Plywood designs as foam core and fiberglass

Hey everyone, I am currently in the material research side of building a sailboat. I am looking at the Wharram Tiki 30 design, but due to the rising costs of marine plywood, and the area that I live in I'm looking at alternatives. Wharram has a Tiki 8m which is a fiberglass version of the 26, but it is a design one of the professional builders has exclusive rights to. The designs call for 9mm(3/8"), 6mm(1/4"), and 4mm(5/32") plywood with 200 g/m2 glass.

My questions are these:

What core material(s) with what weights of glass can I use as the plywood substitutes while still maintaining structural integrity, and if possible with a minimum of final weight deviation?

What will these materials cost? As I am going to be the one doing the work, I don't mind trading material costs for extra labor.

Thanks in advance

 

Welcome to the Forum LF.

I think the basic answer here is 'it depends......' - on all sorts of other things that have to be taken into account as well.

But maybe @fallguy might be able to give you some rough ballpark estimates for foam thickness and number of layers of glass - he is currently building a foam sandwich Woods Skoota catamaran.

 

Nine millimeters of plywood corresponds approximately to 12 millimeters of Divinycell H80 with 1200 grams glass on the outside and 600 grams glass on the inside.
Construction method with prefabricated sandwich panels https://www.boatdesign.net/threads/construction-method-with-prefabricated-sandwich-panels.64651/#post-891098
JS

 

There isn't a way to simply replace plywood with a foam core. For example, the Tiki 30 has hardware attached to the hull which would require a re-engineering of those areas.

 

It depends how you go about it, but it sounds like a lot less work with ply.

 

My plans included a foam/glass booklet. There's a section on substituting materials. I posted the relevant page below. I decided on foam and glass for my build because the price increase was worth it to me. The resale when it comes time for that is much better.

 

The first sentence states: "The conversion formulas presented here are not to supersede your designer specifications". That means that the skin needs to be re-engineered. At the end it also has a statement that makes doubt the accuracy of the manual: "so the extra fiberglass sort of compensates for this inequality" (italics mine).

 

When you say convert to GRP sandwich, you mean pre-fab panels already glassed ?

 

I strongly recommend you suck it up and pay the extra.

As Gonzo wisely points out, the hardware connections in foam are a primary concern. If the hardware points are well understood before laminating, this reduces the issue, but there is so much more to consider...

First if all, good foam is really not cheaper. The skin thicknesses MUST increase as HJS points out. This means more glass cost and more epoxy, but that is not all.

I suppose you could avoid using vacuum, but building with glass and foam core is best done under vacuum. The bonds are better, impact testing better, all of the attributes I can think of are better with the excess resins pulled out. This means a very simple build process of light glass on plywood has become more expensive and more costly.

As to cost, let's use @HJS figures and say you use Corecell 12mm and 1800 grams of glass versus 400 on 6mm ply. The glass cost for foam is 4.5 times as much and so is the epoxy.

I can tell you from experience that the core and the ply are actually very close in price.

Epoxy is roughly about $100 a gallon.

Fiberglass is roughly $10 a yard say, and let's use 400 gram glass as a baseline. To keep it simple, let's also go to English for me. 12 oz glass is 400 gram glass, also really nice to work with, but not as noce as the 200g spec for the plywood... I digress. Anyhow, a 30' boat is 10 yards long, let's say 3 yards wide or 30 yards for the hull.. in 30x12 is 360 ounces of glass for the ply version or 22.5 pounds and you need about the same in epoxy by weight plus losses or about 3 gallons. Roughly $300 in epoxy and $300 in glass or $600 in epoxy and glass.

For the foam boat. Simply 4.5 times $600 or about $2,700 in epoxy and glass.

Let's say foam is $150 for 3 yards, the core is $1500. But you need a jig and can't build it like ply.

Assume the worst and the ply is double, it comes in at say $3000.

So, crude figures, foam hull hand laid somehow?, $4200...ply hull $3600.

And you are forgetting all of the reasons and issues related to foam. A ply hull can be made over timber framing. A foam hull is supposed to be made light with vacuum. A foam hull cannot simply be laid over a jig and glassed as a full unit, seams are typically reinforced with better than wovens, then the hull cannot be built upside down easily.

It is really no competition. If a boat is specifed in plywood; do not deviate.

 

Honorable mention... @Scuff had specs for foam; this is different..

not sure if he used vac or how the build changed, but guessing it did, I would like to know how the boat was formed, in a female jig, over a male, etc and how foam sides didn't misform, etc

 

Buy the book " Foam Fiberglass Sandwich Construction by Ed Horstman " . Hard spot connections for hardware are discussed . Contact your designer and ask about the hard spots . Form ply such as Russian Birch or Finnform , not HDO . Formply is heavy but is as good or better for bulkheads , stringers , and hard spots , and should be cheaper than marine ply . Foam for hull , ply for bulkheads .

 

How to make your mold for foam is in Ed,s book , the web sight Bateau also has a good article of foam construction in their Tech support section . Find some place like Mid South lumber to price your form ply DuoFace Birch or DuroFace 220 , you will have to sand the edges and and anywhere you tab or glass . Don't forget marine grade fir if your on the West coast . Work up your material list and then get bids on a bulk purchase , break it down to epoxy , glass and foam . When buying also keep in mind sometimes buying a roll of glass can be as cheaper than buying almost a roll , and factor in shipping cost in your material price .

 

The sheet I provided is out of Ed's book he provided when I bought my plans. He also provided the layup schedule. The hulls are built on a male mold and laid up by hand no vacuuming. I did a peel test on samples and the foam comes away with the glass. I have been able to get 50% glass to resin ratio. Once the outside is laid up you screw stringers at the sheer and spreaders at regular intervals to maintain shape. Bulkheads will be done on a table and I may try to bag those. I just flipped the second ama I'll snap a pic after lunch.

 

Scuff , I have plans for the 31 , I bought all my material first , got ready to form it but lost my free parking spot when my brother sold his lot . Sooner or later I'll build something I can pull home . Are you using foam on your bulkheads ?

 

Here's that pic. The layup he provided used mat and was geared for poly or vinyl. He was not interested in helping me modify that. In the end I had another NA help me. Knytex has a free program that you can use if you have the skills .. I got help. There's actually a thread on here I started when I was trying to get the layup "modernized". I'm an amateur and don't have mad math skills but my estimate was a savings of 300 lbs on the boat by using epoxy and no mat.