Darryl,

questioning the "why" of things is just human nature so please bear over with us. When Descartes said "I think, therefore I am", the folloup was an immediate "Why?" :-)

OR "say what?"

Well, it is much better than "who cares"..

Wasn't Descartes "beheaded"????

What thuh?

Do the foils affect leeway at all? I'm trying to visualize how they work when the hulls are angled with respect to the water surface (when you're flying a hull, for instance).

How do they work uphill/downhill in medium/large ocean swell (1-3 meter, 5 second period type swell) with wind chop?

I don't have an engineering degree and I didn't stay at a Holiday Inn last night, so a lot of this discussion is over my head right now...

Thank you Darryl for sharing your ideas and results!
Your posts here are most interesting and stimulating and contribute to making this forum worthwhile.

But I think most of us agree that trying to understand why something works is a prerequisite for developing technology further.

The most significant mechanisms of operation have been presented already by Darryl and others:
- Reducing pitching, thereby preventing airflow over the sails from becoming detached.
- Keeping the bows from going down downwind, allowing driving the boat harder.
- Reducing tip-vortex drag - if one gets the wings right.

However, I would like to add one factor that may contribute positively in some conditions:
The horizontal wing may act as a paddle as the boat pitches in small waves, hence the wings MAY contribute forward motion. This effect will be stronger if the wings are allowed to move some degress up and down (as I14-wings do?)

Darrryl: What kind of rudder fittings do you use?
Can you show us more pictures?

Stein

Look at the top

That's what happens for being too cleaver.. Beheaded, boiled in oil, have to drink hemlock.. It goes fast.. I feel he is a necromancer of the dark arts and he must be an evil doer!

The rudder fittings are "the most simply that are practical" I.E the top and bottom fittings on the transom are stainless pins set in an aluminium casting (standard, off the shelf fittings) and the rest is a top and a bottom hole drilled through the carbon fibre/ Kevlar (50/50) that the stocks are made from with a moulded shape allowing for those holes. We like to simplify EVERYTHING as much as practical so that the greatest percentage of our boats can be built "in house". Saves a lot of time and expense. It's also nice not having to rely on multitudes of "outside" suppliers.
The T foils seem to have no noticeable effect other than those already described, the windward and leeward performance as regards pointing/leeward drift, appear to be the same sailing with T foiled rudders or without, the foils work the same with the cat heeled or not, and lifting one rudder completely out of the water has no different “feel” or movements than the same without foils, over “long swells” the foils tend to keep the decks parallel to the “average” water surface therefore traversing slow, long changes to the surface (as in long swells) the hulls will follow parallel to those change, but through “chop” the decks stay level and instead of the bow going up and down, the whole hull rises and falls -level- (but with much less movement overal than with the "pitching" motion of a cat without foils) also without the (nodding dog) rocking.

Scooby,

Thanks for the picture.

Do the foils make the overall beam wider, i.e., extend past the widest part of the hull?

Not in the Stealth F16, but they look like they might on the F14.

Many thanks for sharing first hand information about foils.

Some comments:

- Rudder foils may ride in the back wave. The effect is simmilar to having a longer waterline.

- Your foils seem to be symetric in section and horizontally placed (zero angle of incidence). Still, they generate upwards lift most of the time. Explanation: in order to form the back wave, flow in the stern is upwards. From the foil's point of view, this means a positive angle of attack. As a result, whenever the boat is horizontal or with a bow up attitude, your foils lift the boat from the water, reducing wetted surface.

- The concept of curved corner L shaped foils is to increase effective span. I don't know why and how. Learned that from articles about Cogito and Hobie's foiler. This shape deserves a try in the F14. Please!

I hope to be able to add information to this thread when my boat is launched. My rudder's foils are different, though. See the picture attached.

Cheers,

Now THAT I find really interesting (even exciting), LUIZ (the foils in the photo). There are several things that you have done with those foils that I would like to know the effects of under actual sailing conditions, I would like to know from you just how well they work with the position of the foils so far from the tip, whether there is interaction/interference between the “wake” of the transom and the foils, whether or not their proximity to the surface effects their performance, and by using a “delta” shape for the actual foils, are they more effective at reducing “tip vortex” from the foils. There are several other lesser questions but as the foils that you have decided on are similar to ones that we considered but decided finally not to go with (mainly for manufacturing reasons), and as a consequence we did not conduct any testing on, I am more than curious as to the outcome. THEY SHOULD WORK (and work well). There are several things about your set up that I find very attractive and which I like very much, and I look forward with not a little anticipation to hearing of your “test” out come.
GOOD LUCK from down under mate, great to see an individual willing to put their ideas to the “ACID” test
P.S. They are symetrical foils?

Luiz,

can you send me the articles on Cogito and trifoiler that you have. I'll really appreciate that.

wouterhijink(at)hotmail.com

Thanks

Wouter

Let me agree with Luiz. You need to review the theory on this.

Wouter,

I read both articles in old editions of Multihulls Magazine. They are not available online or in digital format. I'll try to locate them to scan and send to you. Allow me some time - I think they aren't at home, but in the weekend house.

Note: I do not subscribe or recommend multihulls magazine anymore.

Darryl,

After five years of construction, ACID is a rather appropriate word… for my stomach! I knew nothing about foils before purchasing the boat's project and learned what I could during construction. It is still going on, so there’s time to learn more. Hopefully not much (I hope).

All remarks are based on what the designer said, not on research or first hand experience. In other words, I am just forwarding information and elaborating on it.

My boat is designed to reach maximum speed with the central hull just skimming the surface. It is not intended to fly two hulls - which is good for cruising.

The rudder foils are asymmetrical. They are placed close to the surface so that lift is maximized by the back wave flow and goes to zero suddenly, when the central hull leaves the water. Note that there is a “step” in the lift function: from very high lift to zero lift. In comparison, your rudder tip foil generates positive lift most of the time and becomes slower - eventually negative - as the bow points down. But the lift function is continuous, without steps, meaning that they work quite differently from mine.

Analyzing the differences:
- High asymmetric rudder foils are meant to maximize lift until the central hull is raised from the water, when their lift contribution becomes unnecessary. They never generate negative lift. The lift x angle of attack function is discontinuous. They are draggy in light airs.
- Your foils provide a smaller positive lift most of the time that gradually reduces and becomes negative as the pitch attitude requires more and more correction. There is a lot less drag in light air, but extra wetted surface in heavy weather conditions due to the negative lift sinking the stern. This means more drag AND more wetted surface. In other words, it slows down the boat to keep it from pitchpoling. Sacrificing speed for stability is a good trade whenever fore and aft stability is what limits speed. For this reason, it is reasonable to say that your type of foils is a great addition for any boat with more transversal stability than fore and aft stability.

I ignore the rationale behind the choice of each foil shape used in my boat. There are four different formats:

- Symmetric section rudder blade with elliptical tip
- Asymmetric section Bruce foils with elliptical tips.
- Asymmetric section delta shaped rudder's foils with straight tips.
- Swept back asymmetric section rear float foils with straight tips.

All asymmetric foils feature one flat surface and one curved surface. As far as I know, the choice of section was for constructive reasons, convex surfaces being difficult to build both for the plug and to laminate in the mould (for serial construction). I wonder if the tips of the smaller foils are the only straight ones for the same reason (constructive), or if it is because those are the foils that go in and out of the water to keep the boat with a bow up attitude.

More later. Or after tests <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />

Hi Darryl,
I'm curious, how did you determine the incidence you should set your foils at so that they are neutral when the boat is level? I've got a Hobie 14 (AKA, Pitchpole Central!!! {And the H-16 guys complain!!!}) that I'd like to add these to, but I'm not sure how to "Keep it level"!!! Have you got any suggestions?
Thanks, Ryan

My guess: he just positioned the foil parallel to the waterline - but this is not zero angle of incidence because the flow is tilted upwards. If you want zero incidence and the rudder is shallow, place the foil parallel to the nearby hull bottom. If the rudder is VERY deep, horizontal is fine. If it is average, place the foils between horizontal and parallel to hull bottom.

Related info: my rudder foils are positioned with the flat face parallel to the waterline. This represents about 1 or 2 deg incidence,if we consider "zero" to be the zero lift flow direction. However, the flow there runs upwards. Assuming the flow being paralel to the hull's bottom, I must add another 11 to 13 degrees, totalling 12 to 15 degrees incidence, which seems VERY HIGH.

The lift must be significant. The price is paid in drag. I wonder if this incidence isn't too much - and plan to try a plain blade as well, just to compare performances.