The Rot Doctor


Subject: Grand Banks Soft Spots
Date: Tuesday, March 26, 2019

Hi there,

Overall picture of Grand Banks 36'

I was looking for some advice on making spot repairs to a wooden hulled Grand Banks 36’. I had a survey done last year and the surveyor identified a couple of “soft” or hollow sounding places in the hull below the waterline. At least one was around a through hull and the other at a hull butt joint. If I was to replace the planking, these would be very expensive and involved operations. I have been reading that it may be possible to drill a lattice of small holes (size?) from the inside of the hull into the soft spots and inject CPES into the holes to fill, harden and strengthen the wood in those soft spots. My questions would be:


Thanks for the images. They make giving useful advice that much easier. I will answer below, interleaved with your questions.

1. Is this a viable repair method?

We feel that it is quite viable. We have used this ourselves, on our tug Delta, on more than one occasion. While we don’t have any testimonials to this exact repair, there are several boat repairs in our Customer’s Projects section.

2. Do you have specific instructions on how to approach a repair (or preventative maintenance) such as these?

You mention drilling holes into the wood. While that is certainly a useful method when required or desired, it isn’t always necessary. It depends on the depth of the rot. Typically, I only use the drill method if the rot is under good wood, and I can’t get to it directly. Or if the rot is so deep that I am concerned about the ability of the penetrating epoxy to get to all the rot just by soaking from the surface. Usually, this is if the rot is deeper than 2.54 cm, or 1 inch. Shallower than this, I will often just let the epoxy soak in from the surface. The wood does need to be dry for the penetrating epoxy to penetrate as much as possible. I always start by checking the depth of the rot. Either with the tip of a pointy knife, like a fillet knife, or a smaller drill bit, stopping when I reach sound wood. Or before I go all the way through, whichever comes first.

3. What are the down sides of using CPES?

The biggest downside that I can think of is that it is difficult to determine just how much stronger the wood has become. Without question, adding epoxy resin to a matrix like rotted wood will make it stronger. In our experience, significantly so. But, as each piece of rotted wood is unique, there is no way to say for certain how much, unless you were to test it to failure. If you are fine with the repair being some significant, but unknown percentage of the original strength, fine. If you absolutely have to know that it is 100% or more, you have two choices. One, replace the wood. Two, strengthen the wood with the penetrating epoxy, then reinforce the area with sistering. Of these two, the second is almost always the cheaper option.

As far as the wood needing to expand, I will say this. Once the wood is treated, that area will not expand and contract at the same rate that it used to. In regards to the surrounding planks, this is of no concern, as long as the hull is single planked, with caulk between the boards. As the rest of that particular plank will be swelling and shrinking, there will be some stress. But it should not be a concern, as the normal stresses of operating a boat in any kind of sea will induce far greater stress to the wood. In short, this should not be a concern.

That is somewhat of a tricky question to answer. The short answer is no. The more detailed answer is there are some engineering concerns regarding the load that the repair is put under. If you would like me to explain further, I would be happy to talk to you via telephone. There is just too much to say to do it via email easily.

While it is possible that there could be enough epoxy which gets to the joint between the plank and the fitting, generally things don’t get quite that strongly bonded.

4. Do you have examples of repairs that were made like this? (websites etc):)

Check out our Customer Projects section.

Can I do the same thing with soft spots on the deck? i.e. drill a lattice of holes and inject CPES into the core. How does one go about repairing the drilled holes then?

As I said, drill holes are not always necessary. If they are, then the holes are usually filled with a thicker epoxy resin, or wood dowels are glued in with epoxy, then cut flush.

I have attached a couple of pictures of the soft spots in the hull and on deck.

A close-up of the hull plank soft spot at joint A view of the deck soft spots

Your help and advice would be greatly appreciated.


I hope this answers your questions. If not, or if you have further questions, I would be happy to help. Please let me know.


Ps. We had some issues with the pictures getting deleted, so when we reached out Cam was nice enough to send them again along with this update.

I’m forwarding to you my original message with the pictures attached. I did treat the planks (soaking as much as I could with a paint brush). Now just need to see how it holds up over the summer season but it seemed to be absorbed and hardened. :) I did not treat the deck soft spots as this was taking alot of time I didn’t have and they really didn’t seem that soft to me.