Hydrofoil design, surface piercing stability

[LevenIce]

Hello, I am currently desiging a conversion of a motor boat (crescent 425) to a foiling one.
I have read Ray Vellinga's book and a bit on this forum and have a question regarding the stability of the surface piercing design.

In a canard configuration with a small surface piercing foil at the bow and a larger submerged foil at the stern (see image), can sufficient stability be achieved without adding any systems to vary lift of the rear foil? My rear foil will provide 240kg of lift and the front foil 100kg of lift.

In my mind, when the boat is transitioning to foil borne, the hull is initially angled upwards, giving both foils a greater AOI / AOA. When the front foil has risen to its desired height it will stay there, which results in it being the new point of rotation. The rear foil should then rise further so that it aligns in line with the front foil, just like an arrow. Will the foils stay in line through disturbances etc or will I need to make the rear foil surface piercing as well?

I am also wondering what angle I should choose for the surface piercing foils, this is something that I feel Ray Vellinga forgot to describe in detail in his book. From my limited understanding, the surface piercing foils should provide lift in a line which creates leverage above the vertical centre of gravity of the boat. Is this correct? Could someone explain this further or point me to a resource which clarifies this and how to calculate the right angle?

I am open to suggestions and thankful for all help. Thanks.

 

[BlueBell]

Will the foils stay in line through disturbances etc or will I need to make the rear foil surface piercing as well?

The beauty of this design is they are self leveling to some extent.
As the rear foil drops (for whatever reason) it's AOA increases,
if it gets too high (again rotating about the forward foil) the AOA decreases and it settles back down.

To calculate your fixed AOA, you need to calculate (or measure preferably) your load vs lift at a given speed.
There are tables for various wing sections showing optimum lift angles.

Ray's book is a little "pedestrian" but it's a good introduction.

 

[BlueBell]

My rear foil will provide 240kg of lift and the front foil 100kg of lift.

At what speed?

 

[Doug Halsey]

In my mind, when the boat is transitioning to foil borne, the hull is initially angled upwards, giving both foils a greater AOI / AOA. When the front foil has risen to its desired height it will stay there, which results in it being the new point of rotation. The rear foil should then rise further so that it aligns in line with the front foil, just like an arrow. Will the foils stay in line through disturbances etc or will I need to make the rear foil surface piercing as well?

How will the front foil know what your desires are?

And how will it manage to stay at one specific height, in spite of the speed and angle-of-attack changes?

Did you get these ideas from Vellinga's book?

 

[LevenIce]

How will the front foil know what your desires are?

And how will it manage to stay at one specific height, in spite of the speed and angle-of-attack changes?

Did you get these ideas from Vellinga's book?

I believe I simplified my explaination a bit too much in an attempt to convey my question, or perhaps I have just understood this wrong.

Here is how I imagine it:
The front foil won't stay at the exact same height after it overcomes the weight placed on it, as it will sink a bit after the AOI decreases (as a result of the aft foil rising).
As speed changes while, so will the running height. The surface piercing foil will "find" a surface area which together with the speed and AOA (Cl) equals the lift required.

At what speed?

About 7 knots is what i plan on being the take off speed.

The beauty of this design is they are self leveling to some extent.
As the rear foil drops (for whatever reason) it's AOA increases,
if it gets too high (again rotating about the forward foil) the AOA decreases and it settles back down.

To calculate your fixed AOA, you need to calculate (or measure preferably) your load vs lift at a given speed.
There are tables for various wing sections showing optimum lift angles.

Rays book is a little "pedestrian" but it's a good introduction.

I have been simulating wing profiles in XFLR5, analysing experimental data and searching the airfoil-tools website and have found the NACA 6412, with an angle of attack around 4-5 degrees of attack, to be great around my given reynold number. (600000-700000 depending on water temp). I selected this angle for maximum efficiency

XFLR5 does seem to give quite optimistic results compared to real life/experimental data, no matter what settings I apply. Simulated drag being half of real life and simulated Cl being about 10% too high. Perhaps someone knows whether this is a known issue or if I have done something wrong?

I have tried using different ncrit values, 1 to 3 is what I found recommended somewhere on this forum for hydrofoils.
I have tried changing transition locations for both bottom and top side, and found someone recommending 2-3% on this forum.

I have also used the 4412 foil, both as a potential candiate and to compare data between real life and simulation. See images, the single photo graph is real life data for 4412. The other image

 

[BlueBell]

Well there you go, you're all over it like white on rice!

What happens as you gain speed?
Do you have a cruising speed you're designing for?

 

[LevenIce]

Well there you go, you're all over it like white on rice!

What happens as you gain speed?
Do you have a cruising speed you're designing for?

I will design the front foil to still be in the water at cruising speed through its variable area. As the front foil rises due to the increase in speed, so will the rear foil. This is why I suppose the rear foil might have to be surface piercing as well? I cannot fully wrap my head around this scenario. Will the rear foil be placed deeper and have to rely on the surface proximity effect and the weathervane effect to not breach the surface or make the boat nosedive as a result of its increase in lift?

 

[emrecantasdemir]

you ca use xflr5 app to calculate about the foil

 

[BlueBell]

I will design the front foil to still be in the water at cruising speed through its variable area. As the front foil rises due to the increase in speed, so will the rear foil. This is why I suppose the rear foil might have to be surface piercing as well? I cannot fully wrap my head around this scenario. Will the rear foil be placed deeper and have to rely on the surface proximity effect and the weathervane effect to not breach the surface or make the boat nosedive as a result of its increase in lift?

Sorry, a little late responding here.
I think it all doable the way you've worked it but you'll need adjustability in order to tune/trim the fore and aft foils.
And, you'll need to get your foil sizing correct to start with.
Be prepared for a lot of sea trialing and realize, you may need to butcher (or add) to the foils.
If it still doesn't work you can always go to height sensors (mechanical or electronic) to control altitude.
The boat is going to be very weight sensitive.

Remember, you need to design to achieve more than your take-off speed.