A jet ski costs $140 an hour on the water, which is more than the boat

A jet ski looks like the sensible way onto the water. It costs a third of what a boat does, it lives on a trailer in the driveway, and there is no slip fee. People buy one for exactly that reason, as the cheap option.

Per year, it is the cheap option. Per hour of riding it usually is not, because a ski gets used in short bursts and a boat gets used by the day. Here is the arithmetic, with every assumption on the table so you can swap in your own.

What a jet ski costs a year

Take a used three-seat jet ski with its trailer, bought for $11,000. It is kept at home, towed to the ramp, and put away for the winter.

$2,800 a year is a number most households can live with. The trouble is the other half of the fraction.

Hours are the number that matters

Nobody spends a whole day on a jet ski. You tow it to the ramp, ride for a couple of hours, get sunburned and thirsty, and go home. Ten outings like that is a decent summer, and it comes to twenty hours. You do not have to take our word for the twenty. Every modern ski has an hour meter on the dash, so read yours, divide by the number of seasons you have owned it, and use that instead.

Now put it next to the boat from the math on splitting a boat four ways. That boat cost $10,500 a year and went out fourteen days. A boat day is a real day, so call it six hours on the water each time.

$140 an hour against $125. The boat costs almost four times as much to own, and it is still the cheaper hour, because it is used four times as long. Count the people aboard and the gap gets wide. Six people share the boat's hour. Two share the ski's.

A jet ski is still a fine thing to want. The costs are nearly all fixed and the hours are few, so the only lever worth pulling is the hours. One household is not going to ride sixty hours a summer. Three households will get close without trying.

Three households, one ski

Same ski, three households, each riding sixteen hours a season. That is forty-eight hours in total, so add a second service to the year and call servicing $600. Everything except fuel now comes to $2,650, split three ways. Each household buys its own fuel.

Each household's share of the fixed costs

$883Each household's share of the fixed costs

Fuel for sixteen hours

$280Fuel for sixteen hours

Cost per hour, down from $140

$73Cost per hour, down from $140

Each household pays $1,163 a year, which is $1,637 less than owning the ski alone, and the hourly cost roughly halves. The ski still spends most of the summer parked. Forty-eight hours is two full days out of a season that runs three months.

The number to run for anything like this is cost per use, and the cost per use guide shows how to do it for cheaper things than a jet ski.

Why a shared ski is harder than a shared boat

The money is easy. What makes a ski awkward to share is that everyone wants it at the same moment. A boat is pleasant on a mild Tuesday evening. A jet ski is for the hot Saturday, and a summer only has so many of those. Three households and perhaps a dozen good Saturdays is the whole scheduling problem.

The second thing is the trailer. Somebody has to store it, somebody has to own a vehicle with a hitch, and the ski has to get from one driveway to the next. Decide where it lives and treat that household's driveway as a contribution, the way you would if they were paying for storage.

The third is damage, which on a ski is quick and expensive. A tow rope or a handful of gravel pulled into the intake can end the weekend and cost a few hundred dollars. Decide now who pays for that, before it has a name attached to it.

Settle these before the first ride

• Where it lives and who tows it. One driveway, one hitch, and a plan for the household that has neither.
• The shared account. Equal monthly payments that cover insurance, storage and servicing, so nobody is chasing anyone in October.
• Fuel. It comes back full, every time. This is the rule that gets broken first.
• Who is allowed to ride. Ask the insurer whether every adult needs to be named on the policy, and check your state's boater education and age rules before a teenager takes it out.
• Damage. Whoever was riding pays up to an agreed amount, and anything above it comes out of the shared account.
• The hour log. Write the meter reading down when you take it and when you bring it back. It settles the servicing schedule and most arguments.
• How the hot Saturdays are divided. Not first come, first served.

If the group is seriously buying together, the boat partnership agreement checklist covers the ownership and exit questions, and nearly all of it applies to a ski.

The Saturdays need a budget

A group chat does not divide Saturdays. It hands them to whoever asks first, and by August one household has had the ski for the holiday weekend twice and the other two have stopped asking. The fix is the same one that works for a boat. Give each household a small budget of weekend days and a larger one of weekdays. Booking a day spends a credit, and the credit comes back once the day has passed, so nobody can hold the whole of July in advance.

That is how AllDibs works, and the guide to sharing a boat walks through the setup, which is the same for a jet ski. Run your own hour meter through the table above first. If the answer is a number like $140, find two households who would ride it too, and put the ski on AllDibs so the Saturdays get divided before anyone has to ask. It is free for your first five items.