Transom Rot

One of my goals as stated in the introduction of this blog was to help people not only learn what getting into a restore could entail but also signs that a restore is upcoming for those who might want to avoid such a project.

As I mentioned in an earlier post my Sea Ray needed new motor mounts and close inspection showed it also needed a new transom.

First, I want to discuss the transom and some basics of boat construction and then wrap up with how to identify transom rot.

Boat Construction Crash Course: In fiberglass boat construction a hull of a boat is created by layering fiberglass (more on this shortly) over a mold or some type of structure.  When the fiberglass cures it becomes a rigid shape.  In modern boat construction a mold is created and the fiberglass is shaped to that mold and a top and bottom piece of the boat are combined to create the boat (that's why most boats have a trim ring around them near the top of the boat... that's where the two piece join).  A lot of the reason in modern construction for this type of approach is that it requires no wood products to be used to mold the boat hence no chance of the wood rotting.

Older boats, like this Sea Ray, used wood to create the form for the fiberglass hull.  Wood planks running from front to back (called stringers) provided a shape for the hull connected to the transom (roughly the back of the boat). In outboard boats the motor attaches to the transom while in larger boats the transom is generally referring to the back of the boat.

Once the fiberglass is cured it becomes the structure and the wood is merely the form around which the fiberglass is poured.  So you might thing... if the wood isn't structural, why do I care if its rotting?  Well, that get back to the properties of fiberglass.

Fiberglass: What we call fiberglass is typically "fiber reinforced plastic" or FRP.  Depending on the application the fiberglass can be chopped, strands, or even woven mats of glass.   By itself the glass is fairly flexible but when a hardener/adhesive is applied (resin) the structure cures into a hard surface that is hard as steel.  Fiberglass is usually composed of many layers of fiberglass/resin to increase the stability of the structure.

However, fiberglass has some drawbacks such as not being  UV resistant and being absorbent (blistering is an example of this).  So why would be make a boat out of this stuff?  Its light weight, its tough, and with the proper coatings you can repel water and UV protect it.

Coming full circle... you can see how rotted wood (IE wood that has water in it) would be harmful to fiberglass.  Over time, the moisture causes the fiberglass to de-laminate (IE the adhesive starts to fail) and the fiberglass falls apart.

What happens with transom rot?  As the moisture de-laminates the fiberglass it allows water and moisture to further penetrate the fiberglass.  Not only does this extend the area of rot but also the amount of water being absorbed into the glass.  While this breakdown takes some time it basically means that if you let your boat sit in the water long enough it would eventually sink because water would be flowing freely through the hull.

The main reason you want to get transom rot fixed (or any rot really) as soon as possible therefore is to prevent further damage (IE a more costly repair) and to reduce the risk of your boat sinking.

Signs of rot: There are a few ways to determine rot on a boat and they are as follows:

1. Wood that is rotted will be soft to the touch, wet, and likely crumble if you handle it.
2. Rotted fiberglass will also manifest itself as soft spots; places on the floor of the boat where the floor gives or feels soft when pressure is applied.
3. Places where the fiberglass looks to be coming apart into sheets (de-laminating)

Screwdriver test: While I don't recommend running around doing this to everyone's boat you can use a screwdriver (or the end of a ratchet) to strike the fiberglass with a light to moderate amount of force.  If the fiberglass is rotted you will likely penetrate the fiberglass.  If you were to touch inside the hole in the fiberglass you might find the back side of the fiberglass to be mushy.

Knock test: Another test which is slightly less reliable is a knock test.  If you knock the hull of the boat you should hear a solid sound but if you hear a hollow sound it means there is a void where something has likely rotted.

Cost of repairs: The cost of fiberglass repairs can vary but expect a transom repair/replacement to cost $3,000 - $5,000.

Repair options: In my case, there were two potential fixes I explored.  The "traditional" fix is to remove the old fiberglass and wood, and add new wood and fiberglass over it; just like the original boat construction.  On smaller boats more so than larger ones a popular repair is to remove the old wood and fill in the void with some type of epoxy; a popular one being Sea Cast.  This would have been cheaper probably but it does add weight and requires there be no moisture in the transom area for it to bond correctly (which would have been difficult to do without taking the transom apart, etc)... so I opted to have my transom redone.

Preventing transom rot: The easiest way to help keep your transom from rotting (or any parts of the boat for that matter) is to keep it dry.  Make sure your bilge is dry... apply an anti-fouling paint if you store the boat in the water... and make sure the paint coatings on the bilge, hull, etc are well maintained.  Those coatings help prevent UV damage and repel water.