Feature spec
Goal: Send a clear invoice after finishing a job.
- Add a client, line items, a due date, and payment instructions.
- Preview the total before exporting a PDF.
- Keep an editable draft if export fails.
App Overview
Apparent Wind Calculator
Apparent Wind Calculator takes the maths out of one of sailing's most important concepts
$0.99 · In-app purchases
PRICE
$0.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
N/A
No ratings yet
Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.
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Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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What's included15 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
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Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.
Screen plan
Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.
States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.
Differentiate and validate
Hypothesis: Contractors need faster repeat invoices more than more templates.
First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.
The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.
Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.
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Free quick prompt For a smaller starting point
Quick prompt
Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.
// RORK SYSTEM PROMPT
Rork plan preview for Apparent Wind Calculator: map the core Navigation workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
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Opportunities
AI-GENERATED OPPORTUNITY HYPOTHESES
These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.
Hypothesis 1
Enhance UI with better animations and visualizations for results.
Assumption to check: Limited user interface and interactivity compared to competitors
Validate with people who use this workflow before building.
Hypothesis 2
Implement onboarding flow to educate users on app features.
Assumption to check: Lack of user guidance or tutorials
Validate with people who use this workflow before building.
Hypothesis 3
Add cloud backup for user settings and preferences.
Assumption to check: No cloud sync for user settings
Validate with people who use this workflow before building.
Hypothesis 4
Consider a freemium model with basic features free and advanced features as optional upgrades.
Assumption to check: In-app purchase for advanced features may deter users
Validate with people who use this workflow before building.
App evidence
Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.
App Store description
Apparent Wind Calculator takes the maths out of one of sailing's most important concepts. Feed in your true wind angle, true wind speed, and boat speed — and instantly see your apparent wind angle and speed, visualised as a live vector diagram. Whether passage planning, under sail or just getting your head around why the wind always seems to come from forward when you bear away, this app makes it immediately clear. BOAT POLAR Load a polar table and the app sets your boat speed automatically based on your true wind angle and wind speed. Built-in presets cover small, medium, and large monohulls and catamarans — or enter your own target speeds for a custom polar. With auto-fill enabled, every diagram you see reflects realistic performance numbers for your boat, not a guess. FEATURES • Live vector diagram showing true wind, apparent wind, and boat velocity as scaled arrows — updates in real time as you move the sliders • Monohull and catamaran boat graphics that respond to your apparent wind angle • Boat polar table with presets for monohulls and catamarans, fully editable • Auto-fill boat speed from your polar for realistic results • Point of sail readout — from Close Hauled to Deep Run • Port and starboard tack display • Works entirely offline, no account required WHO IT'S FOR Sailing students learning the theory, racers checking their VMG angles, cruisers planning passages, and coaches explaining wind shifts to crew. The apparent wind is what your sails actually feel. This app helps you understand it.
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Screenshots
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Build scope
Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.
Define the first version
Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.
Set boundaries
Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.
Check the hard parts
Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.
SUGGESTED BUILD STACK
React Native
Expo
React Navigation
Axios
React Native Paper
BUILD KIT DIFFICULTY
Starter
From the Build Kit assessment.
A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.
