# Total Resistance Formulation of Hydrofoil Craft

Discussion in 'Boat Design' started by leviathan, May 30, 2012.

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### leviathanNew Member

Hi all, I'm a naval architect undergraduate student, and recently I'm doing my final assignment about hydrofoil craft.
I have done a towing tank test and obtain the total resistance, but I need the formulation to calculate it numerically for validation.

thanks and regards

2. Joined: Oct 2008
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Depending whether you hydrofoil is a surface piercing or fully submerged, the forces are given by:

You have to work out the lift and drag of each foil, so i the figure above the total drag shall be = Drag (aft ) + Drag (fwd) + Drag (aero)

where:-
Drag(aft) = drag of the aft foil
Drag(fwd) = drag of fwd foil
Drag(area) = the aerodynamic drag of the hull once foil borne.

There is a very good section in the book "Mechanics of Marine Vehicles" by Clayton & Bishop

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### DingoJunior Member

Method to work out resistance of hydrofoil boat

Let e = Rt/displ where Rt is total resistance and displ is displacement
Also, for the model, we have e_model = Rt_model/displ_model

e = k.e_model where k is a correlation factor and is given by:-

k = 1 - (Cfh_model - Cfh)/Ct_model - [(Cff_model - Cff)/Ct_model][Sf/Sh] - [(Cfst_model - Cfst)/Ct_model][Sst/Sh]

Sf is surface area of foil of boat
Sh is surface area of hull of boat
Sst is surface area of strut of boat
Cfh is hull friction coefficient after ITTC 57 line
Cff is foil friction coefficient after ITTC 57 line
Cfst is strut friction coefficient after ITTC 57 line
For the model, use Kirkman's method "Scaling Problem of Model Appendages" or as reproduce in Principles of Naval Architecture to determine Cff_model and Cfst_model for a given Re
Ct_model = Rt_model/(0.5 rho_model V_model^2 Sh_model) which is based on the wetted area of the hull alone. Rt_model is what you measured in your model test.

Hope this helps.

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### tspeerSenior Member

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