Foil blank construction recommendations

[Corley]

Im thinking I'll get the foil core profile made professionally for my Formula 40 trimaran project (I'll vac bag the laminates on myself) I'm sure they could get a much more even result for the basic cutting than I could in my workshop. If I went the professional route does anyone have any recommendations on who it would be best to contact in Australia? The board is 10'7" in length

[PAR]

If you have 3D CAD files any CNC shop should be able to create a "cut file" from it and carve up your core.

[Corley]

Thanks for the reply PAR. I dont actually have a cad file for this part but I do have a 1:1 foil section and a detailed plan view of the board with all measurements and to scale. I was thinking I might get just the foil shape cut and do the curve on the base of the board in the workshop.

[Munter]

Maybe Ian Dixon in Sydney?

[catsketcher]

After building 6 foils for different boats to fit into CB cases - I would say that if you want to go the high performance route - don't go the normal build.

Usually we build a blank out of cedar or foam with inserts and then put laminate over it. It seems a good way but there are some real problems. The main one is that you need to put on a staggered laminate over the board so the middle gets more glass. So you thin the board down there to cater for this - How much - you guess and hope.

Then you bog the whole thing up and fair it down again. After this you can make the case around the board which is fine but have you checked the board has no thicker sections. You end up with a board of questionable section, indifferent thickness and with a heavy core.

After ten years of stuffing around with Kankama's board I am going to pull the darn things out and make some new ones. They will be built in a steel mould from the outside in. A flat sheet of steel is folded and put in a jig. Then you lay up all your laminate onto the vee of the jig. Then you get some hardwood - or carbon beam - and pull the board around it and glass its back edge up. No fairing - no excess weight and no bulge in the middle.

If you don't like steel you could make the jig out of melamine sheets with a cove for the front curve.

May be food for thought

Phil

[Corley]

I think what your suggesting may be the way to go phil, I hadnt really considered that method but it makes a lot of sense and its probably much easier to set up and fair a female mould for this particular board and carefully laminated it should be symmetrical. I've always dreaded daggerboards, I've built 3 in timber for various boats and never been happy with any of them.

[m3mm0s rib]

If any can help ask me

[mhasting2004]

Hi Corely

I'm on the Central coast NSW might be able to help out if you're local. Got a large format CNC router and been doing bits for the two F85-sr's being built locally. PM me if you want to discuss.

Cheers

Mark

Ps You too Phil..... Its been awhile since you saw the "beast" in the garage.. I might be able to help you out with some bits too

[jamez]

Originally Posted by Corley

I think what your suggesting may be the way to go phil, I hadnt really considered that method but it makes a lot of sense and its probably much easier to set up and fair a female mould for this particular board and carefully laminated it should be symmetrical. I've always dreaded daggerboards, I've built 3 in timber for various boats and never been happy with any of them.

The steel mould method as described by Phil is well proven. Also Malcolm Tennant's prescribed method method going back maybe 35+ years.

[Steve W]

I have built, maybe 10 foils in sheetmetal molds over the last 30 yrs, rudder blades, daggerboards and even a bulb keel blade for a monohull. I like the method a lot, i doubt that you can arrive at a nice fair finished foil quicker or cheaper without a full production mold. However, it does have one problem which is that you layup the laminate with the mold laying open at an angle and then you close it up around the beam which you insert at the point of maximum thickness, what i have found is that the laminate tends to bunch up a bit around the leading edge which you really cant do anything about. Ive never found it to cause a problem with the finished board but i dont like it. I have done some which i have filled with an epoxy/styrofoam bead mix, one rudder blade which i left empty and one with a hot wired styrofoam core where i cut a strip out in the appropriate place and glued the front and back sections to the beam and folded it around that to try to alleiviate the bunching at the nose, i can only assume it worked as you cant check it. I have my molds folded at a local sheetmetal shop and also have them fold out flanges at the trailing edge to which i screw on wood cleats to clamp it shut. I dont know if Malcolm invented the method but hes the one who described it to me.
Steve.

[mhasting2004]

The only disadvantage with those methods and extrusions I can see is that your limited to parallel foils. Where elipitcal and tapers are posible using a cnc or hand carved methods.

I remember reading a post somewhere by I think Tom Speer about how a uni was making amd testing hydrofoils using a metal tray full of machinable wax. They cnc cut the mold out of the wax and when they wanted to test another foil profile they just remelted the wax and recut. Seemed like an interesting way to do one offs.

You don't need a CNC to make perfect foils IMO its just easier. For example I hand ground an 8" mirror for a telescope so its possible to create very accurate shapes by hand... (far tighter tolerances that most foils need or obtain) but you do need a way of accurate measurement... which is the tricky part.

Cheers

[Geoff Harman]

Will finish CNC machining two 4.4m long foil cores for a customer tomorrow.
The cores have a WRC spar 150mm wide with foam either side.
The cores include a tapered groove 160mm wide running along the core for UD carbon fibre.
The taper is zero at the ends and maximum depth (8mm) at the keel.
Previous boards of similar length had hardwood spars.
I often include CNC cut acrylic patterns to help the fabricators maintain the shape.
Both sets of boards go into existing cases.
pm me if you want further info.


Geoff Harman

[Steve W]

Everything is a compromise, for every theoretical advantage there is usually a disadvantage, for example a parallel foil with a closely fitting trunk and the bottom finished to the keel line can block off the trunk smooth when in the up position whereas anything else will leave a dirty big hole with a lot of drag. Transom hung rudders are another example,while we all aknowlege they are less efficient hydrodynamicly they are simply more practical on most multihulls so lots of uber fast multies use them despite any percieved inefficiency. I personally feel that the economy and ease of build of sheetmetal molds make a lot of sense for an item that is somewhat damage prone.
Steve.

[CatBuilder]

Interesting thread. I am currently building a set of 13' x 3' (approx) NACA foil boards for my boat.

The designer suggested the mold method, but I really prefer to shape foam first, then glass a perfect shape.

So... what I have done is cut many (VERY many) sections of the foil out of sheet foam, which I am bonding together, then fairing. It's a lot of work, but for a one off, it does seem faster than building a mold. Easier anyway.

It's another way to do it.

[PAR]

I've used a take off on the lost wax method for making smaller foils. I've used foam, usually cut in sectional lifts, then either routed or sanded to finished dimensions, checking with printed templates. This serves as the disposable plug and can be dissolved or if some planning is used, the mold made in halves and the plug removed.

The female can be made from several materials, depending on how durable and how much pressure it needs to tolerate. Plaster is a favorite, as is polyester resin or epoxy for long term molds that you might want many foils from.