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Water resistance of different types of wood

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albert
Captain

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USA
262 Posts

Initially Posted - 04/09/2002 : 14:35:42

Does anyone have any info on the natural water resistance of different types of wood?

RUMORS AND OTHER STORIES THAT SOUND GOOD:

-I've been told that mahogany will not rot if it is sitting in seawater, but fresh water will rot it. I'm not even sure if this is true--I'd like confirmation.

-Cypress is supposed to be excellent in a marine environment.

-Teak is good, but it tends to split easily along the grain. I assume it won't rot in either fresh or salt water exposure since it is consistently used in marine applications.

-Outdoor grade (glued with a water resistant glue) plywood works fine as long as its not exposed to standing water.

ETC.

Albert Iturrey
al@comhertz.com
<i>Abacus'</i>: http://www.comhertz.com/abacus

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OJ
Master Marine Consultant

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4382 Posts

Response Posted - 04/09/2002 : 15:42:05

Here's my 2 cents on the marine plywood: I have a friend who is a physicist and is building a 28 foot sailboat from scratch. He said the only difference between marine plywood and outdoor plywood is that the former is guaranteed not to have any voids. He's using it for all the decks, hatches, bulkheads and interior cabinetry. He's using Douglas Fir for the structural pieces.

Steve Madsen
#2428
OJ (Ode to Joy)

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 04/09/2002 : 16:58:02

This document on wood from the U.S. Department of Agriculture, Forest Service, Forest Products Laboratory explains various aspects of wood. Chapter 3, Physical Properties and Moisture Relations of wood is fairly comprehensive. The article also lists the different types of wood and their resistance to rot.
http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/fplgtr113.htm

Don Lucier, Northstar
C25 SR/FK

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Douglas
Master Marine Consultant

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1595 Posts

Response Posted - 04/09/2002 : 17:15:25

Realy doesn't matter what you use if you use a product called rot doctor. Its a thin liquid epoxy that will be absorbed by almost any wood. Once the wood soaks it up and the epoxy sets water wont bother it. check out www.rotdoctor.com

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Derek Crawford
Master Marine Consultant

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3324 Posts

Response Posted - 04/09/2002 : 18:54:03

I truly believe this forum is the equivalent of a Master's Degree in Encyclopedic Knowlege!!
Derek

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Greg Jackson
1st Mate

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84 Posts

Response Posted - 04/09/2002 : 21:23:42

There are several issues to deal with
resistance to decay
coefficient of expansion with change in moisture
strength
ease of working
price

Redwood, cedar, white oak, teak, and mahogany are good for rot resistance. There are also a few others. Be aware that most woods resistant to rot also have relatively toxic sawdust.

The coefficient of expansion is perhaps the biggest reason that teak is desired. The other author is correct, the USDA book "Wood as an Engineering Material" is perhaps the ultimate sourcebook for information on this. Under coeff of expansion, teak stands out. Wood below the water line will always be wet. Interior wood will always be dry. Topsides the brightwork is wet-dry-wet-dry ad infiniteum as you sail or it rains. This will induce end checking in lumber, proportional to the coeff of expansion. This is particularlly true with casework where there are glued cross grain joints, such as hatches, combings, or splashboards. Teak is about the only thing that will work in those instances.

The classic lumber for strength is white oak. This is perhaps the most desirable framing lumber for wooden boats.

On price, teak is about the worst there is. Last time I looked it was $15 per board foot. Red oak, maple, ash and other common hardwoods are generally around $3 bd ft.

Additional concerns include ecological issues. Teak and mahogany are still first growth woods and the forests have been devistated. The great teak forests are nearly all gone, more so than the Giant Sequoias.

As to the epoxy treatment, don't count on it. There is a hull construction technique called "cold molding" where wood fiber is used in place of glass in a FRP matrix (fiber reinforced plastic). It is basically a layering of thin veeners, wetted out with resin. That sort of hull will never rot because it is thin enough for the resin to actually soak in. Even the thinnest of resins will not soak in more than about 1/8" cross grain. It can get in pretty deep through end grain, but there will be little penetration on cross grain. If you use the epoxy saturation technique and have lumber of any thickness, you will end up with a rotted out shell. It will look nice on the outside but will end up hollow inside.

Myself, I would prefer teak brightwork outside and cherry inside. The C25 has all teak so I have done all my work, including extensive cabinets inside, in solid teak. Costs a fortune.

G. Jackson

S.V. Compass Rose

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Ellis Bloomfield
1st Mate

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USA
85 Posts

Response Posted - 04/10/2002 : 12:51:46

Albert,
I wouldn't even breathe the words "I'd like confirmation". I went through this years ago and the thoughts of an old Swedish Lutheran pastor attempting to make me memorize all kinds of stuff still give me nightmares.
On the serious side, one of the replies stated marine plywood and exterior were basically the same except for voids and I have been told by the same by lumbermen. They say the glues are all the same today. They also said, and it makes sense, that if you use exterior grade be sure and plug, with a goood wood filler, all of the exposed voids after you cut it.
I noticed another thread on cypress. Most of the old wooden water towers were made of cypress and left untreated. Some are still standing after half a century. We replaced one at a town I lived in and a resident bought the old wood and sawed it down and it was still in good shape.

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