Can't help with that question, but.....
You had asked me to keep in touch about SR marine instruments, I remember you because my brother lives on the Blackstrap Rd in Falmouth and I learned to sail in the 70's on a Pearson named "Irene" in Falmouth, many good memories, anyway, SR marine has a web page under srinstruments.com I mailed my knotmeter on Thursday and heard from them on the following Tuesday, they could repair/replace the part that goes thru to the water for 100 bucks including a new bezel so the face/instrument looks new or I could trade the whole thing in and get 50% off a new one. I opted for the rebuild but the holes are the same if I had gone with new. I couldn't be happier with their customer service. If your instruments fail you or are looking old and you have a few extra bucks dealing with these people is a pleasure, good luck and see you on Casco Bay sometime, jack