The captain wants a number by Friday: what will the interior cost to run this summer? "About the same as last year" is not an answer, because last year the owner used the boat for three weeks and this year the family are aboard for a month with two charters bolted on. You need a unit of measurement that survives a change of programme. That unit is the guest day.
The Guest Day
One guest day = one guest aboard for one day.
Eight guests aboard for thirty days is 8 × 30 = 240 guest days. It is the single most useful denominator in interior budgeting, because most of your spend — food, beverage, amenities, laundry, flowers — scales with guests aboard, not with calendar days.
Cost per Guest Day — Worked Example
Last season, guest-facing interior and F&B spend was EUR 96,000 across 240 guest days:
96,000 ÷ 240 = EUR 400.00 per guest day
That is your benchmark. It is comparable across seasons, across programmes, and — carefully — against sister yachts.
Forecasting the Coming Season
The programme is: owner use 10 guests for 21 days, plus charter of 12 guests for 14 days.
| Element | Calculation | Guest days |
|---|---|---|
| Owner use | 10 × 21 | 210 |
| Charter | 12 × 14 | 168 |
| Total | 378 |
Now the critical distinction: charter guest costs are funded from the APA, not from the owner's operating budget. So the owner-budget forecast uses only the owner-use guest days:
| Line | Figure |
|---|---|
| Owner guest days | 210 |
| Benchmark cost per guest day | EUR 400.00 |
| Forecast owner guest spend | 210 × 400 = EUR 84,000 |
| Contingency at 10% | EUR 8,400 |
| Forecast submitted | EUR 92,400 |
The 168 charter guest days are forecast separately, against the APA, and reported to the charter guest — not to the owner's interior budget.
Provisioning Forecast — Worked Example
For a single charter of 10 guests over 7 days, at a food and beverage allowance of EUR 95.00 per guest day:
10 × 7 = 70 guest days; 70 × 95.00 = EUR 6,650
Add the crew's provisioning separately, at its own per-head-per-day rate, against the crew provisions code.
Onboard Notes
- Some costs do not scale with guest days — uniform, annual laundry contracts, standing subscriptions. Forecast those as fixed lines, not per guest day.
- A preference sheet can wreck a per-guest-day average on its own. A guest who drinks a rare vintage is not a budgeting failure; flag it early and separately.
- Always state your assumptions with the forecast: guest days, rate per guest day, contingency. A forecast without assumptions cannot be challenged, which means it cannot be defended either.