Besides using a pre-made EPS foam "cookie", does anyone know what manufacturer's used to fill the inside voids of rudders? Was it simple resin and micro-balloons? I thought I remember a lightweight product for the purpose, but it escapes me at the moment.

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Chuck C
NACRA 500 Mk2
Hobie 21se (sold)
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All depends on how the rudders were made, but I would not expect a significant quantity of microballoons. More likely a two-part expanding structural foam.

That's kind of what I was thinking on a two part mold, and from pictures I've seen, they looked to be filled with urethane expanding foam - maybe in the 5-8 lb weight. However I know I've seen a lightweight epoxy or vinylester resin that is made for purpose and doesn't exotherm nor expand as much, making cavity filling more precise (though you'd have to look for bubbles). Just can't find that product and I remember it as being white(ish). Cutting cookies to fit perfectly does not seem like a very good production method. Just worried about that 2-part foam expanding so much it ruins the part, even with a vent hole in place somewhere.

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Chuck C
NACRA 500 Mk2
Hobie 21se (sold)
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I've made some rudders from a production mold from a manufacturer. You made each half, then decked them flat with a router on a sled, then bonded them together. That way there is no gap filling so long as you did the previous steps correctly.

Never thought of using a router sled to flatten and trim the parts-cool idea. I've always cut them down while the product was still green, if doing this by hand.

Still, you have a hollow structure you have to fill to keep compression strength up and keep water out...

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Chuck C
NACRA 500 Mk2
Hobie 21se (sold)
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That is a question that probably has a secret answer. I remember when Chris C. was developing his hobiecat rudders to be unbreakable, he tried all sorts of core mixtures. He would mount the test rudder in a vice and pull on the tip to try and break it. He was a big guy and very strong, and when he made one that didn't break, that was the correct mixture. What he put in that bog mixture, i dont know. I know with molded windsurfing fins a regular epoxy bog works, but it is a small space and the rest is layers of carbon carefully cut to size. I made some molded fins and we tried using the cnc machine to cut a wood core after calculating how thick the carbon would be, and how big the void in the middle would be. It made a really stiff fin. I also tried using this special core material used for kayaks that was made of woven glass infused with micro balloons.
The original hobie 14 rudders in the 60's had a foam core and eventually swelled up and froze in the castings. Those got replaced by a larger injected molded rudder which snapped easily, leading Chris to start his rudder company. We could not finish a regatta without breaking a rudder on the reach in those days. We kind of forced hobiecat to let us make our own rudders. I just hand shaped them out of wood and glassed them, but he made a whole company.
So my answer is don't use foam. Use some kind of bog. Make the skins as strong as possible and the core will not matter as much. Use epoxy if you can. -Bog is a mix of resin and fillers like microballoons.-

Chuck, I think I will laminate some blades and finish them with carbon or may go clear for the "cool factor". Easy enough to have them cut out via CNC. I have the template I need in AutoCAD.

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Scott

Prindle Fleet 2
TCDYC

Prindle 18-2 Mod "FrankenKitty"
Tornado Classic "Fast Furniture"
Prindle 19 "Mr. Wiggly"
Nacra 5.8 "De ja vu"
Nacra 5.0
Nacra 5.8
Tornadoes (Reg White)
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Hey Scott - no doubt; and, if you need some help vacuum bagging it and finishing in clear, just let me know since I got that down pat. We just need a good Friday evening to lay up and Saturday to do the hot coats.

I just WANT to try my hand at making a full on, vacuum infusion rudder mold, and will likely either start with the standard Hobie rudder or Prindle rudder, since those seem to be the most useful to friends/acquaintances.

The advantage of molding, is I won't have NEAR the post finish work to do. All the strength will be in the glass and reinforcements - not derived from the fill material. That core material referred to above is Coremat Xi. I did find "foaming epoxy", which looks to be the trick; hard as a rock when cured but can be poured and utilized like urethane foam. Only, it's a bit heavier and a WHOLE LOT more expensive. It may all be for nothing, but that and my giant CNC plotter are my "winter projects". Once I get going, I'll post.

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Chuck C
NACRA 500 Mk2
Hobie 21se (sold)
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There is an article elsewhere here that deals with rudder and centerboard profiles, and has a link to the mathematical formulas for various profiles that tested well in flow tanks. Maximum control vs minimal cavitation, that sort of thing.

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Sheet In!
Bob
_/)_____/)_/)____/)____/)_____/)/)__________/)__
Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
AZ Multihull Fleet 42 member
(Way) Past Commodore of Prindle Fleet 14
Arizona, USA
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Got the profiles, etc. Of course had I known and could find same, might have saved a few hours research... Luckily, NACA airfoils are used, making the construction of a plug fairly straight forward. Nevertheless, if anyone has a link to profiles articles, that'd be cool and useful to confirm.

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Chuck C
NACRA 500 Mk2
Hobie 21se (sold)
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