These are Rainbow’s figures, a Perry 47 ketch and Rainbow’s End Marine’s own boat. Enter yours.
Your boat
Sail area as published, or the rig measurements. With both, the published figure is used.
The sails she sets. These give a second SA/D beside the standard one.
The load
How she compares
Every figure here is an estimate from published formulas. Show the math under any figure to see how it was worked out, and what it rests on. How the calculator works it out has every formula and its source.
- Physics
- Standard physics: the weight of water and fuel.
- Geometry
- Arithmetic on lengths, areas and weights. Nothing is assumed.
- Published
- A published figure, and whose. We name a source only when we have read it.
- Our estimate
- A figure we chose. The calculator treats it as an estimate, and so should you.
Compare boats
Save the boat above, enter another, and save it too. Up to four boats, side by side.
The multihull
Sheet and halyard loads
Range under power
Saved in this browser only. Nothing is sent.
Anchoring works it out the same way
Every formula here is published, and every step is shown. The answer is honest; the decision is yours.
Rainbow’s End Anchoring holds to the same rule. It works out the rode to pay out and the anchor to use from your own boat and the gear in her locker, and its method page shows each formula, where it comes from, and which figures are its own estimates.
What it won’t do
- It won’t tell you whether she is safe offshore. No ratio can.
- It won’t estimate displacement. Enter the published figure.
- It won’t size a planing powerboat’s speed.
- It won’t send anything anywhere. Your figures stay in this browser.
The numbers describe a design and a load. The sea decides the rest. Rainbow’s End Boat Calculator is an aid to judgment, not a substitute for it.
More
- Rainbow’s End Anchoring, with the free MonitorThe anchor & rode calculator that knows your boat — and a free companion for the people who sail with you.
- Rainbow’s End ManifestThe book of your boat.
- Rainbow’s End Float PlanLeave word ashore before you leave the dock.
- Rainbow’s End ProvisioningProvisioning for sailors, from a day sail to an ocean passage.
- Rainbow’s End Ship’s Purse, with the free GuestThe charter kitty, kept honest — and the crew’s view of it on their own phones.
- Rainbow’s End LogbookThe book of your voyage.
- Rainbow’s Arc PositioningA camera sextant, for the fun of it.
Designed by Rainbow’s End Marine. Apps for sailors, by sailors.
May you find a rainbow’s end and treasure found wherever you sail or set your anchor.
© 2026 Rainbow’s End Marine, LLC · support@RainbowsEndMarine.com · www.RainbowsEndMarine.com