I have been a trailer sailor for 50 years or so.
FOR THE RECORD AND STATED IN PREVIOUS POSTS, I RESPOND TO QUESTIONS THAT PROVIDE WHAT IS NEEDED TO GIVE A RESPONSIBLE RESPONSE BASED ON KNOWLEDGE AND EXPERIENCE GAINED THRU WORKING ON BOATS PERSONALLY AND AS A DEALER, REPRESENTING MANY LINES AND SO FORTH. In addition, I became friends with many in my industry over the years being given valuable information.
First replacing, cutting or repairing a mast or part of it requires photos and without that I am not going to give a response until that happens. O'Day closed it's doors in 1989 but there were remaining hulls to be sold after closing of the plant as remaining inventory as I saw a small O'Day come thru my yard in the 1990's as either a 1990 or 1991 I think. Therefore, I am responding back to
@GSBNY thinking this is an O'Day thinking this boat is about 30 years old or more.
So often I see folks saying cut the mast as a solution. It is a case by case as to the direction of what I would suggest and I will not comment until all the requested information is available. Why? First many do not have the equipment and in some cases the know how to cut a mast true and square. If that does not occur will cause the base of the mast to be lopsided putting stress on a pop rivet or screw. This also includes bolt holes in the mast as well putting stress on the bolt holes which are awkward. I have seen where holes enlargened or wallowed out sometimes causing metal fatigue even with cracks in the mast elongating and so on. Cut masts that the cut was not clean and lopsided not seating perfectly as well causing a cracks in the base of the mast sometimes elongating and so on affecting at times standing rigging. There is more but you hopefully get the picture. As for stainless steel screws into the mast, remember aluminum and stainless steel are two dissimaler metals and will corrode suggesting an ounce of insulating the two such as a gasket in between them.
Anytime you do shorten the mast and in many cases the rigging is old and stretched to the point there is not much to tighten within the turnbuckles, you will have to either replace the standing rigging or shorten them putting on studs yourself and many are afraid to do that. These are the two primary reasons but there are others.
many of the older mast manufacturers are gone leaving a handful now. Parts may not be readilby available. If I were going to shorten a mast, I would add an insert that fits the space inside the mast perfectly or if there is one made to fit the mast inside. Welding, I would be cautious for a variety of reasons and one of course is the welder with experience. I recently saw a welded section on a mast which I have reservations but the weld which is holding was performed by an experienced airplane mechanic who welds aluminum for a living on airplanes. That I will be watching closely but the micky mouse rigging has to be replaced. I have seen some make an elongated foot for the mast made out of Delrin that was made perfectly to fit the interior of the mast and worked. Anything short of a perfect match will stress the mast which it is attached to causing problems. It would take a book to write the amount of screw ups seen by me over the years.
When anything is suggested, I would also say to those proposing to supply photos as well to see if the fix worked. The old saying a picture is worth a thousand words. Again, this issue of the base of the mast is a case by case and without photos, I will not respond till then.
@DrJudyB Thank you for advising that. That is an earlier Z Spar part pointed out by you.. Kenyon Isomat was purchased by Z Spar in the 1990's with all employees for both companies let go. Only four from Z Spar went to Charlotte, NC as the only work force. I had Hunters without masts sitting for a while paying interests on them to the banks until Selden Masts came to the rescue. Z Spar left the country but remerged as US Spars later in the same location just outside Gainesville, FL. Parts for Z Spar may be obtained from US Spars but better by the forum store for the Hunters. As for the employees, Kenyon Isomat emerged as Charleston Spar and later as Sparcraft. Parts for Keyon Isomat can be obtained from Rig Rite. Friends of mine included managers at Z Spar and Kenyon Isomat.
Dave