Hydrofoil sailboat project

[Panos_na]

Hi there!!

I am a student of Naval architecture and shipbuilding engineering.

I am about to start a project for my University.

I am interested in designing a sailboat with hydrofoils. I don't know a lot about hydrofoils, but I want to learn about them and understand them.

I have a book about Wing theory, which is very helpfull, but I will need help from people who have already built similar boats.

So, if it is possible, please guide me!! From where should I start?

I have many ideas about the whole design of the boat and the general arrangement plan, but i don't know many things about the Hydrofoils.

I have seen many types of hydrofoils : T foils, Vshaped, etc.
What special characteristics each one has?

Please help me with this!!

Thank you!

[Van Nostrum]

The basics

- Determine the configuration you want, 2 or 3 foils?
- Determine the sailing weight of your boat and crew (how much does the foil need to lift?)
- Determine the windspeed you want to foil at.
- Determine the speed your boat will be able to achieve in that windspeed.
- Determine the size of the foil required to lift the weight of boat and crew at that boatspeed.

Once you know roughly what you need then start refining it.

Read the wing theory book and calculate the lift of a plane with wings pointing in a V shape compare it to the lift of a plane with horizontal wings.

Go to www.foils.org where you'll find a wealth of information including how to calculate lift.
Look carefully at whether you want a multihull configuration(generally three foils) or a monohull configuration (generally two foils) and used on small boats like the Moth so far.
Look carefully at the difference between surface piercing foils that generally don't require an altitude control system and fully submerged foils that do require some form of altitude control.
Did you have any particular size boat in mind? Have you considered whether this boat will be a daysailer , cruising boat, race boat-or a top end speed machine? Have you considered doing a design within an existing class like the Moth for which there is a wealth of practical ,real world experience available?
Spend time researching these questions and you'll be able to narrow your concept down.
Here is one of the best sites in the world on the Moth class:
Australian Moth Class Association http://www.moth.asn.au/forum/index.p...321c902664a9d2

[Panos_na]

First of all, thank you for your reply.

So,you say that if I follow these steps, I will be able to calculate my foils?

It doesn't seem to be too easy though..!

As it consernes the foils sections, which types are most used? Maybe some NACA series? What are the pros and cons about each foil series?

I think that the most difficult, is to choose what foil section I will choose. Is that right?

[Panos_na]

I want to build a top end speed machine.

A really fast hydrofoil sailboat. I konw that it is not easy, but I want to try.

[Van Nostrum]

Originally Posted by Panos_na

I think that the most difficult, is to choose what foil section I will choose. Is that right?

I think building the craft will be harder than choosing which section to use.

Choose an section that you think is right (or get one from the moth site), then do your calculations, when you have it foiling at the required windspeed then you can experiment with different sections. If you start with a section which isn't optimal and can get it foiling then you will be able to foil with the refined section when you don't sail it at optimum.

You might be interested in reading a topic I started about very high speed foils, Harmonic pressure distribution

[tspeer]

Originally Posted by Panos_na

...
I have seen many types of hydrofoils : T foils, Vshaped, etc.
What special characteristics each one has?
...

It helps if you approach the problem a bit backwards. Start with what you want in the end, and use that to develop requirements for the other parts of the system. For example, you need to have some idea of what the minimum lift/drag ratio has to be in order to have a viable design, and how much load each foil has to carry.

As for how to decide between different foil types, here's how I approached that problem: http://www.tspeer.com/Hydrofoils/generic.pdf. Some types of foil, like the V, have performance that is sensitive to how the foil is operated. Others, like the T, can be very efficient at one speed, but picking that operating speed is crucial to the rest of the design.

Stability, trim and control are separate but related issues that have to be solved. You will need a means of trimming out the overturning moments from the sail rig. This can come from the stability or through contol. You will probably need a combination of both for good performance. Static stability will require that the forward surface(s) be more highly loaded and have a greater heave stiffness than the aft surface(s), so center of gravity placement is important. The heave stiffness can come from the foil configuration, or from some klind of feedback control system.

[tspeer]

Originally Posted by Panos_na

I want to build a top end speed machine.

A really fast hydrofoil sailboat. I konw that it is not easy, but I want to try.

If you want to set a speed record, the current record is already beyond the range of a subcavitating hydrofoil. Greg Ketterman's Longshot bascially reached the end of that road.

So you have to decide betweeen different possible objectives:
- World speed record using planing surfaces instead of subcavitating hydrofoils
- World speed record using supercavitating hydrofoils
- Less than record speeds while greater than course-racing multihull speeds, but able to achieve top speed under less favorable conditions using subcavitating hydrofoils, therefore able to achieve them more frequently (possibly a trainer or test bed for the top two categories)
- Top speeds comparable to existing racing multihulls using subcavitating hydrofoils, but able to achieve them in rougher or less favorable conditions and with greater safety/less skill, thus raising the average speed and competitiveness around the race course
- Top speeds comparable to existing racer/cruiser multihulls, but with more comfort (especially to windward) and greater safety (especially downwind)

You have to look at existing designs and ask, "What is it about each of them that is the limiting factor?" and then engineer your craft to overcome those limits without creating new problems that are even worse.

You should become very familar with the wind triangle and the basic sailing performance relationship:

Va/sin(gamma) = Vb/sin(gamma-beta) = Vt/sin(beta)
beta = arctan(Drag/Lift)|hull & foils + arctan(Drag/Lift)|topsides & sail rig

Va = apparent wind speed
Vb = boat speed
Vt = true wind speed
gamma = angle between boat's velocity vector through the water and true wind direction
beta = angle between boat's velocity vector through the water and the apparent wind direction

Lines of constant beta and lines of constant Va/Vb define sets of concentric circles that are orthogonal to each other, and completely define the performance space. Lines of constant Vb/Vt are also circles, and along with rays of constant gamma form a polar grid that covers the same performance space. When you superimpose these two coordinate systems, it will tell you a great deal about what you need in your design to achieve your desired performance.

Achieving low drag in both fluids will rule your design. Lift is easy; drag is hard.

[DSmith]

Originally Posted by tspeer

Static stability will require that the forward surface(s) be more highly loaded and have a greater heave stiffness than the aft surface(s), so center of gravity placement is important.

Could this be expanded upon a bit (particularly the centre of gravity placement)?

I have moved the two front foils (submerged, incidence controlled) and hence rig as far forward as possible on my A Class Cat to increase the fore/aft stability. It was very stable when towed and hope to have it sailing before the end of the year.

Dave

Dr.Sam Bradfield told me the Rave was designed with 80% of the load on the main foils. He used symetrical foils with the main foils at a +2.5° angle of incidence and the rudder foil at 0°. Each main foil has its own wand for altitude control and for developing righting moment. The center to center dis between the main foils on the Rave is substantially greater than it probably would be on the A class.
Good luck with your boat! Look forward to seeing pictures...

[Panos_na]

I have searched the Internet for boats designed foe great speeds.

I saw many different designs. For example, some boats are symmetrical and can sail port and starboard as well, while others are designed to sail only in one directon.

What do you think is best? Symmetrical or asymmetrical boat design?
I think that symmetrical is more practical, because of its' ability to sail on both sides, so you can do easier test runs.

Also, the 2 designs i like the most, are the Longshot (trifoiler) and the Hydroptere, which is an offshore boat. I have in mind something like those.

My target is not to break the sped record with my first boat, as I believe that this very very difficult, because of my little experience.

But I would be happy if my boat reaches over 30knots.

[tspeer]

Originally Posted by DSmith

Could this be expanded upon a bit (particularly the centre of gravity placement)?

Depending on the relative size of your fore & aft foils, there will be a point in between them about which the pitching moment won't change when the angle of attack of the craft is changed. This is called the neutral point. You want the center of gravity to be ahead of the neutral point for pitch stability at a constant height.

I don't know if you towed with crew on board, or where the crew was positioned if aboard. Evidently you were stable for the center of gravity location tested while towing. But moving your weight back when sailing is going to affect the stability.