DIY and Gear

DIY Core Replacement on a Hans Christian 33t Sailboat


This is a detailed blog about how we did this job.

By no means is this the only or best way to do it

How to tell if the core is rotten?

Signs and symptoms that this HC33 was experiencing:

  • “soft” or “spongy” feeling decks
  • the decks would “give” while walking on them
  • a “dull/ hollow/ thud” sound when using a sounding hammer
  • high readings with a moister meter
  • actual “ooze” coming out from fittings or below
  • mold and mildew smell throughout the boat

Choosing a new core material

There are 3 main core materials to choose from:

1) A type of plywood or Endgrain Balsa

2) A type of closed cell foam

3) A plastic structure like Honeycomb

Along with the owner, we decided to go with Nidaplast8 a Honeycomb core.

Here is more information about this product from

Nidaplast polypropylene honeycomb structures with an 8-mm mesh are used for the core of structural sandwich panels.

Nidaplast 8 is the standard product in the range suitable for use with contact or spray lamination. The surface coating (polyester non-woven material) enables the lamination or bonding of virtually all types of materials, for use in many different areas: construction, transport, yachting, industrial equipments, sport, recreation, etc

The polyester non-woven material on the Nidaplast 8 provides an ideal surface for impregnation with thermosetting resins (polyester, epoxy, etc.). 
Thermo-welding this non-woven material to the honeycomb structure ensures perfect adhesion. 

The plastic film beneath the non-woven material makes the honeycomb waterproof and limits the amount of resin used.

The honeycomb is made of polypropylene, a long-lasting, 100% recyclable material. The honeycomb structure with a 95 % void ratio uses little material while ensuring superior mechanical properties.

You can buy honeycomb in virtually all thicknesses as well it can be scored for making camber or radius bends.


Pros for Nidaplast8

  • lightweight
  • strong
  • non rotting
  • sound and heat insulator
  • affordable
  • easy to cut

Cons for Nidaplast8

  • does not conform easily to radius
  •  lacks good screw retention
Removing Rotten Core

This HC33 needed about 97% of its core replaced. To “save” time during the filling/fairing stage of this project, we decided to remove the core (on the cabintop) from the inside. (The core on the side decks would be removed from outside.) We replaced the cabintop core first, then moved onto the decks. We chose to do the cabintop from the inside for many reasons. The interior headliner was damaged from water and rot, along with the plywood under the Formica laminated side panels. All of these things needed to be replaced, so by doing the job from the inside we avoided extra fairing labor when it comes to the final shaping outside on the cabintop. There is an added benefit of cutting out the inside skin of the core as it is usually a bit thinner and all of your work will be covered with trim and cover panels. A big negative is all the laminating will be done overhead and against gravity.

We did however have to address a few sections of the cabintop from the outside, as bulkheads or plugs blocked access from the inside.

Let's get Started

Interior Cabintop Sections

A) Remove all hardware, the headliner and side panels until the bare fiberglass is exposed.

We had to peel/ grind off the white laminate and white headliner to gain access to the screws holding up the plywood bases.

We had to remove the laminate from the plywood to gain access to the screws. Then we were able to remove the plywood panels from the doghouse
We had to pry off the glued-on scaving headboard to gain access to the screws holding up the plywood headboards
With the plywood removed, we had access to the bare fiberglass
The inside layer of fiberglass is exposed

B) Cut away the inner fiberglass layer to reveal core. 

We had to sometimes pry/ peel this layer from the core after we cut the edges.

Cutting a small section out
Small section cut out, you can see how wet the wood core is
 Wet core in the radius of the cabin top
exposed wet core

C) The fun part, remove the core. 

We used a hammer and chisel. If the core is wet enough it will literally fall out. The drier sections required a bit more prying.

Outside Cabintop Sections

Almost the same steps as above, but now we were not working above our heads! We had to remove any hardware that was in our way, then we went straight to cutting the glass away.

Forward Pad Section:

This was the first section we attacked, and because a large piece of teak lives on top, we worked on top of the cabintop. We followed the same ABC steps as above.
Prepping the Surface for New Core

Once the majority of the wood core was removed, a thin layer of resin and wood remained somewhat adhered to the lower layer of fiberglass. Using a grinder, Jon ground down the remaining core until the fiberglass surface was free and clear of all wood and old filler. 

Not only did we prep the surface where the new core would be laid in, but also a 3″ border around the section. We did this so the new fiberglass could be structurally tabbed back into the surrounding glass.

Jon is doing the last bit of prep with a die grinder
grinding fiberglass makes a mess
Port side had far less rotten core. Pictured is the section above the dinette.

Dry Fitting the Core and Fiberglass

A) Fitting the Core

Honeycomb: Fitting the core material was easy enough, as we could measure then cut it to fit.  The areas that were not flat or had a radius, we used the precut scored honeycomb. The scored material allowed us to make the bends easily to help keep the original shape. In the areas where the portlights go through the doghouse, we installed a plastic “plywood” product made by Aquaplas. It is a synthetic material which has strong screw retention. We also used this product in 1″X1″ blocks spread throughout to add extra screw strength when installing the plywood siding of the cabintop.

fitting the core into its section
Dry fitting a bulkhead section of the cabin top, using some davinacell foam in conjunction with honeycomb for a tighter bending radius
We used tape to dry fit all the honeycomb in place. The more solid looking piece is the aquaplas where the portlight will be installed.

B) Measuring the Fiberglass

The 1708 bi-axel fiberglass stencils took a bit more concentration to get right. We needed to make sure that we could achieve the necessary structural strength with 3″ overlays while still keeping the pieces small enough to handle over our heads. Because of the smaller sizes we needed to avoid the layered sections building up too high. We did this by offsetting each layer.

We used 3 layers of 1708 on top of the core material.

We used brown construction paper to stencil the layout for the fiberglass.

Installing Core and Laminating

We would have everything laid out and ready to go so that each step could be started and completed without any hiccups. We did all these steps in the same day to achieve a chemical bond and to cut down on sanding.

We used Laminating Epoxies from US Composites and from Boat Building Central. All together we used about 30 gallons of resin during DQ’s refit.

We installed the aquaplas material in the portlight areas a few days before. We would prep the areas around it for the core and lamination steps.

A) Clean and wipe down the surface with acetone.

B) Apply epoxy to clean surface.

C) Apply epoxy to the “membrane” skin of the core material

D) Apply thickened epoxy to the base of the core material and the prepped fiberglass surface (this helps prevent sagging and air-bubbles being formed)

E) Install core material

F) Apply thickened epoxy to fill in any gaps and the seems 

G) Saturate the 1708 with epoxy then apply on top of the wet core. Use a fin roller to work out the air bubbles. Finish the first layer, then move on to the next.

Applying epoxy to the prepped surface
New core and glass laid up
Rolling out air bubbles from the first layer of 1708
Rolling out air bubbles from the second layer
finishing touches on interior lay-up


We knew that we were not going to reuse the teak on the decks nor the existing fiberglass as a “skin” to put back in place after the core was replaced. Because of this, we did not remove the teak off the decks, Jon simply cut the teak and top layer of glass off to expose the core with an angle grinder and a metal cutting wheel, a messy but quick way to cut glass. To make our lives a bit easier, Jon used a peeler to  remove the exact amount needed for the 3″ tab sections. 

We then followed the same A-F steps as above. With the addition of apply fairing compound to the top of the laminate while it was still curing. We do this all in the same day so that we do not have to sand the laminate before fairing.

To achieve the needed strength and thickness, we applied 3 layers of 1708 on top of the core material. We took time and care to make sure the layers would over lap in different sections allowing the surface to stay “level”.

We used plastic and different colored sharpies to stencil out the fiberglass layers.

Wet core exposed

Removing the wet core

Core removed, prepping the surface.
Core removed, prepping the surface
the bow section ready to be dry fitted
Core for the bow section fitted
stenciling the pattern and layout for the 1708 on the bow section
the cut layers of 1708 for the bow

connecting the bow and side decks together

Products & Materials

Core Materials

  • H8PP Honeycomb 4X8 sheet – 13mm (1″x1″ scored)
  • H8PP Honeycomb 4X8 sheet – 16mm (1″x1″ scored)
  • H8PP Honeycomb 4X8 sheet – 16mm
  • Aquaplas 3 Transom foam


Epoxy & Cloth

  • Gallon MarinEpoxy Kits – Slow Hardner
  • 1708 Biaxial cloth with mat

Disposable Materials & Tools

Prism Blog Post: Indispensable Tools

Time and Cost


Approx. 450 hours

These hours are counted as if Jon and I were a single person. In a yard it would have been 900 total hours as we are 2 persons working.

*Note : This is the time it took us to gain access, remove the rotten core, prep, replace and laminate the new core in place. 

This DOES NOT include the finish work ie: the sanding, filling, fairing, painting and re-commissioning

The entire refit took roughly: 1400 hours 

Material COSTS

Approx. $6100.00

Tent Rental: approx. $1600

Core, Epoxy and Laminates: approx. $4500

Tools & disposable materials: approx. $1500 


7 thoughts on “DIY Core Replacement on a Hans Christian 33t Sailboat

  1. Great post!! Awesome work guys, way better than any yard here in PR where we are!

    Seeing how rotten the cores were on pretty much everywhere scares the crap out of me!! 😳

    Was this HC built in the original yard or the second yard?


    1. You guys went deep, the old core was so rotten. Very interesting to see the replacement and the materials chosen. I have watched a few boat construction videos, and the core is always a wild west situation. Thank you for showing the process. Keith Ciminello

  2. Thanks for sharing. I’ve got a couple of soft spots on my boat … I think I’ll pay someone to do it😄. I’m sure I could do a functional job from what you posted, I’d just rather not. I’ll stick to filling cracks.

  3. Well written and holy shit you earned your sundowners! Nice work 👌 I’m sure it was a tough decision but now that it’s done the piece of mind is priceless. We could learn a thing or two from your organizing skills 😉

  4. My thinking was that individual blocks of wood would isolate any leaks. How did the water get to all the wood? De-lamination from the fiberglass?

    1. Water intrusion came mostly from hardware installs, but migrated because the yard did not encapsulate the core with enough resin.

Leave a Reply