Missing rudder foam? revised

[Dubo]

I am in the process of restoring foam and shape for H340 rudder. It has sunken in on lower starboard side and has been dripping water on haulouts. I routinely drain about 8 oz water. Port side has retained proper shape. My confusion is, what is the explanation of the foam shrinking? Looking at the pic you will see a gap between the FG and foam. Does foam shrink in salt water? Does heat cause shrinkage of the foam? Being closed cell, I would expect foam to be stable. I know rudder delamination can occur, but I wouldn't expect it to be inward. Is this weird? As a side note, one thing I discovered is there is no SS rudder post which was a surprise to me. I guess no corrosion issues with a composite post can be a good thing.

Updated: I was able to verify this rudder was lacking foam at the base which would explain the sunken side. This would have been a manufacturing problem. The sun heating the rudder on the hard could explain the concave area since it was void of foam. The second pic is the bottom of the rudder cut open.

 

[twalker H260]

My understanding is that foam rudders should be painted light colours in order to avoid delamination and deterioration of the foam due to heat - primarily caused by solar gain when on the hard. A search on the forums will bring several results discussing this and I believe is also the recommendation of Foss Foam who manufactures many of the Foam rudders used on Hunters, Catalina’s etc.
Don’t think the water is a huge issue as long as it’s drained before being exposed to freezing temps. Many owners in more northern areas have mentioned in site postings of drilling a hole in the rudder each year to drain the rudder for winterization purposes.

 

[dmax]

There should be no gap as you suspect - the foam they use doesn't shrink once it sets, it was probably that way when it was made. I would drain all the water out and let the rudder fully dry and then seal the rudder so it doesn't take in more water (at the top of the rudder post and where it enters the fiberglass). Then I would look at injecting some foam into the gap and fairing out the depression if necessary.

 

[Don S/V ILLusion]

There should be no gap as you suspect - the foam they use doesn't shrink once it sets, it was probably that way when it was made. I would drain all the water out and let the rudder fully dry and then seal the rudder so it doesn't take in more water (at the top of the rudder post and where it enters the fiberglass). Then I would look at injecting some foam into the gap and fairing out the depression if necessary.

Quite correct. Foam doesn’t shrink. This is a delamination Issue not uncommon to Foss rudders.

 

[Dubo]

Quite correct. Foam doesn’t shrink. This is a delamination Issue not uncommon to Foss rudders.

It didnt have any foam to de-laminate from. Foss left a gap there when they injected the foam which I discovered once I dissected the rudder.