Hydrofoils for Solar Boat

Hello everyone,

My name is Pedro and I'm one of the members of Tecnico Solar Boat team.

We are designing a trimaran with hydrofoils and I've some questions regarding the hydrofoils.
We are limited on the available energy (solarpanels + battery) and so the way we use the energy is very important. On the other hand we are designing a boat for competition so its important to get the boat going as fast as possible. And we cannot forget that it must be stable during the change of direction.

1)If we consider a low velocity to "take off" does it reduces our maximum velocity? In other words, are those velocities related or is it possible to have a low take off velocity and an high maximum velocity with the same hydrofoil configuration?

1) Knowing the Lift force needed and the average cruise speed how should we choose the best foil? We are divided between Eppler and NACA.

2)Do we have to pay attention to extra problems besides cavitation and ventilation? Or can we treat HYDROfoils the same way we treat AIRfoils?

4)We know that there must be a commitment between the effective area (S) and the velocity but what can we do when we are not sure about the velocity?

5)The "take off" velocity is also important since we want to go out of the water quickly (less energy wasted). Is there any way to predict the take off speed (free software, theoretical equations etc)

As extra info I will attach some links:

The page of our team:
https://www.facebook.com/tecnico.solarboat D)

Youtube videos:
https://www.youtube.com/watch?v=Dz3c0PvaaQ8 (delft team)
https://www.youtube.com/watch?v=X6IZCqbE7G8 (monaco solar boat challenge)
https://www.youtube.com/watch?v=G2TQ5oNucDs (Clafis team)
https://www.youtube.com/watch?v=m7H3MLnAJYU (Roc team)

Universities are renewing the use of hydrofoils and there are some of them that already reached 50km/h max speed. It may not look as a fast thing but since it is a solar boat, believe me, it is!

I hope you can help us!
Best regards!

 

The takeoff speed is one the most important choices you will make. In principle you can take off at any speed if you give the foils enough area. However, if you choose a low takeoff speed, the excess area will cause drag that limits your top speed.

You essentially have a certain amount of power available, so you need to match the power required to the power available. The power vs speed curve when flying on foils looks like a Nike swoosh. There is a speed at which the power required is a minimum. Below that speed, the lift-induced drag increase as the speed drops will increase the power required. Above that speed, the increasing parasite drag will increase the power available.

You also need to add the drag of the hull as the boat accelerates to takeoff speed. The total drag will increase with speed, but as the boat lifts out, the drag of the hull drops away. The result is typically a hump in the power required vs speed curve. Below hump speed the boat is hullborne with partial support from the foils. From the local minimum above hump speed out to maximum speed, the boat is flying. You need to ensure the power required at hump speed is less than the power available, or you will never be able to take off.

The section choice is one of the last things you need to do. For now, just pick a representative section of the desired thickness that has characteristics typical of your application. Use the section's zero lift angle as the reference for incidence, angle of attack, etc. That way, you can substitute sections later and the lift will be unchanged if you maintain the same incidence of the zero lift line.

Once you have been through the design spiral, you will have a much better idea of the operating conditions for the sections. These include the Reynolds number range, the range of lift coefficients, and the minimum thickness (driven by structural constraints) for different stations along the span of the foil. Then you can use a tool like XFOIL to design sections that are tuned to your requirements.

One difference between hydrofoils and airfoils is the effect of the free surface. Initially, you can use the infinite Froude number approximation, also known as biplane theory. This treats the free surface like a reflection plane, similar to how you would model a solid surface, but the signs of the singularities in the mirror system have the opposite sign compared to what they would have with a solid surface.

Another difference is boundary layer transition has been observed to happen earlier in water than in air. When doing section design, a value of the critical exponent is probably in the range 1<Ncrit<3. Unless you have data on boundary layer transition that supports it, I wouldn't use a value any higher than that. You should also run calculations assuming the boundary layer is fully turbulent with no laminar flow at all. That way, you will be protected regardless of where the boundary layer transition actually occurs.

You need to make some assumptions to start, then see where those assumptions lead. You will probably find out that things don't add up and the design won't work. Then you modify your assumptions and try again. For example you can choose a value for the area and span, and calculate the drag. You can assume that the lift equals the weight and calculate the required angle of attack. You do this for a range of speeds, looking for the speed at which the power required equals the power available. That will set your maximum speed.

You will need to develop your own performance prediction. This can be done in a spreadsheet. At first, the prediction will be very approximate, but as you learn more about the design, you can make it more detailed. Excel's Solver is also an excellent way to do design optimization based on your performance prediction. The spreadsheet will probably be driven by tables containing the section characteristics, propulsion system characteristics, solar power, etc. It's a good idea to script the creation of these tables and their inclusion in the spreadsheet, because you will be repeating them many times.

 

Dear Tom Speer,

I would like to thank you for the advices and knowledge given.
We will certainly take in account all that information during our designing phase.

Best regards,
Pedro Roleira