Hydrofoil Construction Questions

I am looking to build flattened V-shaped dagger-board lifting hydrofoils similar in design to the Nacra 17 Olympic foils for a Supercat 20 tall rig sailing catamaran. I estimate the all up weight including 3 crew to average about 40% heavier than the Nacra 17.

Nacra 17 foils: Nacra 17 MKII Foils preview - Catamaran Racing , News & Design https://www.catsailingnews.com/2016/10/nacra-17-mkii-foils-preview.html

Given the increased weight, ready to purchase foils for the Nacra 17 and also for the Exploder A-cat would both seem to be a bit too small. The foils for the Nacra 20 FCS are the approximate correct size but are upturned J-foils which need to be loaded from the bottom. I have a strong preference for foils which load from the top and so the Nacra 17 Foil design of flattened V-shape foils seem the best solution. I am looking at building or contracting to have foils built that are approximately 20% longer and 20% greater in thickness and chord than the Nacra 17.

I am thinking that hydrofoil T-shaped rudders are less size critical than the dagger boards and I will probably purchase T-shaped rudders for the Nacra 17, Nacra 20 FCS or the Exploder A-class.

John Lindahl showed some success hand laying curved dagger-boards and also L-shaped rudders in molds he had CNC'd. He appears to have assembled them all from 2 halves with a center air space.
NEW RUDDERS https://lindahlcompositedesign.weebly.com/new-rudders.html

I plan to build between 2 and 10 starboard and port matched sets.
I imagine there is some demand for people wanting to convert 19 and 20 foot cats to foiling.

My tool design is to use a single set of matched molds for both starboard and port foils by having a tapered elliptical plan at each end with alternate ends blocked off to construct port and starboard foils to reduce tooling expense. Each foil will be approximately 2.4 meters long and a constant chord of 280 mm. 0.6 meters straight at upper end, 0.9 meters straight length at bottom end with a 0.9 meter long center curved section through a 50 degree arc.

A. My main question is: What are pros and cons of foil construction using the following three construction methods?

1. Hand layup or vacuum layup of carbon uniaxial fiber in foil halves in molds and then epoxied together. (Lindahl method)
2. Carbon prepreg layup of foil halves in molds, baked and then epoxied together.
3. Vacuum epoxy infusion into uniaxial carbon loaded closed matched molds for 1 piece construction.

The vacuum epoxy infusion method would appear to require very precisely matched molds to prevent epoxy leakage. This method seems like it may not be realistic for a home builder or for these low volumes by a custom builder.

Additional Questions:
B. Is a center air space typical with epoxied halves or is it usually filled with expanding foam?
C. Is there a better way to build one-off foils?

The straight foil end sections can easily be CNC'd from solid carbon fiber stock but the center curved section seems to require molding.

D. Would it be realistic and desirable to prototype the design using 2 straight foil sections spliced together at a 50 degree bend join and then loading them from the hull bottom for testing? Foils would need to be loaded in the water because the foil lower ends would drag on the ground when using a dolly.

All advice is welcome.

 

Could you provide a sketch of the foils you believe you need?Details of section (or at least foil family would be helpful).Do you have a 3D solid CAD file that the tooling people can work with?A section of straight parallel foil isn't too hard to make using any of the manufacturing methods that have been suggested.An accurate curved foil is much more challenging and even more difficult to compare with the original design for accuracy checks.Joining foils and successfully transferring the loads is no trivial matter.Without wanting to be discouraging,I would guess,from almost no information, that you will be spending about twice the cost of the boat before you have a set of foils.

 

The foils will be nearly identical in appearance to the Nacra foils which can be viewed in the link above, however they will be 20% bigger in every dimension.

Yes, the cost and/or the difficulty of this job is the main question and that is why I am posting here to better understand all the issues involved. I am competent in engineering and CAD design and will create the needed 3d files when I am convinced this project has a chance of success at some reasonable cost.

John Lindahl had some success making foils at reasonable cost making me think my project may be feasible with the proper planning. Probably my best chance of success is to emulate Lindahl's methods.


Nacra 17 Foil:

View attachment 188223

 

To be blunt about it, inserting those daggerboards from above won't be practical.Consider the width necessary to permit the curved section to pass through the hull and the consequent lack of support at the half height point.Inserting from below will be more feasible and also ,as you have described,much more difficult to launch with.Banana boards may be a less complicated solution in that respect.
Those rudders don't appear to have any advantage other than fitting within a defined width limit for the complete rudder installation.It would be a better structural solution to use a symmetrical T foil and reduce the bending load imposed on the main rudder section,while also allowing the use of one set of tooling for both rudders.The structural argument holds if you make a male rudder and sheathe it.You will need to create a sharper trailing edge or there is a strong possibility of a musical rudder.

 

Hi TJ Cameron, Could it work to have the wider centrecases needed to take the foil's curve from the top, (rules and regs), and then the vertical tops of the foils are widened to fill the wider gap down to the hulls bottom shape, so they are fully down and secure, but not really adjustable thereafter to a useful amount, (big underside cavity). Or use an additional long wedge or gap filler in the slot alongside the foil's top vertical section, which narrows the foil slot once the curve is inserted past the bottom, so you do have some depth adjustment for the top vertical sections at least. Probably not completely lifting up for a down wind run, easily. More weight, components, and complexity. You'd probably tack down wind anyway, flying. Do foilers even need to raise their centreboards in deep water for any reason ?

 

Thank you for the thoughtful replies. FYI; the Nacra 17 and A-class sailors have been inserting hydrofoil dagger boards from the top for the past 5 years without problems. The foil cases are wider and typically there is an adjustable bearing at the top which stabilizes and adjusts the angle of attack of the foil. There are no problems there to be solved. Foils, both dagger boards and rudders are always in the full down position when foiling.

My main question concerns the most appropriate construction methods for a home builder or a small manufacturer for ease of construction, quality and cost.

 

CNC cut the 2 half molds from thin MDF material or similar . If you use a material which is somewhat flexible you should be able to bend the material around a slightly curved forma to end up with your foil profile which is also curved in the transverse direction .
Some material options could include thermoplastics like acrylic which are heated in oven before being draped over a forma or even timber which could be bent naturally or with steam etc . None of these will work if thr curve radius is very tight , in which case you should CNC another split mold from a solid block of material for the tight curve / change in direction if it’s more like a V, and your straighter molded sections can glue into and join each other at this transition mold.

for a one off - just hand wet lay each peice into open half molds - You can infuse etc later if you’re familiar with the technique.

the air gap in the middle can be filled with low density thickened epoxy / micro balloons mixture when you glue the 2 half’s together . It’s a very simply an Quik way to join the 2 half’s together and very strong .

 

Hi TJ,

I did use all three methods to build foils.
1. hand layup is the right method for me. It requires very good preparation, since you are working under the ticking clock of the setting resin.
2. didn't work for me, because I don't own an autoclave and keeping the vacuum going inside an oven, keeping the mould from warping in those conditions,... was just too much effort for me.
3. Vacuum infusion doesn't work for foils imho because you need very high vacuum for the process and unidirectional fibres for strength. If you combine these two, the fibres will compress so tightly that the resin can not be sucked through. I did several runs and I always ended up with laminates that had not been fully soaked. You can of course use fabrics for better permeability, but that will just add weight for all the fibres running in a non load direction.

hope that helps.

Marcus
Carbon Pirate (@carbon_pirate) • Instagram photos and videos https://www.instagram.com/carbon_pirate/

 

Thank you groper and Carbon Pirate for your thoughtful replies! This interview with Mike's lab about building kite and wing foils has great information.



I believe they use closed matched aluminum molds with infusion. They bake overnight for one set per mold a day.

I am thinking to start smaller with a foil set for a wing board. The goal would be to achieve the highest performance while still allowing for glider disassembly for travel which is currently not available as far as I can see. The two makers of the fastest foils for kite and winging are Mike's Lab and Chubanga Foils in Italy. Both make one piece gliders (foils and fuselage combined) which cannot be disassembled.

If I make temporary molds from MDF, How should the molds be prepped to avoid release adhesion?

 

Hi TJ,
I am using mostly MDF female moulds. At the beginning I had been using Ureol, which is superior to make moulds, but quite expensive. For MDF I have tested different techniques. Depending on how many times I want to use a mould, I use a more or less complex process.
If I want to use it 3-4 times, I am using the following process:
1. fine milling with 0.5mm oversize
2. cover the mould with a low viscosity epoxy resin that gets sucked into the pores of the MDF to stabilize it. Otherwise you have a good chance of parts breaking out.
3. repeat with less viscosity resin before the previous layer is completely set to build up a resin coat.
4. fine millling with 0.25 oversize
5. cover the whole mould with a mould resin. These resins have super high viscosity and can be applied up to 1mm thick and more. Also they have more hardness and can be milled better and polished afterwards.
6. super fine milling in the final size
7. Sanding
(8. Sometimes a thin layer of varnish)
9. sanding
10. polishing.

Depending how serious I am about the mould, I reduce the process sometimes down to steps 1., 2., 5., 6., 7.. This means, I sometimes don't even polish the result afterwards, if I plan to paint the laminate afterwards.

On some moulds that I intent to use only once and that don't need to be super precise, I mill XPS and cover it in resin and varnish. Gets destroyed when releasing, but is cheap, fast and easy to handle.

I am using a molecular film releasing agent. That's some chemical that is not a wax, but that you treat similar. You apply it 4-5 times and polish it until it glosses. Works like a charm.

best,
Marcus

https://www.instagram.com/carbon_pirate/

 

If you think Ureol is expensive,try costing epoxy tooling block such as Aeroboard! It does have the advantage that you can machine it directly to the finished size with no need to apply coatings during the process and then you can seal the block and apply a semi-permanent release agent such as one of the Frekote range.