A Shipwright in Training

When you double plank a boat, you put something in between the layers of planking to seal the two together.  The traditional method is to use a heavy cut of shellac.  Some builders use canvas soaked in varnish, some have used 5200 sealant.  

At IYRS we’re using shellac to seal in between our inner and outer planking layers.  In case you don’t know, shellac is basically bug poop dissolved in alcohol.  Ok, maybe not exactly bug poop, but here’s Wooden Boat’s Harry Bryan attempting to describe it:

Yeah, bug poop.  But enough on that.  You can find out a lot more about the marine uses of shellac in a couple of excellent Wooden Boat articles (#200 Jan/Feb 2008 & #76 May/June ‘87).

If you shellac in between your planking layers, you need to have the stuff be really thick.  We’re looking for a viscosity somewhere around thick molasses.  This comes out to about an 8 or 9 pound cut of shellac.  When you buy shellac in a can ready to go, it’s usually a 2-3# cut.  The cut is a measure of the concentration of shellac.  A 2# cut means that 2 pounds of shellac flakes have been dissolved in a gallon of alcohol. 

You can make up your own shellac by buying the flakes and mixing them in alcohol, or you can buy it pre-made. The flakes are expensive, and it takes a good bit of time to dissolve them if you’re trying to make a thick concentration.  An alternative is to get the pre-made stuff and boil it down until you have a thicker concentration.  That’s what we did.

I was concerned about the fire risk of boiling off alcohol.  It seemed a bit, um, risky, to have so much volatilized alcohol floating around a heat source.  Then I realized, hey, we’ve got tons of steam to work with.  Steam is 212 degrees, and alcohol boils at 173 degrees.  That means we can get alcohol to boil off with just water vapor.  Nice.

This is our industrial strength 40 KW (yes, 40,000 watts) steam generator.  More than sufficient for the job I’d say.  Like using a bulldozer to move your couch. 

I made up a little mini steam box and used one of the hoses from this monster to power it.

It’s labeled because person after person would wander by and say, “hey what are you making there?  What’s it for?”  There are a couple of holes on the left side to allow the steam to escape, and a few drain holes underneath to allow condensed water to flow out.  It also sits on slightly angled feet to flow the condensate towards the drain holes.

Here it is in action.

I particularly like how you can see the distortion from the boiling alcohol right above the can.  You really don’t want to breathe a lot of this stuff by the way.  Yug.

The boiling was pretty vigorous, so I used a vise on the hose to regulate how much steam got into the box.

It didn’t take long to get shellac that started out with a milk viscosity down to this:

Mmmm.  Who’s got an apple?

Of course, the point of all this is to seal your planking.  To prep, get your planks all fitting nicely and drill all our screw holes now.  If you try drilling the holes while the inner face of the plank is covered in shellac you just get goo all over your drill bit, and who needs that kind of aggravation? 

When I fit the broad (the garboard didn’t need shellac because it’s a full thickness plank… the broad is half-thickness and sits on top of another plank) I started by putting in 2 screws to hold it as I fit along the length.  Once it was all snugged in, I drilled and countersunk all the fastener holes.

So, when it’s all set, you slather this stuff in between the planks, and screw them on.

Did I mention that this is a sticky mess?  Here’s the butt joint at the end of the forward section of the broad. 

Luckily, it cleans up easily with a scraper and alcohol. 

One of the side effects of this is that cleaning up the shellac also spreads a thin sealer coat on the broad. 

It gives it a nice warm color I think. 

The ends of the broad go to a feather edge (i.e., a thin point) that’s too small to take a fastener.  So, you have to hold it in place wiht some sort of support while the shellac dries.  Later we’ll wedge it in tight using the caulking.

And that’s a day or two’s work.   Nice to have those planks on at last. 

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This entry was posted on Thursday, March 13th, 2008 at 6:27 am and is filed under Classes. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.