GPS vs everthing else. & where to site paddlewheel

[jimbouy]

hi folks,

A few more weeks of dayskipper theory and researching to try and find out where best to site the log paddle wheel on my 17ft bilge keeler have raised some more questions in my mind./forums/images/icons/frown.gif

Firstly on a practical basis.../forums/images/icons/smile.gif

My boat is a Leisure 17, I expect to be bay hopping and coastal pottering in the solent and although I think i will be piloting rather than navigating i want to put the dayskipper skills in to practise.

But where am i going to site the Log paddle wheel. The Leisure 17 is double skinned along about the first 2/3 of it's length...No Bilges.. so the aft 1/3 is the practical area to install a through hull fitting. But being bilge keel where will be the area of least turbulence./forums/images/icons/crazy.gif

Secondly on a more theoretical nature./forums/images/icons/smile.gif

GPS versus other navigational methods./forums/images/icons/wink.gif

Now don't get me wrong I agree with those who say you can't navigate by GPS alone. But I find peoples views on the reliability and use of GPS variable and contradictory.

Whilst trawling these forums on the subject of Logs I found people saying to use GPS to calibrate your LOG but that you should still have and use a Log rather than GPS.

More contradictory are some of the day skipper leasons, but this may just be our teacher..what do you think.

e.g Question 1 from an exercise.

You have entered bouy x as a waypoint on the GPS which tells you you are xdegrees and y N miles from the way point. Plot your position and suggest how you might confirm your position.

Answer. x marks the spot and take hand bearings from light house and church tower.

Straight forward enough and implies GPS is reliable enough to use as 1 of 3 fixes.

Question 2 from exersice.


You have taken bearings from light house A and Monument B, plot the position and suggest another way to confirm that position. Now x marks the spot is on a contour line and the answer wanted is to check your echo sounder.

When we suggested GPS we were shot down in flames after all GPS is an electronic and battery driven device that can not be relied upon. But surely the echo sounder is all of the above.

So what are your views?

How do you use or rely on your GPS?

How does it compare say to the accuracy of your log or even your echo sounder?


Thank you all

Jimbouy



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[hisw]

The answers to your questions may be:


Question 1

If the buoy is charted a bearing could be taken from this thus giving you your third fix.

Question 2

You can test the depth with a lead line, as many people still do thus not relying on any electrics!

Im no expert, no RYA exams!

Hope this helps




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[chriscallender]

1) Probably on the centreline of the boat or close to (to minimise the effect of heel), where it will be in clear water flow between the two keels. Other than that, a position where it is out of the way but still convenient to remove for cleaning the paddlewheel, which will need done every time, if you want to have any faith whatsoever in what it says. One idea I have heard of is to mount it on a pole on the transom rather than through the hull, as long as the pole is long enough to put it into deep water even when you are heeling and its clear of the rudder that should be OK, and make it nice and easy to keep clean.

2) Personal experience is that the log is heavily affected by weed growth on the hull. I have calibrated so that with a clean hull I would say the error is <10%. However with a heavily fouled hull at the end of the season, despite removing the paddlewheel and cleaning all the weed of it, it will only be reading something like 50% of the true speed. I assume this is because it is basically in "still water" close to the hull, protected from the water flow by all the weed & slime that is on the hull. OK when things get into that state its time for a scrub but there must be some intermediate stage when its perhaps 20%-30% out enough of an error but not too noticeable. Basically I don't trust the thing at all.

A trailing log towed behind the boat on a cable in clear water can be much more accurate. I use one on long trips like channel crossings where navigation is of more significance, and its needed to be able to work out an EP (out of sight of land). My experience of the through hull log is that it is not accurate enough for meaningful EPs. Maybe I don't keep my boat clean enough ... or maybe modern antifouling isn't powerful enough!!! Unfortunately I don't think anyone manufactures trailing logs anymore, or at least none of the mainstream marine instrument manufacturers do, although 2nd hand ones do come up on Ebay etc from time to time.

As far as use of GPS for navigation, I think when I was doing coastal skipper theory the instructor put it quite well by saying that it is just a source of information like any other. The person who relies on it completely and the person who refuses to make use of it when its available are both equally wrong. The problem is that the extreme accuracy it shows can lull you into a false sense of security. As mentioned on your course, you need to find a way of cross checking the information by whatever other means are available, bearings or whatever.... so if you are out of sight of land, then there is still a need to be able to add up distance through the water, tide and leeway and come up with an estimated position from your last known position. And distance through the water needs an accurate log.

Thats my thoughts on it.

Chris

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[jimbouy]

Now I like the idea of on a pole on the transom, in terms of accessability.

Particularly as accessability on the L17 will not be good

I have seen transom mounting logs for sale.

Does anyone have experience or has anyone tried the pole mount.


Jimbouy

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[jimbouy]

Oh I have no problem with the answers, it's the varying attitude to GPS that interests me.

Is it just traditionalists that poor untold scorn on to GPS?

Again I stress that i am in no way saying I will be heading on to the water with nothing but chart and GPS in hand.

Jimbouy

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[duncan]

firstly with an uptodate chart and a working GPS you would unfortunately be better equipped than many out there!
secondly you <font color=red>can</font color=red> navigate perfectly well with GPS and chart or even just a chartplotter but it is not recomended. The reasons are as much to do with the general issue of overreliance and also the ease with which big mistakes can be made - misplotting a position from a GPS midchannel onto paper could result in an inappropriate decision to bear away to your destination as an example. This is a much bigger 'error' in my experience than any false or incaccurate position from a unit. I have never been aware of any positional error on the units on my boat and, as they have constantly rolling logs on screen, and most of my trips are inshore routes, I would have a reasonable opportunity to become aware of any if they occured.Yes the units can fail, electrical power can fail and teh batteries and spares for the handheld can die but where do you draw the line!
I have a couple of reasonable sized colour and b/w screens in view all the time any they, together with mk1 eyeball, form my navigation tools en route.
However I think it is also a misnomer to consider GPS anything more than a navigation tool - tide tables, tidalstreams, notices to mariners and weather information are all equally important to the overall task of navigating from A to B - and for a sailing vessel travelling at 5-7knots they are more significant than for a power vessel travelling at 25knots (unless fuel/distance are a factor).

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[chriscallender]

People that pour scorn on GPS are probably the same people that say the internet will never amount to anything... any new technology will meet some resistance and it was probably the same when the fangled magnetic compass was introduced in navigation.

I use traditional methods as well as GPS for three reasons...

- so that I still get practice at other techniques from finding the way home and I'd stand some chance in the quite unlikely event of not being able to get a GPS fix from either the main GPS or the backup

- so that I catch the silly errors where I plot lat and long on a chart 1 minute away from the correct place through carelessness.

- Also sometimes the traditional methods are quicker and easier - for instance checking you are on a transit is easier than watching cross-track error on a GPS display or even having your head down looking at a plotter.

Chris

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[vyv_cox]

Mounting the log paddlewheel anywhere aft of the forward end of the keel(s) is likely to induce turbulence effects, either all the time or one one tack. Every log I ever bought (actually, quite a few) recommended a position about 6 - 12 inches ahead of the keel, on the centre line. I bought a boat in which the log had been mounted aft of the keel, offset to one side, and it was utterly useless.

No experience of the transom mounted ones but I would guess that it would need to be well out of the way of prop, keel and rudder wash, whether the engine was on or not.

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[Evadne]

Re: GPS vs everthing else. & where to site paddlew

Sounds like you need to think about a trailing log. I've used one for years and apart from helpful people telling me I am trailing a rope in the water it's fine, never grows any weed and if it fouls you just haul it in and pick off the kelp. On any underwater body, the least turbulence is in the forward third of its length.

Your two questions sound like someone is being a bit bloody-minded. The reason for taking 3 fixes is because it is statistically unlikely that all three will be wrong and still give you a good fix, regardless of where they are from. In fact two manual lines and an electronic one from any source is better than three bearings from the same compass. The important thing if you do get a bad fix, either a big "cocked hat" or 2 position lines and a contradictory electronic fix is to try and resolve it. If your hand-bearing compass is wrong, your steering one may be as well.

In real life I use eyeball pilotage or take hand bearings inshore, but do a DR on the chart and check this with the handheld GPS on longer passages or out of sight of land, or recognisable marks. The echosounder is only as accurate as your estimate of the tidal height, which you have to do there and then and is not only weather dependent but less accurate the further you are from a primary port and the older your charts are.



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[chriscallender]

Re: GPS vs everthing else. & where to site paddlew

Dave,

Do you know if anyone manufactures new trailing logs these days - I did a search a year or two ago and came to the conclusion that the answer is no so I fitted a through hull NASA transducer, which is about as accurate as using a piece of seaweed to tell the weather. I agree with you that trailing has plenty of benefits - I have an old Stowe one that I use but unfortunately only the speed display works, not the accumulated mileage. I find that on long trips speed is close enough to constant I can come up with a guesstimate on average speed.

If you do know of someone that makes/sells brand new trailing logs (as opposed to finding one on Ebay or something), I'd be very interested in replacing the broken Stowe.

Chris

 

[Evadne]

Re: GPS vs everthing else. & where to site paddlew

I will have a look as well, but I have a Stowe, which was taken over by Simrad, purchased in 1985. I believe Walker and Wasp are still around, but they don't match the Stowe for convenience.
Simrad supplied a new impeller for me a few years ago and are worth approaching if it's spares you're after. There may even be a few circuit boards around. Actually my counter turned up its toes last time out, so I'm going to have a look at it this winter. It's not a complicated circuit so any electron familiar with transistors and CMOS logic should be able to fix it (he says, hopefully).


Just found this: <A target="_blank" HREF=http://www.marinedirect.co.uk/pages/electronics/general/easy_trail.htm>http://www.marinedirect.co.uk/pages/electronics/general/easy_trail.htm</A>
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[Capt_Marlinspike]

1. Don't both with a log they hardly ever work, except the towed sort.
2. GPS is outstandingly accurate and reliable for position, don't rely on it for speed as the computational method used is useless for yachty purposes.
3. If you are only trolling about in the solent Mk 1 eyeball and a handbearing compass is all you need.

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[vyv_cox]

Rubbish

1. Logs work perfectly well if they are installed correctly. On average mine fouls about twice per season and I clean it maybe once per month. It is extremely accurate. A towed log is a pain in the bum in coastal waters.
2. Suggest you consult your manual to find how to set your GPS up correctly. Install the correct averaging rate and it will give you perfectly acceptable speed readings, except possibly for 15 seconds after a sharp turn.
3. The next person who can judge distance and speed accurately with his eyes, over a period more than about 60 seconds, will be the first.

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[roger]

Views on instrument accuracy

What a lot of stones this thread has turned over!
My experience of in hull logs has been very poor. They do weed up and definitely need calibrating. I would guess that each time you clean them you replace them in a slightly different position - so they need calibrating again. Certainly they are unlikely to be reliable over the course of a cross channel trip.
Trailing ;ogs weed up too. Will you notice quickly that the thing isnt behaving normally when its at the back of the cockpit? I wouldnt guarantee that especially in the dark.
If you dont clean them then your total mileage is useless.
Now how about compasses. I'm extremely doubtful if the average Plastimo Compass (corrected or not) could be relied on for an estimated course over an hour for better than +/- 10 degrees.
Remember there are still alive people who swore they could find position with hand held RDF. Oh yes there are fairies at the bottom of my boat yard too.
Frankly I dont think that in the average yachtsmans hands DR was ever very accurate or reliable. For the ordinary punter, myself included, GPS is accurate and easy to use. You do need to keep your paper charts in case the systems go down. You do need to keep a good log and its VERY important to check the plot. However in case of disagreement between EP and GPS I would be much more likely to accept the GPS position unless some real measurement vilently disagreed with it.

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[Capt_Marlinspike]

Re: Rubbish

I guess we all have different experiences, but:
1. My Raymarine ST60 log, new and proffessionally fitted in 2001, I find to be useless. It either under reads or reads 0 on almost every trip even though I clean and free it each time I get home.
2. First from observation of my GPS speed against elapsed time between known distances I was dubious about what it was telling me. Some information I got from a colleague in the defense industry indicates that the VELOCITY not SPEED shown on GPS uses an averaging algorithym on time taken between a number of past fixes. So this is telling you a speed over the ground average without regard to direction. I fail to see how this is helpful in navigation.
3. Anyaway the point is if you are only planning to cove hop around the Solent then I say again using a chart and a hand bearing compass should be sufficient to keep you located and safe.

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[jimbouy]

Re: Rubbish

Pretty much what i thought.

I might still fit a Log out of interest but it is not high on my list.

Jimbouy

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[MainlySteam]

Re: Rubbish

Hi Captain, sorry to spike your claims -

I have an ST50 log (which I believe uses the same transducer as the ST60) and have no problems with it at all - I suspect you have some sort of problem with yours that needs fixing if you want to use it.

Also, I have just returned from an overseas yard where I am managing some new boats being built for a client. While there I was a guest on another new vessel undergoing sea trials. It relied entirely on its GPS for speed ie it had no other speed transducer of any sort. So I would say from my own experience and using that vessel as a practical example, your dismissal of the GPS's use as a log is misplaced.

John

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[vyv_cox]

Your 2.

That's sort of true (although velocity and speed are the same thing in my vocabulary.) The averaging period can be adjusted on most GPS sets. This controls both the speed and the direction. With no averaging, both functions are near useless, as every wave crossed causes wild fluctuations in them. On the other hand, a long period, say 5 minutes, is so long that reasonable fluctuations are not seen by the set. I have mine set on an averaging period of 15 seconds and this seems to be a good compromise.

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[Evadne]

Re: Your 2.

Sorry to get over technical but it might be of help. Velocity is a vector but speed is a scalar, which means that velocity includes direction but speed doesn't. As already pointed out, the instantaneous GPS position jumps about quite a bit. I would surmise that a "speed average" over 15 seconds would take the velocity between many closely spaced points and average their magnitude (speed) whereas a "velocity average" would take the start and end point and divide it by the time taken to get there. If these two positions are themselves averaged, the latter should be a close approximation to the result you'd get with a chart, dividers and pencil. Hope this is clear.
I'm not sure if my little hand-held Etrex runs to such sophisticated calculations.

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[andy_wilson]

Install it in the least disturbed water. Most are mounted in the front third for ease of mounting. Try right in the middle, just behind the inner skin, given you have no keel bolts there.

Question 2 should have mentioned your GPS was kaput!

I think the idea is to get students thinking about alternative means of putting a position, or at least a position line onto the chart.

I try to practice taking fixes whenever I am well 'crewed up', simply because I have been in a position of having no power to the GPS, and in testing conditions. No harm getting a bit of practice in is there?

To the contributor who cleans his paddlewheel after every sail, but finds it unreliable, I suggest cleaning it beforehand instead.

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