Hydrofoil design - choosin sweep and dihedral angles

[Mikaelhsm]

Hi all.
I m at the midway of designing a hydrofoil boat, but i don know what to do with choosing appropriate sweep and dihedral angles. Does anyone have a good reference about that? Or hopefully anyone can help me?

Thank you guys.

[Doug Lord]

The best reference for designers of small hydrofoils-power or sail-is probably the new book by Ray Vellinga "Hydrofoils Design Build Fly" published by Peacock Hill Publishing, Gig Harbor Washington
ISBN number:9780982236116
e-mail contact: patvel@peacockhillpublishing.com

[Tim B]

Two questions:

What do you want to achieve with sweep?
What do you want to achieve with dihedral?

The former is really about controlling the location of the aerodynamic (or hydrodynamic) centre, making effective changes to the tip and wake geometry, and for large sweep angles (in excess of about 20 degrees) controlling stall behaviour.

The latter is about stability.simplistically, V dihedral gives greater resolved lift (upwards) on the foil which is lowest, however, there is still a rolling moment from the other foil. However, the effect is slightly more subtle, because in having dihedral you get a different reaction to SIDESLIP, which is what actually provides the controllability, by providing a larger cross-coupling term between yaw and roll. The same effect is seen, but with greater efficiency, with polyhedral, where just the tips have a dihedral angle, but the centre-section is flat (or close to flat).

So the trick to this is to uderstand what happens in aircraft stability (which is a deeply immersed hydrofoil), then understand what surface-effects are present.

I would doubt that the stability benefits would be significant from dihedral, unless the hydrofoil is of significant size. Using surface-piercing hydrofoils will provide good stability, but have a drag penalty.

Using sweep will cause structural problems. Perhaps not insurmountable, but a significant headache.

For more detailed suggestions, you'd need to provide more data, but I would consider a foil with the 1/4 chord unswept and straight taper of both leading and trailing edges to 75% of the root chord at the tip. If you're thinking of surface-piercing foils, then in the first instance, parallel and unswept will not be a bad starting point.

Once you've proved it'll work, then look at the efficiency of the foils.

Hope this helps,

Tim B.

[Mikaelhsm]

First of all thank you for your replies.

I am going to use hydrofoils for a high speed planing boat, i.e almost works above 50 knots. thus both of sweep and dihedral have functionalities of delaying cavitation and fulfilling sustainable dynamical stability. About some effect like sideslip I m not sure if they are matter of considering in designing hydrofoils for boat which are working on the free surface of water. Besides I even get confused how to choose the appropriate size for span or chord length (I think both of them will be determined if the loading area of hydrofoils can be calculated). I read some documents about wing when they are used in aircraft, but my case is more complicated.

[Doug Lord]

Originally Posted by Mikaelhsm

First of all thank you for your replies.

I am going to use hydrofoils for a high speed planing boat, i.e almost works above 50 knots. thus both of sweep and dihedral have functionalities of delaying cavitation and fulfilling sustainable dynamical stability. About some effect like sideslip I m not sure if they are matter of considering in designing hydrofoils for boat which are working on the free surface of water. Besides I even get confused how to choose the appropriate size for span or chord length (I think both of them will be determined if the loading area of hydrofoils can be calculated). I read some documents about wing when they are used in aircraft, but my case is more complicated.

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In that case you might be interested in Eugene Clements booklet on designing high speed planing boats with a small step and a hydrofoil for pitch control. PM me or e-mail if you would like a copy of the booklet...
Good luck!