Luanan Plywood-good or bad?

[holly]

Hi Gang,
In checking out plywood in my area, I found the 1/4 " exterior BC, which loks rather poor and some 1/4" Luanan that looks excellent with all edges showing no voids etc.
Has any one experience with this plywood? I intend to glass and epoxy it on the outside, and maybe just epoxy on the inside of my cat with 22' hulls.
Appreciate your input in advance!
Regards, Holly

[messabout]

Luan is more nearly aimed at the construction industry. Many boats have been built of that material but it is not as desireable as Okume or Sapelle. If you are building a boat or doing extensive repairs go for the good stuff. Look for BS1088 designation on the wood that you buy. Quarter inch or 6mm ply is a bit thin for a 22 foot boat.

If you are building on the cheap them Luan will do but it will be a compromise. Your decisions will also depend on the end use of the boat. Generally, the amount of labor that goes into a boat is so extensive that it does not make sense to settle for any material short of the best you can get.

[PAR]

In plywood, the count of veneers is a very important consideration, especially in thin sheets. Typical construction grade lauan will have three veneers, which is not very strong. To compound this issue is the tendency of the two outer veneers to be literally paper thin. These outer layers in a three ply sheet, provide all of the longitudinal strength of the plywood panel. If you have two layers of paper thin wood, it can't be very strong.

Sometimes, you can find 1/4 lauan with relatively equal veneer thicknesses. These are much better in comparison, but no match when compared to a marine grade which will have 5 veneer layers, which provides three longitudinal veneers and two cross grain plys.

I agree with Messabout's suggestion that 1/4" is quite thin for your hulls. If you're looking at a lightly loaded hull, you can engineer it with 1/4" plywood, but you'd be best advised to use the strongest 1/4" you can, use a substantial sheathing or both. Lowes/Depot 1/4" lauan is substantially weaker then a marine grade 1/4" lauan.

[JEM]

are you building from plans or just doing some repairs?

If building from plans, then the designer should be able to provide good advice about the plywood you're looking at.

[alan white]

A suggestion--- to go with douglas fir marine ply (five ply) in 1/4" and sheathe inside and out----- inside after scarfing but before wrapping around frames/bulkheads. outside after all fairing as usual.
10 oz cloth will add some stiffness inside (which would be sealed with epoxy in any case, so all that's being added is the cloth, and conveniently too if it is done while still on the flat).
Fir has good strength, suffers only from problems associated with painting without epoxy/cloth, which is not the case here.
Otherwise, if this boat is meant to be sailed and not just raced, 3/8" plywood for the hulls would add about 50 lbs to the boat, yield twice the stiffness and 1 1/2 times the impact resistence.

Alan

[mydauphin]

I built a disposable boat with Luanan Plywood. Epoxy outside, fiberglass inside. I didnt seal inside all that great. It basically delaminated. I cut fiberglass out and had to brace skin with 2x4. I used boat to repair docks and seawall. It is falling apart but still working... but the Luanan wont last long if it is exposed to water.
By the way, I went with Luanan Plywood because I want something light.

[alan white]

Sounds like maybe you used an interior-glue ply, a three ply. I rebuilt a '53 Chris Craft runabout this spring. The boat was in surprisingly good shape. Most of the structure was solid lauan.
Lauan is actually a cedar, and from my experience, like other cedars it holds up well in a marine environment. Because it is a prolific wood crop, it has found it's way into many home products such as doorskins and underlayment. What makes a good marine plywood, besides the wood itself, is the glue, the number of plies per thickness, and face plies that have enough thickness to contribute to the ply's strength.
Some three-ply home-type plies have very thin outside plies, and one thick inner ply. These are absolutely useless on boats. Lately manufacturers have even been using wafer-board cores, which should not even be used in homes for sub-flooring.
Lauan is not a real strong wood, but it isn't bad for its weight. It is very paintable, which makes it a good choice when it's not being glassed over.
Fir is stronger, in my opinion, and in cases where the ply will be glassed over, I would choose it over lauan.

A.

[Pierre R]

Luanan plywood comes as exterior grades in thicknesses of 1/4 inch to 3/4 inches in thickness and up to 9 plys. I have used it for interior boat work and it has held up just fine. I have not used it for hulls. With hulls I would stick with BS 1088