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Old 13-07-2017, 10:06   #121
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

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Originally Posted by Wotname View Post
...I can't see how...
Ded reckoning requires one to estimate distance (speed x time) traveled (in a certain direction) from last known position. Simple and easy to see unless one simply refuses to see.
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Old 13-07-2017, 16:05   #122
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

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Well Jim, it isn't often that I disagree with you but...

Sir Francis seems to be talking about his ability to deduce his dead reckoning rather knowing his boat speed at any given moment. As I understand the term "dead reckoning", it working out one's position using methods of deduction (of historical data and time) rather than using observations of immediate data. Thus I can't see how this could extend to him knowing his current boat speed at any given moment.

Further, it would seem pointless to me to even attempt it as there would be no way he would know if he was correct or not, given that he had no way measure his instantaneous speed once his speedometer stop working.

Just as your "little game" would be pointless if you had no independent way of knowing the boat speed in order to confirm your estimate; well not pointless, it would be a useful way of whiling away the hours
Wottie, I think we are more or less in agreement here, but misunderstanding each others writing.

When I used to do celestial, we made hourly speed and course and distance run log entries. From those we would construct a DR position, usually used as the assumed position for celestial plotting but in the absence of an observation plotted on the chart as a DR position. The key to accuracy of the DR was how well we integrated the data we had available. Instantaneous speed was far less important than the distance run, taken from our paddle wheel log.

It has been many years since I read Chichester's book, and I don't remember if he had a Walker log as well as his knot-log instrument. If not, then he must have been doing a mental integration of boat speed in order to calculate distance run. He (and you) may not have used that term to describe the process, but how else would one be able to estimate that figure? And the years of practice with a speed log must have helped generate that skill.

Anyhow, we've strayed from the original question about the need for STW data. I continue to find it useful, others don't, and we'll never know how Sir Francis would have dealt with a GPS. I suspect he would have embraced the accuracy with pleasure!

Jim
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Old 13-07-2017, 17:27   #123
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

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Wottie, I think we are more or less in agreement here, but misunderstanding each others writing.

When I used to do celestial, we made hourly speed and course and distance run log entries. From those we would construct a DR position, usually used as the assumed position for celestial plotting but in the absence of an observation plotted on the chart as a DR position. The key to accuracy of the DR was how well we integrated the data we had available. Instantaneous speed was far less important than the distance run, taken from our paddle wheel log.

It has been many years since I read Chichester's book, and I don't remember if he had a Walker log as well as his knot-log instrument. If not, then he must have been doing a mental integration of boat speed in order to calculate distance run. He (and you) may not have used that term to describe the process, but how else would one be able to estimate that figure? And the years of practice with a speed log must have helped generate that skill.

Anyhow, we've strayed from the original question about the need for STW data. I continue to find it useful, others don't, and we'll never know how Sir Francis would have dealt with a GPS. I suspect he would have embraced the accuracy with pleasure!

Jim
Yep, we are on the same page
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Old 13-07-2017, 17:33   #124
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

I am not going to read all the comments to this thread. GPS is not accurate enough for sail trim... it's speed and course is not very useful to sailors except to help with the effect of currents.

My B&G speed is accurate to 0.01 knots...wind to 0.1 knots.

I used GPS to guestimate TTG so wifey has some idea when we will arrive... My guess is usually better.
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Old 13-07-2017, 17:44   #125
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

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Originally Posted by Terra Nova View Post
Ded reckoning requires one to estimate distance (speed x time) traveled (in a certain direction) from last known position. Simple and easy to see unless one simply refuses to see.
I guess I was not clear enough with my previous post. The point I was trying to make was that Sir Francis could not have confirmed his estimate of "instantaneous boat speed" without reference to an external speed measuring device.

Thus while his speed log wasn't too important to him to calculate a daily DR but one can't extend this argument to him knowing his exact boat speed in the moment. Of course he would have a good idea of his average speed say over a few minutes or more and clearly over 24 hours.

Anyhow, you and I both know what DR is (and it's cousin the running fix) and I am straying off topic here (as Jim has noted ).

And I'm sure we both agree That Sir F was an extraordinary navigator.
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Old 13-07-2017, 18:13   #126
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

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I am not going to read all the comments to this thread. GPS is not accurate enough for sail trim... it's speed and course is not very useful to sailors except to help with the effect of currents.

My B&G speed is accurate to 0.01 knots...wind to 0.1 knots.

I used GPS to guestimate TTG so wifey has some idea when we will arrive... My guess is usually better.
And how do you know this to be true?

I suspect that your instrument's display has that many significant figures, but nowhere near that sort of absolute accuracy.

Jim
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Old 13-07-2017, 19:25   #127
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

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I am not going to read all the comments to this thread. GPS is not accurate enough for sail trim... it's speed and course is not very useful to sailors except to help with the effect of currents.
.
I disagree. When racing, we have an SOG display which I frequently study when tweaking the sails - I can clearly see a 0.1-0.2 knot speed change over 10-20 seconds after making an adjustment.
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Old 13-07-2017, 23:07   #128
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

I must be missing something. I don't know what you guys are looking at, but my GPS, on my phone, while I am sure it is accurate, has a bit of a lag to it. So I don't consider it more accurate instantaneously than what I see off the gunwale. For trimming sails I am using my desired compass heading and then trimming for speed and changes in wind direction with tell-tales. For me tell-tales tell it all. With GPS, my speed is, I am sure, very accurate, if I am holding a steady course, after a few seconds, but it is just for determining arrival times (and bragging rights) now these days.
So Stu I see what you are saying but my GPS will read say 6.2 knots then jump to 6.5 then fall to 5.9 pretty commonly depending on I'm not sure what, probably the nature of the swells I am in. Certainly not helpful for sail trim for me. Or maybe I am just a really lousy helmsman... also possible!
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Old 14-07-2017, 01:31   #129
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

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I disagree. When racing, we have an SOG display which I frequently study when tweaking the sails - I can clearly see a 0.1-0.2 knot speed change over 10-20 seconds after making an adjustment.
A subtle point.

But you're both right --

SOG is irrelevant to sail trim if there is even a slight current -- and there always is in the ocean.

HOWEVER, the GPS is often more responsive than speed logs and better shows delta SOG. I used to watch the GPS when doing sail trim just like Stu.

Now I have ultrasonic speed log (Airmar CS4500) and don't do that any more.
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Old 14-07-2017, 01:34   #130
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

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Originally Posted by Don C L View Post
I must be missing something. I don't know what you guys are looking at, but my GPS, on my phone, while I am sure it is accurate, has a bit of a lag to it. So I don't consider it more accurate instantaneously than what I see off the gunwale. For trimming sails I am using my desired compass heading and then trimming for speed and changes in wind direction with tell-tales. For me tell-tales tell it all. With GPS, my speed is, I am sure, very accurate, if I am holding a steady course, after a few seconds, but it is just for determining arrival times (and bragging rights) now these days.
So Stu I see what you are saying but my GPS will read say 6.2 knots then jump to 6.5 then fall to 5.9 pretty commonly depending on I'm not sure what, probably the nature of the swells I am in. Certainly not helpful for sail trim for me. Or maybe I am just a really lousy helmsman... also possible!
Don, it depends on the GPS. What you're saying is true of the older ones, but the newer ones have very high update rates -- 10hz on my Simrad GS25.

If you have a paddlewheel speed log, a modern GPS will show instantaneous changes of speed better.
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Old 25-09-2017, 18:23   #131
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

Wind speed is very on our cruising boat with just a crew of 2 on long legs offshore. We AGREE ON A NUMBER FOR WAKING UP THE OFF-WATCH PERSON. wE ALSO HAVE AN UPER LIIT FOR THE KITE.
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Old 28-10-2017, 11:56   #132
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

I believe the Speedo is better for saving/recording polar files for your boat and of course you want the least current possible.

Both ground and water referenced parameters are needed to determine set and current (along with knowledge or assumptions about leeway).
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Old 28-10-2017, 11:59   #133
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Re: Are Speed Logs and Wind Speed Instruments Obsolete?

In short, no they are not obsolete.
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