Foil assisted multihull design

OK, I'd say just design and build in your hard points where the thing would fit, but not design or build the actual foil(s), that way you will have a performance base to work with, after your initial foil-less trials, and design from there.

 

OK so i came across some info... as per doug lords advice from tom speers, it was suggested that for foil assisted multihull design, one should size the foils based on a reduction of wetted area from the lift generated.

So to explain how i understood it, which may be wrong of course, firstly you need to know the WSA of your plain hull at its design displacement. Then if we assume, say a 50% load fraction on the to be added foils, we also then need to know the wetted area of the hull in this lifted condition. The difference in wetted area is then divided by 4 to arrive at the planform area of the foils to be added. In the readings i did, it wasnt referred to as to how this was worked out, but i assume there was a mathematical relationship which was the basis of tom speers advice - as thats what tom does it seems, a mathematician of sorts...

So if this holds true, then we now have the plan form area of the foils. All we need do from there is assume an operating speed and from that we can work out our required Coefficient of lift based on foil section and angle of attack to arrive at the lift force for our original 50% displacement load. Then presumably picking a section which is then closest fit curve for best L/D ratio. This area, is the total area of the all foils, so needs to be divided up between all foils and there respective loads etc.

Does this sound appropriate so far?

Next i need to work out foil drag- which seems a bit more difficult/inaccurate.
Tspeer again, offered this simplified mathematical formula for foil drag, D = Cdo*density/2*V^2*Sref + L^2/(pi*e*b^2*density/2*V^2)
I dont think this allows for wave drag of a shallow foil however...

 

I blame the America's Cup racing for this !

 

Hi Groper - So summarize what you are trying to do and what you have learned! Cheers Peter S

 

Fnd = 2.20 is based on a large databases of model tests with lots of different hulls - some of them quite similar to yours. Believe me that you will will be wasting you time fitting foils for lower Froude numbers unless there are very exceptional circumstances which I do not see with your boat.

 

Konstantin's spreadsheet is reliable. It is based on well proven and very well researched mathematical methods developed in Russia. They come out of a standard Russian textbook on hydrofoil design.

 

I dont really understand the sense of matching the planform area of the foil to WSA of the hull. The is size of the foil is more closely connected to displacement than anything else. What I think you are trying to do is find a simple rule of thumb to size the foil - but it is not that simple. You need a proper mathematical model to calculate the foils+hull hydrodynamics and structure. From there it is an iteration process to spiral down to an an optimum. I use a genetic algorithm to sort through the thousands of variations to come close to the optimum. Form there i tweak things further based on practical experience...

If you are not going to tackle problem rigorously I propose you tackle it by trial an error on a model. You can try an bunch of difference hydrofoil sizes and locations on a model and they will give you a pretty good idea of the fsetup you need. But when you build the foil I propose you design the connection to the hull in such a way that allows you angle and position change as you will need that to get it working!

 

Think of this as a feasibility study... thats all im doing at this point.

I have the hull resistance predictions from Michlet software - i know its only theoretical and the limitations of that. I have a resistance prediction of my hull at full displacement and also at 50% displacement.

All i was looking for was a starting point to begin the iteration process, i know i need 20kN of lift, and would like it at 20kts - which is Fnv = 2.5 - so this should be ok based on your minimum Fnv = 2.2. There should be available power to drive it upto Fnv = +3.0, but this is not where i want to optimize, i would like to optimize at Fnv = 2.5.

If there is no improvement at Fnv=2.0 or lower, i understand this and is acceptable. Lets not forget im also interested in the motion damping seakeeping improvements the foils might offer - at any speed, or at anchor...

So the structural considerations are to;
design the thinnest section possible which will handle the bending moment generated by the lift + a safety factor. So this means using a high modulus material and trade off thickness and aspect ratio for required strength and stiffness.


So to begin an iteration process, ill simply run a few different configuations and add the foils drag component to my hull resistance and thus estimate any potential gain or loss compared to my boat with no foils. However, i do not completely understand the foil drag calculations, this is from Konstantin Matveev`s site;


But I dont understand all the algebraic symbols used in these equations, nor do i know reasonable values for some of the same...?

 

I think this is based on the planform area of the foil and also assumes that the induced drag of the foil will be equal to the viscous drag, which is an ideal case if the foil's drag law is of the form Dtot= Do + D * Cl ^ 2. I'm not sure that is the best drag law for an end plated tunnel foil near the surface, though, so you'd need to check on that. The closeness of the free surface will probably make Do, D and the power to which Cl is raised depend on velocity and depth, and that would complicate matters.

So basically, the "4 times the area" rule equates drag of the hull to an ideal foil operating at its max design efficiency cruise. It also follows that at liftoff speed, you would want more than four times reduction to avoid a big drag hump and a region of net drag increase at a speed not far from the desired cruise speed.

 

Okay, cross posted with you. The drag equations you posted still reduce to a form of Drag = Do + D * Cl^2 if I read it right. The bit in Eq 3 that depends on Cl to the first power appears to be a compressible flow correction which would be zero for water. Are those equations developed for hydofoils? At first glance, they look more like a supersonic flow equation (compressible flow).

 

I should have included the part which explains the equations... so here it is, available on K. Matveevs`s website, hydrofoils.org;

Foil Drag

Resistance of a hydrofoil can be found by equation(1), where 'rho' is the liquid density, V is the speed, C is the resistance coefficient, and S is the area of the hydrofoil. Resistance coefficient is the sum of profile, induced, and wave drag coefficients (2). Profile drag appears due to the liquid viscosity and includes friction and form resistance. For small positive attack angles of flat-convex profiles, it can be computed by equation (3), where 'ksi' is the resistance coefficient of an equivalent plate determined by Prandtl-Shlihting-Nikuradze diagram, m is depended on C and (varies from 0.4 to 0.7), 'fi' is the relative decompression on the upper side of a foil, and k is taken from expression (4) of the Lift section. In the flow around a foil with a finite span, upward leak of the fluid occurs around the side ends of a foil, accompanied by a fomration of free vorteces. The reason for the appearance of these vorteces is the pressure difference between two foil sides. The vortices force the flow downward, so that the hydrodynamic force deviates backward, and the actual attack angle decreases. The induced resistance coefficient can be calculated by equation (4), where 'delta' (5) is the correction for a rectangular foil (in comparison with an elliptic foil), 'lambda' is the aspect ratio, and 'dzetta' is taken from expression (7) of Lift section. When a foil is moving in the vicinity of a free surface, a wave system appears downstream. For practical calculation of the wave resistance coefficient, one can use the simplest formula (6), where Fr is the Froude number based on a foil chord.

Note that the wave drag can be calculted for high speed only, otherwise expression (6) will give negative values. Note that this method is applicable only for a single foil in calm waters.




So at least this accounts for all forms of drag and probably gives a reasonably accurate result, however the garbage in and garbage out still applies and i dont know reasonable figures for some of those greek symbols...

 

Then this may be of assistance:
http://www.boatdesign.net/forums/bo...ulation-hydrofoil-craft-43304.html#post558405

But the golden rule is, if you're not sure what bits mean what....don't use it or trust the results. Since you won't know if its right or wrong. Classic GIGO...

 

This boat might be named the "Dugong Sweeper".

 

There are some cheap or even free airfoil programs out that produce good lift and drag results as long as you understand how to adapt/tweak them and modify the results a bit; when to mirror a foil span to simulate endplate effects; using a bi-plane model to approximate free-surface effects, etc. Fortunately, there are some good user forums out there to walk you through that stuff. Xfoil and xflr5 are two that come quickly to mind.

 

Groper I get that you understand that we can make the boat foil or not foil. You are trying to establish whether this boat "should" foil or not. To establish this you have to define its mission and operational requirements. It clearly will go faster on foils so if speeds to ticket then thats one answer. If you are trying to make it more fuel efficient then that will lead to another answer etc. I don't think the foil between the hulls is the answer, you will need to look at V type foils that self trim and provide greater take off forces and taper off as the sped increases. So... what are you really trying to acheive with the foils? cheers Peter s