Fibreglass or exotics in layup with wood core.

pmagowan

pmagowan

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I am doing some research at present and was wondering if anyone had any personal experience they could share. I am wondering about the best layup for a cold moulded boat construction. The wood core is likely to be cedar stip with 3 layers of veneer on top. I am most interested in what should be in the fibre and epoxy on either side.

One purpose of the fibre and epoxy layers is to encapsulate the wood so that its strength can be harnessed without its weaknesses such as rotting. The fibre on the outside has to be good at absorbing impact and abrasion. On the inside I presume it must be strong in tension to stiffen the hull. Initially I had been thinking about rather standard glass fibre layups but there are so many exotic materials I want to know if any of them would add to it (kevlar, carbon etc)

For a circa 45 foot blue water cruiser intended for some high lattitude sailing. Any thoughts?
 
Undoubtedly not what you want to hear, but if you're high latitude cruising is likely to mean ice, build it in steel.

Not quite the same thing, but as a young whippersnapper, I spent Christmas and New Year aboard a wooden narrowboat, on occasions crunching through ice on the Oxford Canal. Later we discovered that we had succeeded in routing out a groove along the waterline at the bows - in 2 inch oak. That was just plain old UK winter ice, not gnarly high latitude stuff.
 
Surely any impact will be transmitted directly to the wood core? So the wood core will either need to resist or survive the deflections implicit.
Kevlar has some appeal, but if you were designing for minimum weight, you wouldn't be using wood for the core, so why not just use glass?
Kevlar is also hard to repair sometimes.
It's all a bit guesswork unless you are going to do proper detailed analysis.
 
Twister_Ken said:
Undoubtedly not what you want to hear, but if you're high latitude cruising is likely to mean ice, build it in steel.

Not quite the same thing, but as a young whippersnapper, I spent Christmas and New Year aboard a wooden narrowboat, on occasions crunching through ice on the Oxford Canal. Later we discovered that we had succeeded in routing out a groove along the waterline at the bows - in 2 inch oak. That was just plain old UK winter ice, not gnarly high latitude stuff.
Click to expand...

I understand the pros and cons of the different materials and have decided against steel for a number of reasons (this will not be an exclusively high lattitude boat). I do not have a great fear with regard to the ice. Wood has very poor abrasion resistance but the wood will not be exposed but will be protected by the fibre layup. I have also considered a sacrificial plate for the bow and shoulders of the boat but I question the neccessity of this. I see other fibreglass only boats have done this but many have not and have had no significant damage. I suspect that the purpose of protection will be cosmetic in this context and I am more interested in the strength for significant impact.

I suspect standard glass fibre layups will be more than adequate but I have to get all the possibilities in my mind first as I don't want to skimp on something as fundamental as this if it is just a marginal matter of cost.
 
lw395 said:
Surely any impact will be transmitted directly to the wood core? So the wood core will either need to resist or survive the deflections implicit.
Kevlar has some appeal, but if you were designing for minimum weight, you wouldn't be using wood for the core, so why not just use glass?
Kevlar is also hard to repair sometimes.
It's all a bit guesswork unless you are going to do proper detailed analysis.
Click to expand...

The wood core gives superior strength to weight ratio than solid fibreglass. The core acts like a sandwich and thus most stresses get deflected to either longtitudinal compression or tension of the fibre layers with some sheer forces on the core. Think of it a bit like an I-beam. The strength increases hugely with each mm of core material for only a small increase in weight. The other benefit is that this form of construction allows me to work mostly in wood, something I am comfortable with, and to produce a monocoque structure which has even greater strength characteristics. I am not used to working with the fibre/epoxy layups especially for the exotic materials so it is this part that I need the most information on.
 
Search for GRP tubing for oil wells. There has been much research on construction methods to resist wear and improve properties. There are GRP pipes with a steel wrapped and bonded for abrasion resistance and strength; in high pressure applications the GRP would be too thick. Perhaps there are methods and materials that could be used on your project. For example I read that the optimum angle they wrap the GRP at is 55 degrees for maximum strength.
 
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Andrew Simpson (PBO contributor) built a cedar strip boat called Shindig -AFAIK he is still sailing it to this day.

He wrote a number of quite detailed articles for PBO covering the construction and the design choices.

It would be worth trying to find copies of them.
 
Strip cedar can by used to create a structure with considerable longitudinal stiffness, so this generally isn't a cored construction in the same way as a foam or balsa cored hull is. There have been a large number of one-off and small scale production designs of all sizes - from canoes to the Jubilee Sailing Trust's Tenacious successfully produced in this method. The sheathing is mostly for impact resistance, ensuring the timber stays dry, and is almost uniformly done with standard epoxy resins.

Here's an example of another Andrew Simpson design: www.sailboat-cruising.com/cedar-strip-boat-building.html
 
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I took a passing interest in the cedar strip canoe construction as I had bought one to plunk along the canal. The two bookls I read on the subject both stressed that the wood had little strength and the construction was indeed a sandwich with the GRP skins offering most of the strength and rigidity. One writer who regularly runs his boats on whitewater sometimes uses a Kevlar skin below the waterline to prevent abrasion damage and advocates a wrap of kevlar around the ends to take beachings, etc. Another technique is to add carbon granules to the final epoxy coat to give a smoother fimish (Bucky balls?), but I wonder whether Coppercoat would be more appropriate to a sea-going craft?

Incidentally, Kevlar is ridiculously expensive and hard to use as it doesn't stretch to fit the curvature of the hull like normal cloth and is a devil to cut!

Rob.
 
I've never built anything with a cedar or wooden core, but I have seen the results of water ingress into the core which has either gone un-noticed, because it's in a hard to reach place and covered in paint, rather than varnish, or it's been neglected and not repaired quickly. It seemed to me that the core soon degenerates into a mush, with absolutely no strength. The repairs seemed complex and expensive. So for that reason I've always steered clear of cedar cored boats.
I'd add that these boats are often one-offs and the one's I have experience off, where interesting and innovative designs.
If you're going to high latitude, have you considered Aluminium? Strong, light, generally corrosion resistant (providing you have the correct galvanic protection) and quite resilient .
 
I have considered various options including aluminium but I am settled on wood. Cedar is very resistant to moisture and rot but it should not have to deal with it as it will be completely encapsulated under the fibre/epoxy layers. It is a form of construction favoured for one off high spec boats as the mould is part of the construction.
 
Yes if you want a one off to your own design particularly hull shape then best approach is a wooden basis with GRP covering. In my experience GRP does not do all that well at bonding to wood. Especially if moisture gets in. So if you have a thin wood basis then cover with thick GRP you end up witha GRP boat which happensd to have the mold still in place. As opposed to a wooden boat with GRP skin. Of course with GRP on the inside as well.
Yes Kevlar is very abrasion resistant. I was involved with the building of a Carbon fibre small aircraft which had Kevlar layer on the outside for abrasion resistance. No problem in this case laying Kevlar inside the female mold. Not quite so easy on a male mold because the kevlar would tend to overlap in places and is extremely hard to sand down to smooth perfection. In fact easier to build up iregularities with bog than sand down the kevlar.
With GRP boat weight is not so much of a concern. What is of concern is the overall stiffness of the hull. ie from bow to stern to take rigging loads. So dimension is the key to stiffness. Hence make the hull thicker with a foam or balsa sandwich. Or in your case a wooden mold. To make it thicker the cheapest approach is polyester resin with Chopped strand mat or even a chopper gun.
Carbon fibre can make a thin lay up very stiff but I think you still need that dimension of thickness.
Having said all that really people only build one off boats because they want the satisfaction of the design and build. Your cheapest quickest approach would be to buy a second hand boat or hull in GRP. Yopu could then take the risk of ice damage on the thick GRP or add kevlar as an additional layer with epoxy with any shape changes you want in the under water profile or add errosion plates.
Incidentally kevlar and carbon are both OK with polyester vinylester or epoxy resins. Vinyl ester is more like epoxy but easier to use. good luck with your cogitations olewill
 
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