Polycore for flat sheet catamaran

Thanks for that clarification Rob

 

Oram cat builder here, it's a big no to polycore from me. A friend built a 40 ft cat from polycore and he won't do it again. It has its uses as rob said but i would go with Infused panels in pet foam. It is Foamed polyethylene plastic and has good compression but lacks a little in sheer, going to 110 kg3 gives similar properties to h80 and it is much cheaper. It is almost competitive with ply epoxy if you use vinylester for price per m2.
My friends cat needed to be reinforced several times with uni to stop the whole boat bending as he could not maintain rig tension, incidentally it was a mast aft head sail only rig Brian. Even with several extra layers of uni on the mast beam the rig tension could not be maintained and the boat kept flexing. Also the weight of polycore is much heavier than h80 foam. The quoted weights are usually without the scrim on each side and the extra laminate required to make it nearly as stiff as h80 defeat the purpose of using it in the first place in both cost and weight. Pet foam with grid score and perforations for infusion is the cheap option for the future I think, I'll be trying it out shortly and I'll post the results when I'm done. I am not using it for a boat, I know that's blasphemy / heresy / lunacy take your pick, I deserve to be banished, but it's still going to be an engineered composite monocoque structure so similar principals and loads etc.

 

I assume you are referring to the vessel I linked to with the first posting of this subject thread?

 

It could be Brian I'd rather not say. The boat has a new owner and i would not want to comment. Where are you based in thailand, I spend a lot of time in kanchanaburi.

 

I'm living in Khon Kaen when I am there,.... but Florida most of the time

I visited Kanchanaburi about 2 years ago, preliminarily to look at a lot of the houseboats they have there.

http://www.boatdesign.net/forums/boat-design/retirement-houseboat-floating-home-23987-11.html#post679692

http://www.boatdesign.net/forums/boat-design/retirement-houseboat-floating-home-23987-12.html#post692195

http://www.boatdesign.net/forums/boat-design/retirement-houseboat-floating-home-23987-12.html#post692196

 

We have a few friends in kon Kahn. My brother in laws brother Andy from australia and a pommy guy Dave and his thai wife detoy whom we met when we travelled through a couple of years ago. We spent a few nights at the aussie expat bar in town just down from the market.
we have a motorbike in kanchanaburi and will be moving there in the next couple of years. I have an oram cat to sell before we move. I am looking at a canal boat in Europe to get my water fix and northern hemisphere winter in south east asia somewhere.

 

I have mentioned the aft mast polycore boat plenty of times as evidence of its lack of stiffness. I have been on the boat and the owner is a nice guy. Even when he had it up for sale he was very honest about the lack of stiffness and issues it caused. But then again correct engineering to begin with would have prevented the issue. The added weight and cost of other materials to keep it stiff would probably not be worth the hassle.

Sailhand this is the first I have heard of PET foam. Sounds promising and seems to have instantly made PET honeycomb redundant.

https://www.corelitecomposites.com/corelite-pet®2.html

 

PET foam

https://www.corelitecomposites.com/files/CoreLite PET Data Sheet.pdf

Vs PVC foam

https://www.corelitecomposites.com/files/CoreLite PVC Data Sheet.pdf

The 110kg PET shear strength is 1.32 mpa
The 80kg PVC shear strength is 1.13, 100kg is 1.49

So shear strength is lower, however the shear modulus (stiffness), the Achilles heel of the PET polycore is still quite low. Probably just a PET issue.

110kg PET 29mpa
80kg PVC 29mpa, 100kg PVC 37mpa

Compressive modulus is much higher for the PET though even though compressive strength is lower. Not exactly sure if that is good or bad when it comes to backing plate requirements. Still not sure if the benefits are there over plywood other than being able to say "no plywood".

 

Hi Dennis hope your legs better, like I said the 110 kg pet is similar in properties to h80 pvc but much cheaper. I think the main problem with the polycore is the honeycomb structure the thicker the polycore the less sheer strength it has and it deforms easier. Used 6mm polycore in a lightweight dinghy a few years back and it still seems ok . The other plus for pet is that it's recycled pet bottles ground up and foamed so a good way to recycle all the empty soft drink bottles. Ian that used to own the polycore boat used to come and sit under our oram cat when we were doing our refit whilst he was building that boat. Ian is an engineer as far as I know and designed the boat from scratch. I helped him launch it and one of my mates did most of the build work on it and the subsequent refit. Like i said before it's a big no to polycore for me, pet might be OK it's my next little experiment for some road based fun.

 

Sailhand, where are you getting your PET foam?

Dennis,
The benefits over ply are definitely there if you infuse the foam. Assume two 12m cats have 12mm core with stringers and frames to support it. (the foam one would probably have thicker core and less framing, but that complicates the arithmetic)

The foam boat would have 600 gsm glass inside and out, the ply one 200 outside, 3 coats epoxy inside. Assuming H80 foam, the infused foam weight per sq m is 2.8 kgs (1:2 resin glass ratio), the ply about 6.5 kgs assuming Okoume/gaboon ply at 16 kgs per 3 sqm sheet. If H100 foam was used, the weight goes up by 0.25 kgs.

The cost of the foam (H80, not sure about the H100) will be higher, but there will be less waste. It is less effort to cut and join foam and glass with knife, scissors and hot melt glue than cutting, scarpphing and carrying ply around the shop.

The labour involved in fitting out a ply boat is huge, up to 50% of the total. With Intelligent Infusion it is a smaller percentage of a smaller number.

Not building in ply is a good reason, but for a lot more reasons than low resale, rot and maintenance.

Are you back in Bris? Still got your drone? Bucket List has had a little set back (busted rudder mount), but should be ready for test sails and photos in a couple of weeks if you are still interested.

 

Rob not sure where i will get my foam yet there are a few options available still looking at this stage. I have a house build to finish before i get to play with composites again unfortunately. I agree with you about ease of build and fit out with foam also much much less weight. Easy might not be so easy after all. Pricing at this stage is around 60-70% of pvc foam. Actually not that much different to the cost of good ply and the resin is cheaper. I won't be surprised if overall cost is lower with pet foam, this could be a game breaker for the ply lobby. Pete schwartzel from carbon works put me onto it. He is a great guy and very helpful. He is engineering my next project and so far has been fabulous to deal with. Bob oram recommended him and he was right, he did a free standing carbon rig for a 60 footer for bob a few years back.

 

I assume you are going to look at a canal boat that already exist over in Europe?...there should be a pretty good selection of used ones on the market?

So you will spend the really hot Thai summer months in Europe, and the cooler winter months in Thailand?

Did you see those photos of the houseboat under construction on the waterfront there in Kanchanaburi?,...pretty quick and easy construction, ....and no land purchase necessary, which can be a real problem for foreigners in Thailand

 

Polycore Boat's Stiffness

Hi Dennis,
I would like to make note that it was NOT just the polycore construction that contributed to Lyra's lack of stiffness. I have addressed some of these issues in another forum discussion HERE

I re-emphase the fact that this vessel had very little, if any 'longitudinal stiffness helpers' such as the nacelle structure I specify on most all my cat designs.

If you look closely at how he attached his backstays/shrouds you will find them all attached to an aft step structure, and that structure can readily bend upwards via the flat deck 'plate' forming the aft deck floor.








Neither the backstays,... nor the shrouds are attached to any major bulkhead of the vessel.

On top of that look at the construction of any of the 'major bulkheads' of the vessel. Do you see anything that resembles an extra stiff/beefed-up bulkheads. I don't, ....all I see is a single this plate of sandwich construction glass/polycore material as utilized all over the vessel.







Summary: Way under engineered structure for this catamaran vessel and its higher tension rig requirements,.....not necessarily the fault of the polycore material.

 

Vessel Substructure to Support Rigging Loads

Just found an old posting of mine that relates to this 'stiffness' discussion.

Vessel Substructure to Support Rigging Loads
http://www.boatdesign.net/forums/sailboats/sail-loading-rig-rig-loading-vessel-2293-4.html#post114487

 

Interesting posts Brian, but did you miss the part where I said "But then again correct engineering to begin with would have prevented the issue."