... but it's not feasible to make big production plans initially to satisfy designer payment terms under those conditions.
That's only necessary if you want to convince the designer to do the initial work for free.
In most cases you pay for the initial design commission, then agree on a model for the production run. There are several common models:
1. You buy all of the production rights outright. How expensive that is depends on what the designer thinks about the market potential of the boat. The design is now yours and you can sell it or license others to produce it.
2. You buy exclusivity and agree on a licence fee per unit. You are the only licensed builder for the duration of the contract, but you can't sell or license the design to others. Exclusivity price can be a one time payment up front, a yearly fee independent of the production numbers, a higher license fee per unit, a percentage of the sale price per unit, etc.
3. You only agree on a (fixed or not) fee per unit. The designer is free to offer the plans to others.
Boats aren't sold from the back of a truck, you issue HIN's, generate financial paperwork, etc. The designer can always sue you if he feels you didn't payed for all the units.
Contracts are a matter of negotiation. If he wants to stipulate that every year an outside expert checks your books to see how many orders you had, it's on you to accept or not.
Bottom line, you won't find out what it is until you actually negotiate with a designer.