Best foam for rudder

[Tranona]

Thats interesting.


To me having a foam filled cavity with welded stainless steel fittings inside is a recipe for crevice corrosion and tang failure if ever water gets into the foam around the tang area.

If the rudder is properly constructed water should not get in. However many are not, even from so called quality builders. Preventing water getting in is simple if a bit of thought is applied to seal where the stock enters the blade and the two halves of the shell are joined.

As mentioned earlier by others, aluminium stocks and tangs are common and better techniques used to inject the foam and bond the shells.

 

[Daydream believer]

What happens when a boat with foam that is saturated is lifted out of the water in the winter & the water freezes? Will it expand & split the rudder or will the foam take up any expansion by compressing? if it does will further years of saturation/freezing then split the blade when the foam can no longer take the compression needed?

 

[Seasick Ian]

Thanks for all the input so far. My rudder was originally just glued together, no tape. The glue was distinctly visible as a strip about 5mm wide running around the perimeter of the rudder. It was showing some cracks along this seam in various places and rust was weeping out. I butchered it getting the two halves apart, and found a fair amount of water inside although it wasn't pouring out and sections were still pretty dry. The lay-up was really heavy in places and I cause a lot of cracks spitting the two halves, as the cut I made with a dremel didn't go deep enough. I should have gone for an angle grinder with a thin cutting disc in hindsight. The foam has gone hard as concrete in places, presumably where it has been soaking in seawater, although maybe that happened during curing?

The turning force is distributed from the tangs to the rudder shells by direct contact with the GRP moulding which has been built up to accomodate the tangs on both sides. I am going to strip all the paint right off of both halves, as I think i'll need to sheath it in an epoxy barrier coat as I flexed the shells to the point of cracking in a few places. I had considered building an entirely new rudder by shaping some foam sheet to a suitable foil shape and then laying-up GRP on top, but the would have required a different rudderstock design I reckon.

The boat has a full skeg and the rudder is attached at the bottom via a pintle/gudgeon arrangement. This should minimise the bending forces on the rudderstock. The new rudderstock is the same design as the last one, which presumably has lasted the 40+ since it was built with no failure of the tang welds due to torque, so should be strong enough in that regard.

As for foam, I am tempted by the screwfix builders option. I wonder if i could clamp the two halves temporarily while the glue expanded and filled the voids, and then epoxy the the halves together permanently. The big problem there is pulling the two halves apart to glue them once the foam has set. Will the foam adhere? I've never used to stuff. If I had to use some sort of release agent, that would really mess with the epoxy glue....

 

[Neeves]

You could attach one half to the rudder stock and fill with foam then remove. Attach the other half and fill with foam and remove. Take a sander and grind off the foam until its flat then glue the two halves together on the stock. You could wrap the stock and tangs in cling film or packing tape to make sure the foam does not stick to the the tangs and makes removal of each half easier (remove film before glueing together). You can then add very thin tape round the joint, I have found it stretches and can be applied quite smooth. I think, if I understand correctly, you are going to grind off the paint - so I'd do that before you glue together but you will need to fair the joint so this will need to be done once glued together. Then epoxy the lot. If you are concerned about any cracks I'd glass the whole thing as well - I'm assuming you are not too concerned about the extra weight but a layer of glass, if the rudder is cracked from your removal then it will give it integrity. if the cracks are more serious that I interpret then glass patches on the inside before you foam would be a good idea - and if you are really concerned you could make a whole new rudder but that sounds a longer task.

Hindsight is great - but the angle grinder with a fine cutting disk was the way to go. If I'd known you were using a Dremmel I could have told you - but its far too small.

Good luck

Jonathan

 

[peteK]

When I filled mine with foam I just drilled a hole in the completed rudder as the foam comes out the tin as a liquid and expands inside. Left it to set and then filled the hole after cutting off the excess.

 

[Neeves]

If the tangs are hard against the outer skin, which seems logical to me, then it is going to be difficult to ensure the foam goes to the opposite and of the rudder from where you inject. I'd be inclined to consider the possible routes the foam might flow and have another hole at the extremity, opposite where you inject, and if foam does not extrude I would inject from both ends.

As you need t make up some epoxy if you inject from one hole there will be plenty left over for the other hole.

I'd be foam filling after you glue together and before taping, then you can tape over the holes.

Jonathan

 

[geem]

We repaired rudders on our catamaran a few ago. They were hollow and the skins were quite flexible. They had osmosis. We peeled them and used a wall paper steamer to get the moisture out. Once they were nice and dry we cut a hole in them and poured epoxy in to the bottom to cure a persistent leak from the bottom s/s pin. We then drilled a few more small holes in the skins and filled the rudder with high quality two pack foam. The extra holes helped us judge how far we had filled them as we did it in several small batches of foam. Once they were foam filled we added two layers of six ounce cloth and epoxy to the whole rudder and then filled and faired with epoxy filler. I then spray painted them to match the hull as the rudders protruded up to deck level on the cat. They never got water in again and they were considerably stiffer with the foam.