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Anchoring Mathematics – again!

Jeff Seigel, founder of ActiveCaptain, writes…

This week we decided to reach into the archives and pulled out one of our most popular topics, an article from almost two years ago. Of the 250 newsletters we've written (which amazes us), the subject of anchoring math was in the top 5 for comments, arguments, and discussions. Since there have been so many new ActiveCaptain members in the last 2 years, we thought we'd revisit the subject to get everyone thinking about what really happens when you anchor using an anchor alarm.

Some of you will think there are logic and mathematics errors in this article. There aren't. Read it all and study the linked reference document. If you want to debate it, make sure to reference the linked graphic document and show how it's not correct (it is correct though!).

It should be simple. Pick the spot to anchor; come to a stop; drop the anchor and set the anchor alarm. Then pull back until the anchor sets.If you pull away further from the anchor set point than the distance you specified, alarms should go off. Simple, right?

Well, not exactly. The mathematics are surprisingly a lot more complex.

We know. It seems easy and obvious. We've been involved in many debates until the pencil and paper come out and then, "oh yeah" is heard.

Here's the missing magical point. You've got to notice that the point where the anchor position is set in the alarm is the position of the GPS and not the position of the bow/anchor. That one small point ends up bringing a whole bunch of trigonometry into the calculation.

When the boat swings 180 degrees, the error created by that offset equals twice the distance from the bow to the GPS. (Honest, twice.)

Let's take an example for a typical 42 foot sailboat with a GPS on the stern rail. This is the worst case scenario but is very typical and demonstrates what happens very well.

Assume you're anchoring in 10 feet of water with a bow that's 5 feet off the water's surface. A good scope for a night without much weather expected would be 5:1. This means 75 feet of rode will be let out and pulled back to set hard (we call that power setting). Then the anchor alarm is set at 125 feet, far more than the 75 put out. And since you power set the anchor, you couldn't possibly move 50 feet, right?

At 3 am, because these things always happen at 3 am, the anchor alarm goes off. You're 127 feet back. You remember that you way over added to the 75 feet so you start planning what you're going to do in the total black of night with the moderate wind that's now blowing. But in reality you don't need to do anything. Your anchor is not dragging.

What really happened is that the tide changed at 1 am. During the next 2 hours you slowly swung around and moved back. Not knowing this new math for anchor alarms you didn't realize that the GPS displacement caused 84 feet of position error in the anchor alarm. Your alarm went off after moving back only 52 feet. In reality, your anchor alarm should watch you move back another 32 feet without your anchor moving 1 inch on the sea floor. The anchor alarm should have probably been set at about 75 + 84 + 10 + 10 = 179 feet. The two 10's are for GPS accuracy error and slop since the anchor doesn't set immediately. Can you imagine setting an anchor alarm at almost 200 feet with only 75 feet of rode out? And yet, that's the right number for this boat.

This unexpected error is the reason we wrote DragQueen (available for free in the Apple app store and Google Play). Since the anchor alarm is on a phone, the GPS position is the phone itself. When deploying the anchor, we stand with the iPhone at the bow to eliminate one half the GPS position error. There's still another position error based on where the GPS is located while we sleep at night (25 feet back in our stateroom).

Remember too that this positional error happens at all angles. Swing about 90 degrees to the side and the error is about 1 times the GPS displacement distance. Even that can be significant. Given a heading/fluxgate sensor and a few configuration settings, 100% of this GPS positional error could be eliminated (DragKing?).

If you're still saying, "wait a second – there's not a 2x error in the position" – check out this graphic proof of what happens. We'll wait to hear the "oh yeah":

https://activecaptain.com/articles/misc/anchorAlarms.php

Happy anchoring!

By Jeff Seigel

Guest Author & Founder of ActiveCaptain

ActiveCaptain

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NMEA announces 2014 Product Award winners

Marine electronics experts nationwide named 11 products in nine categories as winners of the 2014 NMEA Product Awards.

Once again, Furuno USA was named Manufacturer of the Year for Support.

The winners were announced at the National Marine Electronics Association (NMEA) International Marine Electronics Conference & Expo. Nearly 400 leading marine electronics manufacturers, dealers, and distributors, as well as media and others from the United States and abroad, attended the event.

Category winners are chosen by regional panels of NMEA members from around the country. These experts are hand picked by the NMEA Awards Committee for their

…….Click NMEA announces 2014 Product Award winners …. to continue reading

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WiFi on boats … beware the ‘Auto-Update’

Jeff Seigel, founder of ActiveCaptain, writes…

This week we ripped out all of the WiFi components on our boat and installed all new equipment, antennas, and wiring. We changed the way the main router is powered and now use one Power-Over-Ethernet 12v injector for the main router and the outside high-gain modem. We also re-positioned everything to allow for easy changes without ceiling tile removal in the future.

Every setup has different requirements so the specific products we're using might very well be all wrong for you. Still, we know you probably

…….Click WiFi on boats … beware the ‘Auto-Update’ …. to continue reading

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WiFi Devices for Boats – Part 5 of 5

Jeff Seigel, founder of ActiveCaptain, writes…

Continuing with our summer series about WiFi on boats, we're going to turn our attention to the central display most of us use when viewing information – a laptop, or quite commonly today, a tablet. We surely don't need to talk about iPads or Android tablets again. If you have a pulse, you know all about them.

This week we want to discuss an important WiFi concept for laptops and tablets – how you should connect them to the internet over WiFi. There are some new

…….Click WiFi Devices for Boats – Part 5 of 5 …. to continue reading

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WiFi Devices for Boats – Part 4 of 5

Jeff Seigel, founder of ActiveCaptain, writes…

The previous segments of this WiFi series have tried to provide some WiFi background information. The devices to use today on your boat for WiFi are changing and there's no one set of products that will meet everyone's needs. We'll write more about some background technology later but it's time to start showing some of the incredible things you can do today with WiFi. We'll be writing a few segments throughout the summer describing some WiFi devices that you might not know about.

Today's segment is

…….Click WiFi Devices for Boats – Part 4 of 5 …. to continue reading

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