Filler inside Hobie fiberglass rudders?

Posted elsewhere, but putting a second one here, just in case someone else knows. Losing Matt's insight stinks (plus great guy), therefore casting a wider net -

So, Hobie fiberglass rudders aren't hollow, for obvious reasons, but does anyone know what they're filled with? They are 2 clam shells of a mold reattached, but I can't recall if it's just a mix of resin and micro-balloons or some kind of special filler. It's not expanding urethane foam... Ideas? I think I remember it being white-ish.

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ChuckC
H21SE- 408

I could be wrong, but I've always been under the impression that it's some kind of (dense) foam. When doing a repair, that's what it looks/feels like to me. I would think they'd be a lot heavier if it were resin/microballoons. (But I could be wrong about that, too!)

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Jerome Vaughan
Hobie 16

I have to admit I only glanced over the rules and regulations for this forum. I'm about to post the thoughts of the Claude AI on this topic. If the use of AI is not allowed on the forum, I certainly understand. I tried to get the AI to format for the phpBB bulletin board styles, but it's kind of whacked up. Anyway I think for someone familiar with fiberglassing, this will make sense. Here's the prompt I used.



Best Foam Choice: Rigid Closed-Cell Polyurethane, 2–4 lb/ft³

For a fiberglass-over-foam rudder core, 2 lb/ft³ is the industry standard and what most boat builders use. Here's why:

Why 2 lb Works Well

• Light enough to keep the rudder's weight and swing momentum low
• Strong enough to hold its shape during glassing
• Closed-cell structure won't absorb water if the fiberglass is ever punctured
• Easy to shape with a hot wire, rasp, surform tool, or sandpaper
• Bonds well with epoxy resin
• Cost-effective

Why You Might Step Up to 4 lb

• A catamaran rudder takes significant lateral loads — especially at speed
• If the fiberglass layup will be thin (2–3 layers), denser foam provides more backup strength
• Better resistance to compression if the rudder strikes something
• Easier to fair and sand without denting the surface before glassing

Recommended Approach for Your Build

Core density: 2 lb/ft³ rigid closed-cell polyurethane is sufficient if you plan a solid fiberglass laminate schedule (e.g., 4–6 layers of 6 oz cloth, or biaxial cloth). Go to 4 lb if you want extra peace of mind or are doing a lighter glass layup.

Mold considerations:

• Pour in stages if the rudder is thick — foam generates heat (exotherm) during curing, and large pours can warp or crack
• A two-part mold (clamshell) will give you the cleanest foil shape
• Lightly wax or use mold release on the mold so the foam doesn't bond to it

Shaping after demolding:

• Even with a good mold, you'll likely do some fairing with a long board before glassing
• The foam sands easily but be gentle — you don't want to hollow it before the glass goes on

What to Avoid

Option - Why to Avoid
Open-cell foam - Will absorb water through any crack in the glass
EPS (beadboard styrofoam) - Dissolves with polyester resin; also fragile
Very high density (8+ lb) - Unnecessary weight for a rudder core
Flexible foam - No structural value as a core


Note on EPS: If you ever want to use expanded polystyrene instead of poured foam, it's fine — but you must use epoxy resin, never polyester, as polyester will melt it.

Practical Product Examples

• US Composites 2 lb pour foam – widely used by DIY boat builders
• Foam-iT! series by Smooth-On – reliable, available in multiple densities
• Foamular / Dow Styrofoam – if you go the pre-shaped route instead of poured

For a 20-ft cat rudder, poured 2 lb rigid urethane in a shaped mold, covered with biaxial fiberglass and epoxy, is a well-proven and cost-effective method.