Formulas and percentages of net worth to determine how much to spend on a boat? That's a good one. Hope it works out.
You should know if you can afford a boat or not, how much to spend, what to save for maintenance before buying. Do the math with your household cash flow.
With experience, one knows what is affordable and what is not. Too many people getting into boating with no experience, buying what the formula, magazine article or broker suggests.
If you've had several boats, hopefully starting small and working up over the years, you will know firsthand what it will coat to keep a boat. You will have decided what you can and can not do yourself, what you will hire out etc, what it cost to have things done and what maintenance is required.
With people getting into boating later in life with little experience, it's a difficult decision process figuring a budget for the acquicision, moorage and maintenance of the boat. And most of the time buying unnecessary equipment or incorrect ones for the application.
Most new buyers rely on brokers to guide them. If they are fortunate to find an experienced broker, they are in good hands. Most brokers I've dealt with were inexperienced, got into boat sales as a retirement job or part timers doing it so they can say they are.
There are a few experienced boating consultants out there, who are not brokers selling boats. They spend time with the clients focusing on their needs and type of boating planned. Come up with budgets, types of boats to look at and guide them along on their quest for the ideal boat. The consultant oversees the showings and points out features or areas that may be problematic. Some consultants are also boat handling instructors and give instruction to the new boater to satisfy insurance.
They assist the new owner during equipment installs and refits, after purchase to insure proper work. Often trip planning for the first few outings are also fine tuned by the consultant.
These consultants work by the hour.
They are usually hired by inexperienced boaters planning to purchase large 50' plus yachts. But sometimes by prudent buyers of boats less than 50'
I retired from being a boating consultant a few years ago. I saved many new boaters from headaches, heartburn and unnecessary spending. And buying the wrong boat.