Minimal weight epoxy fillet

[Lulworth]

Kind folks,

I've been pondering this question for a while, and I have developed opinions, but I'd like to hear other (informed , if possible, but not necessarily), opinions. I am about to put epoxy fillets on the transom(s) of a wooden opti (7.5 ft pram) that I am building for my kid. The goal for this boat is for it to be class competitive in the local regattas -- it will be one of only a handful of wooden Opties(one or two) out of hundreds of mass produced white gelcoated 'glass ones that make up the majority of the fleet. To make a competitive hull requires minimum wieght (easy to do in wood), maximum stiffness, and careful wieght distribution (out of the ends). My thinking for the fillets is that glass is not really necessary if the filler is adequate and the radius is, say 1" or so. Anyone care to debate this?

David.

[JimD]

How well is the boat framed, David? 2" tape on all the corners would only add a pound or so anyway.

[Cuyahoga Chuck]

On most S&G boats the 'glass/epoxy is necessary to strongly lock all the panels together. The fillets on the inside are only necessary because the 'glass tape cannot follow the hard angles on the interior. For light weight construction keep your fillets as small as is practicle.
I used a 1" radius on my D-4 but think you could probably get away with the radius of a 5 cent piece.
If you must leave something out make it the fillets and not the 'glass.
Charlie

[Keith Wilson]

How about a fillet of epoxy and microballoons, with the lightest glass tape you can find, or even - get out you wallet - carbon fiber? This might be stiffer and lighter than a fillet of epoxy and harder filler. I've never done it; just an idea.

[paladin]

Use the epoxy and microballoons with the radius of a dime (use a eskimo pie stick) and cover with one layer of veil......

[NormMessinger]

To much guessing. I'd run some tests. Microballons will weaken the fillet but the glass would restore some or all of the strength. The glass spreads the stress but I;m not sure it is necessary. My guess is that on a small boat the fillet alone will be sufficient and with a smaller radius that you propose. Run some tests.

[mmd]

Charlie is right - the fillet is only filler - there is very little strength in microballoon/epoxy when in tension (its forte is in compression). Minimum fillet radius should be about 1/2". Make your filletting putty very dry, just enough resin to bind it together. It should be the consistancy of cookie dough. Use 2" tape and be very fussy about how you wet it out; I'd suggest pre-pregging it on the bench so it's almost resin starved, then lay it in on the fillets that have been lightly painted with thin resin. Cutting weight is an exercise in fanatic attention to detail - go slow and be very, very, fussy.

[John B]

use micro balloons as glue fillets?
seriously?
How much weight are you going to save using balloons over ( instead of)fibres in a 7ft dinghy. :confused:

[ 10-05-2004, 06:18 PM: Message edited by: John B ]

[Buddy]

NO balloons in structural fillets PLEASE. Limestone dust, talc, micro chopped cotton or glass, but microballoons have no strength. For eash to sand fairing putty only.

[paul oman]

Hello

I sell epoxies and have build a number of stitch and glue, including a D4 dinghy.

I think you can skip the fillet completely. Soak the seam with a brushload of epoxy. If there is a gap, use a tube of bathroom caulking to fill.

It is a little boat and the glass on the outside of the seam should be enough!

paul oman
progressive epoxy polymers
www.epoxyproducts.com/marine.html (http://www.epoxyproducts.com/marine.html)

[paul oman]

I should add (see post above) I personally glass the inside seam (if a bubble or two under the glass no big deal) and use the fillet on top more for looks.

paul oman

[JimD]

One of my kayaks, 1/8" plywood stitch and glue is not fileted but is carefully taped. Been knocked around quite a bit over the years and no sign of coming apart. I've often wondered just how important all that messy fileting really is.

[George Roberts]

Lulworth ---

It looks like ...

2"x6oz cloth on both sides of 2 8' seams will only weigh 8oz.

If I was doing this, I would mix up 4oz or so of epoxy, put a light coat of epoxy where the tape will go, add some wood fiber to the remainder of the epoxy and put a bead of the thickened epoxy in the joint, and press the cloth in place over it.

Wipe up any epoxy drips as they leak out.

[NormMessinger]

"If there is a gap, use a tube of bathroom caulking to fill."

HOLY SMOKES! Be sure it contains silicon so the epoxy will not stick to it. Stick to the selling side of things Paul, or my preconcieved misconceptions are seriously awry. There is little strength in unsupported glass. But, as Burt Rutan used to say, if he still doesn't, chase after to ounces and the pounds will take care of themselves.

Is there a better way to save a couple of ounces? Oil rather than heavily pigmented paint perhaps. 11/16" thwarts rather than 3/4", one size smaller bolts, no inwale, Helium ballons for floatation, ....

Edited to add: Fillets on top of the glass? That's a unique method, Paul. Seems like I'm picking on you this evening. Otherwise, what George said, as if he needs my cooberation.

[ 10-05-2004, 08:00 PM: Message edited by: NormMessinger ]

[Cuyahoga Chuck]

Most thickening agents have been rated for strength. On the high end are fibers. Particularly chopped 'glass. In the middle is wood flour. At the bottom is microballoons. It's used for ease of sanding.
West Systems has profiles of their thickeners on the boxes.
Choosing the right one will enhance your construction.
Charlie

[Cuyahoga Chuck]

Most thickening agents have been rated for strength. On the high end are fibers. Particularly chopped 'glass. In the middle is wood flour. At the bottom is microballoons. It's used for ease of sanding.
West Systems has profiles of their thickeners on the boxes.
Choosing the right one will enhance your construction.
Charlie

[mmd]

Buddy, John B. - Yes, microballoons! The fillet joint we are talking of here is so small (if the fillet is 1/2" radius, the fillet legs will be effectively less that 1/4" wide) that their strength is marginal to non-existant regardless of what filler you use. The strength to hold the panels together will come from the 'glass tape on top of the fillet, so the fillet is there just to provide a smooth base for the 'glass to lay into. If the fillet serves no structural purpose and will need to be sanded fair to accept the 'glass cleanly, why worry about its strength? If it must be sanded, why not make it easy to sand?

[JimConlin]

I certainly haven't done the structural calculations, but I like the notion of high-strength skins over lightweight cores. So, my first guess for max lightness structural corner joint of 1/4" ply would be a 3/4" radius fillet of epoxy and microspheres topped with a 3" strip of 6 oz. glass cloth ON THE BIAS. Use two layers of same glass on teh outside for a bit of toughness. Lay it all up at once, top the mess with peel ply and squeegee as much resin as you can out of the glass.
Make some test corners and reduce the size of the fillet and the width of the glass 'til the joint is no longer stronger than the ply panel.

[Wild Wassa]

Originally posted by NormMessinger:
"Microballons will weaken the fillet but the glass would restore some or all of the strength."

Good comment ... without glass.

The thixotropic effect comes into play when the epoxy and macros are put under stress, otherwise macros and epoxy are as strong as any compression strength filler that I've used (under working conditions). The ability to sand the epoxy/macro fillet is much easier to finish, when fairing, than other compression strength fillers like BCP's compression strength filler or Smith and Co's Fill-It. I'd go with the macroballoons ... in fact I do (and following on from the comments of others).

Warren.

[ 10-06-2004, 01:19 AM: Message edited by: Wild Wassa ]

[Dave Carnell]

I have made ¼" radius fillets filled with 3M's glass microballoons and found they pulled the plywood apart when broken. These were right angle joints in ¼" plywood, and I was a bit surprised.

[paladin]

Ayup...the fillets izz there for appearances and not for primary structural strength....and one of the reasons some cloth should be added over......First you makes your structural joint, then you makes it purrty....

[ 10-06-2004, 08:34 AM: Message edited by: paladin ]

[Buddy]

I thought we were talking about light weight construction without glass tape, reinforcing joints only with the epoxy mush fillet on the inside. That's why I wouldn't use microballoons myself. True, if it's taped both sides, then the function of fillet is to get a bigger "moment" separating the stressed skins and as said microballoons are capable enough in the compression loads inside the angle. Dave Carnell has offered a lot of experimental evidence on fillet only destructive testing he's done that's worth reading.

[Lulworth]

Wow, thanks for all the discussion. In the end, testing is the only way to be sure but for this project I don't think that strength is the issue. Here's the deal. These boats have very tight tolerances including allowed dimensional variations of +/- 2 mm (in places) and a minimum weight of 35 kg (or 32, I don't quite recall and the specs are not handy). Taking the usual precautions, it is difficult to make this boat out of s+g plywood and reach the minimum weight! So the game here is weight distribution not total wieght. With this in mind I will probably make unglassed minimal fillets (say 1/4 -1/2" radius of curvature) at the ends (wood flour/cabosil maybe) and, along the sides, 2" 6 oz tape on microballoon/wood flour fillets getting beefer near the center where the energetic 75 pounder kid hiking-out will be loading the side panel.

I'm probably being nutty about all this. At the moment, with gunnels, bow and transmom, bottom panel, hog, and stringers all glued together, I've discovered that at least two (of about 50!) critical measurements are out of spec by more than 5mm. Ouch, after all that planning! I've got some wood hacking, bending, and scarfing to do before I go any farther!

I'll just add a little caveat along the lines of (in my opinion), "people at home shouldn't try this" Meaning, if you just building a boat to play around on, use glass and proper filler and don't fool around with this kind of nonsense.

Thanks again for the fun discussion,

David.

[Buddy]

I teach juniors in Optis and we have about three woodies out of the 46 in the fleet. All real "coffee table" furniture grade and boy do the dad's gripe about the dings they get. Not sure that's a practical limitation/ expectation to put on all the kids, not just their own. A certain amount of bumper car is going to happen out their with learners.

These joints are not highly loaded. They need no tape except for scrape on the chines hauling out if any at all. The rudder attachment, the hiking stick swivel and the daggerboard leaks are the trouble spots.

[Lulworth]

Buddy (and everyone else),

Thanks for the input -- I have not seen many wood optis up close so your info on fillets is good to hear. Scapes and dings I can deal with. You have, no doubt, seen what happens to a fiberglass Opti when the corner of one bangs into the side (or either transom) of another? Yes, a huge triangular rip in the flimsy layup needed to meet the minimum wieght requirement (C-fiber is not allowed by the class). Wood ones have superior impact/puncture resistance -- of this I have no doubt!

By the way, this is what I'm talking about (my 9 year old speed demon)
http://www.imagestation.com/picture/sraid142/pf274e16f8cdcb5f49faa7596170e1c06/f6b83eac.jpg