INTRODUCTION
In order to clear out space in the hovercraft shed, it was decided to dispose of the hull from the first hovercraft. Prior to disposal the hull was sectioned in multiple places to examine how the interior had held up after several years of use.
TOOLS USED
- Electric chainsaw
- Sawhorses
- Camera
PROCEDURE AND FINDINGS
The hull was laid on the sawhorses and the first cut marked on the hull, just aft of the seating ply. The chainsaw was used to cut the hull.
Massive Great Stuff expansion was found, similar to other areas of the hull examined previously while the craft was in service. No meaningful moisture was found.
Crack formation in the foam core was noted where the plow plane blocks lined up with the top panel edge. Not good.
It was also frighteningly easy to separate the Great Stuff bond. This proved to be a common theme throughout the demolition.
Equally frightening was how easily great swaths of fiberglass skin were pulled away from the foam core.
Original Glassing Procedure:
The core surface was lightly gone over with a rotary sander to take off the “skin” that forms on top of the panels. The fiberglass cloth was laid down and positioned on the core, and the epoxy was applied over the cloth and “squeegeed” down and evened out. The obvious error here is inadequate surface prep.
Places that had been repaired later adhered to the core much better.
This shows a section cut down and through the in-hull wood fittings that held the prop cage. You can also see how an entire piece of the top layer was easily peeled off. Also visible are screw holes protruding through to the lower layer of foam, which provided a water entry point deep into the core of the hull. Some slight moisture was noted in the Great Stuff layer.
Another example of a structurally failing Great Stuff bond. This was the story all across the hull.
Showing the aft prop cage attach strip. Unlike the forward strip, this one was showing some fairly advanced rot. Evidence of some slight water penetration is visible along the aft edge of the hull where the fiberglass has been removed. The skin that has delaminated here (from cutting with the chainsaw) is 3 layers of 10 oz. fiberglass; no difference was noted in how it could be separated from the hull.
These images show the mounting block for the anti-sway brace for the Thrust Engine Mount and its mounting plate. This was added at a later date, over Christmas 2006, when the craft was outfitted with the more powerful thrust engine. The backing plate was bonded with an epoxy slurry made with microballoons, and I was unable to separate it from the hull – the bond was extremely strong.
Upgrade Fabrication Method:
The original skin was sanded away with a belt sander, and the piece of plywood epoxied directly to the core. I do not remember what degree of surface prep was done, but I suspect the surface was roughed with the belt sander to some degree. Then 2 – 3 layers of 6 oz. fiberglass were used all around the edges of the plywood base plate. Again, this piece was firmly bonded to the hull.
This image shows a cross-section of the plywood block for the cleats. These were glued (epoxied) down with a piece of Kevlar felt in between to strengthen the bond, and that worked very well: this bond was still very strong.
This image shows the second large cross-section cut on the hull, across the seating ply. Extreme Great Stuff separation is readily apparent. The fiberglass under the seating ply was still firmly bonded to the hull and showed much less inclination to delaminate than the rest of the hull. If I was not already planning on it, this would convince me to skin the top of the hull of the next hovercraft with plywood.
These three images show how frighteningly easy it was to pull apart the entire section of hull! It took almost no force to simply peel my entire core apart. This is what really, truly convinced me never to use Great Stuff again: it has no chemical bond with the foam, it absorbs water, is weak in tension and shear, and compresses easily.
Great Stuff Surface Prep:
As you can see in the images, the surface is aggressively scuffed and sanded with very coarse sandpaper (60 grit range). No pinholes were punched in the foam, but in retrospect that would have been a good idea.
Another mistake made here was the sheer amount of Great Stuff that was used in the joint. Even with 4 large garbage cans full of water on the hull plus other random heavy items, it still expanded over an inch in some places.
This image shows water penetration under the port side legs that held the lift engine to the hull. The foam had pockmarks in it from water sitting there for long periods. The fiberglass was heavily delaminated around the lift engine mount legs.
The starboard side lift engine leg mounting area. Gasoline was spilled on this area on 4 July 2006, and ate into the foam underneath the mounting leg. It was repaired using foam chunks and an epoxy slurry. This repair held up reasonably well. Again water penetration is present.
Sectioning the lift duct to look for moisture or damage. Besides the shoddy construction, no glaring delamination, structural issues, or water was found (the piece of foam with water damage on it in the foreground is from the hull, not the duct).
CONCLUSION
This hull was in extremely hazardous condition. The core had mostly separated from itself, and the skin was, at best, tenuously bonded to the core. It is, frankly, a miracle it did not fall apart sooner with the larger thrust engine installed. Specific key structural points on the hull were very close to catastrophic failure. It is a good thing this craft was decommissioned when it was.
The Great Stuff failed from the beginning to form a chemical bond between the two layers of foam in the core, and the skin was never strongly or correctly bonded to the core. Yet despite these glaring shortcomings in construction methods, the hull managed to hold together for the better part of three years. This is a testament not the skill (or lack thereof) of the builder, but to the incredible strength and forgiving nature of composites.
Lessons Learned:
- Don’t use Great Stuff!
- Surface prep is KEY to getting a good bond between foam layers and between the skin and the core.
- Use more than one layer of fiberglass to seal the core – one layer has lots of holes, even with an epoxy topcoat.
- Skin the top of the craft with 1/8” plywood or doorskin.
- Epoxy slurries seem to bond MUCH better than straight epoxy.
- Be careful of water penetration points, such as screws.
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