Go with the Luan or is there a better alternative?

Hello all,

Whilst repairing the damage inflicted by Hurricane Frances to our 23 year old ketch, we found that the entire coach roof and sides had become saturated over the years. Since the rigging is off for inspection we have decided to replace all of the ply (existing is 3 layers of 3/16" Luan). It is obvious that most of the water intrusion occured over the years around the fittings and such that are all through bolted into the coach roof with metal backing plates inside. I intend (regardless of material used) to overdrill all of the fastening holes in the new material, fill the oversized hole with a structural filler and redrill the proper sized holes. This should eliminate 99.9% of the potential for future intrusion.

My question is should I go back with the three layers of Luan finished off with a couple of layers of glass, or 5/8" marine ply with a couple layers of glass, or is there another alternative I am not thinking of. I think because of the numbers of fittings on the coach roof that closed cell foam would require many cored areas to be fitted. The surface area including the sides of the cabin is approximately 420 sq. ft.

I have also toyed with the idea of placing kevlar between the layers of Luan to improve its ability to repel ballistic intrusion. Anybody worked with the stuff.

Thanks in advance,

Darr Palmer
SmallYachts
S/V Ishtar

 

Luan Replacement

Indeed, you can take quite a few options in consideration:

The main problem with Asian hardwoods is that they have lower bonding specifications than for example the Mid-American and African Mahagony/ like Entandrofragma & Swietenia;
Lauan or Meranti has bonding problems due to its secretion (bleeding); it is hard to paint, to varnish - to coat at all.
There are a number of other woods you may better use: Iroko (after degreasing with MEK (methylethylketon); Abarco (Venezolan Mahogany) Okume, Sapeli or similar timber;

making your own plywood with Kevlar layers in between can be done: actually
it is a very good way to give a classical looking boat a very strong but own appearance. The "but" is that Kevlar has bad bonding specs, use therefore a hybrid woven: glass/kevlar, not too heavy; 300/grs approx. Such a combination will ad immense strength to your construction. Early '90s Bruynzeel and I were experimenting with it already, so I know.
You can also make a sandwich construction of Airex as core material (or similar product) and 2 x 4mm mahogany layers (or similar class II wood) on the outer sides.
The core should be approx 20/25 mm thick. It depends a bit on the dimension
of your cabintop. It should be emphasized that you shall not use any kiind of PU foam.
If you glue, use epoxy of good quality.West systems in your area, I believe.
Make sure that you work with an epoxy impregnation resin and a topcoat of
a two-component PU varnish with an UV filter included. Never leave the raw
epoxy untreated against UV radiation. Very important!
Lauan is a non-standard harwood that is actually only used in Asia itself for boatbuilding purposes, The area you mention in your Post is bigger than I thought so consider to increase the dimensions of the woodlayers you are using. The idea of putting kevlar/glass between the layers is in any case a good one and not only to repell ballistic intrusions.
Good Luck!

 

A simple flat surface and your favorite foam core will work to rebuild the coachroof.

Simply lay the core (slightly oversize) in the flat surface and glass ONLY one side.

After a day the glass will have hardened , yet still be "green" soft and flexible.

Lay your new roof over the old one and it will sag easily to match any curves.

Glass the top outer surface , and when hard , remove , then remove the rotten wood and install the new custom built coach top.


Easier than messing with rot.
You can build anything formerly of ply with this concept , cheaply and easily.

WE have built whole decks, deck houses and coach roofs from sheet made this way.

FAST FRED

 

To specifically answer your questions,

It's likely that the cabin roof was made this way to overcome the crown. In other words, the thicker material wouldn't bend over this shape easily and the thinner material was used. The 3 layers of ply would also make a stiffer panel then the single layer of 5/8" and the seams could be over lapped to prevent leaks. This is a tried and true method and works well, provide you stay after bedding and penetrations.

The bonding of all penetrations is a very good idea and well recommended (it's the only way I do it) This leaves you with only keeping up with the bedding.

Kevlar is a wonderful product, but it has it's issues. It's very expensive and requires special tools and methods. Unless the craft is a competitive racer in need of this weight/strength advantage, then I couldn't justify the cost and hassle of it's use on an aging ketch. If you really want this stuff, then go talk to my friends at FGCI (FiberGlass Coatings Inc.) over the Gandy, in St. Pete. They sell the material, at least the last time I was there they had some 5 oz. stuff hanging up at 27 bucks a yard of 50". If you have 420 sq, ft. this is over 13 sheets of ply per layer, so that's 840 sq. ft. that will need covering (two layers) at 2.66 yards a sheet x 27 bucks a yard. That's a lot of cash (over 2 grand) before any goo hits the surface.

Those three layers of ply, bonded with epoxy and sheathed in cloth will provide a surprisingly strong roof, especially if there is much crown. Look, I got hammered by the hurricanes too. Hell, in 100 plus winds a popsicle stick can get shoved through a cabin roof. I've spent most of the fall and winter thus far fixing lots of this seasons damage. Personally, I got off light, but my friends and clients didn't. Try not to over build for the next storm of the century.

Unlike D'ARTOIS (what did you think of SwordFish) I like meranti, which has reasonable rot resistance. Lauan has low rot resistance, but does finish well. I don't use it in structural areas when I can use something better. Lauan is a generic name given to a number of lumber species used in plywood, which are generally called "Philippine mahogany" and all sheet goods coming from this area of the world should be carefully checked as the manufacturing controls and grading standards can vary dramatically.

 

Lauan and its Purposes

Hi Paul, thanks for mailing me Swordfish, what a beaut! I had something about her but will mail you about that separetely.

My father in law (ex) has a construction & building company and I used to check his timbershop on a regular basis to look for pieces of wood I could use for my boat.
According to the Dutch hardwood importers Lauan is similar to Meranti, a common used wood in the B&C industry, however not used in yachting.

Instead of Lauan we use the African mahogany types like Okume, Sapeli, and lesser good species as Makoré, all from West Africa.

What surprises me is that you are so close the the Carib Isles and Mid America,
like Costa Rica, Honduras and those other Rebel Counties. There must be plenty of highly qualified hardwoods like the Swietenia's and Abarco's, next to that you have the top of the bill woods like Port Orford Cedar, Lolbolly Pine,
Slash Pine, not to forget Sitka, so what I am asking is why are you looking up those hard-to-bond and hard-to-glue stuff like Ironwood (Merbau) and Lauan (Meranti)?
With your American Oak and Sitka, I would have by long built my own wooden
Philip Rhodes or a Buchanan or a Fife.
We rely here in Holland entirely on imported timber, our local timber is only good to feed the fireplace, so we have to buy everything at import prices.

So I think it's a matter of how you grew up in the business, leisure or whatever you call it.
Although I think we can learn from each other this way quite well. And therefore I adore boatdesign.net that I browse through whenever I cannot sleep, remember we have 6/7 hours difference in time between us.

Cheerio!
Brien