Polypropylene honeycomb panels for hull construction?

[harry tams]

Having searched for marine plywood to construct a Jay Benford 'Friday Harbour Ferry' I'm beginning to think of materials other than plywood. The price of plywood in Australia makes polypropylene honeycomb an attractive alternative. Has anyone used this product to replace plywood for constructing hulls?

Impact testing shown in a 10 minute video www.polycore.com.au appears impressive.

What also impresses me is the large sheet sizes available by some suppliers (2.4wide x more than 9 metres long).

Skin materials are thermally bonded rather than glued and it appears that many different skin materials can be used.

Does anyone have any experience with these products? Would you trust PP honeycomb core with GRP skin to replace plywood for hull construction for a 25' boat?

Harry Tams

[CDK]

Polypropylene is one of the few materials always excluded on instruction sheets of glues and sealants. When used as a core material in GRP you have to provide another means to attach the inner and outer skin to each other. Using compartments surrounded by GRP ribs might be a solution.
Thermal bonding is an industrial process using rf energy or a laser. It will only work for thermoplastic materials, not with GRP.

[bertho]

do some parts of accomodations, some deck parts not really critical in structural, this is nice (but heavy compare to foam, not to ply... ) material, but not reliable with thermic variation, , just compare with a empty drinking water bottle...!. it's the same material..frozen, will crack, hot, will melt.. i use this material 25 years ago...
cheer's
bertho

[harry tams]

Originally Posted by CDK

Thermal bonding is an industrial process using rf energy or a laser. It will only work for thermoplastic materials, not with GRP.

I'm sure they are doing it. The video on the website shows impact testing of both 600 & 800gsm resin infused fabric thermally bonded to a 15mm pp core. Perhaps I made an error using the 'term' GRP. Did I?

I also saw reference to the core thermally bonded to polyester and the resin applied later in the assembly process. Is polyester fabric used in fiberglass?

I believe the heaviest pp core weighs 80kg per cubic metre. The impact testing I refer to also makes a comparison with foam panels. Foam did not perform near as well.

I'm inclined to think that today's technology would have improved polypropylene properties over the past 25 years. I believe I also saw reference to the product being useful in the range from cold to hot as well as testing with regard to resistance to flames.

Harry

[AndrewK]

Harry,
PP core has a polyester veil thermally bonded to the core and you use glue to bond skins to the veil. Skins can be ply, metal, grp etc, in case of grp normal practice is to laminate the glass directly on to the veil in one process.
PP core is mainly used in decks and internal fit out and not so much in the hulls for the fear of water getting into the cells. But have heard that entire boats have been built with this core.
PP core has lower sheer strength than 80kg foam which is less then balsa etc.
You need to take this into consideration when changing from ply plans.
Although you say marine ply is expensive I think you will find that it is still the cheapest option when you take everything into consideration.
Search the forum as there have been a number of discussions concerning PP core, there was a good one about two years ago.

Cheers
Andrew

[harry tams]

Thank you Andrew

I did search the index before I posted the thread but only came up with my post in 'Boat design- coastal cruiser' and I think one other. I'll try again to find further information.

Can you explain sheer strength? Wouldn't sheer strength be tested to some degree in impact testing?

I did get some further detail from 1 supplier. Cost for a 10mm 8x4 sheet is $90.The polypropylene honeycomb core has a 200gsm polypropylene film thermally bonded to each side. 45gsm polyester is thermally bonded to that and it is used as a bonding film. The core itself weighs .08kg per mm in thickness per square metre.

With regard to price... its not only about price for me, I'm also looking for a lighter and a material that requires less maintenance and caution than plywood.

I have a trailer sailer with a foam sandwich cabin top that has delaminated so I'm also concerned about that issue with regard to foams (is that relevant to the topic of sheer)? The supplier tells me that PP core is stronger than foam in this area as well.

Harry

[teakcell]

Hi Harry,
What Andrew says is right. At the end of the day, plywood construction will be cheaper than any HCB. I'm a hcb manufacturer. When it comes to hull, hcb must be used only if lightweight is what drives you. Not the price. Go for plywood of cold molded plywood or even cheaper cylinder mold (kurt hughes) or others. Cheers

[Alik]

We use PP cores in many our designs in topsides, decks, cabins and interiors. For 4-5 years we have never been reported on delamination or fatique issues (besides some of those boats are in commercial use in tropical conditions).

For shear strengh - yes, strength of PP is lower compared to foam of equivalent density, but still enough for most tasks. Pls note that usually for PP honecomb structures (designed to ISO12215-5 standard) safety factor for shear is still 3...7. Why should it be stronger than nessessary?

In terms of weight - yes, PP honeycomb structures are about 20-30% heavier compared with similar structures in foam, mainly due to higher resin absorbtion and heavier material. But taking costs and weights together I still believe that honecombs are good choice for some applications, specially compared with plywood.

[AndrewK]

Harry, plywood, timber strip plank, PVC & SAN foam, balsa, PP core, you can produce a durable boat from all of these materials. You need to create your own list of pros and cons to make that choice. Don't just look at weight and cost, include ease of build and resale value.
Have a look at these two good discussions for a start.
Hexacor compared with other composites?
Foam cored 20 footer?
There have been others, try searching plastic honeycomb cores, plascore, nidacore , hexcell, polycore etc

Regards
Andrew

[harry tams]

AndrewK
Thank you for that.
I've read the information in the first link. Very informative. I'll read on and try some of the recommended 'descriptors'.

Harry Tams

[Herman]

I made some racing sailboats with Nidacore.

In terms of production it is a very easy material, as it can bend in 2 planes, so the panels used can be large in size. I laminated the outer layer, then put the 5 pieces of Nida in, and bagged the lot. After cure I laminated and bagged the inner laminate.

In retrospect, today I would probably have used balsa. Fatigue strength for small racing boats is not a big deal, stiffness is more important.

One thing that is something to keep in mind: The inner laminate of my boat was so thin, that it was porous for water. This filled the cells with water, after the boat got flooded and was left unattended that way. It took a great deal of effort to get rid of the water again. (I wonder what would have happened to the balsa option...)

Anyhow, shear strength is a bit low (which makes for good impact strength) and weight is a bit high. But altogether the product has nice properties.

[harry tams]

Originally Posted by Herman

I made some racing sailboats with Nidacore.

In terms of production it is a very easy material, as it can bend in 2 planes, so the panels used can be large in size. I laminated the outer layer, then put the 5 pieces of Nida in, and bagged the lot. After cure I laminated and bagged the inner laminate.


The inner laminate of my boat was so thin, that it was porous for water.

What would you recommend as the minimum for the inner laminate?
What would you recommend as the minimum or the outside laminate for a displacement hull designed for 12knots?
Harry Tams

[Herman]

I have no idea, as I do not have details of your boat.

For my boat a 200 grams carbon biax outside, and a 120 grams carbon biax inside would probably do, as the bare minimum. But with laminates so light, it is a bit stupid to use a (relatively) high weight, low shear strength material. If weight saving is a factor, then a thin balsa, allowed to flex somewhat could also fit the need, for probably half the weight.

There are so many options both in laminates as in cores, and in panel size (stringer placement) which all have benefits and drawbacks.