conditions are unfavorable for the asymetric ones in the floats. I am not sure if they are ever used in combination. I guess some foils may interfere with others (?).

The light air solution in the Catris is called "engine" It was designed to be the first foiled cabin cruiser series in the World... a speed oriented cruiser, but still a cruiser. It doesn't seem to be so slow in light air, though. According to race reports in San Francisco, it beats most other small tris in light air. It is slower then the faster stripped out racing competitors in light/moderate air. But then, I wasn't at the helm...

For an A class, I think the ideal would be something similar to the Moth class: asymetrial foils in heavy air replaced by symetric foils in light air. I don't know if it is legal and have no insight as to what motivates the class rules/evolution.

One detail: the Catri stabilization system is also patented - it consists of automatic boat pitch control to adjust angle of attack using rear "wings" and NO moving parts at all. The A class would never pay royalties.

By the way, does anyone know how they keep the foiled Moths stabilized at flight? Stability and seaworthiness in heavy/moderate seas are critical aspects for foilers.

Cheers,

Hi Luiz,
I recently watched the Australian Moth nationals. The two foilers present used foils constructed by Fastacraft (www.fastacraft.com) in Western Aus. They had bow mounted sensor arms, ala rave, operating trailing edge flaps on the main T foil via a specta loop. The Rudder T, had a flap controlled by twisting the tiller arm, as used on the I14s. This takes care of pitch angle, and allows the skipper to have control of takeoff and gybing. Lateral stability requires the same hiking and mainsheet/steering activity of non-foiler moths and dingies, though the foilers are actually a bit easier to keep upright.
The series was won by Rohan Veal on his foiler, who literally ran away and hid from the non-foiling competition upwind and down. He said it was about the same speed as a Tiapan 4.9 upwind and faster down which I can believe because he smoked past my little 14ft Alpha Omega Cat. Incidently, for those who have'nt seen them, the modern Skiff moths are only 11ft long, 13 inchs beam with 86 sqft of sail. In displacement mode, they still sail faster than many 16ft cats despite their waterline disadvantage. All skiff moths have T foil rudders which allow them to exceed their pitch pole induced hull speed limit, and also trim the hull to gain dynamic lift at speed.

Cheers

Simon

Cast my vote for the "need to lighten up" commitee...

This is a forum not the US Supreme Court!

"Forums like this"

What do you think happens on beaches and in bars?

MacDonalds were # 1, not because they sold "health giving, nutritional food", but because their ads portray a "kindler and gentler society".

I think that Hobie makes good boats. No one doubts that, but to say that Hobie makes no errors is a bit silly.

If you can't take the pressure of working for a large company that sells boats to a wide variety of people and it is therefore open to a wide variety of discussion...

I find it a bit unnerving that you have a team of lawyers. I remember years ago a discussion with some other pilots in which one company was described as reacting to lawsuits with a team of lawyers, while another reacted to lawsuits with a team of engineers. Company # 2 became company # 1 because their products improved.

Thanks!

None of us would like to trail a Moth around the cans in a Taipan...
If foiled Moths are sailing as fast as first class multihulls, then it shouldn't take long for some development multihull classes to accept foils.

Cheers,

bear in mind though that the moth class is almost "anything goes" as far as class rules IIRC.

What's so bad about the Moth rules?

If "almost anything goes" enables a singlehanded monohull to outsail a Taipan 4.9, then I'd like to see similar rules in at least one multihull class.

Alan,

Not even sure you were refering to one of my posts, but you mentioned Hobie Cat... so...

No, we don't have a team of lawyers on staff.

My post is out of context as the post that IT was refering to was changed / edited a number of times after I posted.

I didn't claim anywhere that Hobie Cat doesn't make mistakes, so I'm lost on that one and the bit about pressure of the job.

"All skiff moths have T foil rudders which allow them to exceed their pitch pole induced hull speed limit, and also trim the hull to gain dynamic lift at speed"
ok, so why not have T foil rudders on cats when we reach ou pitch pole induced hull speed limit? Has this been tried? Would this not add a margin of safety and/or allow for even bigger spinnakers downwind?
Al

Cogito had a set of "L" shaped foils but it seems that they didn't have time to test the configuration enough to use in the series.
Yves Parlier's new 60 ft twin rig planing cat also features rudder foils. According to sailinganarchy.com, the boat topped 38 kt during the tests and averaged 30 kts in 8 hours. Unfortunately the article doesn't tell fi there were waves or how big they were.
Rudder foils seem to be a good start for any class.

Eh Al

Stealth marine uses em -

http://www.stealthmarine.co.uk/

Carl

Very interesting! That endplate or horizontal stabilizer on the end of the rudder reminds me similar devices used on aircraft.

Wing tip (whatever you want to call them, tipsails, endplates, etc) curved up and down stop the flow of high pressure air under the wing, migrating to the top of the wing. This prevents wing tip vortices and makes control surfaces(ailerons) much more effective. This leads to a more efficient wing. Somewhere in the vicinity of 5 to 7% less Induced drag.

Induced drag is drag that is created by creating lift.

All this to explain that by putting the foil on the bottom of the rudder you are making the rudder more efficient, so it could conceivably be smaller and thus lighter and less draggy.

However the tip foil might create structural issues in that the rudder would have to be kept strong so it would not distort when under load. (probably not an issue)

The other thing is that the added rudder tips would probably be susceptible to damage and dragging weeds.

This issue could be pacified by placing the foils up from the bottom of the rudder say a few inches and making the foils sacrificial, in other words if you hit a rock they might break off and be replaceable as a cheap part or a do it yourself, carve one out and screw it on type of repair. Also by sweeping the foils back more they would hook less.

Makes me wonder if the same endplate could be used on the daggerboard, so it wouldn't have to be so long....

Easy to find out...Has anyone tried an endplate on the daggerboard?

Hello Frozen,
The commercial aircraft you have seen the wing tip plates on have swept back wings, leading edges. This promotes spanwise flow toward the wingtip and hence fences and tip plates help reduce the losses. It all started from the swept back leading edge. Sweep the wing forward slightly, 5 degrees or so, and the spanwise flow is gone. Also the foil will not ventilate, pump air down the leading edge, at high speeds. This is the best solution to maximise the span effectiveness of an airfoil. Check out a high performance glider with slightly swept forward wings.
Bill

Bill

Interesting reply! Boats and airplanes are very similar.

One burning question I have is... "How would you compare the characteristics of air, as opposed to water in regards to how it acts on a (air or water) foil? (I know that's a huge question)

Also do beachcat rudder, centreboards, cavitate, like skegs on windsurfers?

The aircraft that I am familiar with that has swept wings was a German made jet called the "Hansa Jet". I've only seen one in person and it was a very nice looking aircraft.

Apparantly the reason why sweeping forward the wings never caught on has to do with the structural difficulties associated with the spar.

Although having said that, the spar being more aft gave the aircraft an advantage, in that passengers did not have to climb over it.

Lots of straight winged aicraft including our Beachcraft KingAir 350...

http://www.raytheonaircraft.com/beechcraft/king_air_350/photo_gallery.htm

have Tipsails or Winglets. They make a huge difference in the aircraft handling characteristics. The ailerons are considerably more solid, partly due to the lack of spanwise flow and partly due to the rudderlike tipsails.

We also put on the "Tail Strakes" kit which makes the airplane Dutch Roll in turbulence.

Gliders have such fascinating designs. Quite often the Radio controlled gliders come out with great and sometimes crazy ideas designwise.

Motor boat engines have used stabilizers for years for the same reason that "Stealth" uses them.

Interesting stuff!

Hi Alan,
Air and water at low speeds behave very similarily. The same equations and laws can be used. As far as we are concerned the air flowing across our sails is incompressible.
I doubt that sailboard skegs cavitate. It takes horsepower to make a propellor in water cavitate. I think they probably ventilate, pump air down the leading edge, because of a swept back leading edge and the foil becomes ineffective when this happens. The aerodynamics that are most closely applied to sailboats are those from other slow moving ships of similiar size like gliders. The highest performance gliders in the world are all carbon with slightly swept forward wings, foils, and they are built in Germany.
Bill

When foils are added to the rudder, an extra vertical load stresses the gudgeons. Those are usually loaded mostly in the horizontal plane. The new load is equal to the rudder foils lift, I guess about 1/3 of the boat weight plus a safety margin in a Rave - probably more in a cat. This is not an issue only if all is calculated beforehands.

If it is a big boat designed to use an outboard, the engine's weight can be used to REDUCE the maximum vertical load in the gudgeons.

The foils can also be raised to work closer to the boat's bottom. The flow there is moving slightly up, so even horizontal symetrical foils will generate lift, at the expense of the back wave height.

The only problem is that higher foils are easier to take out of the water, in which case their lift would disappear. I guess they can be placed there when the boat is not supposed to fly that specific hull or when taking the foil out of the water is desirable (ex: if the windward hull is flying and its rudder foil is better out of the water then generating drag).

Cheers,
Luiz

Interesting!

So to take it a bit further, would dihedral or anhedral be appropriate? Stealth seems to have a slight anhedral.

There are virtually no weeds where I sail, so I suspect it would be appropriate to have very little or no sweepback. Does that sound correct?

What about the aspect ratio, tip shape?

Since the foil would be used to keep the nose from digging in and flying up I assume it would be symmetrical.

I envision a foil about half way up the rudder, to prevent rock rash and reduce the rudder side bending moment arm. The foil shape (airfoil is the name in an aircraft) would be symmetrical.

I also think that a temporary mounting plate so that angle of attack could be investigated (adjusted) would be in order.

Any thoughts?

I think the answers depend a lot on your specific project.

The boat I am more familiar with (Catri) uses dihedral swept back asymetric rudder foils for lift only. They are not intended primarily for pitchpole prevention. The main foils are not swept back and the rear float foils are not dihedral. There's a picture of my boat attached. All depends on what the design requires from the foils.

It seems logical that rudder foils dedicated to pitchpole prevention should be symetrical and dihedral, but I am not sure if they must be swept back.

The geometry tells us that the angle of attack of symetrical rudder foils in a boat about to pitchpole will never exceed (roughtly): arctan [rudder foil depth / water line length].

At the design pitchpole speed this angle of attack may or may not suffice to avoid pitchpoling. Do the math for your project and you will have more elements to decide if the foil depth is enough or if it needs additional (negative) angle of attack.

Cheers,

Thanks Luiz for your patience.

Your picture answered my vague question. I was speaking about a rudder foil for pitchpole prevention. (as per the stealth rudder)

I haven't had the pleasure of a pitchpole, (H14) although on the first time out I came within a hair of one. The cold water temperatures here make the idea an uninviting one!

I had the time this afternoon to check out all the websites on the posts and now am much clearer on this fascinating subject. (took awhile on my dialup service)

I remember years ago a picture in a windsurfing book of a guy on foils in Hawiai.

I can throw in my 2 cents. I've been a Rave owner for a year and have crewed my friends Rave for a year (so total of two seasons foiling). The Rave has a few small flaws - but fast, responsive sailing does not appear to be one of them.

I wish I could try the Hobie Tri-Foiler for a day, just to compare the two - I have no first-hand comparative analysis to offer.

However, I have enjoyed some very fast hydrofoil sailing on the Rave. Controling height with "wands" coupled to the bungies sounds goofy, but it actually works really well - and is totally automatic. They allow the boat to ride over chop like a fighter pilot on "terrain-following autopilot", the boat actually bounces up and down a bit in response to waves. Chop can slow you down to get up on foil, but once foil borne, you're flying.

Controling the rear rudder-foil via joystick works very well too. Despite only having a narrow trailing "flap" it is surprisingly responsive. I think it is an excellent system overall, for controlling a foil-borne boat.

One negative issue for the boat is lobster traps here in Maine. I like to call them "foil-mines". A hydrofoil craft is at risk of snagging these. At 20kts it's no laughing matter - my friend has done damage to his boat catching one by accident.

Pitchpoles happen too, usually at slow speeds with a loaded reacher. I'm not sure a rear foil would prevent a pitch pole - it certainly is no aid at slow speeds.

I have enjoyed watching this thread though and seeing links to other foil boats. I've always been facinated with foil boats (why I bought the Rave). Maybe foil-borne craft, in general, need a league of their own for racing? Hobies VS the Rave VS ?? - SHOW-DOWN! Bring on your foil craft.

We can settle this in the water.

>The light air solution in the Catris is called "engine" It was designed to be the first foiled cabin cruiser series in the World... a speed oriented cruiser, but still a cruiser>.

This post does not mention Dave Keiper's Williwaw, a foiled cabin cruiser that used surface-piercing foils on a trip from California to Hawaii to New Zealand... and back....successfully. Pictures are available for viewing on International Hydrofoil Society archives.