laminated plywood keel on a weekender?

[dustywaves]

I can't remember the site but i saw piccys of a weekender under construction with the keel laminated out of 19mm ply. I had thought of doing this myself given the difficulty and expense of suitable DFir or other timbers in my area. Aussie hardwoods are not only HARD but dang heavy too so i'm looking for a reasonable alternative. I thought of sheathing the ply keel in Dynel or fiberglas and epoxy to avoid water ingress and/or maybe adding a hardwood capping to take the bumps that go with beaching on unknown shores. The inevitability of damage that lets the water and rot into the inner core still puts some big questions over the idea for me.

Anyone tried this with success or otherwise?

Cheers

[dustywaves]

On second thoughts, scrap this idea. did some more research and found the guys here using hoop pine (arakaria) successfully. i can get this by the truck load.

cheers

[PAR]

You'll still be laminating a keel from hoop pine (good stuff, if it's reasonably clear), much like a plywood version, just thicker layers. Don't be tempted to use one big, thick piece. It'll be stronger and less prone to warping or checking if it is laminated in layers thinner then 250 mm. I also don't recommend sheathing the keel proper, but do suggest a grounding shoe of hardwood, which is bedded in polysulfide (3M 101 or similar) or an oil based bedding compound. This shoe is sacrificial and will need to be replaced from time to time, so it has to be removable, but still have a good seal against the keel.

[dustywaves]

Good plan - the last thing i want to do is to be replacing a keel any time in the future, soon or later, so the sacrificial hardwood is going on.

thanks.

[Ken_Potts]

Hey PAR,

Did you mean layers thinner than 25mm?

[PAR]

Keep them an inch or under in thickness.

[JeremyM]

So, yes or no on the plywood laminated for the keel?

Laminate the keel and then run hardwood on the exposed edges? Epoxy up like crazy?

[dustywaves]

apparently it has been done Jeremy, i've read a few references to guys that have laminated the keel out of plywood. No idea how durable this will be regarding water ingress. If I have to go this way i'll be epoxying a hardwood sole onto it and encapsulating the whole shebang pretty thoroughly. some guys talk about simply painting it but I put a lot more confidence in epoxy and some kind of fabric ( glass, dynel, etc) to seal it up tight.

have fun.

[dustywaves]

"Normoose" plans to use plywood for the centre lamination as well as adding some ballast - check out the discussion under "keel" on the main forum - it makes interesting reading.

[boatmik]

This shoe is sacrificial and will need to be replaced from time to time, so it has to be removable, but still have a good seal against the keel.

No reason not to sheath or epoxy the whole thing, if that is what is wanted.  The hardwood shoe is a good idea - it can be glued with the 'pox too if wanted.

I often use this idea and if the shoe/edge gets damage I just plane it off and glue a new one on - which can be easier that trying to prise apart a sikaflexed joint.  Neither way is better really.  Just whatever you think is best for you.

Best wishes

Michael

[Frank Hagan]

Sheathing the keel in the Weekender has been problematic with solid wood ... too much wood movement and shrinkage, and you get a crack in the sheathing.  This leads to trapping water, which leads to rot.  No one has expressed any problems with leaving the keel unsheathed as per the plans.

With a plywood keel, you do have to protect against water intrusion along the edge grain.  I'm not sure I would trust epoxy alone.  Its my understanding plywood has much less wood movement than solid wood, so a sheathed plywood keel may not have the same problems.

[PAR]

Plywood has considerably less movement then solid wood. Softwoods also move less then hardwood. A softwood laminate (with reasonably thin layers) wouldn't move much either.

Plywood is much less strong in tension, compared to solid wood (half the grain is going the wrong way). It is considerable stronger in tension across the grain. Plywood is also heavier, on average then solid wood, given the same dimensions.

What makes plywood versatile is its cross grain strength, which can be used to advantage in "box" or "beam" engineering elements. Also this dimensionally stable material (a result of the veneer orientations) can be employed in "egg crate" structures, which mimic solid ones, but lighter and stronger.

An example for a keel would be a hollow box made with plywood, which has similar dimentions as a heavy solid white oak keel, but lighter. Another keel could be an "I" beam structure, maybe with solid wood flanges and a plywood web (houses use floor joists with this exact construction method). These are good applications for plywood in a beam assembly (which is what a keel is, okay well sort of, it's actually the lower portion of a truss).

In other words, a solid wood keel will be lighter then a plywood sandwich and stronger too. The plywood sandwich will be more dimensionally stable, but this is a minor consideration with the materials and coatings we have currently.

You could employ plywood in a keel, but I'd recommend a biax sheathing set in epoxy, so you'd have the fabric elongation that matches the resin properties. This means it'll stay stuck longer.