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Old 09-03-2014, 16:20   #31
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Re: Is a compass your primary navigation instrument or back up?

N58

I was referring specifically to N77's Autohelm Personal HB compass, which has a fantastic user interface but (in my case) is so inaccurate as to be useless.
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Old 09-03-2014, 16:36   #32
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Re: Is a compass your primary navigation instrument or back up?

Now that's a nice binnacle.


But when using a compass it depends where are, almost useless here at times. The chart notice says Magnetic Anomaly .... reading may range 27 Deg West to 3 Deg East. GPS on the other hand is spot on.

When in fog with just compass you would want to be extra careful.
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Old 09-03-2014, 17:31   #33
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Re: Is a compass your primary navigation instrument or back up?

The Autohelm HB compass (like other fluxgate compasses) must be level to work accurately. When using one to take a bearing on another floating object, it is easy to accomplish this: if the compass "sights" are on the distant object, and you are very near the sea level, the compass will be level. If using it to sight a land object, which may be well above sea level, there will be error introduced.

In practice, when at sea on a moving vessel, extreme accuracy is difficult with this or any other HBC that I have ever used. Worrying about a degree or two is not helpful, but with the Autohelm, it is easy to take a series (up to 9) of sights in succession and do an eyeball average... this has worked well for me for many years.

Cheers,

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Old 09-03-2014, 17:55   #34
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Re: Is a compass your primary navigation instrument or back up?

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Originally Posted by OceanQuest View Post
Unfortunately it doesn't hook up to the autopilot so requires manual helming, but if you keep the arrow and compass direction lined up and adjust your heading whenever the current changes you are always taking the shortest course to your destination. The nearest thing to a straight line.
I presume you are describing a course with 0 cross track error.

This is not the quickest, or most efficient route in conditions of changing current.

This thread explains it best:

CTS vs Following Rhumb Line - DILUTED THREAD
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Old 09-03-2014, 18:18   #35
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Re: Is a compass your primary navigation instrument or back up?

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Originally Posted by Andrew Troup View Post
N58

I was referring specifically to N77's Autohelm Personal HB compass, which has a fantastic user interface but (in my case) is so inaccurate as to be useless.
Mine certainly has high precision. I usually fire 3 readings (which is sort of a force of habit) and its not uncommon for all three to be identical. They are certainly always close.

It has been some time since I checked the accuracy but a couple of years ago I used it to find a suitable place to move the autopilot compass and one criterion was looking for a spot where the reading was identical to steering compass over several different directions and it behaved as expected. I should check it again.

The Autohelm HB is now quite old. My guess is that they have not made them for 20 years. They seem very reliable and I have not heard of any defective units, but it's not surprising some are starting to show up.
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Old 09-03-2014, 18:33   #36
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Re: Is a compass your primary navigation instrument or back up?

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Originally Posted by noelex 77 View Post
I presume you are describing a course with 0 cross track error.

This is not the quickest, or most efficient route in conditions of changing current.

This thread explains it best:

CTS vs Following Rhumb Line - DILUTED THREAD
That seemingly interminable thread was mostly concerned with the specific instance of crossing the English Channel, a body of water that has strong tidal currents, extraordinarily good tidal data available, and passage times that often encompass a full tidal cycle.

For the rest of the world, there is less (frequently zero) tidal data, poorer charting, weaker tidal flows and often shorter passages. In many cases, the only really useful approach is to attempt to maintain the rhumb line, and here the use of a gps and steering to minimize CTE is a practical and safe methodology. I don't think that it makes much difference whether one uses a conventional magnetic compass or some more modern device -- if you don't have info about tidal set, following the rhumb line is a good means of safely reaching your destination (even in the fog... actually, ESPECIALLY in the fog!).

Cheers,

Jim
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Old 09-03-2014, 18:48   #37
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Re: Is a compass your primary navigation instrument or back up?

Can anyone recommend a good book about how to really utilize what GPS has to offer? I have Weekend Navigator by Bob Sweet and it looks to be pretty good. ANy other ideas? Thanks.
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Old 09-03-2014, 18:50   #38
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Re: Is a compass your primary navigation instrument or back up?

Jim, Noelex 77,

Thanks guys, I guess my unit must be faulty. Even on land, staying away from ferrous masses and taking care with levelling, I'm lucky to get 5 degrees repeatability. At sea it is double that, even in good conditions.

A hockey puck, for me, gives more like 2-3 degrees, in the latter circumstance, and about 1, on land.
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Old 09-03-2014, 18:55   #39
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Re: Is a compass your primary navigation instrument or back up?

I broadly agree with what Jim wrote about crossing tidal streams, but as a point of clarification:

What he wrote about strength of those streams is on the button, but I would point out that the strength needs to be evaluated relative to the boat's speed, not against any absolute yardstick.

... So a Robert Tucker Silhouette bilge keeler, which goes to windward the way thistledown doesn't, and downwind like a dumpster (but they are a lovely boat all the same!), needs to think of crossing a cyclical 2 to 3 knot tidal stream in the same way someone on a 48' multihull might think of an 8 knot stream.

The strategies necessary to rise to these challenges require looking into the future with some accuracy. The rhumb line strategy, even with GPS and all the trimmings, will not 'cut it', nor will unreliable tidal stream data.
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Old 09-03-2014, 18:56   #40
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Re: Is a compass your primary navigation instrument or back up?

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Originally Posted by Andrew Troup View Post
Jim, Noelex 77,

Thanks guys, I guess my unit must be faulty. Even on land, staying away from ferrous masses and taking care with levelling, I'm lucky to get 5 degrees repeatability. At sea it is double that, even in good conditions.

A hockey puck, for me, gives more like 2-3 degrees, in the latter circumstance, and about 1, on land.
We had our W&P serviced by a compass service shop. They do aviation instruments most of the time. The cost was 170 - well less than a new compass and that included a new card, vacuum refill and certification. Well worth it.
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Old 09-03-2014, 19:11   #41
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Re: Is a compass your primary navigation instrument or back up?

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We had our W&P serviced by a compass service shop. They do aviation instruments most of the time. The cost was 170 - well less than a new compass and that included a new card, vacuum refill and certification. Well worth it.
Sorry, N58, if I was unclear. The Autohelm Personal Compass we are discussing (fluxgate, handbearing electronic compass with a neck lanyard and a memory for nine bearings) is somewhat unique, no longer in production (I wondered at the time if there were issues with reliability) and not (AFAIK) serviceable.



I kind of like the 'body language' of a magnetic hockey puck compass with a floating card, because it will generally signal to a perceptive user if it is unhealthy.

And perhaps that's part of my nagging mistrust of fancy electronics. Like certain people on the internet, (NOBODY on this forum of course ;-) they are just as authoritative and plausible when they are wrong as when they are right.
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Old 09-03-2014, 19:29   #42
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Re: Is a compass your primary navigation instrument or back up?

Ahh it's hell to be old. and haveing to depend on a compass for your main navi tool!! But ya know it's worked flawlessly for over 40 years ! with a few charts and a depth sounder we have found our way just fine !! But I will admit I like my hand held GPS real well also !! Different strokes and like that !!
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Old 09-03-2014, 19:55   #43
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Re: Is a compass your primary navigation instrument or back up?

I have one of those and I use it as soon as I see another vessel at sea to determine if we are on a collision course. Binoculars with a built in compass would be way more convenient but I haven't gotten a pair yet so for now I will continue using it. A hand bearing compass is very important to have so you can determine whether or not a collision course situation exists.
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Old 09-03-2014, 20:09   #44
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Re: Is a compass your primary navigation instrument or back up?

Quote:
Originally Posted by Jim Cate View Post
That seemingly interminable thread was mostly concerned with the specific instance of crossing the English Channel,
Quote:
Originally Posted by OceanQuest View Post
I'm from the UK
Quote:
Originally Posted by Jim Cate View Post
For the rest of the world, there is less (frequently zero) tidal data, poorer charting, weaker tidal flows and often shorter passages.
Jim I think your point that there needs to be a reasonable and variable current (relative to boat speed) is well made. I agree on many passages steering the rhumb line is perfectly satisfactory, but there are other times when it can make passage times much longer.

These passages are not confined to English Channel. Sailing around the opposite side of world, (in Queensland waters) in my early days, showed that passages could be very long and uncomfortable if you don't get this right. Admittedly I was cruising in small yacht which exacerbates the differences.

I also disagree that you necessarily need very predicable tides. A simple example is sailing across across a tidal stream that you know is going to strengthen. Rather than sailing the rhumb line an initial offset upstream is simple to incorporate and need not necessarily be precise to provide worthwhile gains. I am sure you instinctively do these sort of things, but many inexperienced navigators do not and even worse think the Rhumb line course is optimum so never even attempt to make these corrections.
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Old 09-03-2014, 23:55   #45
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Re: Is a compass your primary navigation instrument or back up?

Puget Sound, charter fleet.

Primary is iNavX and SeaIQ on an iPad mini, followed by whatever plotter the boat I happen to be on has.

On a typical cruise, there are four or five people on board, three of whom will have an iPad and all of whom will have a phone. So there's usually at least six independent GPS systems running besides whatever electronics the boat has.

When we take shots to practice manual fixes, we use Steiners, not the compass. When we practice paper, we go from paper to Steiners and Steiners to paper.

I've never used the boat's compass for anything, and I'm not sure what I'd use it for in Puget Sound. It's too easy to look around and see where you are. In fog, there's no possible way I'd trust the compass over the electronics.

Fog is not the time to learn new skills, fog is the time to go with what you know. So in fog, I use what I use the most-- the iPad.

In Puget Sound, we don't really worry about the rhumb line, we set the destination according to the current, and then sail to it according to the wind.
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