Wet stringers: What you may or may not want to know

Posted by on Jun 19, 2014

Wet stringers: What you may or may not want to know

There are a lot of myths and mystery surrounding wood cored stringers which have been found to be “moist” or “wet.” before you skip past this article under the assumption that your stringers are dry, you might want to give it read. Chances are, if you have wood cored stringers and your boat has been in the water, they probably have moisture in your coring.

I am going to throw some caveats in, lest I perpetuate some of the myths or create panic among boat owners. First, we are talking about wood cored stringers. A lot of builders are going to FRP (fiber reinforced plastic, aka fiberglass) grid systems which are not cored. These can create different problems related to hull coring materials and hard spots, but that would be another subject.

Second, throughout the years of development of structural systems, materials, and building methods, there are materials which can give false readings on a moisture meter. Moisture meters are excellent tools in the hands of the experienced (another discussion topic if you have heard moisture meters are unreliable). Between the moisture meter, percussive testing, and visual inspection, a surveyor forms an opinion basis about elevated moisture and that might justify tapping some small pin holes or taking core samples to determine stringer core condition.

I may throw in some more exceptions or caveats along the way, but I also want to keep this a reasonably simple discussion for now. Every boat is different. A boat you have had, currently have, or may buy in the future is very likely to have wood cored structures. It is a tried and true method of construction and nearly unavoidable. Also unavoidable, those structures contain moisture. Yes, I can hear the chorus of denials streaming in. From the minority of boats with wood cored stringers which may still be dry, it is just a matter of time. But, don’t panic. Here are some things to know and things you can do to prevent the real problems, rot and delamination.

To know:

  1. Start with understanding that wood contains moisture when it is installed. The wood is generally dried, but only to a point. Wood devoid of all water content would be brittle and weak. So, the wood is actually stronger with the appropriate moisture content.
  2. Wood, by its nature, is designed to absorb and move water. Before it became part of your boat, it was the trunk of a tree and was responsible for taking water and nutrients from the soil and moving them throughout the tree. It is very efficient at it and does so without a “pump” of any sort. For you science geeks out there, you can read up on capillary action. Seems obvious, but is often forgotten.
  3. Rot is the true villain and generally requires water AND sufficient air (oxygen most importantly) to be present. Bilge water can contain chemicals which can also chemically break down the wood, but again, that is another topic.
  4. Delamination has many causes or a combination of causes. Again, another topic. But, excessive moisture can be contributory if it breaks the bond between core and laminate through hydraulic action (during flexing), chemical break down, or bad layup (construction).

Prevention / Mitigation:

  1. Keep your bilges clean and dry. This maintenance detail is valuable for many reasons. Not only will it limit the water in your bilge which will find its way into the stringers and bulkheads, but it will alert you to issues with water coming in the bilge, and to mechanical problems if you find oil, coolant, or hydraulic fluid in the bilge. With a little effort, the bilge can be kept dry by routing scuppers and deck drain hoses overboard and securing packings and thru hull seals. Look for the places that rain water is getting in and seal them off or route them overboard. NOTE: don’t seal off ventilation sources to the engine compartment or you’ll have bigger problems.
  2. Seal limber holes. What are limber holes? you will find holes drilled trough your stringers which allow water to drain to the center bilge. Of the hundreds of boats I have inspected, I have only seen one or two which had limber holes which appeared well sealed. To clarify, don’t seal the limber hole from allowing water to pass to the center bilge. Seal the edges to prevent water from soaking into the exposed wood.
  3. Avoid using your stringers to fasten wires, hoses, or other installed items. If the best route for securing them requires use of the stringers, attach them toward the top (but not on the top), and make sure you use a sealant around the fastener and use it liberally. Renew sealant on fasteners anytime you remove them, or if you see they may have dried out or don’t appear to be providing a good seal. Don’t assume the manufacturer sealed the fasteners to structural components. It is very common to find the builder went to great effort to keep the stringers or other structural members sealed and then the system install crews hit them with screw guns.

Myths:

  1. Myth: Wet readings on stringers mean the boat hull is junk. If that were the case, then most every boat with wood stringers would be junked. The fact is, most have moisture levels which are higher than when the stringers were installed. Take care to prevent rot and the boat can live on. If you see signs of rot at the limber holes or through the laminate, have your qualified marine tech inspect them and determine the best course of action. Arrest any issues as soon as possible to prevent the dreaded and expensive stringer replacement project.
  2. Myth: The wood stringer frames do not have structural responsibility to the boat hull. The wood stringers are merely forms for the laminate. This may well be true on smaller boats (under 35′). The naval architects and engineers may have designed them to be sacrificial. I rarely see separation of the laminates or tabbing that would indicate imminent hull failure from loss of structural integrity. However, I have found hollow (rotted out) stringers at the engine mounts. When the builder’s argument is challenged about the stringers at the engine and transmission mounts, they tend to back away from this argument and suggest inspection and repair. Most bilge water, limber hole and moisture issues I find are in the engine room bilges and at the inboard stringers…where the engines and transmissions are mounted. If there is anywhere on the boat where the wood may have still structural responsibility, it is there.
  3. Myth: Encapsulated stringers don’t get wet. Among the many mysteries that I hope to address with the engineers / architects is why they encapsulate the stringer and then punch limber holes and install fasteners without sealant. Also, while I can speculate why they don’t seal the ends of the stringers, I would like to hear how they consider those stringers sealed when the bulkheads end up feeding the moisture to the end of the stringer and they still get wet (see again capillary effect). I might change that myth slightly to “encapsulated stringers don’t get dry.” Good idea, poor execution.
  4. Myth: The moisture meter read dry so I don’t need to worry about any of this. Hopefully they are dry. But they won’t stay that way without observing the maintenance tips above. Moisture meters won’t detect moisture if there is no longer any wood attached to the fiberglass. They also become less sensitive with thicker laminate. The builder may have really strived to build thick and strong stringers. They may have done so due the size of the boat or to allow the wood to be sacrificial. If you do not have sealed limber holes, or tabbing to bulkheads, you can be sure there is moisture in your stringers. The only way to be certain is to take core samples or get down to the wood and test it.

There are certainly more myths about stringers and we could get into foam cores and grid construction hollow stringers. But since most boats still use wood coring to build stringers, I wanted to hit on some common discussion points I have with clients regarding most boats I have inspected. In my experience, 90% of boat owners think they have dry stringers. Also in my experience, 90% of wood stringer boats older than 5 years have elevated moisture in the stringers or cored structures. Instead of living in denial, address the causes to prevent the complications and your boat hull can have a long useful life.