Yellow poplar

[Bob Gray]

    I was researching a good core wood for my next plank in the USDA "Wood Handbook" . It has the mechanical specs for most of the wood grown in the US. I wanted a wood as close to Sitka as I could get without the expense. Of the readily available woods , yellow poplar is a " very" close match in just about all the parameters. It is slightly heavier (another source shows it identical in weight) and is as strong, stiff and tough as Sitka except in tension perpendicular to the grain and that isn't that big a concern to me. This wood is found under several names such as tulip poplar  and tulipwood. I got mine locally rough sawn for $1.68 a board foot. I will probably use this wood if I build another hull. Bob

[eric_anderson]

Be careful with nomenclature.
Tulipwood is usually associated with a  dark, hard, expensive  Brazilian Hardwood

Why not just use sitka?  The cost of wood is a small fraction of the cost of a sailable boat.

eric

[Bob Gray]

c Liriodendron Tulipifera or yellow poplar is also marketed domestically as tulip poplar or tulipwood. The very beautiful and expensive tulipwood that you spoke of is imported from Brazil.
    I've built several boats out of Sitka and its okay but not everybody can readily get it and to some people the extra $100 is a factor and the woods are so very nearly the same. If you're using it for a plank core, boom etc why spend the extra money.

[Ken Smith]

Bob,

Tension perpendicular to the grain will be an issue to consider on the sideboards above the plank and near the seat.  This can be overcome by adding unidirectional fiberglass vertically (perpendicular to the grain) from the seat back to a foot forward of the plank, installed prior to the bottom and seat.  I believe the Goodwin plans call for this and it a good practice on any boat.  Search the archives using the search bar above, or in the Collective Wisdom section, and you'll find several strings on fixing cracked side boards.  Better to avoid...

The other downsides to this wood:

  1.  It doesn't bend as nicely as the Sitka, an issue near the seat back.
  2.  It isn't nearly as pretty.  Sometimes it ha a green cast.
  3.  The grain is usually wide, an appearance issue.  On purpose, to get many wood crops in fewer years.
  4.  It is hard or impossible to get quarter sawn, as the trees just are not big enough.

That being said, it is great for plank cores, structural and non structural internals, Optis, and DN hulls, if you intend to paint anyway.  It is the wood on the longitudinals, plank cores and internal structure on my land sailor, No Sheet Sherlock.

[Bob Gray]

  I never said it was a pretty wood, just a good substitute . Sitka, Doug Fir and basswood are all pretty close in strength in tension perpendicular to the grain. Poplar is about 45% stronger.

[Geoff Sobering]

I was researching a good core wood for my next plank...

Does anyone know how the stresses in the core of a typical ash-skinned DN plank are distributed?

I an "ideal" cored panel (ex. carbon over foam), the skins carry all the tensile and compressive loads, while the core only sees shear and perpendicular compression (i.e. compression trying to crush the structure).
Since a "conventional" wood DN plank is not nearly ideal (thick skins relative to the core, and typically a core material with non-negligible stiffness) the core should see some tensile/compressive forces, along with (mostly?) shear.

It would be interesting/helpful to know the load/stress/strain on the core to help choose the right wood.
One would think that the ideal legal material would be end-grain balsa, but its almost complete lack of stiffness might change the loads in the skins (ex. making them need to be thicker).

FWIW, my favorite core is basswood: cheap, light, and easy to work with.

[DN5135]

I have made about 8 DN planks, 2 J14 planks, and 2 J14 springboards all using Poplar cores and never had any issues. No broken planks to date. With Poplar, it is important to choose the boards closely as some are heavy and some are light. I have used both ash and yelllow birch faces.

[Paul Goodwin - US 46]

Geoff,

To answer your question on the stress in a plank core, the simple answer is the "strain" is distributed linearly, and stress is a linear function of strain.

Stress and strain are pretty easy to figure out for the plank, as long as the material and thickness are the same on both skins.  Strain is the amount of stretch, and stress is a linear function of strain.  Strain is much easier to grasp.  When a plank deflects downward, the top skin is in compression and the bottom skin is in tension.  The compression causes the wood fibers to shorten, and tension cause them to elongate (this is why the plank bends).

In "pure bending", the Neutral Axis is in the middle (halfway between the skins).  At the Neutral Axis, compressive strain is 0, only shear stress exists.  I don't believe sheer stress is a big worry for planks, wood usually fails in compression.

As you move away from the neutral axis, compressive/tensile strain increases, and shear strain reduces.  At the outer fibers, compressive strain is a maximum, and shear stress is 0.  Between the Neutral Axis and the outer fibers, the strain varies linearly.  At the boundary between the core and skin the strain is the same in each material.

Now look at stress.  Stress is determined by the material, each has a stress/strain property.  High modulus material has a high stress/strain value.  This is why low modulus cores have very low compressive stress levels.  In wood the stress/strain values are much closer.  Because of this, the choice of core material has a much greater influence on the stiffness of the plank.  Also, a thicker core cause higher stress in the core, and core material becomes more critical.

Figuring out actual stress and strain gets much more complicated.  One way is to study the inner workings of the Plan Design spreadsheet

[Bob Gray]

Paul, Are you saying that a softer wood like basswood makes a better core then say poplar or Sitka.

[Paul Goodwin - US 46]

Bob,

I wasn't really trying to make a statement about any particular core material.  The takeaway is that with a wood core, the material does make a significant difference in the overall stiffness.  Also, I don't think whether a wood is hard or soft matters for suitability as a core material, except that soft woods tend to get damaged easier.  I had a plank I loved with a redwood core, but it was constantly getting chunks ripped out by hitting stuff on the ice.

My core material of choice these days is clear Pine.  It has a fairly low modulus and can be purchased at most lumber yards.  Unfortunately it is also very expensive, close to the price of Sitka Spruce.  I save the Sitka for more critical stuff, and use the Pine for plank cores because it is so easy to get.

[Bob Gray]

Thanks.  Bob