How to Repair a Split Plank

by Mike Stobbe

There’s probably nothing a wooden boat modeler dreads more than a split plank on his or her prized classic yacht. Invariably, the damage occurs in a location where there is no possibility of gaining access to the inside. And while the yacht may not sink, its owner experiences a sinking feeling indeed. Well, take heart. Repairing a split plank need not be a difficult or lengthy process. I’ve been giving the subject much study, and I believe I have demonstrated a quick and simple method which will provide a structural integrity equal to the predamaged condition. Furthermore, no tools are required, no access to the interior is needed, and with a little luck there will be no immediate need to repaint the boat. The boat will be fit to sail immediately after the repair.

The method I am proposing relies upon capillary suction to draw a special epoxy into the crack. It will draw the epoxy into the furthest reaches of the crack and through to the full thickness of the plank. The epoxy will saturate and seal all exposed wood inside of the crack and glue the plank edges together with a strength that exceeds that of the wood. Epoxy is also gap-filling, needs no clamping pressure, and needs no surface exposure to “dry” as it is catalytically hardened. Capillary action works best if the fluid is thin. Ordinary epoxy resin is as thick as honey. This repair utilizes penetrating epoxy, marketed as a dry rot repair. Penetrating epoxy is thinner. It is designed to be readily drawn into the crack, even if it is no more than hairline width. There is no need to gouge out or widen the crack to expose the edges of the plank.

As proof of concept, I prepared a small panel of edge-glued 5/32 in thick planks. The panel was 6 in by 8 in. To simulate a hull, I varnished the “inside” and painted the “outside”. The finished panel would then be cracked in half, and the two parts reassembled closely but not clamped, to create a hairline crack. The epoxy would be applied from the “outside” only. The objectives of the test were:

1. To confirm that the epoxy would reach all the way through the length and depth of the crack, reaching to the inside surface of the planking.
2. To confirm that the panel could be immediately immersed in water; i.e. that all exposed wood was sealed without any subsequent painting or varnishing.
3. To confirm that the strength of the repair was at least as strong as the wood.
4. To confirm that no additional damage would be done to the painted surface. Indeed, that the surface wouldn’t need to be repainted at all, if so desired.

All four objectives were confirmed successfully.

After the repair, the test panel was soaked to see if any exposed wood would soak up water (there was none).  The panel was rebroken to judge the strength of the joint. It broke not on the glued joint, but at a new location 1/8 in away.  It was also cut transversely to evaluate how far the epoxy had penetrated away from the crack. It was found, however, that there was negligible epoxy penetration into the sound wood on either side of the crack.  The test was performed with the panel horizontal, resting above the tabletop on two small wood strips (which could be a simulation of two ribs, structurally split apart from the planking).  In theory, however, capillary action should be strong enough to let the repair be done vertically or even upside down if necessary.  Capillary action ceases at a free surface, so once the crack is filled, it will stop drawing.  Epoxy will not keep flowing and drip into the inside of the hull, unless the crack is so wide that the epoxy will simply run through it by gravity flow. The following are step by step instructions to make the repair.

Step 1: Determine the Cause of the Split.

All split planks are likely to be caused by either moisture penetration or by overstressing, such as direct impact by a collision with another boat (even if not noticed at the time) or inertial loading by the lead keel when a boat is abruptly stopped by the lake edge.  The primary evidence is the location:  cracks above the water line are likely due to collision; cracks below the waterline are likely due to moisture.  Also, moisture penetration will usually cause the paint to lift adjacent to the crack, whereas the paint will remain tight adjacent to an impact crack.  Leaving standing water in the boat during storage, or a closed design without hatch ventilation may be the culprit.  Inertial loading by the keel will cause cracking along the garboard plank adjacent to the keel fin.

This repair method will work with either type of crack, but the important point to remember is that with a moisture penetration crack, you will only be treating the symptom, not the cause.  I’m talking about moisture inside the boat here, generally while the boat is in storage.  Moisture penetration causes cracking because wood swells when it absorbs water and shrinks when it dries out.  It will swell and shrink repeatedly with every wetting/drying cycle.  The stresses set up by these dimensional changes exceed the strength of any wood and any glue joint.  Thus, after the crack repair (not before), it will be necessary to ensure that the interior of the boat is sealed.  Swirling a quart of varnish around inside the boat (including the underside of the deck), then pouring it out is the traditional cure.  Storing the boat with the hatch cover off is an important preventative measure.

Step 2: Let the Boat Dry Out.

Before repairing a split plank, the wood must be allowed to dry out long enough to be sure it has dried through the full depth of the planking.  For a collision crack, detected promptly, this may be 2 or 3 weeks.  For a moisture penetration crack, I recommend the boat sit all winter.  If you have a moisture penetration crack and a closed, hatchless design, you can never be sure the interior is drying, no matter how long you wait.  For this condition, I recommend that you cut in a hatch or remove the deck.  Epoxy will harden in contact with water, but moisture in the wood will reduce or eliminate the crack’s ability to generate capillary action, rendering this repair method ineffective.

Step 3: Tape the Crack.

Carefully apply a strip of masking tape to the paint on both sides of the crack, as close to the crack as possible, and extending about 1 in beyond each end of the crack.  For a collision crack with intact paint, you should be able to place the two strips of tape about 1/32 in apart or even closer.  If it is a moisture penetration crack with flaked or deteriorated paint edging the crack, you may have to place the tape slightly farther apart but not more than 1/16 in.  Don’t remove any flaking paint at this point.  To stay within the 1/16-in limit, tape directly over bare wood or loose paint if necessary. The object here will be to preserve the paint as much as possible, and promote a clean tear in the epoxy when the tape is removed, as described below.  Be sure the tape is pressed down tightly or else the epoxy will creep under the tape.

Step 4: Mix up a Batch of Penetrating Epoxy.

As described in the first page, the repair medium must have as low a viscosity as possible to be drawn into the crack.  Other types of glue might work with this method.  I have chosen penetrating epoxy because it is harmless to the existing paint, it hardens slowly enough to permit plenty of time to apply, it is gap-filling with no need for clamping pressure, it will not in itself cause the wood to swell as it is absorbed, it is plenty strong for the purpose, and it cures at depth catalytically without air exposure.  Also, unused epoxy has an almost infinite shelf life.  While its viscosity is not as low as some other glues, it nevertheless performed well in my test planking.  Disadvantages are its expensive price and potential incompatibility with certain paints.  Regular epoxy, normally quite viscous, can be used if it is heated up first.  Heating it in a pan of simmering water will dramatically thin it out.  It will also dramatically shorten the pot life, but as the repair will only take a few minutes, this shouldn’t be a problem.

I used West Marine Penetrating Epoxy.  A comparable product is “GIT”-ROT.  You will need only a tiny amount, but may have to mix more as a practical matter just to be able to judge the proportions.  Unlike polyester (i.e. fiberglass) resin, where a greater or fewer number of drops of catalyst will influence the working time but not have much effect on the ultimate strength, epoxy’s strength is strongly affected by the proportions of resin to hardener, so measure as carefully as you can.  The working time is mostly influenced by temperature not proportions.  Read the instructions on the container.  Generally speaking, mixed epoxy thickens very slowly.  It doesn’t “kick-off” suddenly the way polyester resin does.  When I conducted my test, it was 74 degrees in my shop and the mix remained thin for about 1 hour.  Mix thoroughly, scraping the sides of the container as well as the stir stick.

Step 5: Apply the Epoxy.

Now listen up, this is important.  DO NOT SIMPLY PAINT ON A SWATH OF EPOXY THE FULL LENGTH OF THE CRACK.  For this method to work, capillary action must draw the glue into the crack.  If you simply paint on the epoxy with a single stroke, you will trap air inside the crack under the epoxy.  Trapped air will prevent capillary suction from developing, and little or no penetration of glue will take place.  Sure, you may get lucky, and if the crack extends through to the inside of the hull, the air may escape in that direction.  However, we are assuming that the crack is inaccessible from the inside, so you can’t be sure what the inside condition is like.

Place a row of single drops of epoxy along the crack about one drop width apart. The epoxy will enter the crack at discrete points and the air in the crack will be free to escape between the drops.  If the crack is hairline, it’s volume will be so small that you will not be able to tell that anything is happening but be assured that within seconds the entire crack will be filled, from one end to the other and to full depth.  For a larger crack, you may see little dimples form in the drops.  Do not worry that the crack may be “too tight” for the glue to enter.  The beauty of capillary action is that the smaller the available space is for the fluid to enter, the stronger the capillary suction becomes.

After a couple of minutes, you can apply more epoxy to form a continuous bead of glue.

When the epoxy reaches the inside surface of the hull, capillary action will cease.  The bead of glue will stop sinking when the wood adjacent to the crack becomes fully saturated.  In my test, with a crack 8 inches long, this took less than 5 minutes.  No epoxy dripped through onto the tabletop.  However, epoxy was drawn beyond the crack to fill the joint between the back side of the planking and the “ribs” supporting the test piece. Stop for coffee.  Check it again in about half an hour.  Fill any low spots if any have appeared.  Set it aside.

Step 6: Thicken the Glue Line.

Wait until the epoxy in the container is about as thick as honey.  In my test, this was about 3 to 4 hours.  Paint on a thick bead, lapping heavily onto the masking tape.  Mound it up about 1/8” thick.  The objective here is to build up a thickness that is several times as thick as the width between the two strips of tape.  Stop and wait.

Step 7: Allow to Semi-Cure.

Epoxy gains strength quite slowly.  It will be several days before it is fully cured.  At this point, however, we are looking for a semi-cured state, where one can still easily dent it with a fingernail.  The time will be temperature dependent.  In my test, I waited overnight, about 16 hours, which in hindsight, was too long.  I could still dent it, but barely.  Try about 8 hours.  We want the epoxy to be just solid enough to tear as opposed to stretching like toffee, yet still weaker than paint.

Step 8: Tear Off the Tape.

If you have judged the strength correctly and gotten your two strips of tape close enough together, you should be able to peel off both strips simultaneously without breaking the 1/8-in thick layer of epoxy that joins the two strips.  The epoxy between the two strips is only 1/32 in wide, so the epoxy should tear right at the paint surface, leaving the crack completely filled and the paint undamaged.  In my test, unfortunately, because I had waited too long, the epoxy was strong enough to rip off the paint.  All of the paint between the tape strips came off: a line 1/32 in wide.  The paint under the tape was undamaged.  I’m sure that had I removed the tape sooner, the paint would have been undamaged.

For a collision type crack repair, if the paint adjacent to the crack is intact, the boat is ready to be used immediately.  Aesthetically, the repair is hardly noticeable.  From a moisture penetration standpoint, the hull is fully sealed inside and out.  From a structural standpoint, the hull is serviceable:  even in contact with water, the epoxy will continue to cure until it reaches full strength.  Painting is optional, but see the warning below.

Step 9: Painting.

For a moisture-penetration type crack repair, tape removal may rip off some loose paint, in which case repainting will be required before the boat can be used.  Also, at this point, the inside of the hull should be sealed so the problem doesn’t reoccur.  Also, please be aware that some paints will not harden over some epoxies.  Try painting over the hardened epoxy left in your mixing container as a test to see if it will dry properly.  If it doesn’t, change to a different kind of paint.  Alternatively, polyurethane varnish will harden over epoxy and can be used as a barrier layer to keep paint from touching the epoxy.  If there is a compatibility problem, even a hairline of epoxy is enough to cause trouble, so be sure to test your paint first: you won’t be able to simply “bridge over” a hairline.  Some paints harden just fine over epoxy, but when they’re incompatible, the problem is permanent:  it doesn’t matter how long the epoxy has had to cure.  In my test, I used West Marine Penetrating Epoxy, and West Marine Brightside Polyurethane paint.  I found no incompatibility.

Also, please note that epoxy will cure over polyester (fiberglass) resin, but polyester resin will not cure over epoxy.  But if you are planning to fiberglass over your entire wooden hull anyway, then the split plank problem will be taken care of at that time and there will be no need to undertake the repair described above.

There you have it.  In a nutshell: allow to dry, tape carefully, apply epoxy slowly, rip off the tape, go sailing.