Rainbow’s End Anchoring — Support

It works out how much anchor rode to pay out for the conditions you’re in, picks the best anchor-and-rode combination from the gear you actually carry, and tells you plainly what to watch — thin holding, a bottom your anchor won’t set in, not enough water under the keel at low tide. It covers regular anchoring, mooring balls, Med mooring, lines ashore, and Bahamian moors, plus a storm mode. It runs offline, except for the optional map under the watch plot.

Your boat, on the Boat tab: length, beam, draft, displacement, the height of your bow roller above the water, how your depth sounder is calibrated, and the ground tackle you carry — each anchor with its weight and model, each rode segment by segment. Ten minutes once, and every answer afterward is about your boat rather than a generic one.

Because it’s the most common source of anchoring error. A sounder can read from the waterline, from the transducer, or as water under the keel — three different numbers for the same anchorage. Tell the app which one yours shows and you can always type exactly what the display says; the app corrects it to true depth.

In the app, open the Settings tab and scroll to the end of About. The line under the support address reads, for example, “Version 2.6.2 (79)”: the version, then the build. Tap the support address and the email it opens carries the version in its subject.

Versions before 2.6.2 do not show that line. For those, open the iPhone’s Settings app, then General, then iPhone Storage (iPad Storage on an iPad), and tap Rainbow’s End Anchoring; the version is near the top. The App Store page for the app shows the newest version under Version History.

On the App Store. Open the app’s page in the App Store app and tap Version History, under What’s New: each version is listed with its date and what changed. To have new versions install on their own, turn on App Updates under Settings, then App Store, then Automatic Downloads. The larger updates, and new apps, are also on our What’s new page.

The classic ratios multiply the water depth. But your rode doesn’t start at the waterline — it starts at the bow roller, several feet up, and the tide may rise after you set. The app multiplies the full distance from bow roller to seabed at the worst point of the night. In shallow water the difference is dramatic: 8 ft of water under a 6 ft bow roller needs scope on 14 ft, nearly double what the sounder alone suggests. The old rule isn’t wrong; it just forgets where the rode begins.

Yes, and the app shows you how much. Set What I paid out on the Anchor tab to what you actually put down. With less than tonight wants, the chain lifts off the bottom and starts pulling the anchor upward, and the steeper that pull, the less it holds; with too little, the anchor never reaches the bottom at all, and the app says so. With more, extra chain lies on the bottom and the pull stays flat, but the boat swings wider, and the anchor watch’s fence grows with her. The drawing is to scale along the bottom, with the depth stretched so the curve can be seen.

Because an anchor holds less when the rode pulls it upward. With less rode out than the app recommends, the rode meets the anchor at a steeper angle, and an anchor holds best pulled along the bottom. Past about 7° it holds less, so since version 2.5.0 the margin allows for the angle and says how much rode it is for, such as “4.5× margin at 85 ft”. Chain lies flat far longer than rope, so an all-chain rode loses little, and Danforth and Fortress anchors lose less than scoops and claws, as published short-scope tests found. Tap the ⓘ beside the margin to see the angle. The reduction is our estimate from those tests; the figures are on the method page, under “The angle at the anchor”.

It’s the estimated holding power of the recommended anchor in tonight’s bottom, divided by the estimated pull on it from wind, waves, and the boat’s yawing. Green means the margin meets what your kind of stop demands (a lunch stop needs less than leaving the boat unattended); amber means it’s thin — anchor with care and set a watch. Both sides of that ratio are estimates, which is exactly why the app insists on a margin instead of calling 1.1× good.

Setting and holding are different failures. A fluke anchor has enormous holding power in sand — and skates uselessly across a weed mat, because it never gets to use it. The app scores every anchor you carry on both questions for the bottom you chose, and when the ground is wrong for the anchor it says so instead of quietly suggesting more rode. More scope does not fix the wrong anchor.

It also checks size against the boat. For a night aboard, a storm or a boat left unattended, the anchor must hold, even in good sand, the load ABYC gives for a boat of her length in 30 knots of wind. A smaller one is marked fine for a lunch stop and is not the pick while a bigger one is aboard. That is a minimum, not a recommended size.

It tries every anchor you carry on every rode aboard and recommends the pair that scores best. The score weighs the anchor’s estimated holding in tonight’s bottom against tonight’s pull; whether it will set in that bottom; whether it will reset when the pull swings round, which counts for more when a front is due or the tide turns; and how much chain will be out, up to one boat length. A rode too short for tonight’s scope, or an anchor wrong for the bottom, loses heavily; an anchor that has to be moved to another rode loses a little. For anything but a lunch stop, an anchor too small for the boat ranks below every one that is not, unless it holds more than all of them in tonight’s bottom. Each rode is asked for its own scope: 4:1 for all chain, 7:1 for chain and rope, 10:1 for mostly rope, with more for an unsettled night, an unattended boat or a storm. Tested, those three pull at about the same angle at the anchor. On a chain-and-rope rode the app works from what is paid out. If the chain alone covers the all-chain scope, only chain goes out. If rope must go out too, the scope moves from 4:1 toward 7:1 by how much of what is out is rope. In a current, it keeps 7:1. If every rode is too short for tonight, the one that comes closest leads.

Chain helps you by sagging: its weight keeps the pull at the anchor horizontal, which is the direction an anchor holds best. The app computes the exact wind at which tonight’s chain is pulled bar-taut and the sag is gone. Beyond that point the chain’s weight has stopped absorbing anything — which is not an emergency, it’s the moment your snubber, or the nylon in a rope rode, becomes the only shock absorber, and the app tells you the pull angle so you know how much that matters.

Both are right; they measure different things. About 8° is the angle of a straight line from the bow roller to the anchor at 7:1. The app gives the angle at which the rode leaves the anchor, which is what pulls the anchor up out of the bottom. Chain sags into a curve, so it leaves the anchor flatter than the straight line; as the wind rises the pull straightens the rode and the angle at the anchor climbs toward 8°. With all rope and no chain the two are nearly the same.

Yes. The surge back on a gust is the moment a short rode lifts the anchor’s shank, so gusts raise the scope — the app sizes it on a figure between the sustained wind and the gusts. Peak loads (what actually parts gear) are computed at the full gust figure.

Because the app never counts on the sail in that wind. Above 40 knots, sustained or gusting, the load figures give a riding sail no credit, so the margin and scope you see are already worked out for the boat yawing without it. A mizzen, or any sail not built for a storm, should come down then: it flogs itself and its gear apart, and a blown-out sail is wreckage aft. A purpose-built riding sail, made and rigged for heavy weather, may stay up, but the figures do not count on it.

Yes. Set Current on the Anchor tab: steady for a river, reverses for a tidal stream, and its speed. The pull is added to the load, the scope is sized for wind and current together, and the app warns when a turning tide will make the anchor set again.

It stops recommending anchors — none of yours is in the water — and answers what is left. Your swing circle, which is smaller than at anchor because a mooring’s riser is short: about 1.5:1 for a helix ball like the BVI’s, 3:1 for chain to a block, against the 4:1 to 7:1 your own rode goes out at from the bow roller. The circle is the ball ranging on its riser at low water, plus your pennant, plus the boat. It also gives a pennant length for your bow (two and a half times its height above the water) and the pull your boat puts on the ball. What it will not do is vouch for the mooring: look for the posted rating, look at the pennant and the shackle, and back down gently to feel it hold.

Yes — as many as you like, each with its own tackle, on the Boat tab under Saved boats. Add a charter boat for the week and delete it when you fly home; keep the dinghy as its own entry. The Anchor screen always names the boat it’s calculating for, right at the top.

Yes. Rodes and anchors both take names — “Storm rode,” “Lunch hook,” “Big Mantus” — and the app uses your words everywhere it refers to them. The weight-and-model details stay visible in the boat profile, so naming loses nothing.

Always. Open “Why this anchor” and tap any combination aboard — the entire plan recalculates for what you’re actually putting down, including the warnings. The app never blocks your choice; it just refuses to pretend a poor one is a good one, and it will tell you what it would have picked instead.

Yes, in soft mud. The app works out an adjustable fluke anchor at 32°, the normal setting. With soft mud or silt chosen as the holding ground, a switch under it lets you say you have reset the flukes to 45°. The figures then use the 45° holding, about twice the 32° figure in Practical Sailor’s 2014 soft-mud test. An earlier Practical Sailor test, in soft mud over a firmer bottom, found 45° held far less, so reset the flukes only where the soft mud is deep. In any other bottom the switch does not appear, because at 45° the anchor will not set there. The Fortress Guardian’s flukes are fixed at 32°, so it has no switch.

Yes. Wind on the bow is what sails a boat about at anchor, and a scoop adds area right there, so she ranges more and a riding sail aft has more to undo. No test we know of measures how much. The app has no setting of its own for a scoop: count it in Windage on the Boat tab, one step higher than the boat alone, or take the scoop down on a windy night.

Depths in feet, meters, or fathoms; rode answers in any of those plus shots (shackles) or your own rode markings at whatever interval you’ve marked. Input and output units are set separately, because you read depth off an instrument but pay out rode by your marks.

Mark the drop on the Watch tab and the app draws a fence from tonight’s plan — the swing circle plus a margin for GPS scatter when you lie to one anchor, the corridor between two anchors for a Bahamian moor, and a tight ring around a boat held by a quay or by lines ashore. It then reads the GPS and plots the boat against the fence, north up or heads up as you choose, with the trail of everywhere she has ranged. You can mark from the bow, midships, or the stern; point the phone toward the bow and the app places the anchor where it actually went down. Allow location “always” when you arm and the watch keeps running with the phone locked or another app in front; decline and it runs with the app open, as before. Plug the phone in either way — the GPS is what uses the battery.

Yes. On the Watch tab, tap The anchor is already down. Go to the bow, back down gently until the rode is straight ahead, and mark with the phone pointed along the rode. The app puts the anchor that far ahead and widens the fence by how far the mark may be off. Once armed, tap Move the anchor under the plot, drag it, and tap Set the anchor.

Not after the drop: that mark is taken at the bow, with the phone along the straight rode, the truest line to the anchor. From the cockpit it would start a boat length short, and prop walk swings the hull off the rode’s line. Back down from the cockpit, then go forward and mark while the rode is straight. To mark from the cockpit, mark as the anchor goes down and choose Stern; the app uses the compass to place the anchor a boat length ahead.

By design. GPS scatters, and one wandering fix at three in the morning is not a drag. The alarm needs several consecutive fixes clearly outside the fence, beyond the fix’s own reported accuracy, so it takes a few seconds to be sure. An alarm that cries wolf gets disarmed for good, which is worse than one that takes a breath. If the GPS goes quiet or the app is closed, the screen says the watch has stopped rather than pretending to guard you.

Yes, once the watch is armed: − and + on the Fence row step it by a twentieth of the recommended radius, and the alarm watches the fence as you set it. The plot shows both — the fence you set as a solid line, the plan’s fence dashed beside it. Tightening spends the GPS margin and stops where the fence meets the swing itself; a boat lying to her rode reaches that line by doing nothing wrong, so a fence inside it would not be a closer watch, only a false alarm waiting for the wind to shift. Widening is for a GPS that scatters in this anchorage, a long snubber, or a fence that cried once tonight; it stops at twice the recommended radius. The setting stays with the watch and returns to the recommendation the next time you arm.

The app’s fence is the swing circle plus 50 ft, for GPS scatter and snubber stretch. You don’t need to add the GPS’s own error on top: the alarm sounds only after three fixes in a row lie outside the fence by more than 15 ft, or by half the accuracy the fix reports when that is more. Widen it if this anchorage scatters, up to twice the recommended radius; what a wider fence costs is the distance a dragging boat covers before the alarm sounds.

Yes, from version 1.10.0. On the Watch tab, under the plot, switch on Night view: the whole app turns red on black, not just that screen, so opening the Anchor tab at three in the morning does not flash you white. Red is kindest to eyes that have adapted to the dark, which means nothing on any screen is any other color — so green, amber and red can no longer say calm, warning and alarm. Brightness says it instead: a quiet boat is the dimmest thing in the cabin, an alarm the brightest, and the word beside it says the same in plain English. Apple’s satellite imagery is daylight photography and cannot be made kind to night vision, so switching the night view on switches the map off; switch it back on if you want it. The setting stays where you put it on that device until you change it.

From version 1.10.0, on a screen wide enough — an iPad held wide, or a 13-inch in portrait — the armed watch splits in two: the plot on one side, as large as the glass allows, and the controls on the other, each side scrolling on its own. Mute and Disarm sit below the controls and stay there, so they are never scrolled away while the alarm is sounding. Above the plot a band says where you stand in one word — WATCHING, EDGING OUT, EARLY WARNING, SWING ALARM, GPS QUIET or DRAGGING — with the sea room left beneath it, large enough to read from the companionway. On a phone the watch stays one column, with the same controls in the same order.

From version 1.10.0 the armed watch keeps a short record under the controls, on the phone as well as the iPad: the farthest she lay from the mark, her least sea room, and how many times the early warning chimed, each with the time it happened. Every figure is judged against the fence as it stood at that moment, so widening the fence at two in the morning does not rewrite what happened at midnight. The record lives with the watch and goes when you disarm.

Switch Map on under the plot and Apple’s satellite imagery of the anchorage appears under the same fence, hull and trail, in the same place, so the shore, the docks and your neighbors sit around the picture. It needs a signal and comes from Apple; the plot, the alarm and everything else work exactly the same with it off. It is not a chart — no depths, no marks — and the imagery may be years old.

North up, like a chart, or heads up, with the boat’s bow at the top. At anchor she lies to her rode, so her bow points at the anchor and the picture reads the way the view from the cockpit does. Auto chooses north up with the map off and heads up with it on; you can choose either. The phone’s compass is not used, so carrying the phone about the deck does not swing the picture.

The anchor alarm catches a boat that drags out of her fence. The swing alarm catches a boat that stays inside her fence but swings round her anchor to where she should not be — a wind shift that lays her over a neighbor, a shoal or a mooring on the other side. Switch it on once she is lying to her anchor. The alarm sector is an arc of the circle, set by two true bearings from the anchor and read clockwise; aim it at the water you want guarded by dragging the two handles on the fence line, or by stepping each edge round 5° at a time. It holds those bearings while she swings, so the same water is watched all night, and Guard the other side swaps it to the opposite arc in one tap. Three fixes in a row clearly inside it sound the same alarm, and the card says which it is. Near the anchor a bearing means nothing, so the sector counts only past an inner radius, half the fence unless you change it. It is offered where the boat swings — regular anchoring and a mooring ball.

Yes. A boat sharing with crew sends the sector with every report; the Monitor draws it and, with the app closed, a swing alarm arrives as its own notification, worded so the crew know it is a swing and not a drag.

The early warning. Inside the fence lies a band the width of the GPS margin — what is left of it, if you have tightened — and a chime sounds once when three fixes in a row put her in it by more than her own GPS uncertainty. At the recommended setting the band begins where the rode’s reach ends, so a chime there means she is either scattered by GPS or beginning to drag; watch the trail for a minute and it tells you which. If you have widened the fence, crossing the plan’s original line chimes too — that is the alarm you would have had, demoted. Tighten the fence all the way and there is no room for a warning, and the Fence row says so. The chime is soft and sounds once; the alarm is still the only alarm.

Yes, from version 1.6.0. When you arm, iOS asks to use your location always; allow it and the card says “Background watch — you may lock the phone or use another app.” The GPS keeps coming, the fence is judged by the same rule, and the alarm sounds through the lock screen — on the phone’s own speaker, which ignores the silent switch and Focus for as long as the app is running — with its words on the lock screen as a notification. If fixes stop arriving, you get a notification within two minutes: “The watch is not watching.” Decline “always” and the card says “Foreground watch — keep this screen open,” with Settings one tap away if you change your mind. Either way, plug the phone in: the GPS is what uses the battery, and a locked screen saves only the screen.

Three, all made on the phone, none involving a server: the anchor alarm with its words while the phone is locked; the early warning’s chime, the same way; and the watch-stopped warning, a notification the app keeps two minutes ahead while the watch runs so that a watch iOS terminated is announced rather than silent. The alarm’s notification and the watch-stopped warning are Critical Alerts if you allow them when iOS asks: they sound through the silent switch and Do Not Disturb. If you decline, they still break through a Focus mode but not the silent switch; while the app is running, its own sound ignores the silent switch either way.

Yes, from version 1.3.0, using a second device on the same Apple ID. Aboard, arm the watch and switch on Share to my other devices. Ashore, open the Watch tab and tap Look for a shared watch; the shore device shows the same fence, boat, and trail, and sounds its own alarm if she breaks out. The boat’s reports travel through your own iCloud and nowhere else, so both devices need to be signed into the same Apple ID with iCloud Drive on, and the boat needs a signal. Keep the app open on the shore device too — it checks for a fresh report every half minute. Only the boat can disarm the watch; silencing the alarm ashore silences it ashore only.

Yes, if both are on the same Apple ID. On the iPhone, switch on Share to my other devices. On the iPad, open the Watch tab, tap Look for a shared watch, then Take over this watch here. The iPad arms on the same fence. Disarm the iPhone. Then switch on Share with crew on the iPad and enter its code in the free Monitor on the iPhone.

Yes, from version 1.5.0. Aboard, arm the watch and switch on Share with crew; the screen shows a ten-character code, with Copy, Send and New code beneath it. Anyone you give the code to opens the free companion app, Rainbow’s End Monitor, enters it, and sees the same fence, boat and trail with the same alarms — and can tap Find the boat for an arrow that points at her with the bearing and distance, which is how you get back to the right boat from a dinghy dock full of identical charter yachts. The record is encrypted with a key made from the code, so treat the code like a key; it is deleted when you disarm or switch sharing off. The code stays until you choose New code. Crew can watch and find; only the boat can disarm.

From version 1.6.0 the Monitor hears the boat with the app closed: a breach, a quiet GPS or a stand-down reaches crew as a notification within seconds, and silence after fifteen minutes. That needs the crew phone signed into iCloud; watching with the app open does not.

Not necessarily — but the app refuses to guess. If no report has arrived for a minute and a half the screen says the link is stale; after three minutes of silence the monitor sounds an alarm, because a dead link must never look like a safe boat. Silence usually means the boat lost her signal, the phone aboard died or went to sleep, or the app was closed. The watch aboard may well still be running; the alarm is telling you to find out, not that she has dragged. The one silence that is expected is a watch the boat disarmed itself, which the shore device reports in those words and then goes quiet without alarming.

Not for the calculator: Rainbow’s End Anchoring works offline except for the optional map under the watch plot, which needs a signal to draw Apple’s imagery and is off unless you switch it on; nothing you enter leaves your device. The anchor watch asks for location permission — and, from version 1.6.0, to use it “always”, so the watch can keep running with the phone locked — and reads the GPS — which is satellites, not internet — on the device alone. The remote watch and sharing with crew are the features that need a connection: switch on “Share to my other devices” and the watch, and only the watch, is copied to your own iCloud so your other devices can monitor it; switch on “Share with crew” and it is published encrypted under a code for the people you give the code to. Both are off by default, and we never see what either sends. The app also asks to send notifications, for the three things listed under the anchor watch. See the privacy policy, which is still short because there is still little to disclose.

Every formula the app uses is on How Anchoring works it out, with what each one rests on and the numbers for one boat worked through, so you can check them yourself.

No, and be wary of any anchoring tool that claims otherwise. Holding power varies between anchorages and between one set and the next in the same spot; the coefficients in the app are estimates drawn from independent testing, not manufacturer claims. The app is an aid to judgment — if its answer disagrees with your seamanship, trust your seamanship.

Please tell me — seriously. Every calculation in this app got better because a sailor said “that doesn’t look right” and included their inputs. Email the numbers you entered, what the app said, and what you expected, and I’ll trace it through the math and either fix the app or show the working.

Purchases are handled by Apple and Google, so refunds are too — through reportaproblem.apple.com for the App Store, or your order history on Google Play. I never see your payment details and can’t issue refunds directly.