Epoxying plywood to Aluminum?

[davef]

Any success epoxying plywood to aluminum? I'm working on my rudder blade and I'd like to shape it from a blank that is made by epoxying two sheets of 5/8" sapele plywood to either side of a thin sheet of (1/8") aluminum. In this way the aluminum would provide protection against running the rudder aground and would also provide strength along the feathered trailing edge. I did a small test piece and it came out fine but I'm wondering if there is any risk that the larger rudder blade (48") would delaminate due to glue line failure as the blade flexes. Any thoughts?

Thanks as always...

-Dave

[John Bell]

ONe solution I've seen work with putting metal tips in wooden whitewater kayak paddles is to drill the metal plate full of holes. Then thread thick fiberglass strands pulled from 20 oz. cloth all through the holes, stitching 10 oz glass to the metal. You can then bond the metal/glass to the wood with epoxy and be darn sure it wont come out of the end of the paddle.

How well it will work on a larger piece like a rudder, I don't know. But I'd be willing to try it if I had an appropriate design.

[Ian McColgin]

I'm not sure but what this is a bit over-engineered. A purely plywood rudder barrior coated with epoxy, final epoxy coat maybe beefed up a bit with colloidal silica, has worked well on a local boat. You can readily see if the wear on the lower leading edge is getting to the point where a little touch-up is needed.

Any metal would be pretty hard to bend around the leading edge - especially assuming a nice foil shape to the rudder - and hard to hold together at the trailing edged. Aluminum seems a bit weak for the job in any event.

For pure abrasion protection, how about making a bronze or even iron U shape over the vulnerable part with both glue under and let in flat head screws, maybe a bit of thicked epoxy to further fair the trailing edge of the skid patch.

G'luck

[G. Schollmeier]

I my experience epoxy will fail when bonded to aluminum especially if allowed to flex. If you’re worried about wear on the edge, insert the aluminum just along the edge and use 3M 5200 to bond it in. If you want to increase the stiffness you can sandwich some glass between the two plies.

Another consideration is the addition of aluminum could be a cause for electrolysis, depending on the type of rudder hardware.

Gary :D

[bheys]

I cut a trench using a slotting bit on the edges of my rudder and centerboard. This was done prior to any shaping of the foils. The slot is approximately the width of the trailing edge and is about 3/4" deep. It is filled with epoxy and cab-o-sil. Along with their fiberglass sheathing, I'm hoping the rudder and centerboard will be needing less frequent repair.
http://www.imagestation.com/picture/sraid102/p73c2eb45052069d3d072a5310eb0d5e2/f9ba2c10.jpg

[Pernicious Atavist]

i've made two rudders, both of which are retractable.
the one for my canoe is red oak (shush, all, i know what you'll say) with a 1/2" x 1/2" x 10" lead piece inset to the lower leading edge. it's not inlet, but a segment of the wood was cut away and the lead is screwed into the space. it acts as ballast and a bumpguard. no epoxy; it's all varnished and is holding up well after five years.
the rudder for my thistle is 3/4" exterior grade ply wood, epoxy and 'glass, with a lead insert into the body as ballast. the lower leading edge is three layers of 'glass.
both have held up well in very shoal waters where running arground is part of the deal!

[John Meachen]

I see no indication of your boating or building location so the potential problem I am about to mention might not occur.My concern is that repeated temperature changes may break the bond as the metal will expand and contract much more than the wood.The suggestion given elsewhere of an epoxy sheathed plywood rudder might be a better idea-and less work.Thinking on another note,if you have abullet proof rudder,what is likely to break when you hit an obstacle?

John Meachen

[davef]

Ian,

I don't think I explained myself well. The following image of my test section should help. I'm not planning on forming the aluminum sheet at all - just on using it as a flat laminate between the plywood. I want to finish the plywood bright. I don't think the aluminum adds appreciably to the bending stiffness of the rudder and the strength would probably be fine without it, however it does a couple of other things for me. The weight of the aluminum will provide most of the ballest I need to prevent the rudder from floating excessively. In addition, when the rudder is shaped, the aluminum will form the leading, trailing and bottom edges and will be the first surface hit if the rudder strikes bottom.

My test sample seemed to work well but as you can see, the plywood sands to a feather edge at the trailing edge. I'm just a bit worried worried about the whole stack delaminating as the rudder flexes.

http://www.imagestation.com/picture/sraid102/p81fd32cec981370900e6c2adc0bdaf89/f9b98058.jpg

[Paul Scheuer]

Looks good to me. I'd say go for it. Maybe rough up the metal. If it comes apart, all the loose pieces will float. If you have to rework it, you could shoot some screws in from each side.

I'm thinking of the same thing for my other boat. Foil rudders seem to be all the rage for CamPacs. The stock rudder on my 23 was 1/4 inch flat stock. I saw the new Eclypse at Strictly Sail, with a molded plastic foil shape added. They say the performance inprovement is worth it.

What's your plan for the hinge area? Metal bushing maybe ?

[davef]

Bheys,

That's a slick method and had not occured to me. Might make more sense than my AL plate. I've got a hunk of aluminum sheet hanging around and was inclined to use it but, as usual, I have some chin scratching to do here.

Re the pivot, I dug a 5/8" bronze bushing out of the scrap bin and trimmed it to length. I then tapped both ends of a brass rod that fits through the bushing and is flush to the sides of the rudder cheek. I plan on countersinking some brass plates and then screwing flush to those plates with some flat head screws to hold the whole mess together. The tiller pivot is done and seems to work nicely that way. Drives the pivoting between the busing and the pin so there is no pivoting on the wood to wear out. We'll see I guess.

I think I'm going to chicken out and CNC machine the rudder blade. It's a NACA 63 foil and I think if I try to do it by hand I'll screw it up.

Wish me luck.

http://www.imagestation.com/picture/sraid102/p007bd70baac64776cd3c790696860d27/f9b90c54.jpg

http://www.imagestation.com/picture/sraid102/p9eb6469022d4bbc7b3ed1ff8a0734be5/f9b90fe4.jpg

[Todd Bradshaw]

As I remember off the top of my head, the Gougeon Brothers literature stated that it was possible to get a decent bond to aluminum but that you need to overcome the problem that immediately after prep-sanding the metal's surface it starts to oxidize and that tends to weaken the bond. They solved this by using an acid-etching solution on the metal, just prior to coating it.

I've heard that some fast-oxidizing metals can also be abraded in a manner similar to wet-sanding by using mixed (but uncured) epoxy, rather than water. You mix up a small amount of resin, put on some gloves and basically "sand" the metal, grinding wet epoxy into the scratches the paper leaves. The idea is that the thin film of resin limits the exposure of the freshly abraded metal to the outside air and reduces it's ability to oxidize. Sounds like it could be a messy process and I've never tried it, but it makes a certain amount of sense.

[JimConlin]

I'm not comfortable with the use of aluminum for a loaded underwater component. It's hard to bond to without etching first, Being thermally active, temperature cycling will make it want to peel apart. It presents the(unnecessary) nightmare of electrolysis. If it's at the neutral axis (in the middle) of the foil, its weight is excessive. It'll yield when bumped, probably opening the bonds to adjacent material.

If you really plan on beating the foil up, for ends and leading edges, it's hard to beat extra glass sheathing. For the trailing edge, i'd consider a strip of G-10 epoxy-glass laminate, (available from McMaster-Carr), set in a slot, like BHeys solution. Use G-10 tube for the bushing or cast the bushing with epoxy and milled glass.

[John Blazy]

Good ideas above, but I would never laminate ply to alum with epoxy. I make a living in glued laminates and have two patents in polymer science in the areas of gluing dissimilar materials, and my experience proves that delamination would occur due to flex, thermal expansion/contraction cycling, and moisture damage. Epoxy is too hard, and once a micro edge is chipped and delaminated, there isn't any flexibility to "hold" the delam from creeping further.

3M 5200, as sugested earlier, is a superior adhesive for this purpose due to the elasticity. PL Premium polyurethane construction adhesive is every bit as good too. Not only the flexibility, but the actual ADHESION is incredible. Hole drilling / roughing up is great too.

[davef]

John,

I have never used 5200 although I've certainly heard it referred to on here many times. Is it stiff enough to securely hold the plywood to the aluminum while the rudder blade is shaped? I guess I assumed it was pretty rubbery - kind of like a caulk but I may be way off base.

Dave

[Todd Bradshaw]

Unless thic boat is a lot bigger than I think it is, a good old slab of mahogany with a little routine maintenance and a skipper who generally avoids purposely running aground with the rudder down is pretty hard to beat.

[davef]

Todd,

Comments well taken. Ironically, my original approach was to be a slab of Mahog but during a previous post on the topic I was persuaded to switch to the plywood due to comments and suggestions from this forum. I suspect either would have worked.

Regarding the need/desire for something different at the center of the rudder blade, I can only say that I elected to implement a true foil shape - partly out of curiousity - i.e. to see if I could both design and build it. I used a couple of technical references to develope the foil shape and profile.

One characteristic of the true foils is how thin they become as they lead into the trailing edge. If I recall the references correct, the trailing edge has one of the highest contributions to rudder performance as a blunter trailing edge creates trailing vortexes. As a result, the difference in drag between a 1/8" trailing edge thickness and a 1/4" trailing edge thickness was pretty extreme. To keep the trailing edge true to the profiles and minimize the radii at the end seemed very fragile and not a real good candidate for all-wood.

I know I know... there is NO real reason to implement a foil shaped rudder on this boat. I mean what are we talking about here?? the difference between 4.7 knots and 4.75 knots???? Who am I trying to kid?

Regardless, it was one of my objectives when I designed the boat so I guess we'll follow through and see where it leads. If nothing else, it's been fun. I guess the worst thing that happens is the rudder fails and I drop in a slab of wood that I took a router to.... and live with 4.7 knots.

-D

[John Meachen]

You mention a NACA 63 foil section.IIRC this is a laminar section and may well stall more readily than is likely to be convenient.Would a 00xx type be more appropriate?Your test section looks impressive and shows clearly that you will make a good job of the rudder by whatever means you eventually choose.
If the object of the exercise is to hold the rudder blade down, a system that I have used successfully for several years uses a clamcleat CL257 to fix the rope downhaul.The cleat in question has a hinged base that can be adjusted to ensure that the cleat flips and releases the downhaul if an object is hit.Take a look at www.clamcleat.com (http://www.clamcleat.com) to see what I mean.

John Meachen

[swingking]

This guy has done some testing and likes Sikaflex 292
http://tinylink.com/?ofQ081oeaD

[davef]

The internet is an amazing thing huh? Thanks.