# Hydrofoil - modelscale to fullscale

Discussion in 'Boat Design' started by Nyman, Sep 9, 2009.

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### NymanStudent

Hello

I have performed some modeltests on a hydrofoil-model, but I do have some problems for calculating/predicting the fullscale resistance.

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3. ### Guest625101138Previous Member

If you are not familiar with something like javafoil or Xfoil then you should look at one and get to know how they can be used. I find javafoil very easy to use. This will give lift and drag coefficients for your chosen foil section. Javafoil also has empirical compensation for aspect ratio. It will also give data for different Re#.

Obvious change will be Reynolds number. This will generally be beneficial as you scale up.

As you scale up, cavitation will become more likely. This will reduce the lift and needs to be considered as it is a completely different regime. This is an area I have not looked into.

There are papers around that give correction factors for wave making. Typically having the foil closer than 3 chords to the surface will mean you need to consider the foil wave drag. You end up balancing wave drag with strut drag. Going deep reduces wave drag but the reduction is offset by extra strut drag.

Normally with something like this I validate the theory against the model data and then apply the same theory to the scale up. I cannot think of any other significant factors that need to be accounted for but there may be. Others might have more to offer.

Stability control could require more consideration at full scale.

Rick W

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### masrapidoJunior forever

I love javafoil, but when compared with xfoil the results can be dramatically different. Sometimes in favour of javafoil, sometimes in favour of xfoil. And both can be unreliable, but one never knows which and when.

There was a spreadsheet somewhere, years ago, in this forum, which had pretty good parametres for calculating the hydrofoils.. I tried to find the thread but for now no luck. I may be wrong, but I think that a general rule of conversion is 1:4 when upscalling the results for a model into a real size foil. Am I wrong?

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### Tim BSenior Member

I would suggest using your model tests to validate a computational method, then progress using a computational method.

The computational method could be Xfoil, or CFD, dependant on proximity to the free surface, cavitation behaviour etc. etc.

If you're dealing with fully (and deeply) submerged foils, then you need to be looking at changes with reynolds number. If it's surface-piercing (or near the free-surface) then you really need to think about both Rn AND Froude number. At this point you really need to be looking at a computational solution.

Tim B.

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Nyman

Can you explain further what you mean by:
"..I have performed some modeltests on a hydrofoil-model, but I do have some problems for calculating/predicting the fullscale resistance.."

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### ancient kayakeraka Terry Haines

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### tspeerSenior Member

There are several ways of scaling up a hydrofoil, depending on what you want to keep constant. If you scale the hydrofoil in proportion to the rest of the boat but keep the speed the same, then the foil loading goes up because the foil area increases as the square of the scale factor but the weight will go up approximately as the cube of the scale factor. if you want to keep the same foil loading, the area must go up as the 3/2 power of the scale factor. And if you want to keep the same span loading, the span has to up with the cube of the scale factor!

If you want to keep the same Reynolds number, then the speed must decrease, which really isn't realistic.

My favorite hydrofoil scaling is Froude scaling. With this method, the foil size is proportional to the scale factor. But the speed goes up with the square root of the scale factor. In other words, the hydrofoil operates at the same Froude number. The increase in speed accounts for the fact that the weight goes up as the cube of the size, so the foil operates at the same lift coefficient.

The drawback of Froude scaling is the cavitation speed does not change with scale factor. So the difference between takeoff speed and the onset of cavitation shrinks as the hydrofoil is scaled up.

Since Froude scaling operates at the same lift coefficients as the model, this makes it straightforward to scale up the model scale forces. Since the forces are proportional to the area and the square of the speed, and the speed goes up by the square root of the scale factor, the forces will go up with the cube of the scale factor. Moments will go up as the fourth power of the scale factor.

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