When we purchased Shamrock, we noted salt crystals and what appeared to be some minor corrosion at the base of the mast and mast step. When we replaced all the mast lights and repaired the wiring last year, I began to clean and scrape most of the loose flaking paint, salt crystals and corrosion from the base of the mast, bottom 2".
We had the boat surveyed last week, only item of note was the mast base and step, noted as minor corrosion, nothing serious.
As Paul Harvey says, “now for the rest of the story.” As noted earlier, near the base of the mast on the port side was the DataMarine knot log, no plug to install while cleaning, this must have been intersting trying to service this with water gushing in? We replace the knot log with the new knot log sender as well as the depth sounder, still new in the box, they tried to adapt the Data Marine depth sender to the new West Marine Multi-data display, naturally it didn’t work properly. There is also a Dyna Plate for SSB, to which the mast, chain plates, stem and boomkin fittings are bonded.
I got into the cleaning mode today while the Svendsens guys were applying the last coat of barrier coat and bottom paint. I scraped and wire brushed the mast step and base of the mast, the hardest area to reach required laying on the port seatte and working on the area of the mast where the bonding wire was attached to the mast step. Suprise, suprise, surprise, as Gomer Pyle would say, I found a 1" diameter corrosion depression in the mast near the base ajacent to the bonding wire, 2/3 the depth or the mast thickness.
We are planning on pulling the mast next week after we launch, we are planning on cutting the last few inches off the mast and adding a shim under the mast. While the mast is out, we are planning on touching up the paint and get ready to sail.
I talked to the guys in the yard, they tend to believe the bonding of the mast is often found to cause a corrosion issue in this area. I tend to agree in part as the corrosion spot is directly above where the bonding wire is attached to the mast step, however the depression is not at the very bottom of the mast in contact with the step? I tend to believe allowing saltwater to pool around the mast after cleaning the knot log sender, possible drainage from the chain locker and lack of a proper drain in the mast for rainwater was probably more of a contributing factor to corrosion.