When the rubbing strakes are continuously welded along the top edge, but just stitch welded along the bottom edge, water ingress between the rubbing strake and the hull plating can cause corrosion which goes deep into the hull but is not immediately obvious. This is especially dangerous near the waterline as the hull shell can corrode through and the vessel can be flooded.  

This was the only available option for a haul out for this narrow boat to be surveyed pre-purchase. The vessel was lifted out of the canal, over the hedge and in the adjacent farm field. Unusual, but worked fine.

This propeller had been painted over with antifouling. Light hammer sounding returned dull sound. When the blades and the boss were scraped, a dezincification in advanced stage was discovered. The blade tips were porous and chipped off even at light impact.