Foil Boats, Which Way From Here?

[Bill Roberts]

OK, We have talked about the AC72 boats and how they controlled their foils. For beach cats that system requires a skipper on the wire doing his normal things plus trimming the foils on the rudder. The crew is out on the wire doing his norman tasks of trimming the hull out fore and aft, trimming the jib, calling the puffs and watching the traffic. Now somebody has got to actively trim the main foils. These foils require constant attention because boat speed is constantly changing. I guess we have just added another person to the boat to sail it properly, a main foil trimmer. So if we go with a version of the AC72 system we are talking about a three person boat and probably a boat in the 25 to 30ft length range.
So, this all doesn't sound so good to the average beach cat sailor. Is there another way? Yes there is. Dr Sam Bradfield has been developing hydrofoils for the past 20 or 30 years. He has built several foil boats that get up and fly and the main foils are automatic, self trimming. His best foil system according to him is the T foil. His T foil has a trailing edge eleron on it that is variable and this varies the lift just like an airplane wing. The eleron is controlled by a wand that drags in the water and measures the speed of the water, the drag force of the water, passing the hull. This force travels up the wand and creats a torque at the top of the CB. This torque is apposed by a spring and levers and finally a linkage travels down the inside of the daggerboard and this ends up being connected to a small external bell crank which moves the eleron. This system is totally self trimming. Dr Sam Bradfield passed away last year. His efforts are being continued by Mike McGarry who lives here in Florida. Mike worked with Dr Bradfield for several years and is very knowledgeable on the system. Mike showed up at the Buzzelli Race with the beautiful foiling trimaran. I'm sure he would have showed us all the way home in the distance race if the wind had been slightly stronger. The foiling Moth uses the Dr Bradfield's foil and control system. Moths have been flying for a few years with this system. It works and it works well.

[havliii]

here's one solution!
http://vimeo.com/83829320

[J Drew]

Bill,
So could an optimistic person assume there might be a design like this available for a SC20 in the future?
John
Refering to Dr. Bradfield's design

[Bill Roberts]

The answer is NO! If you want a foiling system check with Mike McGarry and he can build the parts for you.
One important question to answer for yourself is, do you live/sail in an area where the wind blows 15knots and up on a regular basis?

[J Drew]

I might have entertained the thought for a while, but I don't think I'll change the fine boat I have. Of course it might be nice if it was about 7' bigger

[Bill Roberts]

Some foil things not understood:
1. The size of the foil is determined by the weight of the boat plus sailors and the selected "lift out speed".
Bigger foils lift out sooner, at lower speeds.
2. Foil size determines top speed for a given boat design. The larger the foils, the slower to top foiling speed.
So, everything we are seeing in pictures/videos etc are "all carbon light weight boats with small people sailing them".
To think that one could take an old 450 pound boat with two 200 pound big guys on it and put some foils under it
and go flying, that just insn't going to happen. The foil size to lift 850 pounds is so large that the boat with normal sail
plan and max righting moment can't get to lift out speed while dragging the large foils through the water. For foiling,
skipper and crew weight and boat weight are critical and lighter is better and smaller foils are faster as long as the
boat can get up on the smaller foils.

Here's one, how about using a power boat to get a foiling sailboat with small foils up to lift out speed
and foiling and then this foiling sailboat will have a higher top speed as long as it doesn't fall off the foils.
Is this any way to race??? There needs to be a FOILING SAILBOAT Racing Association. At this point in time the
foiling boats are out of control because there are no foiling boat racing rules.
Foiling boats need to have their own classes based on boat weight, sail area, overall length and width, etc .
Maybe some box rule scheme with critical demonsions spelled out for each class would work well for foiling boats.
Foiling boats should have their own shaped race courses to balance out the upwind and downwind point of sailing times
and distances etc.

[Bill Roberts]

Hey Guys,
What's up?? I write all this stuff and explain about these foils and how things work and not a peep out of anybody.
What's going on? What are you thinking?
Remember this foil stuff isn't new. Hobie Cat produced a foil boat. Where are they?
The Rave produced a foil boat. Where are they?
Why aren't these two designs large one design foil boat classes today ?????????

[J Drew]

Bill,
Actually I'm a little confused as to why you say foils couldn't work on one of our boats. Perhaps I'm missing something?
The AC72 weighs 5900kg or 13000lbs, and is +/- 4 times the size of a SC20 in each dimension. In comparison the f18 phantom weighs 165kg or 370lbs and has a mast height of 9.5meters or just under 31'. Let's assume the crew weight is the same for the f18 and a SC20.
Please don't misunderstand me, I have no desire to argue or contradict, and I am always glad to learn from you.
This is somewhat of a moot point for me as I have fallen off the idea and decided that my boat is just fine the way it is.
While simply discussing foils is definitely interesting, I can only look at a beautiful woman for so long before I want to go up and say hello, even if I fall on my face doing it.
John

[Bill Roberts]

Drew,
It is highly questionable whether or not an 800 pound 20ft cat can be made to fly or not on foils. The foils are going to be large. Can the boat reach lift out speed while dragging four large foils through the water is unknown. That is the hard point, the difficult task, associated with foiling especially if sails are your power plant. Lighter weight makes everything easier and faster.
I think that carbon Phantom weighs about 100 pounds less than your number. Those guys sailing the boat look like 140 to 150 pound people about 5ft 6 ins tall. The main lifting foil is fixed. What I think I was watching was a boat being sailed to a fixed foil. When the boat/hulls got too high above the water, I noticed the boat bore off slightly which supressed the speed and the boat came down to a lower height off the water. There were times when the hull got close to the water and the boat steered up slightly which increased relative wind speed and boat speed and the boat elevation increased. The skipper was steering the boat to hold a given height off the water. This can be done with a fixed foil but the direction the boat steers is not totally the skipper's choice and the boat speed, the dynamic pressure, is constant while flying, matching the fixed foil and fixed angle of attack.
Contrast this to a variable trim main foil where as the boat goes faster and the hull begins to or would begin to fly too high, the control reduces the foil angle of attack, lift coefficient, and the boat accelerates to a higher speed at constant lift. This control loop continues in this direction until a speed is reached where sail thrust equals foil drag and that is terminal speed. In a lull in the breeze and the boat slows, the control increases the foil angle of attack as the boat slows but it keeps flying unless it slows to lift out speed and then it settles back to hull born velocities with draggy foils. During all of this foiling at various speeds, the boat direction can what the skipper desires and he does not have to steer up or down to keep from flying too high or too low.

[J Drew]

The weight of the phantom was taken from their specs on their website, 10kg equals 22lbs .73965oz
So the actual weight according to my calculation is 363.762 lbs
I have no idea what those two guys weigh, and the video I saw was too vague to show much zig zagging, but as I recall the AC72 didn't have a very straight course.
The question is are the foils designed by Dr Bradfeild that much larger than the fixed foils used on the phantom? If so how do they work at all?

[Bill Roberts]

Drew, The foils can be what ever size the designer calculates they need to be. They are sized based on lift out speed and gross weight. The Bradfield foil with variable eleron would have eleron down for max lift at lift out . Then as the boat begins flying and accelerating the eleron comes up to something like a cruise position or best lift to drag ratio and the llft coefficient is reduced as the speed, dynamic pressure, increases. Velocity**2 and Cl must move equal and in opposite directions to keep Lift constant during flying.
Remember Lift = Cl x Area x Water density x Velocity**2/ 2 x g . While flying lift is boat total gross weight and is constant at all flying speeds.

[J Drew]

So give me an ideal number for foil area?
I'm not sure that my # for area is correct, but, if the weight is 700lbs and the velocity is 13.6, and everything else is calculated then lift is 194.53150686 ???

[Bill Roberts]

Hi Drew,
You can use 1sqft for area and then you ca calculate how much weight 1 sqft can lift at various speeds. Use a lift coefficient of 1.0 and velocity in ft/sec. Do it again for 2 sqft. You can generate a table of lift and speed for various speeds and areas. You will be surprised to see how fast lift changes with speed and then you will understand why you have to modulate Cl to hold lift constant while flying.
PS The water density is 64lbs/ft**3 and 2xg is 64.4 ft/sec squared so just let these two terms cancel each other. What the equation boils down to basically is Lift per sqft of foil area = boat velocity squared in ft/sec.

[J Drew]

Bill,
Now I see what you meant about weight. If I calculated it right, the difference between 650lbs and 850lbs is .5 sq ft of area. With the wind blowing 15+ and the std rig up with no jib, it would take a great deal of courage to fly solo.
Thank you, you taught me something, this was an interesting exercise,

[Bill Roberts]

You have it right , Drew.
Now, where are the Trifoilers? Where are the Raves??
These foil boats had modulating main foils and were easy to sail as long as there was enough wind.
The Phantom with the fixed foil must be sailed to the foil. In the video notice that when the boat gets too high, foil about to come out of the water, the skipper bears off and slows the boat down and the boat settles down. When the boat gets too low, the skipper heads up slightly and the relative wind goes up slightly and the boat speeds up slightly and the hull flies slightly higher. The skipper is steering a slight S path through the water to try to keep the hull the correct height off the water. What if that is not the direction you want to go??? What if the direction to the bouy is 10 degrees higher or 10 degrees lower than the path the boat must sail to keep it flying and not jumping the foil out of the water? With a modulating foil, a smart foil, you can sail any direction you want to as long as the boat will have enough speed to fly, like the AC72s. For example, on a windy day with modulating foil the skipper can sail just about any path he wants to sail off the wind, downwind and the foil will adjust to the speed holding lift constant. Not so with a fixed foil. The sailor must sail the boat to constant speed whatever that direction is.