CNC molds are best because they reproduce the foil section exactly but they are very expensive. Another way to produce an excellent fin is to get 3 pieces of absolutely straight, untwisted 3/8" aluminum that is longer and wider than your fin by 1" (min) on all sides. Make sure that one piece of aluminum is precisely square cut. Then use thin aircraft plywood to make your “plug”(an identical half of your fin) The more time you spend on the section so that both halves are identical the better off you are. Do all the shaping and then use CA CAREFULLY to glue each half to the precisely cut aluminum piece described above.
Then use CA to seal the wood and after it is dry sand it with 80grit dry paper(do not wet sand using this method)fill with something easily sandable like bondo.Final sand down to 400-600.Use dry paper or wet and dry w/o water. Double and triple check that each side is precisely in the same place on the aluminumm"plug support". After you’re satisfied with the shape wax with partall paste wax then coat one side first(cover other side but not the edges of the “plug support”) by POURING not spraying liquid partall over the one side then stand on end to allow the part all to set up. This produces a perfect gloss with no pulling problems that could otherwise be expected from using bondo as a plug surface.Downside: shape the wood as much as possible before filling as the bondo and wood are different densities so the more uniform the shape is before filling the better. You can also coat the wood repeatedly with cyano with no filling at all-sandingbetween coats.Be sure to use a respirator when using cyano and don’t get cyano on the aluminum surrounding the wood plug;if you do use a blade to get rid of it.
To layup you have to understand that one side of the resulting mold must have little “tabs” on it to receive the other side so that both new molds remain precisely aligned. So do that side first by cutting 10 layers of 3/4 oz glass cloth about 1" by 1.5" for each of six tabs: one on each end ; two on each side.Cut 9 layers of six oz. cloth to make the glass part of the mold. Grind one side of the other two aluminum pieces so that the surface is rough. Then layup the first half starting with the tabs.Using 4/1 epoxy from Fibergalss Coatings you can lay up all nine layers at once. Keep room temperature 68 degrees. After finishing the layup mix up some epoxy and cabosil and apply to your just finished lamination and to one aluminum piece and set in place on the lamination and squeeze a little.The aluminum will disapate the mild heat build up from exotherm.
When this cures(overnight) you can pull it and do the other side. You’ll have to carefully grind the tabbed half to neaten it up and the other half will need carefull grinding along the clearly marked edge (DO NOT TRY TO TRIM BEFORE CURING!) in the cured laminate so that it fits perfectly in the tabbed half.
This is a cool method since it can be done relatively quickly and because you used cyano to hold the plywood plug on the plug support you can use a thin blade to remove the fin plugs from the support and re-use the plug support for another set of fins with a diffrent section or whatever. It is a relatively inexpensive method to turn out a nearly precise section with precisely matched molds that can be clamped together using the massive force from 6 c-clammps without fear of distorting the molds. The precision of the section and planform depends entirely on how much time you spend doing that phase of the work.
I mentioned the resin that I use for this work; other resins may require different time and temperature. Yes bondo does bond to CA treated wood after it is sanded.Bondo is just a word: there are a number of easily sandable fillers that will work; stay away from epoxy fillers because they can bond the plug to the plug support so it can’t be safely removed.The best course is to buld up layers of cysnao so tat the finishd surface is 100% cyano but that is fume intensive and takes much longer-but the more tme on the shapingand finishing the better the resulting part. Do’nt use epoxy resin on the wood-polyester might work but it goes on relatively thick compared to cyano. Cyano ,all inall is probably best toat least seal the wood because while it bonds the wood to the aluminum satisfactorily the wood is still removable.
If you were to build a fin with a foam core, 2 layers of unis and one layer of +45/-45 cloth on the outside, I doubt that you would get much weight difference between that fin and the ones made by the pros (the guys you mention) and I would be willing to bet that the stiffness would be pretty similar as well.
I mistook your original post. I thought you were saying that the +45/-45 cloth orientation that doug proposed was not optimal. But it appears you were actually talking about the solid versus cored laminate techniques. If you were simply trying to point out that Doug’s “solid carbon fin” is much heavier than a foam core fin, well, I cannot argue with that…
Sorry for creating so much confusion…
Doug,
I built some molds very similar to what you describe for some windsurfer fins I was manufacturing 10 years ago or so. I used tapered aluminum pins to locate the two halves of the clamshell mold relative to one another. It was relatively easy since you could place the pin in the first layup and glass around it to “capture” it in that half of the mold. Then when you lay up the second half of the mold, you could wax the pins and glass around them and they would form a nice tapered hole in the second half of the mold.
BTW, have you considered using PVA instead of wax? I have found that it washes off the finished parts much easier than the wax does. That way you can attach molded pieces together when they are done and not worry that the mold release is going to cause problems at the joint.
BTW, have you considered using PVA instead of wax? I have found that it washes off the finished parts much easier than the wax does. <hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Another alternative, which I want to try is to use several coats of any cheap, water based hair spray for release. Heard/read about that in WEST System newsletter - or …somewhere. Maybe on taxidermy forum? Damn - brain fade ![B)]
Just a caution: don’t use a water based release on a wood plug. I’ve always found it better to use partall #2 with PVA and in my method below I describe using both…
Why would anyone use a wood plug that wasn’t sealed with some form of barrier coat? (plastic tape, polyester resin, epoxy, wax, PVA, WD40, Silicone spray, etc.)
[:-angel]Dick I would say don`t worry about going OFF TOPIC too much in a context like this.
It is great to follow the discussion and learn so much from those with experience.
IMHO it should not be stifled.[:-dunce]
YEAH! THIS POST BRINGS UP MY PROMOTION. You can now call me CUR I mean SIR[:-jump2]
[:-captain]
nerds of the wold untie
Its not a million miles off topic, so I’ll let you off! Best way to make a wood mould ready for laminating that I have found is to cover it in brown plastic parcel tape, 3 or so thin coats of mould relese wax, then a smooth coat of pva mould release, sounds overkill but always works. 3M have just come up with a teflon material thats sposed to do this and be reuseable x number of times, we had a bit for testing, dunno how it came out in the end, damn expensive anyway…
3M Stuff is number 249 Tape. Comes in 3/4, 1 and 2 inch widths. A low tack adhesive with brown crepe back. Designed more for masking off parts of a mold than for release.
If you have high caliber mold, and want the best, look here. At $35.00 a can, there is enough for at least 2 molds [:D] although my guess it will handle a “few” more.
Re Will’s earlier post–Yes, I was suggesting that a fin with 14 layers of material per side as suggested as mandatory by Doug was not an optimal choice; is not a method adopted by other builders; and would be extremely heavy.
Re: Doug’s suggestion for carving fin molds out of wood and attaching to an aluminum backing. Again, this system is not the common practice among those who regularly build fins. One method other than c&c is to simply make a plug out of aluminum and build a mold off of that. Another method is to build a mold around a shaped wooden core as a plug. Once again, those interested in other ideas should contact any of the many people I have listed below.
However, there is a huge advantage to a “pro” built fin from an aluminum c&c cut mold. People who build these kinds of parts regularly know how to optimize resin and cloth to get a significantly lighter, thinner stiffer fin than the home builder.
One thought, given the difficulty of building great fins out of carbon for the home builder doing a one off boat, if it isn’t necassary to have the stiffest, lightest fin it might be easier to simply build one off fins out of aluminum.
Alot of us are always looking to carbon to be the work horse, however in our club I have seen a couple one off fins made from aluminum preform quite well. I believe Bobby Twombly had one that was quite good.
Having been playing with this fin thing for a while, I have finally conceeded and fallen in line with the fact that the only way to get a really high quality fin is from a production grade machined aluminum mold. However, home made rudder molds using any of the above methods are very do-able.
~tb
PS: Roy, c&c is a half way decent cruiser/racer that I have a decent amount of experience sailing on. CNC stands for Computer Numeric Controlled and often refers to the pieces of machinery used to machine 3 dimensional production grade molds and other various metal parts.
The system I described below is simple ,cost effective and can produce repetitive sets of perfectly matched molds that can then be used to layup foils to any level of stiffness/ lightness using cores, high strength resin and high modulus fibers or just standard carbon and excellent quality epoxy resin. The choices are wide open; the resulting foil can be nearly perfect depending on the effort put in.
The method is NOT dependent on using wood to shape the fin halves -almost any material that can be hand shaped can be used. The key is that perfectly matched molds capable of being used at very high pressure are produced.
I’m quite bored tonight so I was reading some old posts and topics. This one had be quite interested to think that the expense that some people go to to build light strong fins. I have a boat builder friend who has been in the business for 30 odd years and he has built fins for alsorts of boats ranging from 420 dinghies to 35 foot bay racers(centreboards).
I know the lay-up and the method used to build these fins/centreboards. All the moulds are made from plugs. These plugs are made out of pine or something similar. The moulds are then layed up and joined when still wet.
I have witnessed him laying a 420 rudder on two saw horses and hitting it with a 40 pound sledgehammer and the hammer bounced off. Most on here if not all will not believe this, but as is always the case if it sounds like crap then people won’t believe it unless they see it for themselves.
Yes, but, I’ve snapped two foam core, glass skinned daggerboards in the last two seasons while righting my boat, I doubt you would dent them much if you ran over them with a truck but they snapped with 12 stone on the end and they’re only 5 foot long. Also they deflect a hell of alot with that weight on them. The core does stuff all really, when they broke the core crumbled. If you wanted a stffer fin you would have more layers on the skin and less core material, if you want a really stiff fin you would have practically no core. The big winsurf fins are carbon all the way through so they dont twist or bend, the last thing you want is a bendy fin causing you to spin out at 25 knots! (ouch!![:D])
You clearly need to design the fin for adequate stiffness for the loads that it is likely to see. Windsurfer fins are very highly loaded which is why they are solid carbon. In fact most of the shapers I know buy autoclaved blocks of either G10 epoxy/glass or carbon so that they have really high quality layups and then the grind the shape of the fin out of the block.
For your dinghy daggerboards, it sounds like you need more skin material. The core is really only there to take up space and provied some transfer of the shear loads from skin to skin so that the two skins can do their job correctly.
For RC models, the type of construction you want depends on the class. For a class that uses deeper keels and more ballast, you probably want more layers in the skin. for a class with low displacement and less depth, fewer layers will suffice. As you decrease the thickness, you need more layers of skin to resist the bending loads. A thicker foil will require fewer layers for the same ballast and length. Thinner is generally better up to a point (if you go too thin the foil will be prone to stall… ) So once you have established how thin you want (6% - 9% t/c depending on aspect ratio and fin area) you can figure out how many layers of skin you need to react the bending loads and the remainder of the thickness can be a lightweight core.
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Matthew Lingley
I doubt you would dent them much if you ran over them with a truck but they snapped with 12 stone on the end and they’re only 5 foot long. <hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Matthew - as noted by Will - you are using the wrong “SKIN” on your boards! Change over to this type of “SKIN” and you should be fine ! If they hold up on a 20 foot cat, they should hold up on your boat.
You see, the angle of the hair on the “skin”, lays in unidirectional pattern. This imparts a great deal of strength and resistance to “bending” - and is the next wave of the future for “skins” for boards, rudders and perhaps even hulls. I understand there is a possible shortage on these type of “skins” - but if you can find some, give then a try - just rememebr to keep them lined up opposite the direction of bend!
OK - just foolin’ wit cha - we made up a set of these using black tissue paper that was covered with clear polyester resin. After cure, sanding and fairing, the resin was polished to allow the pattern to show through. Last I heard, owner was considering a sail pattern to match !
Will, The funny thing is I didnt make the boards I snapped, they were designed and made by the pro’s! Mayby I just get too angry when I wipe out mid race!
Dick, Like it! tissue paper eh? have to try that one sometime, cool effect!
they do it at Hockey events all the time - all designs are of very light paper, (or sometimes colored powder) embeded in/under the ice. Same or similar principle applies. Can also use rice paper and overcoat with epoxy, but only looks good on wood with varnish as it turns a golden yellow color. Neat effect on "stripper kayaks and canoes, though.
Ok I cracked it (well good enough for me) by getting 2 pieces of balsa laying up a layer of uni (e-glass) on each then plaing the fastenings coated in vasoline(petrolium jelly) about cenre of the chord length then clamping the two parts together (creating a simmilar effect as vac bagging) - so at this stage the laminate is balsa,uni,bolts,uni,balsa. The bolts wind out very easily after the cure but be sure to leave them in for the next step…
then after that all went off i shaped the balsa to shape then layed up a layer of uni on both sides of the fin then got 2 pieces of laminex (yes laminex the stuff on your kitchen workspaces) coated the non face side in resin and placed them on the wet uni each side of the fin (careful to line it all up). the final construction is laminex,uni,balsa,uni,fastenings,uni,balsa,uni,laminex. then by clamping the leading and trailing edges together it exserted an eaven force over the uni as it cured and the whole fin was exposed to a “clamping force” simmilar to that of vac bagging. the finised fin was fair, smooth and straight due to the clamping process and the laminex outer suface, the laminex also adds alot of stiffness have you ever bent a bit of laminex to a curve (ie around the airofoil shape of a fin) and tried bending it the other way? the resulting fin was extremely stiff, light, no twist possible and had fastening holes “cast” into the integeral strength of the whole fin.
My apprentice has recently made a fin for his by making balsa ply by gluing 2x 1.5mm pieces of balsa together then shaping the ply and vacuum bagging two layers of uni carbon to each side. the resulting fin was thinner and as stiff (although did twist a bit) as mine. (smart arse).
though vac bagging is out of reach to most hence i put my non vaccing method above which worked fine.
welcome to input and suggestion if anyone has any.