How to waterproof over space?

[Dale B.]

I have a stainless steel rudder that has significant plates cut out of it. It was filled with foam, wrapped in metal tape and encased in fiberglass. It was poorly done, very heavy and unsightly to put it nicely.

I removed the fiberglass and the tape (see images).

I'd like to remove the foam and then encapsulate the whole thing with something very light and waterproof. Any suggestions for a very thin, light weight waterproof coating that can span the cut out spaces while holding shape? Is fiberglass the lightest option? What other options might there be?

Thanks.

[chrisr]

carbon is the answer to everything...

cheers,

[Cheechako]

I used a product called "rudder cast" many years ago to fill a rebuilt rudder that had foam previously, but I dont think it is available any more. Word was it was used to make quick rudders for the Viet Nam river boats by just casting them, no glass etc involved. I never did confirm if that was true but it worked great as filler. It was blue in color.
I would think epoxy with a light weight filler would be about the same. I would still want a outer encapsulation with glass .
Remember, every thing weighs less in water (Relative weight)

But another option is to put nothing inside the rudder and glass it. What good does foam do? It just soaks up water any way. Put a drain plug in and every year at haul out drain it!

[Dale B.]

Thanks for your replies


chrisr, carbon? say more. what product?



Cheechako, "put nothing inside the rudder and glass it."


Yes, my preference is to remove the foam and have no filler. Unless there is some kind of carbon coating I'm unaware of, glass will have to do I guess. With this approach how do I keep the glass tight while applying the resin so it doesn't sag into each cavity. I think that is why it was wrapped in tape before.



Aside from cleaning well, do I need to do anything to prep the stainless for best glass/resin adhesion?

[a64pilot]

carbon fiber is very similar to glass, just much stronger and lighter, but it’s not really all that much lighter as it’s the resin is where the weight is.
Gut says that the way it was is about as good as it’s going to get, it shouldn’t take much glass as it’s not really structural, just a waterproof covering.
Water weighs about 9 lbs or so per gallon, fresh water is 8.34 lbs, so salt must be heavier, the 9 is just a guess.
assuming you rudder isn’t thick steel, it may be weightless in water, depending of course on what it weighs.

On edit, carbon fiber gets its weight reduction largely because it’s much stronger so less can be used, but as you don’t have a structural need for it, it’s not likely to help all that much

[Spot]

8.56 pounds per gallon 'average' salt water according to the interwebs...

Having the metal tape between the rudder and glassing ruined all hopes for the original resin to have bonded to the metal. In fact, clear packing tape is a quick way to turn a plug into a mold for epoxy.

What is the texture of the metal now?

I would be inclined to sand/blast the metal, fill in the missing foam with epoxy and glass spheres (made up like lightweight spackle), fair the repaired foam to match the metal, and vacuum bag a new skin onto the works, adding a couple strips covering the whole leading and trailing edges and bottom. I have 4 years on my lee board and rudder blade of my small boat, outdoors 7/24/365 higher latitude, and the only place compromised was a non-reinforced leading edge where cloth layers did not overlap.

If you do not support the new layers of cloth, they will sag into the metal cutouts.

[mlydon]

Foam is available in different densities. You can get 2lb/cuft - that rudder probable wouldn’t use a whole cuft.....
You can also do a LIGHT skin of glass - a few layers of 2-4oz cloth - this will SEAL the rudder, but add no structural strength. The glass over the bays will be easily punctured, as well, BUT, you will have the lightest possible rudder. Toughness requires a thicker skin, added weight. Carbon adds stiffness, but isn’t needed in your case, as the rudder is practically all stainless to begin with.

Sanding the stainless just before glassing will get the best bond - it’s a purely mechanical bond. And use epoxy, not polyester resin.

I’m also into giant-scale radio controlled planes. We strive for the same qualities - high strength at minimal weight, with smooth, accurate surfaces. Many wings are skinned foam with some spars set into the foam, then glass vacuum-bagged to them. It’s the exact same process you would use on your rudder. Search the web for howtos and you’ll have all you need to do a great job.

Btw, I get foam and glassing supplies from Us Composites - pretty much one stop shopping.

Matt

[Calaban-Alleria]

If you haven't already I would suggest that you have a watch of of the sequence of Sail Life videos detailing a complete rudder rebuild very much like what you are undertaking. He is a meticulous craftsman.
Cheers

https://www.youtube.com/watch?v=_IjN9Sgm2qs

[Nani Kai]

Originally Posted by Dale B.

I have a stainless steel rudder that has significant plates cut out of it. It was filled with foam, wrapped in metal tape and encased in fiberglass. It was poorly done, very heavy and unsightly to put it nicely.

I removed the fiberglass and the tape (see images).

I'd like to remove the foam and then encapsulate the whole thing with something very light and waterproof. Any suggestions for a very thin, light weight waterproof coating that can span the cut out spaces while holding shape? Is fiberglass the lightest option? What other options might there be?

Thanks.

The wooden planks below the rudder in your photo appear to be 6" wide. Just a guess, but if that is the case then your rudder is approximately 48" tall, 12" wide and 4" deep at the rudder shaft. Assuming these numbers are roughly correct, the volume of the rudder blade would be about 1.3 cu ft if it had no curves and were a rectangular cube. But it does have curves so let's further estimate the loss in volume due to its curved shape is roughly equal to 25%, or about .3 cu ft. That would put the volume of the foam at roughly 1 cu ft. A cubic foot of ordinary expanding foam not in an enclosed mold weighs approx 2 lbs. which is not worth fretting about, especially since once it is submerged in water it will be weightless.

With that in mind, I would suggest leaving the foam as is, filling the cut-outs with epoxy filler and fairing it to the level of the SS. Then carbon fiber or fiberglass the surface of the blade to your desired strength.

My guess is that the heaviness of the rudder assembly is due primarily to the weight of the solid SS rudder stock, not the blade itself.

If you are still determined to do away with the foam, after filling, fairing and fiberglassing as suggested above, you might try dissolving the foam with acetone or lacquer thinner after giving the glass significant time to fully cure.

Hopefully you got to this point where the actual useful information resides. Apologies about the extreme speculation in the beginning of my post based whole heartedly on assumptions. Probably due to boredom from Covid sequestering.

Good luck with your project.