A Ford "T" Foiler?

Sorry to start yet another foiler thread, but I am looking for a sound design approach for a reasonably safe and easy-to-fly foiler for recreational use on small lakes, something that my grandson's grandfather could use for example. Definitely not performance oriented like most of the other threads, but something that may attract additional people to the sport.

There's a lot of information on foiling theory and practice in the forum that I am trying to put together in my mind. I must admit I am getting a bit swamped in detail and don’t always see the wood for the trees first time. I think I have the basics; please tell me if any of the following assumptions are wildly (or dangerously) off.

The lowest practical speed required to achieve foiling is about 10k. To achieve foiling at that speed with fairly low sail power requires a long narrow hull with minimum rocker that can get to foiling speed without needing to plane, say 12 ft x 18". Relatively large foils around 1.5-2 sq ft immersed would be required for low speed foiling. The surface piercing 90 degree Vee type seem the safest because it will automatically adjust flying height and resist heeling. To simplify getting up to speed stability can be provided by small, long and narrow amas that are out of the water when the boat is level and can carry the main foils, which would be about 2/3 immersed at speed. I am considering a foil with a total span about 5 ft, 9" chord, 70% Clark Y profile for thickness 8% of chord. Foils to be mounted to provide about 1 ft clearance between the bottom and flat water and located somewhat ahead of CoG, leaving around 20% of total weight to be supported by a box-kite style ladder foil/rudder combination at the stern. Simple cat-rig sail about 50-75 sq ft area.

Of course, it may have already been done; maybe there’s a foiler version of the Mirror dinghy!

 

Just as a frame of reference( one major difference between these two besides the obvious-the Rave generates its own righting moment,the Moth does not):
Moth
LOA about 12.75',including rudder gantry
Hull 11'
SA 86 sq.ft
Beam incl racks about 7'2''
L/B at w/l-about 10-11/1
Sailing Weight,minus crew 66lb
ideal crew 154-165lb though a 190 pounder just got third at the Aussie Nats.
Nominal foil areas: main 1.08 sq.ft.
rudder .83 sq.ft
Much more info here: http://www.int-moth.org.uk/Worddocs/Foiler Sailing Guide1_Intro.pdf
------------------
Moths with minimum crew will take off in 6-7 knots wind
Ka sails once estimated that a Moth with a minimum crew and a 90 sq.ft. main could take off in 5 knots wind.
I firmly believe that a "peoples foiler" should be designed to take off in the lightest wind possible since so many venues in the US and elsewhere are 10 knots or less much of the time.
-----------------------
Rave multifoiler:
Loa 16'
SA 190(+ screecher)
Allup weight 368lb
Beam 12'cl ama to cl ama
main foil areas-2 @ 1.7sq.ft. each; 3.4 sq.ft. total
rudder foil 1.7 sq.ft.
This boat will take off in 8-9 knots wind with a minimum crew offwind.
A carbon version could cut weight by at least 50% and take off in lighter air.

 

You do not need to go very fast to fly.

I have actually "flown" with my V8 submerged buoyancy boat. It did this at about 6kts with tiny foils on each of the three outrigger. 90% of the boat was supported by submerged buoyancy.

If you have a large foil it will contribute significantly to buoyancy.

The main thing I learnt from this is that foils and submerged hulls need to be mounted quite deep to avoid making waves that sap energy. The recommended depth for a foil is at least three times the chord to avoid wave drag. So shorter foils do not need to be as deep. You can find equations for wave drag relative to depth.

The next thing is that higher aspect foils will have better L/D. There are diminishing returns but AR around 10 is typical.

I tried an 8ft wide foil on a pedal boat and found it impossible to get self stabilising roll control. My next attempt will be two main 'T' foils, each about 4ft wide set at 8ft centrelines.

There used to be some good videos of low speed hydrofoils on this link but I do not know if they are still alive:
http://poisson.me.dal.ca/site2/design_projs/projs.html#0506

If they are there you can see the waves from the foil for the 2004 boat. They went to ladder foils in 2005.

There are a number of disadvantages with foils that I feel work against their practical application. Think about how you are going to launch and board the boat. You need a weed free area to operate in other wise you will spend a lot of time removing weed. They need very strong mounting because one strut might end up supporting the whole boat. They will not glide up a beach when beaching - just a shuddering jolt. You need to have a light boat because extra weight will translate directly to extra drag - much more sensitive than a displacement craft.

Rick W

 

Doug: the Moth numbers suggest my low-speed foiler is achievable. I was afraid 16 ft length would be needed to reach a take-off speed of 10k in displacement mode assuming a critical Froude number of 0.8. If take-off at 6k is practical for my concept, per Rick’s post, then a 12 ft hull length should work (I want to keep it small for low cost and ease of transportation). I noted the Moth is only 11 ft. The Moth hull waterline beam is only 12" from other sources but I think a sit-in cockpit is better for mature sailors so I will plan for a 18" beam, BWL 2 or 3 inches less.

I noticed you provided minimum wind speed for take-off. Is the boat speed at take-off as low as 6k? Knowing the take-off boat speed would help me with hull design analysis.

Referring to the Moth’s gantry, foil adjustment may be needed in a prototype but not a final design with surface piercing foils. Other than that the Moth’s gantry other function is foil separation, not needed with a combination of Vee main foil and ladder aft foil.

Rick: I didn’t find the videos for the Halifoil, I would like to have viewed the waves. It’s foils look very large. I understand it was not entirely successful. It uses a control wand which I wish to avoid, hence my preference for Vee foils. I want to make longitudinal and lateral balance automatic as with the Hobie Trifoiler. Vee foils will be simpler to build than Tee or ladder foils. The structural demands of foils with an aspect ratio around 10 is a challenge for home-built materials and technology; a full width Vee foil helps but may be too deep for some lakes. I need to learn more; I am trying to learn enough hydrodynamic theory from Wikipedia and other net sources. It’s coming but I no longer pick up new stuff on the first read, sometime in the past half century I became intellectually underprivileged!

Your comments on the disadvantages of foils and launching/boarding are important and duly noted! If the hull is narrow for minimum drag it will need amas for displacement mode stability since I don’t see a “mature sailor” scrambling about on racks. The amas would be carried above water normally for minimum drag and I would like them to partially immerse for more primary stability as the sailor moves forward to access the rig. I am not so sure about long narrow amas now; short, wide planing amas may be safer.

Thank you both for your interest and inputs.

 

More Thoughts

Ideally I would like a boat that can be paddled home following failure of wind or crew courage so I prefer a collapsible sail rig with amas well forward especially if a double paddle is used. If a canoe style paddle is used it would take up less space. Retractable amas? Hmmm ... the foils at least should raise, remove or retract for beach launching and so my mature sailor can board in shallow water. Foil mounting can be handled when erecting the rig but accessing them way out on the amas could be difficult; more to think about. The need for a light boat is a given as I am aiming for car-top transportable.

I think a hull design is emerging and DelftShip should give me hull drag. My foil concepts are not yet workable but I am making some foils for another project and can do some lift-drag tests in my pool: I don’t trust my knowledge of hydrodynamic theory. Once I have foil and hull drag data I can determine sail area. At this time I think the sail design can be conventional and simple as this boat is not designed for out-and-out speed. This can be a big cost area and I want to minimize that.

I was speaking earlier of the Hobie Trifoiler: was it widely adopted? I don’t see many around. Perhaps there were cost, weight, handling or safety problems. From pictures it uses 2 waterskis plus a stern foil; looks safe but slow. Expensive too.

I have not been able to find much more information on the Rave multifoiler but it sounds similar to, if heavier than what I have in mind. The Moth has significantly less sail and foil area per unit weight than the Rave, but its take-off speed is about the same so it seems more efficient. That is presumably due to use of fully-immersed centerline foils, but the Rave foils resist HM, reducing hiking requirements on the sailor. The data gives me guidelines on foil dimensions, thanks again.

I haven’t committed to build a foiler yet although it would be a logical extension of several other projects I am working on. However, I think it would be great to popularize foiling and maybe the “People’s Foiler” concept will start something worthwhile.

 

AK, you can have retractable foils for beach launching with either t-foils or surface piercing foils-Hydropteres mainfoils retract and the t-foils on the RS600FF retract.
The distance between the mainfoils and the rudder foils is very important for pitch control and should be the maximum possible.
A good target is to have the mainfoils support 80% of the load and the rudder foil 20%.
Moth takeoff with a minimum crew is at around 7 knots boat speed.
-------------------
The Hobie trifoiler and Raves were attempts to make a popular foiler but because of weight and other issues both suffered with relatively poor light air performance. If you took two people on either boat takeoff was delayed to above 10-12 knots of wind. Both used fully submerged foils but the Hobie's altitude control system was two independent "feelers"(waterskis) directly connected to L shaped foils - the whole foil pivoted. The Raves altitude control system is two independent "wands" directly connected to trailing edge flaps.

 

If you make foils that enable low speed flight they will not be the best at higher speed.

For me on a pedal boat the break even between displacement and foiling is about 7kts. So there is no point trying to fly below this speed other than to say you can fly.

There is some interesting information on the Decavitiator site on how the foils evolved. I think you will be surprised at how small they ended up:
http://lancet.mit.edu/decavitator/History.html

This is the 2004 Dalhousie hydrofoil in action. You could estimate the speed based on length of pool and time:
http://poisson.me.dal.ca/~dp_04_5/images/testing/successful-hydrofoil-test-march-14-2005.wmv
The highest speed they measured was 8.3kph but I believe it is up around 3m/s at the best.

This is the 2006 model test:
http://poisson.me.dal.ca/~dp_05_6/testingvideo.htm

The 2006 boat did not fly with human power:
http://poisson.me.dal.ca/~dp_05_6/pooltesting.htm

There are good design reports on both projects.

You can do most of what you need to do with JavaFoil. If you keep the foils deep enough you do not have to worry about wave drag. However there is an optimum where strut drag becomes more significant than wave drag.

If you are intent on trying a very low speed then the size of the foil will contribute to buoyancy.

Rick W

 

Doug: retractable foil design compatible with other objects is part of the design challenge. Love that kind of stuff (the engineering virus never quits its victims). 7k is doable with 12 ft hull; must be, look at the Moth, but I have the numbers for hull drag computation now.

Rick: looks like the foil alphas needed optimizing for the '06 Dalhousie hydrofoil runs. Peddler was obviously straining so drive chain is suspect also. I think it should have flown. Foil sizes look more realistic here but low aspect ratios.

Low speed take-off is of more interest to me than max speed, at least at this point, but it would be nice to get up say double the take-off speed. Did you mean "size of the foil will contribute to buoyancy" literally or were you referring to lift? A foil large enough to significantly buoy up a boat sounds huge, about 12 ft span, drag permitting it might take off about 3k but that would be a bit dull even for me.

I will checkout Javafoil after I finish my Hydynamics education; mostly I'm hung up on understanding Reynolds number, can't find out how to compute it for a given foil/fluid/velocity combination - will Javafoil take care of that for me?

 

JavaFoil will give you lift and drag coefficiencts at various Re#.

Not hard to find formula for Reynolds number. I expect for the speeds you are considering and the foil size it will be 100,000 - 200,000. The L/D will not vary a huge amount in this range but I usually have it as a variable in any optimisation. You have to factor it in because reducing the chord will lower the Re# for the same speed but it still favours a high aspect foil.

A large slow speed foil like that used on the 2004 Halifoil will displace 5 - 10l of water. This is 5 - 10kg of displacement. If you think of going on a diet to lose this amount of weight to reduce the drag accordingly then it certainly is significant. I am aiming to make next boat 15kg total weight. I have been working hard to reduce body weight under 70kg. All this makes it easier to fly but I cannot count on much buoyancy from the foil.

A big foil is not much point for me because I want to be doing at least 7kts to fly.

Weight on a foiler is much more critical than a displacement boat so you end up getting very sensitive to what things weigh. Part of the reason for the growing development with foilers is the availability of carbon fibre and the stiffness to weight advantages it has.

My latest pedal frame weighs 1.2kg. It is stiffer than steel and about a third of the weight. Something like 4 times stiffer than aluminium of the same weight. Same factors give rise to efficient foils.

These days I take the view that if I am putting time into making something I may as well make it as best I can. It depends on how firm you are with the basis of design. Using the best materials is the quickest way to the best result.

Rick W