Mainship Stringer Moisture

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Feb 16, 2023
Vessel Name
was - Luna Del Mar
Vessel Make
Farrier F-41
Looking at 2001 Mainship 390. Surveyor's meter showed some high moisture in portions of both of the central stringers. They will drill some inspection holes later this week. My reading tells me that stringers can not be dried if encapsulated, as these are (except possible using Dryboat's services). Been told by seller's rep that leaving holes open to the air will allow the stringers to dry. I imagine that they would dry near the hole but not enough to prevent eventual rot and loss of the wood structure. Has anyone ever had any success drying strings by drilling vent holes through the exterior laminate? I know on some boats the stringer's structure is in the fiberglass encapsulation and not in the foam or wood core. On a Mainship I am guessing that this is not the case since they are said to have the stringers built up by layers of plywood.
I looked into this on a boat I was in contract with. It too had wet stringers. The yard that specializes in these issues said the way they do it is lift out the motors, cut the top of the stringer off, dig out all the wet core then refill the core with new wood and epoxy. It was an expensive process. The next step is to find out how the stringers got wet in the first place. Usually standing water in the bilge over the years. Gotta keep that bilge dry through the use of dripless shaft seals perhaps......
Wet stringers can be fixed. The process is labor and time intensive. It is one of those projects that one should not buy into.

If the boat is half of market or a one of a kind then maybe you can justify buying a boat that won’t be usable for a year. For sure I would not enter into the project before identifying the source of water intrusion.
I know on some boats the stringer's structure is in the fiberglass encapsulation and not in the foam or wood core. On a Mainship I am guessing that this is not the case since they are said to have the stringers built up by layers of plywood.

This is true for hull loads but not necessarily true for loads such as engine mounts. These type of loads could cut right through the glass without the support of a solid core.
I have seen where they drill holes in the glass and tap the holes. Then screw fittings into the holes and hook up air hoses to the fittings. Hook the hoses to a shop vac and let it rip. Don’t have any idea how well or how long it takes.
Contemplating the repair of wet stringers on a boat you don’t own seems a severe uphill battle. With requisite skills, money, indoor storage and a hobbyist attitude any boat project is fair game. If you want to cruise and enjoy onboard family and friends buy a nearly ready to go boat.
I guess it would depend on several things. How badly do you want the boat, emotionally connecting to the boat prior to purchase can be a bad thing. What are your DIY skills or do you have really deep pockets. What is it you like about boating? Do you like working on boats as a hobby, I certainly do. Or you more of a I want to go cruising now type of person? If you can answer those questions then you will be on your way to either buying the boat or walking away.
I'll offer another perspective. Condemning a boat based only on a moisture meter is a fools errand. I can measure moisture somewhere in most 30 year old cored boat stringers, particularly if it's a wet bilge. I can get high moisture readings somewhere on the deck of 95% of all cored decks that are 30+ years old. Is there delamination? Debonding of the core and fiberglass? Is it only around an unsealed limber hole? Could something else be causing the increased conductivity besides a rotten core? Would a surveyor not using a moisture meter have any reason to suspect the stringers as bad? What does an infrared inspection of the stringers show?
Talk to the surveyor. Just because the moisture meter is singing doesn't mean the fat lady is too.
Agree with Fleming, all too many surveyors over emphasize the use of moisture meters as the know all, end all. It’s not an exact science. Many things will influence the readings. The meter should be used in conjunction with a good surveyors eyes and ears.
Earlier it was mentioned to put holes in and vacuum the moisture out. This procedure has been used successfully on cored hulls as well. It takes time, but is a good solution.
You won't know the extent of the repair until the wet stringer is opened up. Wood will literally turn to pudding if its wet for long enough. It the core is rotten like that, then just vacuuming out the water and letting it dry doesn't fix the issue. The rotten core must be removed and a suitable core must be replaced for structural rigidity.
Survey drilled stringer at the forward port motor support below the seawater pump which could have been leaking at some point. Stringer wood had turned dark and was moist (saw chips formed balls not dust) but did not yield when an awl was pushed into it. Seems structure is still there but so is the moisture. Stringer fiberglass is 3/16"-1/4" so I would say that the stringer strength is in the wood and not the glass. I am thinking that as long as there is moisture trapped in the stringer that rot will proceed and the stringer will eventually lose needed support strength. My opinion is that there is no sure way to dry them (excepting possible Dryboat) and that they would need to be opened up to be repaired. If anyone has ever had success drying moist but otherwise okay encapsulated stringers I would like to know what was done.

Picture of stringer glass and
wood flakes from hole saw.

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