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James Chilman

Durakore vs Cedar

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I'm thinking about building a mini 650 of my own design, and want to feed back on what the best core would be.

I want to strip plank the boat, having done a little of it before and not being a boat builder.

Also all the leach and a lot of the shaw 650 where built this way.

I'm looking at a these options.

13mm Durakore for the hull with 400gsm E glass skins,

9mm Western red cedar with 400gsm Eglass skins

9mm Cedar with Carbon skins.

 

In you opinion which of the above would be, Lightest, Stiffest and build the best boat.

 

I think I'm going to use flat panels to make the deck just 12mm 80kg foam with 300gsm and 400gsm glass.

 

What are peoples experience with using any of the above?

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For the same skin weight, carbon wins the stiffness contest easily. Reduce the carbon skins to save weight and cost and match E glass strength, you could have a local puncture issue. Better to save money in the hull build and put the savings towards carbon spars.

 

Re cores, have you looked at using Paulownia:

http://www.paulowniatimber.com.au/marine.ph

 

http://www.storerboatplans.com/Faq/paulownia.html

Cheaper and lighter than cedar. It is also a bit softer.

 

I have nearly finished building a 32 ft Proa using carvel butt glued 190x12mm planks, glassed outside and inside in heavy load/traffic areas only.

 

PM me if you want more info.

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The Gougeon brothers have done a lot of research on wood for coring. Chapter 5 of their book goes into some of the advantages of using wood compared to other materials: it's relatively cheap, readily available, really stiff, and resists fatigue well. If durakore is simply balsa core between wood veneers, it may not have the desired stiffness or rot resistance that cedar can provide. Pricing is another issue. Have fun building!

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Paulownia has almost no rot resistance. It's a lovely wood to work with, as is Eastern White, or Atlantic White cedar. Western Red Cedar is more brittle, and more subject to splintering, but undoubtedly, easier to find in long lengths and wider boards in most of the country...... Of course, almost all WRC will be KD, and a lot of the AWC or EWC might be AD, which might make a difference, although by the time you get to skinning the hull the AD Cedar will probably be at EMc anyway, so...................

Personally, I would go with AWC.

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Paulownia has almost no rot resistance. It's a lovely wood to work with......

What rot!!! They use it a lot for solid and even hollow surfboards, usually oiled, not even glassed. From my experience with scraps laying around in the wet for over 2 years during the build, not a sign. Staining yes, warping yes, but that it.... I bought a cheap Chinese chicken coop about 3 years ago.... Stained paulownia, been out in the rain and sun ever since. Not a sign. And it is totally contrary to the suppliers claims, the same ones who supply the surfboard market.

 

It is soft, so just treat it like a core material. Spread loads, reinforce locally for or avoid point loads. Weight is roughly three quarters that of western red cedar. So which is stiffer, skins 9mm apart or 12mm apart???? I know which one is cheaper and more insulative.

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corecell comes in bead and cove.

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Interesting. Both links talk about Australian plantation grown heartwood. My only experience is from wild grown North Eastern USA lumber, and it probably had both heart and sap wood.

I know it grows very fast, so is a species that is plantation friendly.

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Here's a link to some of the panel testing done by WEST System http://epoxyworks.com/index.php/how-tough-are-they/ And you can make your own sample panels & test them, or farm out the testing.

If you want stiffness above all else, go with the thickest cores & carbon skins. Keeping in mind that some cores, & some reinforcements don't fare well when wet. And that with carbon expecially, you need to have a semi-sacrifical layer on top of it, to protect it from getting chewed up (or off) by the sanding that goes on during fairing. So figure that into your weight calc's.

 

There's also a good article on small catamaran building with core cell, & carbon on the WEST System site. It was a father & son building a racing cat with B&C foam. The B&C construction saved them mucho time, & all but eliminated fairing.

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Cedar has the nasty habit of causing allergic reactions in some folks. Myself I love working with it, but have become sensitive to its properties and have to wear a respirator when working with it. Seems to be a cumulative thing, at first the effects were few , now my lungs object to the point where I have to be careful. Not sure how the other products are, but worth thinking about. Just my 2 cents

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I built the hull for SR.-27 #1 out of DuraKore. It was fairly easy to work with, and came in underweight, primarily because so little fairing putty was needed. On Glen's later boats, he made strips by laminating unidirectional S-glass on to foam, and then sawing it in to strips. That was slightly more work, but lighter and cheaper.

In all the construction you've mentioned, none will be anywhere near as light and strong as carbon composite, vacuum bagged. So your design will be at a big competitive disadvantage from the get-go.

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This is just an idle question but how much core material do you figure to lose with strip planking when you start fairing compound curves? I imagine the thinner the strips the less is lost but I dunno for sure.

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Not much core material loss here:

under construction

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There is a lot of focus on weight, rightly in that it affects so many performance variables (RM, acceleration, disp, etc) but I wonder if for many getting a boat built in a timely manner that you can afford without sponsors, and go sailing and develop its potential fully isnt more important. Using a material that minimizes fairing, as noted, is something that I am more focused on now rather han the ultimate core, to me a small scale build using cedar or corecell seems pretty logical, depends on the cost where you are. Female molding is nice for those who can afford it, fairing is wasted cost, weight, time, and effort. So it is now more of a priority than it used to be.

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I am thinking about a large performance cruising cat project, professionally built, and am considering foam or cedar core. Balsa is great until something goes wrong - something always goes wrong.

 

Just rebuilt my saloon roof and some of my deck where the water had got into the balsa core. My hulls are 17 year old strip cedar core and they are solid as as rock, even when butted up to the wet rotten balsa core in the decks.

 

Cedar carries a weight penalty but I like the idea of its toughness over a 10-20 year lifespan and also wonder if the weight gain that seems to happen in many foam core boats (due to water ingress in the laminates which seem remarkably common) would balance out (or lessen) the weight issues over time.

 

It seems that a cedar core layup may retain stiffness and strength (especially in compression) for much longer than a foam core.

 

Carbon instead of glass is an option if there is some benefit on cedar for the extra expense.

 

I am a couple of years away from starting and will investigate more at the time - wondering if the benefits are real and would be worth a 10-15% weight penalty.

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Cedar has the nasty habit of causing allergic reactions in some folks. Myself I love working with it, but have become sensitive to its properties and have to wear a respirator when working with it. Seems to be a cumulative thing, at first the effects were few , now my lungs object to the point where I have to be careful. Not sure how the other products are, but worth thinking about. Just my 2 cents

Epoxy dust is even worse.

 

There is no such thing as good dust. Especially in the lungs!!!

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I've done cored boats in Western Red Cedar as well as Spanish Cedar. Both are lovely to work with and serve well as cores but there is a good reason bugs and rot don't love those too. Your lungs and nasal passages as well.

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Seems to me there are a ton of expenses involved in building a boat and an ungodly amount of effort.

Why would anyone use materials that last jess long than forever??

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I think most of you are overlooking the fact than cedar strip planking is not a non-structural core like balsa or foam. It is a layer of unidirectional fiber (that happens to be wood) within the laminate stack. When engineering the laminate, one should be using unidirectional E-glass fabric in the 90 degree orientation and depending on the size of the boat, +/- 45 degree fabric to help with the torsional loads. The skins can be much lighter with thinner cedar planking than a foam/glass laminate.

 

With your proposed cores, A 9mm Cedar strip with 200 GSM UDR skins @ 90 is 9.428 mm thick with a weight of 4.039 kg/m2. A laminate with 13mm Duracore and 400 GSM Biaxial skins is 15.980 mm thick and weightss 3.550 Kg/m2.

 

Stiffness Properties are:

Cedar with 200 GSM UDR 9.43 mm thick; EI 0.620 N/m2 and bending moment of 499.10 N,mm/mm

13mm Durakore with 400 GSM skins 13.98 mm thick; EI .500 N,m2 and bending moment of 375.22 N,mm/mm

 

So your cedar strip planking ends up being considerably stiffer with only a 14% weight premium.

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Great points^, I would seriously consider strip planking then shooting an indicator coat on it for prelam fairing

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Fox - I was thinking it would be a 10% weight penalty for the total weight of the boat. If we are talking 10-15% weight penalty for the laminate that would work out a increase in total boat weight of 3-5% - much easier to accept.

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I helped build a 40 Formula 40 trimaran that was planked in WRC and was surprised when the inner laminates were done in Kevlar laid 45/45. I would have thought the Kevlar should be on the outside for impact resistance but the builder Jim Godbey explained that the 3/4" cedar would help spread a sharp edged impact and absorb some energy and the Kevlar would be much more effective on the inside and be less likely to suffer a broach. The outside skins were 100 % uni strips bound together by thin hot-melt like plastic beads that kept the tows aligned. The cedar strips provided nearly all the longitudinal stiffness but there were some carbon tows in high load areas such as chainplates and beam mountings. The boat was did very well in a 2005 TransAt but was abandoned during a later TransAt(Route De RHum). It was recovered and repaired and ended up with on of the tech schools on the Fla east coast where she was let get run down once again. I tried to put a bid on it from there but whoever bought it seems to have done a great job getting her back in shape and she is now on the Wooden Boat registry.

 

UpMySleeve.jpg

 

The thing that impresses me the most about this sort of construction is the damage tolerance and relative ease of repair and overall longevity. Up My Sleeve is truly the boat that wouldn't die.

 

http://forums.sailinganarchy.com/index.php?showtopic=122730

 

https://www.woodenboat.com/register-wooden-boats/my-sleeve

 

 

Great article about Etienne the French sailmaker who raced the tri so sucessfully.

 

http://articles.sun-sentinel.com/1992-07-15/sports/9202190690_1_boats-spinnaker-sailmakers

 

http://www.thedailysail.com/offshore/11/58584/0/birch-and-giroire-enter-twostar

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Fox - I was thinking it would be a 10% weight penalty for the total weight of the boat. If we are talking 10-15% weight penalty for the laminate that would work out a increase in total boat weight of 3-5% - much easier to accept.

We were only discussing the hull weight. And for the record, I am not saying that either laminate is or is not adequate. For that one would need to consider a lot of factors and do a proper panel study.

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I built my tri with Core-Cell strips.  I milled my own cove&bead strips, using narrow ones in the curvy parts and wider, even tapered strips elsewhere.  

Core-cell is somewhat sand-able, so we did some fairing before before glassing.  Not much fairing putty was needed.  

 

FWIW, I think that the Eastern white cedars (Northern White and Atlantic) are less toxic than the red ones.

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Get a copy of Dave Gerr's "Elements of Boat Strength", it gives you all you need to calculate scantlings for various options.

 

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