polypropylene honeycomb vs plywood for wharram

I have been thinking about building a tiki 26 and am stuck on whether to use plywood, or to attempt it with polypropylene honeycomb. I am thinking that the honeycomb will possibly be lighter, easier to maintain and have a higher resale value.
Has anyone tried this before? considering much of the strength in the wharram design comes from the ply, would I have to use more glass to make up for it?

 

Honeycomb as core can only carry shear stress, thus thicker fiberglass skins are required.
Plywood is structural material on its own, so fiberglass is only use as protection layer.

Honeycomb as material gives 120kg/m3 density in structure, but shear strength only 0.3-0.5MPa. To compare, 60kg/m3 PVC foam gives 0.6-0.8MPa. Yes, there are high strength honeycombs, but for them I would be careful to trust manufacturer's strength properties unless they are supported by independent testing/certification.

Thus, given low shear strength on PP honeycomb, You need to put stiffeners more often with, and use thicker skins.
But it is possible, and I think better than plywood.

We recently designed a replica of Pahi63 in sandwich, to ISO standards. But we did not use honeycomb - by shear strength, it is banned for boats over 15m in length, see ISO12215-:2008/2014.

 

Alik,
Thank you very much for your helpful post.

 

Alik's expertise is real. I wanted these pics as post scripts (wouldn't place at end?), but basically polypropylene honeycomb has some pluses in certain parts of a build.

Here a catamaran cabin bottom with panel widths wider than 4'.

Here a removable panel 53" wide.

Note the edge requirements. This is a lot of extra work; raw edges of honeycomb are no good; just like ply...sort of. See how we had to dog it in to glue on the high density for fastenings? Then squeezeout, then sanding squeezeout, then prefilling before glassing..


Alik is the expert. I can tell you that the build time for plywood is at least 1/2 that of polypropylene honeycomb, probably more. All of the raw, exposed edges of honeycomb must be cored with something different.

It has a place. I built my cat cabin bottom and sole with it and cockpit panels... see pics. But for a Wharram hull, as a builder, no.

It is also cheap up front, but resin thirsty in hand layup. The veil requires a wetout greater than foam cores. You can buy it for bagging; I have not. A place it would actually be nice in your build is cockpit base or decks. The great advantage I have found is you can buy it in 5x10' sheets. I have a vac table 49" wide only. And, cost. The cost of a one inch piece of plascore 5x10 is about 1/3rd the cost of Gurit M foam. But you do pay in resin and that means weight, too.

The stuff has its place. Your hulls are not it.

sorry for the repeats and grammar..no time to edit firther

 

Fall guy, thank you for your input as well. I am well versed in plywood/epoxy construction and know nothing at all about honeycomb construction, so your estimates for time are probably pretty generous. meaning for me ply might be 10x faster!
For me, my time is worth more than material costs (within reason), so don't mind spending more if it keeps the project clipping along. Do you have a material that you would recommend to work with besides plywood?
maybe a material that works similar to ply with out the potential for rot?
Again, I appreciate both you and Alik"s informed opinions!

 

Yes I agree with fallguy about butt joints of honeycomb, we solved this problem by:
- minimizing joints;
- joints through single-skin zone (if produced in mold);
- joints on another (overlapping) piece of core;
- joint by 'shear tier' type of laminate (Z-shape laminate over honeycomb edges)
We used honeycomb intensively 10-15 years ago, but not anymore. Now there are quite affordable foam cores, look at PET foams for example.

 

Alik, Thank you for that advise.
I am attempting to do this project in Portugal and for this have a couple of supply issues to deal with.
I have found a place in Spain that sells PVC foam. One thing to consider if you are trying to replace ply with another material such as foam is how much fiberglass and resin it will take to achieve the same strength.
There must be a formula for that right?

 

Also, with the foam core density, the denser the foam, the stronger and therefore less glass needed?

 

You are in the single market, you can buy materials anywhere in the EU. The correct way would be to have the scantlings redone for foam by a competent NA. Converting from ply is not just a formula.

 

at the end of the day it sounds like trying to do out of anything aside of the ply is a time consuming expensive risk...

 

There is a version of the Tiki 26 in foam core, designed for series production, it is called the Tiki 8m. The molds are now in the US, the builder is a member here.
Tiki 8m Wharram Catamaran http://tiki8m.blogspot.com http://www.boatsmithfl.com/tiki-8m-2

Maybe Wharram will sell you a set of plans for that version, you need to ask them. Otherwise you hire someone to convert the design for you, but then you can just buy a different design of open deck catamaran.

 

Looking trough the above linked blog it seems the scantlings for the Tiki 8m hulls are 1 layer CSM, 1 layer 1708, 5/8 Divinicell, same on the inside. Vinylester and polyester resins. Joining the two halfs are 5 layers of 1708 tape.

 

Regardless other layers, I would always use CSM both sides of foam core. This is a requirement from Lloyd's Register for sandwich laminate, unless one is using epoxy.

 

For that Wharram, stick with ply John. If you look at the background of my pictures; you'll see a Gurit M foam; Gurit pvc hybrid. It was going to be all M; except I had to hand laminate the deck shoe. So, the deck shoe has a bit of pvc if you look close. Because I had some non perforated pvc, but all my M foam was perfed for wet baggin.

Foam is really a lot more work John. It has no stiffness on its own, so at least one side must be laminated on the table. This then means you might as well use vac. Everything takes so much time with foam. Will it be lighter? A bit. Are my extra bottles of wine worth the extra year of build time? Nope.

these guys are talking about foam; if you can buy half shells or hulls made by D Halliday, that would be a good option, not sure he'd do that..building in foam has been interesting, guess what the epoxy throw away rate is for vac bagged parts?

As for foam densities, different densities are used in different places; for example, I am using 12# density foams in the hull core where loading may occur. The transoms are made with 26-28# density. The hulls 5#. The polypro I showed you is 4# iirc. The glass schedule actually has less to do with the foam than the use of the hull. For example, my hulls are made with 22 oz triax largely on 5# foam (bottoms double both sides), the transom is made with 4 layers of 17 ounce each side and uses 26# core and the 4 layers wrap to 8 on the top..

I recommend you build in ply. Faster build, funner build less epoxy. More less than you imagine. Yeah. More less!

Consider this, if you laminate a large hull panel with wet bagging, lets say you are using 22 oz glass and 10 yards or 220 oz each side. 440 oz of glass requires 440 oz epoxy to wet out and say 60 oz for the core or 500 ounces or 4 gallons or so. Wet bagging removes unneeded resins and about half. So 2 gallons of resin in the bin. With plywood, you sheath less glass; uses less epoxy and even if you bagged it; way less waste of resins. Building with foam will add at least a year to your build time.

 

Thank you all for the information.
As I said before, I am pretty experienced with the ply/fiberglass method of construction, but not at all with foam or bagging. I am a ship's engineer by profession, so that vacuum really calls my name! haha
CSM on foam, thanks for that.
Rumars, where did you find the scantlings? I could not find them.
Fallguy, doing a bit of research it seems that a pretty straight forward way to achieve hull shape is to build a frame with battens and then attach the foam to the battens. that would be less work than the mold, right? In my ignorance I am sure there is a critical step I am missing.
My friend built a honeycomb/fiberglass dingy that is bulletproof. I am thinking to use that as a mold to experiment with a couple of the things that we have talked about here. I would rather have a relatively cheap short failure than an expensive time consuming one.
thanks for all your input