Knot meter vs GPS speed. - Page 3 - SailNet Community
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post #21 of 37 Old 08-08-2017
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Re: Knot meter vs GPS speed.

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Originally Posted by chef2sail View Post
People generally ask questions on here because they have real questions. Ridiculing them doesn't make sense.

If you don't have the patience to answer his / her questions without the snarky quips, just say nothing.

Comments like that aren't necessary.
Fair enough... to me this question shows such a profound failure to understand what GPS speed is measuring that it's scary.

Boating is serious stuff and incompetent boaters are not only a danger to themselves... but to others.

And why don't people use google to ask questions?

"GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed."

GPS is not a transducer...boat speed is!
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post #22 of 37 Old 08-08-2017
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Re: Knot meter vs GPS speed.

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Originally Posted by aeventyr60 View Post
Hey the Dodd Narrows is some big fun. Requires an IPA or after you get. A bit of the fun here:

little tug and big log boom dodd narrows bc - YouTube
Nicely Done! Somebody had their boat driving Wheaties that morning.

Totally agree with BP about finding the sweet spot with feel and sound. There are so many factors at play that you can observe on the boat that we can't observe here. Lots of resonances, prop pitch, bottom condition, current, sea and wind state are just a few.

You could start looking for a sweet spot around 75% throttle, work the throttle slowly up and down until everything feels right. When things stop rattling and shaking and the boat moves nicely through the water, you've likely found the sweet spot.

As far as the error on the knot meter goes, I'd say just go with the GPS, there's a good chance the data from the mechanical log is just gibberish and I wouldn't be bothered with trying to figure it out on a rented boat.
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post #23 of 37 Old 08-08-2017
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Re: Knot meter vs GPS speed.

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Originally Posted by SanderO View Post
Fair enough... to me this question shows such a profound failure to understand what GPS speed is measuring that it's scary.

Boating is serious stuff and incompetent boaters are not only a danger to themselves... but to others.

And why don't people use google to ask questions?

"GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed."

GPS is not a transducer...boat speed is!
Agreed. Google would work. Also it would help sometimes if people would do a search on previous posted Sailnet threads to see if the topic had already be discussed .

GPS speed is not a major safety issue though. I remember 40 years ago when I "graduated" from my days of racing nationally Hobie 16's to my first keel boat how every thing I did was learning. One question led to the next. Despite my racing on Keel boats , owning and maintains one was far different.

There were many patient sailors now that I look back who took some of my inane questions with patience. I understand why now. They were paying it forward as they were in my position once.

There was no internet then to ask and get answered instantaneous questions. There was no sailnet. Damn there were no satellites or GPS them either.

Back to the topic. I use the SOG most of the time for how long to the next Mark. The knotmeter is my bible for trimming the sails and the easiest method for teaching others and my wife how their sail handling affects our speed.

There are many times I don't even put the paddle wheel in.

In the Chessie there is very little current in most of it, so I think we get sedentary about understanding its effect.
Where SanderO sails , it's a big deal. A four knot current can add hours to a long weekend trip if you are fighting it on your return leg. Not even factoring wind and Chop direction. Planning is much more necessary
Chessie sailors for weekend trips really only take wind direction and prediction in their planning.

The more you use it, the better you get at it. Hopefully the less basic questions you ask. However on here you will never have a period when there are no newbies.

As good stewards we need to coax them along with patience till they start answering other newbies questions
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post #24 of 37 Old 08-08-2017
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Re: Knot meter vs GPS speed.

Slightly OT...

This site and others like it tend to be a go to place for noobs. They will often ask questions assuming (correctly) that there are more experienced people who have the answers. The alternative is to get a few books and do some reading... search thru YouTubes which have all manner of instructive how to's or search google and follow the links and use this to inform you.

I think SOME or maybe MANY of the questions indicate laziness... and that is something that can be dangerous out there on the water.

Nothing wrong with intelligent questions from someone who is struggling with new material. But too many questions sort of lower the discourse level of the site.

Maybe

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post #25 of 37 Old 08-08-2017
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Re: Knot meter vs GPS speed.

Back to "speed"

Boat speed is useless if the transducer is fouled, the hull around it is fouled and / or it's not calibrated. When the transducer is clean and calibrated it should be accurate enough to trim your sails... which you can't use GPS SOG for... simply not precise or it's "filtered/smoothed".

The B&G speed instrument I have is precise to 0.01. I suppose the accuracy is limited to the amount of friction... the concept of measuring the speed is basic and fool proof... speed is measured by rotations of the paddle wheel or impellor. Of course it is not perfectly accurate... but orders of magnitude more accurate that the SOG.

There is also a "trend" indicator on the Boat Speed read out.... an "apostrophe" means speed is trending up.... a "comma" means it is trending down... neither means the speed is constant.

You can see ever change in sail trim... as long as the conditions are constant... but you can also see every puff or change in the sea... waves and so on. None of this "shows" on SOG as "instantly" as it does with the transducers.

Boat speed can be damped as well... a sort of averaging

GPS will tell you when you will arrive at a set mark... if you program this...

Remember... you can be anchored in a strong current and your boat speed will tell you that you are moving but the GPS will tell you that you are not. Or drifting and the GPS will indicate speed and the boat speed may be reading 0.

Sailors need to understand THEIR instruments and THEIR boat...

Boat speed is useless when you are not calibrated and clean... and properly damped.

GPS is useless for fine sail trim...

Lookie lookie.... GPS reads 7.0... Boat Speed reads 6.47 with an UP speed trend
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post #26 of 37 Old 08-08-2017
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Re: Knot meter vs GPS speed.

This scenario sounds suspiciously like a charter company trying to fake the solution to a problem they haven't fixed. Did someone whack the prop and they installed the wrong replacement? Is the motor failing? Who knows.

Assuming all is proper, you should run the motor to wide one throttle on flat water and note the max rpm. Only takes a few seconds. Then run her in cruise somewhere between 60% and 80% of max rpm. As poker said, sound and feel matter, so vibration may tell you where in that range she wants to be. Lower, is obviously much better on fuel consumption. Who's buying the fuel, btw? Included in the charter, or do you return her topped up?

p.s. If you're not getting close to hull speed at 80% of max, she is either fouled, has the wrong prop or the wrong engine.
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post #27 of 37 Old 08-08-2017
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Re: Knot meter vs GPS speed.

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Originally Posted by SanderO View Post
Back to "speed"

Sailors need to understand THEIR instruments and THEIR boat...
You do realize this is a charter boat right?

But back on topic FWIW, we lived on and sailed the Salish Sea for a year and I can remember the knot meter and GPS matching only a few times. There are a lot of currents in play! Every time I was convinced that one or the other was broken the would sync up again on a particular passage and make me doubt my sanity again

My 38' averages about .56 gph although we cruise a bit slower than optimum (2200 rpm vs 2400 rpm) and get anywhere from 5.5 knots to 6.5 knots thought the water depending, again, on currents.

If you are truly interested learning about the local currents, pick up a current atlas (CHS - Atlas of Tidal Currents) and the 2017 Current Atlas Tables (2017 Current Atlas Tables - Waggoner Cruising Guide) and start making your self crazy trying to route plan. Personally I think its a bit of a waste of effort, but it will sure explain why your speed drops suddenly as you shoot through Discovery Passage on what you thought was an ebb tide — I thought I'd screwed up something... :-)
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post #28 of 37 Old 08-10-2017
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Re: Knot meter vs GPS speed.

SO what if there are currents and you want to know what the boat is REALLY doing. Use your GPS to determine when it's slack water and motor around and see how the knotmeter and GPS compare. And because sometimes the current is so subtle you won't even get a 0.1 knot reading I set the GPS to display, get this, actual Lat/Long. If you check the 3rd digit to the right of the decimal (assuming it's deg, min decimal min) you can see ANY movement of the boat.

This isn't rocket science. And I can guarantee your 40'er isn't doing 7 kts at 0.5 GPH.
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post #29 of 37 Old 08-10-2017
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Re: Knot meter vs GPS speed.

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SO what if there are currents and you want to know what the boat is REALLY doing. Use your GPS to determine when it's slack water and motor around and see how the knotmeter and GPS compare. And because sometimes the current is so subtle you won't even get a 0.1 knot reading I set the GPS to display, get this, actual Lat/Long. If you check the 3rd digit to the right of the decimal (assuming it's deg, min decimal min) you can see ANY movement of the boat.

This isn't rocket science. And I can guarantee your 40'er isn't doing 7 kts at 0.5 GPH.
This is a fine way to find out if there is current running... Of course you may have to wait until you find slack tide to then do you speed runs to calibrate the boat speed. And you need to know that the bottom around the transducer and the transducer itself are not fouled.
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post #30 of 37 Old 08-14-2017 Thread Starter
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Re: Knot meter vs GPS speed.

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Originally Posted by SanderO View Post
Fair enough... to me this question shows such a profound failure to understand what GPS speed is measuring that it's scary.

Boating is serious stuff and incompetent boaters are not only a danger to themselves... but to others.

And why don't people use google to ask questions?

"GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed."

GPS is not a transducer...boat speed is!
If you read my original post, I state I understand GPS useage. You are just going to have to take my word on this, I know how to navigate.

Somehow you come to the conclusion I am too much of a Noob to be asking questions on this forum and am a danger to myself and others on the water. You use Google to contribute "algorithms in the Kalman filter". Are you really trying to contribute to this conversation or do you just like to hear the sound of your own keyboard clicking?

I am on a charter boat with multiple broken systems in a foreign waters to me, sailing with my wife who's sailing knowledge consists of knowing two knots. With the limited access to internet I had, i came to sailnet to seek advice on how to best operate a new to me boat and you respond with ridicule and bad advice. Do you really think GPS speed is a piece of trivial information only useful for letting the wife know when we will stop for lunch?

For those who responded with sound advice, Thank You.

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Oceanside CA
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