# Foil loading limit to avoid cavitation?

Discussion in 'Hydrodynamics and Aerodynamics' started by S V, Sep 20, 2022.

1. Joined: Jan 2019
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### S VJunior Member

I have read that post more than twice already even before my original post. The bad part is that... I understand ~80% of it. After that post and that graph attached I have more questions than answers, some of them:
- Hydrofoil have to be designed to be "in the bucket" of the foil graph? All the sections???
- x axis is the coeficient of lift of the some regions of the foil, not entire foil? If a foil has a Cl of ~0.5 at the ~1 chord depth but some sections of it can have much higher lift coeficient, like Cl of 0.8 or more and it can be outside of the bucket - cavitating?
- The same as the last question: if the same exact foil operating at high speeds, like 30+ knots, is at 0.2 chord depth has due to free surface only Cl ~0.25 - does that mean that it more or less is always inside the bucket with most of the foil sections?
- y axis - do not get it at al... Is it the difference of the speeds of the liquid flow around the foil? The difference of the flow around suction side vs the flow velocity (boat speed)?

Yes, exactly. There is more or less 2D flow on the suction side, and 2.5D on the pressure side, as it is not terminated exactly with the hulls, but they are like beneficial (?) obstacles to dissipate pressure in the horizontal plane.There will be almost none of the "rotation"of fluid from high pressure to low pressure side which as I understand is the exact meaning of induced drag.
The automatic passive control of lift is big part what this "project" is all about. The structural chalenge could be to avoid central vertical strut, which is also possible.

CFD is still a bit away, share with me if you know any good CFD solver, which will take into account planning surfaces of the hull and the lift of foils and will "paint" and solve the whole picture with the hull lifting up.

I also would be gratefull, if someone explains, why on so many posts there are mentions of some "magical" properties of H105? I have read about it on Tomas Speer personal website, but still do not get whole picture...

Sorry do not understand the question. Not talking about foil limits: the project is for 20-~30 knots speeds. I do not aim for higher, as most likelly I will need relatively big engines and for such a small boat is not realistic to safelly operate in faster speeds. 30 knots in 6 meter boat - more than fast.

Last edited: Sep 27, 2022
2. Joined: Jan 2019
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Location: Lithuania

### S VJunior Member

Very optimistic calculations.
While playing with the excel file I get with this foil @20kt:
purelly theoretical 10 chords depth only ~1200N lifting force, Cl ~0.45
0.5 chords depth ~500N, Cl ~0.18, that is less than 1 passenger weight

That foil is ~0.05 sq m area... exactly twice smaller area than hydrofoils used for personal foilsurfing.
If you adjust speed to lets say ~10 knots, you will see, that this is not realistic

p.s. I would like you to look at the xls I attached in the first post. It kinda adjusts the Cl depending on the shape of foil and other factors also

Last edited: Sep 27, 2022
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### SkyakSenior Member

When you finish this project just tell me the lift and drag of the foils. I have an idea for another project....

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### SkyakSenior Member

Maybe it would help if I outline why and how that graph would be used.
Customer asks an engineer -I want a hydrofoil that goes 30 to 40+ knots but doesn't get chewed up from cavitation.
Engineer starts by evaluating 2D profiles. That graph is the evaluation tool. The vertical axis is the limiting factor ->the pressure minimum that cannot be exceeded without cavitation. Negative pressure is proportional to fluid velocity (Bernoulli). The horizontal axis is the output the customer needs ->lift. Both axis are made dimensionless for convenience -peak velocity gets divided by ambient flow velocity, lift is expressed as lift coefficient.
On that graph the engineer checks that the profile does not run into cavitation conditions in any of the crafts functions.
He also can see clearly where he has room to increase performance without a violation. Minimizing surface area for example.
With the 2D profile or profiles selected the engineer moves on to the 3D design, performance discounting from the 2D.

Your case differs significantly from our typical sail power limited, winner take all. You have significant depth, turbulence, and angle of attack issues but no power issues below max operating speed. I could see your need to cycle through that 2D graph a couple times with the 3D depth and turbulence discounted lift.

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### S VJunior Member

Customer-engineer relations works in similar way in all the industries, there is nothing new to me.
In my case as this is hobby project - customer, engineer and project manager is in one person

-Aplication: 6m (19,5ft) LOA tandem foil assymetric planning or LDL type foil assisted catamaran. Foils at keel depth, a couple cm up for practical reasons. Total displacement of boat fully loaded with cargo and fuel will be 1400kg. That is with overhead, but all the calculations I make for this weight. Protected waters boat, sea state 2 most. The goal is to effciently and comfortably cruise at operational speeds with relativelly low Hp engines. Predictable weight caried on foils - more than 50% @ 30 knots, with stability problems can be less.

Skyak, you had really impressive hydrodynamics teachers. I am glad that you share that knowledge here.

My guess most of the guys (you included) here are engineers by profession, I will clarify my requirements in one list as engineer, customer and project manager for the foil:
-Operational speeds: 20-30 knots. 32 knots max, not aiming for more.
-Simple horizontal submerged foils. No surface piercing elements. Demi hulls as endplates. No active elements.
-Will operate 0.25m-0.1m from water surface. The faster boat will go - the more close to surface foils will be.
-Foil span ~0.8m.
-Required lift on single foil @30kt ~500 kg and operating 0.1m from surface. That is in 2D calculations. I assume the loss in my real 3D application will be in 20% range. Can be less lift if there are reasons to limit loading due to cavitation/ventilation, stability/safety concerns.
-Safety factor at least 2: foil has to withstand ~1000kg weight. Lets assume that I will put on all the span sand bags to measure it.
-Nice to have but not a must: low take off speed, simple shapes so simple manufacturing materials and technologies. A little flexibility from the foil profile - ability to operate in different attack angles, different span legths and so on as not to remake all the project from scratch if some minor tweaks are needed. No borderline solutions.

My application is in some way power limited too, as I do not see reasons for fuel to get cheaper in lifetime or two, this is not quad Mercury 450R type project.

Thanks to you I understand this graph even more. Yes, now the H105 looks like a candidate. Found Mr. Tomas Speer comentaries about its low take off speed and suitability for 20-30kt range. Lets use this profile as example. With my limited English will try to formulate some more in depth questions:

For start simple ones, mostly for clarification:
- If the flow is purelly 2D, then the aspect ratio of the foil does not matter?
- Coeficient of lift Cl for lifting foil is in most cases bigger with the bigger attack angle. Lets agree, that we are talking about 1° to 10° angles where most of the applications are. In most cases 2° will have less Cl than 5°. Then if foil is entering cavitation range with 5° angle of attack, we can simply lower attack angle to 3-4°, loose some lift but there will be no cavitation at the same speed and other conditions. Yes?

And the golden questions:
- The total lift formula is: z = ( rho / 2 )* V^2 * Cl * S , (S - foil area of one side, rho - density of fluid, V - speed, Cl - nondimensional lift coeficient), and all the practical hydrofoil boats operate in depths from ~5 to 0.1 of the chord of the foil. So the the proximity to free surface definitelly has effect, maybe even exponential, on operation, one of the effects - reduced total lift (z). I have not found any other formula for lift, except some of the very complex formulas how to calculate or empirically predict Cl, but as I understood that free surface effect goes into Cl value reducing it. Foil section? For foil board https://www.boatdesign.net/threads/foil-section-for-foil-board.63233/ In that graph, I see that H105 at Cl = 0.8 @25 kt will cavitate, but Cl = 0.4 @25 kt it will not? Then my assumption is that if we choose foil conditions where it is lets say Cl = 0.8 at infinite depth, and at the same conditions but very close to surface is a drop to Cl = 0.3, then the graph Tomas Speer provided still applies? It is the exactly same graph for ALL the depths of the the exactly same foil opperation? Yes? In other words: if at infinite depths the foil would cavitate but due to proximity to water surface foil looses lift it can get out of the cavitation zone. Yes? Practical example: boat is at rest on its hull and has hydrofoils deep under the hull. When it is accelerating and still is at the same waterline level - at some speed foils may start to cavitate if they stay deep under the hull (ship is is overloaded or this is a water basin test with fixed foil), but if the boat is light enough foils can lift the boat up fully foilborne until very low depth, like less than foil chord depth and then there will be no cavitation at the exactly same speed and angle of attack?

- Understanding that graph: Not quite understand section lift concept in the x-axis. For example if for foil H105 @25kt total Cl = 0.62, we see that it is on border of cavitation, but Cl for some sections of the foil may already exceed Cl = 0.62 and the foil will be cavitating in some foil areas. Yes?
While designing the foil application I need to be in the cavitation free zone with ALL the foil sections?

-If above is true, what tool can I use for calculating Cl values for separate foil sections?

-Maximum velocity ratio of 1.7 is better for avoiding cavitation than 1.1? Then we need for speed over suction side of the foil be as quick as we can? As free surface is closer to upper suction side - there is less resistance for the water to flow fast? yes?

With all due respect to author graph is attached.

#### Attached Files:

• ###### Cavitation.gif
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Last edited: Sep 29, 2022
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