Peter, I said 10% rule of thumb and add on a yearly average. That means in 10 years or less you have maintained the boat at the original purchase price.
Fortunately for myself and several others, sweat equity offsets many of those $$$.
Not picking on anyone. Heck, I've probably quoted the 10% number myself. The more I think about it, the number is a brush-off. No one knows what goes into the 10% or how you account for differing valuations. Its an answer that answers nothing but begs a lot more questions - it doesn't advance the conversation a bit.
This really isn't that complicated - there are a few buckets of ownership costs - capital costs (purchase of the asset), holding costs (slip, insurance , routine maintenance required to preserve asset in current condition), variable costs based on usage, upgrades, and extraordinary costs (blown engine replacement). Because the last two - upgrades and extraordinary - are wild cards, people simply won't answer the question even though the purchase costs, usage costs, and holding costs are all very predictable.
For routine holding costs for a GB42, in a reasonably popular boating center, $15k-$20k seems about right, though I confess it may be more given rise on insurance and slip rates so maybe an update is in order. Still it's a much more usable number than "it depends" or the 10% rule.
This is financial modeling 101. You answer what you can, leave place holders for what you can't, and home in on the final number as you move through discovery. Being afraid to even start modeling until you have all the data is a fast track to analsys-paralysis.
The OP hasn't been around for a while but maybe there are others who are considering buying a boat who are still reading. If you plan to leave the boat in the water year round, and it's on the 40-foot range, your gonna need $15k-$20k of free cash flow to keep it afloat. Might be more, especially if it isn't in great shape to begin with. Plus improvements. Plus a reserve for unexpected expenses.
Peter