Hydrofoils

Being thinking over the past 9 months about creating some hydrofoils for my hobie 18, but these hydrofoils are different to the standard that i've seen around. They will be able to change their AoA (angle of attack), so at low speeds the AoA will be increase to increase lift and as the speed increases the AoA will be decreased so in effect at all speed (execpt very low speeds <5kts) the hulls will stay out of the water. Buy doing this i will have an add advantage that while up on the hydrofoils the leeward hull will have a possitive AoA to keep that hull out, while the windward hull will have a negative AoA to pull it down and stop the boat from being blowen over. So in reallity i would never have to reef (not that i do that with the hobie 18, but thinking of down the track when this application is used on bigger boats) and the boat will always stay virtually flat.

The thing that has got me worried is that some one has tried this before and for some reason i can't think off, they have proved it doesn't work. I haven't explained myself into deepth with this post, but have put it out there for you guys/girls to comment incase there is a hugh fundamental flaw with this idea that has exluded me. But on the other hand, if there are poeple out there that want to help me develope this concept, i'm all too happy.

I have alot of idea's and solved afew problems, so over the summer here i'll do some testing and see what i come up with.

Dave,

 

Nothing really new in what you are proposing:
http://www.youtube.com/watch?v=g8Z4-Oh2fDE
You could take a lot of cues from the Hobie Trifoiler.

Rick W

 

Thanks Rick, but in short the trifoiler is only good in small steady waves, i can't see it working in rough weather that you get with big winds that Hydroptere goes out in to break the speed record. The biggest disadvantage Hydroptete has that it needs to reef.

 

What you've described is how all multihull sailboats with fully submerged foils have worked. The main difference is most designers choose to use flaps to change the zero lift angle of attack of the foils instead of changing the incidence of the entire foil. You may have some misconceptions about how foils work, however.

The first is the way drag behaves as the speed changes. You're used to drag always going up with speed and shrinking at low speed. The drag due to lift on the foils behaves in exactly the opposite way. The drag due to lift decreases with the square of the speed and would be infinite if the boat could fly at zero speed. So it's by no means assured the boat would be faster when flying on foils than it would be floating on its hulls. There are components of the drag that still increase with the square of the speed, however, and these dominate at high speed. As a result, a fully submerged foil has a speed at which the drag is a minimum when carrying the weight of the boat. What that speed is, and how much drag it is, depends critically on the wetted area of the foils and struts and the span (width) of the foils. When you choose what speed corresponds to minimum drag, how the takeoff speed compares to it, and size the foils to match, that will have a profound effect on the performance of your boat and the maximum speed it can reach.

Another is that you choose the angle of attack. The boat will seek the angle of attack that brings it into equilibrium. That will happen through the pitch attitude, rate of climb or descent, or the influence of your control system. The angle of attack is generally controlled indirectly by controlling the flying height of the boat. After all, it's not enough for the boat to come into equilibrium - you care where it reaches its equilibrium.

Stability and control are also very important for a hydrofoil. A fully submerged foil ("T" and "J" foils) has only a weak variation in the lift as the depth changes. To be statically stable (the tendency to return toward equilibrium when disturbed), the lift (actually the pitching moment) from the forward foil must vary more strongly with flying height than for the aft foil for heave (vertical motion) stability. And the aft foil must have a lower loading (lift per unit area) than the forward foil for pitch stability. You can design these conditions into the foil configuration (the surface-piercing main foils plus fully-submerged rudder foil of Hydroptere' is a good example), or you need to get the same effect artificially through your control system, which is what the Bradfield system (used on the Rave and Moth classes) does with its wand to provide mechanical feedback of height to the main foil flaps.

Dave Carlson has long experimented with a Hobie 18 on Keiper foils. I'm sure he'd be happy to give you advice on your project.

 

Big boat big waves. Little boat little waves. A matter of scale.

Rick W

 

Tom: Had to re-read your reply several times to make sence on what you where trying to say. You hit the alot of key points i had to address in my design and found solutions for all of them. I do want to point out there are two forms of drag, Induced and Parasitic.

Induced drag is created through the lifting action, and like you said it will drop off with speed, because the foil will have less AoA to lift the hull to the same hight. To minimize this, various foil shapes will give different results.

Parasitic drag is caused by the friction of the water over the hull, and as we all know, the design of the hull effects this.

By lifting the hulls out of the water onto a set of foils, i drop the parasitic drag to a minimum, and with increase in speed the induced drag drops off reducing my drag even further.

I have some foil shapes in mind to try to get the best lift to drag ratio from low speed i.e. being able to lift effectively at 5kts, but not having the time to make these foils as of yet, no testing has been done.

Dave