Foil Boats, The Big Picture

[Bill Roberts]

At the Sarasota Race a few months ago there was a foiling trimaran there. It was 20ft+ or so long and beautifully done. It was carbon everything. At registration I noticed the PN was 66.7. I have no idea where US Sailing gets so much accuracy on the PN for a new boat. When foil boats race with displacement boats, the race outcome is obvisous before the race starts. If the foil boat can foil, it will go over the horrizon on the fleet. If the foil boat cannot foil, it will be the tail end Charley dragging the foils through the water for the entire race and not being able to benefit from them. What if the foil boat could benefit from the foils on one leg and not on another, what is the correct PN? As you can see, racing foil boats against displacements boats makes no sence. There is no race to it. At Sarasota the beautiful foil boat could not get up in 8 to 10 knot winds so it was the last boat to finish in the distance race. The race results of foil boats vs the displacement boats certainly do not reflect the sailing skill of the sailors and that is what our sailboat racing is supposed to be all about.
So, how do foil boats race??? The boats should be sold in groups of at least three and five is better and then they can race in their own fleet and no problems about foiling vs not foiling or partial foiling do not matter. It all comes out in the wash. To make this happen boats would have to be sold at a large discount. Today an all carbom foil boat would cost around 75K and that assumes some company has a foil system that works well, foils easily. Where are they going to sell them? It must be a windy place. How about San Francisco Bay? Any place else? As you can see, there are many many problems/questions for foil boat sailing to answer.

[J Drew]

Bill,
My question is, what if there was a boat that could foil effectively when the conditions were right, but when there wasn't enough wind to foil it could use regular dagger boards? Obviously it would have separate setups, like having two separate masts for different wind conditions.
I think that if such a boat was on the market at a reasonable price, that it would be popular.
While carbon is nice and it saves valuable weight, would it be necessary to make the boat foil?

[Mac M]

From some reading on different cat sailing forums it sounds like some of the A Class guys are wanting to move towards foiling. I've seen video of some A Cats doing it, not sure how well, looked like one was 'jumping' a good bit. Some of the people against moving in that direction are arguing that it could 'kill the class' by putting it out of reach for the everyday guy. Saw estimates of it costing $4000 plus to upgrade a new boat that already costs $30,000, and making many of the older boats uncompetitive. There is a company that is about to launch a foiling F18, I believe I saw that it is supposed to cost somewhere around $30-$40,000. I have enough of a challenge with my current boats, so I'm out!

http://youtu.be/_iwhdfbhT1w

http://www.sail-innovation.com

[Bill Roberts]

Drew,
The lifting foil is much more complex than simply putting a T foil or an L foil on the end of a dagger board. The lift is proportional to the dynamic water pressure which is a velocity squared function. So the vertical lift is going to vary with boat velocity squared.
Suppose we sized a foil for our boat that will let it climb out of the water and become foil born at 20 ft per second or 13.6 mph. At this speed the boat is going to become foil born and begin accelerating. The dymanic water pressure at 20 ft/sec, lift out, is proporational to 400 and the vertical lift equals the weight of the boat plus passangers. At this point the drag is greatly reduced and the boat accelerates to 18 mph, 20 mph, maybe even 25 mph in the next few seconds. With a fixed foil the vertical foil lift which is proportional to speed squared becomes much greater than the gross weight of the boat and the boat jumps out of the water and crashes in a big ball of spray.
Example boat: At 20 ft/sec the boat became foil born and lets use a boat plus people weight of 700 pounds with big foils, a fiberglass boat. and the dynamic pressure is 400 lbs/ft**2.
At 20 mph, 25 ft/sec, the dynamic pressure is 625 lbs/ft**2 and the foil lift is 50% greater than the gross weight of the boat and the boat continues in a climbing attitude.
At 25 mph, 31.3 ft/sec, the dynamic pressure is 976 lbs/ft**2 and the foil lift is over twice the weight of the boat and by now, which has been only a few seconds, the boat has jumped completely out of the water and then crashes.
Major conclusion: The foil system must modulate to keep the lift constant once the boat is foil born. This control system must sense the dynamic water pressure and adjust the foil to keep the lift constant. This is the complex part of foils, "the control".

Mac, I think now you can understand why the boat you saw in the video was "jumping".

More Basics: Lift from a foil = Cl x A x Water Density x V**2/2 x g
Where Cl is the lift coefficient,
A is the foil lifting area,
V is boat velocity,
G is the gravity constant at 32.2 ft/sec**2.

Once a boat is up on the foils, we want the lift to remain constant.
But, boat velocity is constantly changing.
So, what can we change to make LIFT remain constant while boat velocity is changing?
The only term left that is not a constant is Cl.
How do we make Cl change?
Well, Cl is a function of angle of attack.
So, we have to change angle of attack.
The way the AC72s did it was to move the top of the dagger board fore and aft at the deck to change the angle of attack
on the "L" part of the daggerboard. The AC boats had big hydrolic rams pushing the board fore and aft at deck level in a long CB slot at deck level.
A crew member was assigned the responsibility of trimming the foil in each hull.
How are we going to do this ON A BEACH CAT with the cew out on the wire?

Another thing, Boat Inertia. Inertia of a body is related to its mass times length**2.
An approximation of an AC72's inertia is 13,000 pounds x 72**2.
The same expression for a beach cat is ~700 pounds x 20**2.
The inertia of a system in rotation in this case has to do with how quickly
the boat is going to climb up on the foils and come down off the foils.
As you can see the inertia of a beach cat is very very very small relative to an AC72 boat.
This means that the beach cat is going to react very very very fast to becoming foil born or coming down off the foils.
Remember Mac's "jumping A Cat?
The beach cat is going to jump up and down like a "jack in the box".
So you can see there are many problems to solve to make a beach cat fly with something like the foiling system used on the AC72 boats.

[Bill Roberts]

Mac, On another point, foil boat price. Your 30-$40,000.00 price range reminds me of the people who said the M20 was going to cost half as much as the Tornado because the M20 weighed half as much as the Tornado. How about that logic?

[J Drew]

Bill,
I was under the impression that Oracle and Artemis came up with an adjustable wing on the rudder to stabilize, similar to an elevator on a tail wing. Am I incorrect?
John

[Bill Roberts]

John,
The main foil is the heavy lift foil located near the main hull CG. It requires constant trimming while foil born. The trim on the aft end of the boat is necessary but it is a minor trim supporting a small load, aft end of the boat, and it keeps the boat/hull level fore and aft. AS I understood responsibilities, the skipper trimmed the rudder foil with a twist grip on the end of the tiller.

[Bill Roberts]

Here are examples of A Cats trying to foil on fixed foils. Check out this video.
http://www.mysailing.com.au/news/ashby- ... reak-havoc
As you can see the boats get up on the foils and then fall/crash. Why?????
When the boat reaches enough speed so that the lift from the foils is greater than the weight of the boat, the boat becomes foil born. (I'm talking in seconds here or fractions of a second.) Then the boat accelerates quickly and the lift increases with the square of the speed.In a second or less the lift becomes much greater than the weight of the boat and the foils climb to the surface of the water/waves. Here they ventilate in an air water mix and the lift goes to near zero and the boat falls back down into the water. This is what you see happen over and over in the video. How do we fix this? Let's look at the basic equation, L = Cl x Area x vel**2. Once the boat is foil born, we want the boat to stay foil born and accelerate. Vel**2 is going up fast so something else in that equation must go down just as fast. How about Cl, lift coefficient? The AC boats controlled foil angle of attack which varies lift coefficient to hold lift constant while flying. The San Bradfield scheme varies the eleron position at the trailing edge of the foil to vary Cl and hold lift constant. This is what airplanes do also for example when they go from climb to cruise. They reduce Cl and thrust to hold lift and speed constant.
As you can see and understand, what to do is simple. How to do it is more complex. The Sam Bradfield method is tried and proven. This is what the Moth Class uses. I don't understand why the beach cat factories that want to produce foiling boats
don't use the Bradfield method. It works!

[Mac M]

http://youtu.be/ikjdzwPo1jI

Thought this was an informative video, explains some of the systems on the boat. Looks way too hard for me!

[Bill Roberts]

In this video notice that when the boat is foiling, the skipper is playing the foil control line. Here is an example of foil modulation and the boat/sailor can get up and fly for some distance. It is not just up and down like at the Worlds. The sailor is not adjusting the mainsheet or the mainsail traveller. The foil control system is now the most important control line on the boat. Notice how big those foils are for a 165 pound boat and 165 pound sailor, 330 pounds total. How big would they be for a 400 pound boat and 340 pounds of sailors? Ans: Over twice as big as these foils are, twice the lifting area required and over twice as strong and twice as heavy.