Speed Through GPS Versus Old Fashioned Paddle Log

[valhalla360]

Originally Posted by Sailor Doug

"You don't know what you don't know" aircraft have multiple navigation, air speed is another issue.
With the price I paid for my boat the knot log was very little more. Keeping a paddle wheel clean is quick and simple. I don't understand why you would not want one. Redundancy is great.


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A literal knot log (ie, rope with knots and stick attached to it) is also simple and cheap. Should you have one of those made up and at the helm because redundancy is great?

It's about cost vs benefit and cost isn't just monetary. It's amazing how many people I've met who dream of sailing off into the sunset but never seem to get away from the dock because one more back up to the backup needs to be added.

If you have a paddlewheel and don't mind cleaning it every few days, have at it. If you are into racing, it's probably well worth it but the point of this thread is if it provides substantial improvement for crusiers. It does eat up time to keep clean and they do break costing money to repair, so is it the best use of limited time and money.

[Sailor Doug]

Originally Posted by valhalla360

A literal knot log (ie, rope with knots and stick attached to it) is also simple and cheap. Should you have one of those made up and at the helm because redundancy is great?

It's about cost vs benefit and cost isn't just monetary. It's amazing how many people I've met who dream of sailing off into the sunset but never seem to get away from the dock because one more back up to the backup needs to be added.

If you have a paddlewheel and don't mind cleaning it every few days, have at it. If you are into racing, it's probably well worth it but the point of this thread is if it provides substantial improvement for crusiers. It does eat up time to keep clean and they do break costing money to repair, so is it the best use of limited time and money.

Some people are better off not leaving the dock, as long as they enjoy their boat great!
Redundancy is not cost vs benefit, but fortunately I use mine for sail trim. Have you ever heard surf without seeing it?
Do you have a knot log transducer, or have you had bad luck or a dirty harbour? I have only replaced mine once due to travel lift and cleaning is not to frequent. Bought a lot more gps's for a lot more money.


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[Ericson38]

Originally Posted by valhalla360

There is still nothing about your scenario that suggests any significant advantage to STW.

Coming down from Reedville Va, heading back to Lake Jackson Creek, after sunset. Tide is running out of the bay, as predicted by the chart plotter and confirmed by the set/drift out of the B&G Hyrda, which is using the GPS SOG and COG, the flux gate, and the STW sensor. Chart Plotter agreement is nice, haven't not seen the content and process using its internal current/tide database ! New Garmin 741, had it a month.

Pass Windmill Point Light 1 mile to starboard, steer directly to Red Nun '2R', water is deep between these two, no channel to maintain. Having a nice 8 knot SOG ride down with 6.5 STW until then. Now taking a line to Jackson Creek inlet, turn to 230 and directly towards R '2'. Plotter gives me bearing to R'2', but that is not the course to steer, as current is going to set me south. Down to 5.5 knots STW, current drift according to B&G is 1.5 knots, set is 160 true, which is a 90 degree cross current heading to R'2', which marks just about the 1/2 way point to the creek entrance. Course to Steer (CTS) is in the 250 deg range at this STW, as this is now effecting by the drift rate and my STW rate. B&G applying the law of sines for me. The width of this section of the channel is 100 yards. I pass directly south of R'2' and observe the plotter's course for flashing Red '6' post. Current still running out of the Rappahannock River here, as well as the Bay itself, so set and drift are about the same on B&G display. Knowing distance to Red '6' and my STW and Drift to the south, maintain the same course to stay in the channel. At arrival of R'6', turn to about 340 true, drop STW down to 4 kts to head to day board R'2', between two duck blinds on shore and a couple of clam farms on either side. Current at 'R' is down to 1 kt, running out of Jackson Creek. At this dayboard, the usable channel width is around 25 feet (daymarkers either side at 50 feet separation) and is shaped like a snake for the next 1/3 mile, still we get to Deltaville Marina, where it widens out to an anchorage in front. At this point, just steer between dayboards the whole way, note how close their shadows look to where the Garmin places them, make mental notes for future reference. Garmin shows current out of Jackson Creek close to my STW-SOG difference, so again, Garmin is getting a high grade tonight.

The whole purpose in the above was to stay close to the right side of the channel, and even if I could have drifted out of it, resulting in a last minute hook to starboard to regain it, the practice itself is important.

Could have just plugged in the waypoints of these nav aids and put the ST70 in track mode, which will keep XTE within 300 feet or better, but that is too much lateral distance for these particular confines.

BTW- ST70 Control Head Manual claims the AP does compensate for tidal drift in track mode to attempt to minimize XTE error, using STW and SOG, HDG and COG.

[Robin3]

Originally Posted by valhalla360

A literal knot log (ie, rope with knots and stick attached to it) is also simple and cheap. Should you have one of those made up and at the helm because redundancy is great?

It's about cost vs benefit and cost isn't just monetary. It's amazing how many people I've met who dream of sailing off into the sunset but never seem to get away from the dock because one more back up to the backup needs to be added.

If you have a paddlewheel and don't mind cleaning it every few days, have at it. If you are into racing, it's probably well worth it but the point of this thread is if it provides substantial improvement for crusiers. It does eat up time to keep clean and they do break costing money to repair, so is it the best use of limited time and money.

Over many years of getting away from the dock much more than most, I have been irritated when the paddlewheel log stopped working, but generally all that was/is required tp correct it was/is to withdraw it and give it a brush with the old toothbrush I kept/keep in the bilge by it and the supplied plug used to close the housing. Oft times simply raising the transducer in its housing and pushing it back down was/is sufficient. I do paint a thin film of antifoul onto the paddlewheel and it's housing once a year, no real sweat to do that. We are not permanently cruising these days and have a diver clean our hull periodically in between haul outs and the diver always give the paddlewheel a gentle clean, but then our diver is himself a sail sailor so knows about doing such things. When my GPS stopped working it was a trip to the electronics repair shop and lots of beer vouchers before it came to life again. At the time I had a backup DECCA set so still had instant Lat/Long available to plot. My 'plotter back then was a 'Yeoman' that put a position onto a referenced paper chart, taking it's position data from either the GPS or the DECCA via a two way switch selection. On my next boat I went fully electronic on the plotter. When later that plotter failed it too was an electronics repair man's dream in beer tokens and 3 months waiting time too. In the meantime we cruised using another GPS and manually put positions onto the paper chart if we were offshore or didn't bother as we were in very familiar local waters in good visibility inshore and could simply use our eyes to plot position in our brains. These days on a local cruise up/down the ICW I don't think we would worry too much about loss of the GPS as navigation in the ditch is pretty much only map reading car style, even the distances are measured in miles not real nautical ones, but I would miss the STW readout although I suppose if there were no true (real) wind blowing I could always read the (apparent) windspeed

IN the words of the back seat kiddos, ' Are we there yet?' or is this thread destined to go on forever?

[valhalla360]

Originally Posted by Sailor Doug

Some people are better off not leaving the dock, as long as they enjoy their boat great!
Redundancy is not cost vs benefit, but fortunately I use mine for sail trim. Have you ever heard surf without seeing it?
Do you have a knot log transducer, or have you had bad luck or a dirty harbour? I have only replaced mine once due to travel lift and cleaning is not to frequent. Bought a lot more gps's for a lot more money.


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Agreed, if they are happy tinkering that's great but I've known enough who get frustrated thinking they need the perfect boat and give up. Maybe they just weren't cut out for cruising or maybe they got scared off by people telling they needed backups for the backups.

We'll have to disagree on redundancy being a cost vs benefit item. By that logic, you should have a dozen paddlewheels mounted all around the hull as that provides more redundancy. Of course that is silly and expensive to little purpose.... and a cost vs benefit descion.

The paddle wheel never worked when we bought the boat and it never moved up enough to be a priority. The blank plug has been in since we bought. I've had them on other boats but they never read correctly.

GPS cost in todays market is what you want to spend. Our helm is protected with a place to secure a laptop, so a $10 GPS puck is our primary GPS. We do have 2 tablets and 2 smart phones with GPS and chart software available as backup but those are essentially free as we bought them for other purposes and they happened to come with GPS capabilities.

As I've said a number of times on this thread, if you are performance oriented, it is good for sail trim. We are cruiser first and find no shame in cranking up the engine if the wind isn't favorable.

[David M]

I use a speed through the water instrument that uses sound. I use it for towing nets and for towing oceanographic instruments. It's really not very expensive to set your boat up with a Speed Through Water (STW) instrument that is very accurate....down to a tenth of a knot.

[Dockhead]

Originally Posted by David M

I use a speed through the water instrument that uses sound. I use it for towing nets and for towing oceanographic instruments. It's really not very expensive to set your boat up with a Speed Through Water (STW) instrument that is very accurate....down to a tenth of a knot.

I think you have the Airmar CS 4500 ultrasonic log, don't you David?

I have the same one. I bought it because I highly value STW data for all the reasons stated in this thread, plus this data is needed for true wind.

It is not as accurate as I had hoped, but seems to be a good deal better than paddlewheel transducers, and besides that, much more resistant to fouling.

[Seaworthy Lass]

Originally Posted by Wotname

A reasonable understanding of high school vector maths will easy demonstate that what Dockhead and others have posted is in fact, correct.

I suggest you dust off your old school books and revise vector maths or if necessary, learn it for the first time otherwise you will just continue to deny hard solid fact.

This is a subject I have a keen interest in .
I add my voice to opinions expressed by OceanSeaSpray, Dockhead, Wotname and Stu Jackson. Dockhead's lengthy explanations are spot on.

CTS was all discussed at length a couple of years ago and made a very interesting discussion. It was condensed in this thread (links to the original threads are provided in the first post):
http://www.cruisersforum.com/forums/...ad-115361.html

This is not just a "personal opinion" any more than saying 2+2=4 is a "personal opinion" .

The understanding of shortest course through water as opposed to shortest course over ground is vital to the understanding of CTS. Imagine the boat being a magic carpet with the carpet pushed about by wind, instead of a boat in the water with the water pushed about by current. A constant heading of the flying carpet will navigate a straight line through the air and give you the quickest time between two points. A constant heading of a boat through the water will have you navigating a straight line through the water and this also will give you the quickest trip between any two points for any given boat speed. Those two points just happen to be over two fixed points on the seabed (your departure and arrival points).

I think you need to get your head around this before the light bulb is turned on and it all makes sense.

If the current is constant then the CST will take you along the rhumb line. I think this is the most frequently encountered situation for short trips and why a complacency with the technique of travelling along the rhumb line has set in. However, if the tide is variable, then using the one CTS may take you significantly from the rhumb line and not necessarily either side of it. While you are deviating from the rhumb line the SOG and VMG may be fluctuating wildly and tell you NOTHING about whether or not you are following the fastest route.

If you sail/motor along the rhumb line with variable tide during any journey then you are taking the slow route. The current does not have to be symmetrical, nor does it have to be reversing for it to be inefficient to follow the rhumb line. It just needs to be VARIABLE for the journey. If the current is not variable, then then calculated CTS will simply take you along rhumb line .

All this of course needs frequent reassessment during the journey, as STW and current and leeway will not be precisely as expected.

SWL

[Seaworthy Lass]

Originally Posted by goboatingnow

I say again Dockhead, multi hour CTS is complex tides , is nonsense , you cannot easily determine ground track and you cannot hence decide on safety of the route. Single or 2 hours is acceptable , especially where tides dont significantly change as you will in fact proceed along the ground track.
......
Yes Yes, lets not do the suck eggs issues here. The fact remains that in a complex non reversing , angular tides, the ground path cannot be easily computed using a single CTS and hence ITS NOT A SAFE WAY TO NAVIGATE
.....
The reality Dockhead is that multi hour complex CTS ( i.e. a combination of tides resulting in a combined CTS vector) is a complete nonsense because you cannot determine in advantage the ground track and hence evaluate the dangers
........
IT is NOT good navigation to deviate into an unknown ground track

Well, I agree with your last sentence, but I beg to differ about the rest . Dave, you have stated over and over in this thread that "cannot determine the ground track in advance". Determining your ground track beforehand is in fact dead easy and is vital to make sure your course is not going to take you over hazards. Simply because the RYA do not teach you to plot this does not mean it is not possible or not easy.

See the examples of ground track that I plotted in almost all the CTS examples I gave in this thread:
http://www.cruisersforum.com/forums/...hod-96579.html

Furthermore, you are not following a CTS "blindly" and unsafely. In practice you are constantly keeping an eye on your position on the chart plotter and deviating as necessary for hazards.

SWL

[rwidman]

Originally Posted by OceanSeaSpray

My friend, the problem here is that you don't appear to even have the faintest clue about how little you actually know and understand.................

As soon as you are sailing against the tide, some or all of it, so it doesn't take you where it is going, you are bucking the tide. ..............

If you know so much, how come you don't know that you don't "sail against the tide", you sail against the current?

[Seaworthy Lass]

Rwidman & El P etc, an even easier way of thinking about it is to imagine a tablecloth. You are an ant and want to get from point A to point B on the tablecloth (ants and tablecloths are a nice analogy Andrew Troup used a couple of years ago ).

You can slide the tablecloth (ie the "water") anyway you like over the table (ie the "ground"), but the quickest way to move from point A to point B on the tablecloth is in a straight line over the tablecloth. As the tablecloth is being slid around the table, your track relative to the table and speed relative to the table can in the meantime be all over the place. If the tablecloth is being slid in a constant and direction and rate then your track relative to the table will be a straight line (ie the rhumb line).

Does that make it any easier to understand?

[Sailor Doug]

Redundancy: is not have multiple gps receivers it is having things that don't rely on the same format.
1 gps the best and not to be without it.
2 DR supports by compasses, knot log, depth and paper charts.

I assume the people who chucked their knot log also chucked their compass, let alone swing and calibrate very often. My electronic compass is nicer than my binnacle one but I keep it along with a compass that I can take a bearing with.
The reason we all have so many gps's is we keep updating them because of newer features. Cell phones and tables add to gps's. To get redundancy you need a different format. Just about every body knows a gps can just do about every thing but not quite.

Redundancy is not doing the same thing over and over.


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[MarkSF]

Originally Posted by Sailor Doug

Redundancy: is not have multiple gps receivers it is having things that don't rely on the same format.
1 gps the best and not to be without it.
2 DR supports by compasses, knot log, depth and paper charts.

I assume the people who chucked their knot log also chucked their compass, let alone swing and calibrate very often. My electronic compass is nicer than my binnacle one but I keep it along with a compass that I can take a bearing with.
The reason we all have so many gps's is we keep updating them because of newer features. Cell phones and tables add to gps's. To get redundancy you need a different format. Just about every body knows a gps can just do about every thing but not quite.

Redundancy is not doing the same thing over and over.


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You forgot one thing that's essential to do DR. An accurate timepiece. So I guess redundancy means at least 2 watches.

[Sailor Doug]

Mark you are right forgive a old man,


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[Wotname]

Originally Posted by Sailor Doug

"........ Redundancy is great.

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Originally Posted by Sailor Doug

Redundancy: ........... To get redundancy you need a different format. Just about every body knows a gps can just do about every thing but not quite.

Redundancy is not doing the same thing over and over.


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SDs point on rudundancy is (IMO), very important and I belive is often overlooked when determing risk reduction strategies.