First, The 10% rule makes no sense at all, how that myth is perpetuated is one of the great mysteries of boating to me.
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You miss the point entirely. It's a newbie number: it's intended solely to give inexperienced people contemplating buying a cruising boat a sense of reality. Boats cost money to buy, and then they keep costing money to own.
The newbie's first question after they're told that is, "How much does it cost to own?"
So what are you going to do? Flood them with the bazillion varisables that they don't have a clue how to relate to or even what half of them are? They'll be more confused than when they started.
The ten-percent number--- which has been shown to be at least in the ballpark of average ownership costs--- is a single number the prospective buyer can be given as way of a reality check. This gives them a number right off the bat that can help them determine if they are capable of affording a boat without biting off more than they can chew.
Of course there are tons of variables. Of course a newer boat may cost less per year than an older boat. Of course ownership costs vary with how much work a an owner can or is willing to do on his or her own. Even my dog knows all that.
But the person new to this kind of boating most likely does not. We certainly didn't. And when we seriously began contemplating buying our first cruising boat, the broker we enlisted gave us that figure as a "be aware a boat will cost you a lot more than the purchase price" reality check.
It was an extremely valuable piece of information for us to have at that time. We could have afforded a boat that cost twice as much as the one we bought. But knowing that a boat--- any boat--- never stops costing its owner money injected a sense or reality into our purchase and our decision. We bought a boat we could afford, and that we could afford to continue to own year after year after year.
THAT'S what the ten-percent figure is good for. People who've been in boating for decades don't need it. People who are contemplating getting into cruising for the first time but know nothing about the ongoing costs involved do.
Bombard them with all the details people with a lot of experiece tend to bombard them with and you get a deer-in-the-headlights look. I know: I've seen it happen many times and so has the fellow who was our broker and who is the lead broker at the GB dealership in our harbor.
Give them a nice easy, representative number they can understand right off the boat and tney will go into their first boat-buying experience with their eyes open.
As they gain experience or as they start asking more detailed questions about a specific boat they will soon learn the realities of owning that particular boat. But what won't happen (hopefully) is that they won't have buy a boat that costs the amount of their boat budget and then start to get whacked with expense after expense that puts them into the negative financial aspect of boat ownership.
For our boat, while we have not kept strict track of every expense, over the last seventeen years, the ten-percent figure has not been all that far off. It's getting higher these days because labor rates have gone up considerably. We do as much of or own work as we can, but when it comes to things like engine mounts, new exhaust systems, new cutless bearings, and so on, we hire that out.
But it's a very simple concept and from what I have experienced and what I have heard from acquaintences in the boat sales business and from individuals we have known over the years who decided to get into cruising as newbies, it is a very appreciated piece of information to have at the outset.