Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 16-02-2009, 16:44   #31
Registered User
 
RBEmerson's Avatar

Join Date: Jan 2008
Location: SE PA
Boat: Baba 35 - One With The Wind
Posts: 392
Er, weighted line with knots? That's called a lead line. I think the beast you want has a wood plate on the end (for the drag needed to pay the line out) and then the knots show up.

Or, to keep it simple, toss a wood chip over the side, near the bow and the foremost stanchion, and time how long it takes to reach, for example, the aftmost stanchion: voila! a chip log! R=D/T, where R is the rate or speed in knots, D is the space between stanchion converted to a length in nautical miles (stanchion space in feet/6076), T is the time, expressed in hours (or seconds divided by 3600), for the chip to cover D. It's cheap and needs no holes in the boat - whatta deal!
__________________

__________________
S/V One With The Wind
'85 Baba 35
"You and I may agree, but we could both be wrong."
RBEmerson is offline   Reply With Quote
Old 16-02-2009, 17:23   #32
Registered User

Join Date: Sep 2007
Location: Colombo
Posts: 1,059
Lots of modern power boats now, both pleasure and commercial, have no paddle wheel log but rely entirely on GPS for speed. Of course, many of them are much faster than sail boats so drift is of less importance to them. Also, in my experience they are not so hung up on tradition especially as real time electronic nav on a chart plotter or PC adds so much to safety when cruising at high speeds so has been strongly adopted.

For sail boats the importance of a paddlewheel log for determining set, drift and leeway is to me very over rated. That because of the errors involved.

To determine actual sum of set, drift and leeway using a log we rely on a vector diagram dependant on fixes, course steered, speed through water (or distance through water) and time. If we assume that the error in time and the fixes is too small to worry about we are left with any errors in the log and in the course.

First the log - they are notoriously inaccurate (as another has pointed out) and in my experience they seem to be often calibrated high, if calibrated at all (look, for example, at the speeds some report for their boats which one knows are very unlikely given the boat type and WL length). Most are intended to operate in the laminar zone against the hull and that is difficult to achieve on a small sail boat due to the changing profile of the turbulent zones from wave action and heel and due to the proximity of the appendages. They may also not be linear (due to axle friction?) so accuracy changes according to actual speed and they are also subject to fouling.

Then the course steered - most small sail boat compasses are only graduated in 5 degree increments for a start but in the end helmsmen will not be able to sail an accurate course. Wave action, pinching into and falling off the wind according to the point of sail on (I know I tend to pinch up in lifts if sailing a close hauled course - I think a habit from growing up racing dinghies needing to gain to windward), distraction, boredom, etc will all contribute to inaccuracy. I would suggest that in other than calm conditions maintaining a course within 5 degrees on a small sail boat is maybe all one can hope for - an autopilot may or may not do better.

So, in the end the calculation of set, drift and leeway will not be very accurate at all.

Then we need to consider that the result is a historic one based over, say, an hour of sailing. When coastal, and especially in pilotage, there may be no reason whatsoever that the set, drift and leeway are going to be the same over the next hour. In fact in my home waters they can vary enormously (like reverse ) minute by minute even 6 miles out from land - but that is not what we all face, of course.

So back to the GPS - in the end when calculating set, drift and leeway we are wanting to know what course to steer in order to end up where we want to go. If one uses a chart plotter or Electronic Charting System on a notebook there is no problem doing this with no log. One just extends the look ahead vector (now officially known as the tennis raquet thingie ) and adjust the course steered so the extended look ahead vector lies over where you want to end up. That automatically allows for tide and leeway and that in real time, not based on some calculation based on the previous hour.

If one doesn't have a chart plotter or ECS but a GPS instrument that allows entry of waypoints (I think pretty much they all do) one just has to create a waypoint at the place you want to end up or somewhere along the route from your current position to it, and then adjust the course steered as one goes so that bearing to waypoint remains constant as you proceed (or so XTE is maintained close to zero). That is you in effect sail up the bearing line from your start point to the destination and required corrections in course due to changing set, drift and leeway you can correct in real time. The closer you place the waypoint to your current position along the desired route the faster the bearing to waypoint will change should you stray to one side.

So in the end I think a paddle type log (or towed one) is pretty much irrelevant to safe navigation on a sail boat with a GPS - it certainly is on small power boats because in the countries I am familiar with the statutory requirement for speed measurement on commercial vessels is met by having a GPS alone.
__________________

__________________
MidLandOne is offline   Reply With Quote
Old 17-02-2009, 01:31   #33
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,331
Quote:
Originally Posted by MidLandOne View Post
<snip>
So in the end I think a paddle type log (or towed one) is pretty much irrelevant to safe navigation on a sail boat with a GPS - .....
Gee Mid1, you really taking the fun of sailing now.
What, no more messing around trying to calibrate the paddle wheel or even the Walker trailing log, finding a known set distance and slackwater; no more trying to maintain fixed heading bewteen a some three bearing fixes using a wonky handbearing compass near a pile of anchor chain wondering why the "cooked hats" are almost as big as the chart.
Coming up on deck and saying "just a couple degrees to starboard will correct our increased leeway due to the wind change".

OK, I will buy that chartplotter and do something else.............
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 17-02-2009, 02:10   #34
Long Range Cruiser
 
MarkJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2007
Location: Australian living on "Sea Life" currently in England.
Boat: Beneteau 393 "Sea Life"
Posts: 12,828
Images: 25
Quote:
Originally Posted by MidLandOne View Post
So in the end I think a paddle type log (or towed one) is pretty much irrelevant to safe navigation
Yes, but anyone who gets a go at the wheel wants to see a number in the speed box not a ---
And how would one go selling their boat will a --- instead of a 6.4 or whatever. You did give me a great idea for when we come to sell this boat: Recalibrate the log: "We're doing 16 knots!!!!!!!!!!!!!"


Mark
__________________
Notes on a Circumnavigation.
OurLifeAtSea.com

Somalia Pirates and our Convoy
MarkJ is offline   Reply With Quote
Old 17-02-2009, 03:58   #35
Long Range Cruiser
 
MarkJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2007
Location: Australian living on "Sea Life" currently in England.
Boat: Beneteau 393 "Sea Life"
Posts: 12,828
Images: 25
YAY!

YAY!

My dad fixed it, because he is amazing!

He is brave enough to try and do something that seems impossible!

He fixed it! YAY! We are happy! Sea Life is happy!

Smiles

NICOLLE
__________________
Notes on a Circumnavigation.
OurLifeAtSea.com

Somalia Pirates and our Convoy
MarkJ is offline   Reply With Quote
Old 17-02-2009, 04:32   #36
Registered User
 
Tempest245's Avatar

Join Date: May 2008
Location: Piscataway, NJ
Boat: 34 Sabre Tempest
Posts: 937
Give that Man an Ice Cream Cone!
__________________
Tempest
Tempest245 is offline   Reply With Quote
Old 17-02-2009, 07:15   #37
Registered User
 
RBEmerson's Avatar

Join Date: Jan 2008
Location: SE PA
Boat: Baba 35 - One With The Wind
Posts: 392
Quote:
Originally Posted by MarkJ View Post
YAY!

My dad fixed it, because he is amazing!

He is brave enough to try and do something that seems impossible!

He fixed it! YAY! We are happy! Sea Life is happy!

Smiles

NICOLLE

Isn't it neat to have a dad that amazing? Smiles back at ya! Rick
__________________
S/V One With The Wind
'85 Baba 35
"You and I may agree, but we could both be wrong."
RBEmerson is offline   Reply With Quote
Old 17-02-2009, 13:29   #38
Registered User

Join Date: Sep 2007
Location: Colombo
Posts: 1,059
When you guys knocked the top off the log sensor what did it look like inside or did you not get beyond the plastic top? - is a bit hard to see in the photo. Am interested as I have the same one.

Thanks

John
__________________
MidLandOne is offline   Reply With Quote
Old 17-02-2009, 15:16   #39
Registered User
 
RBEmerson's Avatar

Join Date: Jan 2008
Location: SE PA
Boat: Baba 35 - One With The Wind
Posts: 392
The assembly is basically just a paddlewheel with a magnet in one of the paddles, and a hall effect sensor in the housing. Every time the magnet passes the hall effect sensor, a pulse is sent to the display. The number of pulses in a unit of time is converted to knots and displayed. At least with ST-60 speed displays, the unit can be calibrated to the GPS speed (assumes VTW equals SOG, so pick a time where current isn't an issue).

The top of the housing contains nothing more than the four wires used by the sensor.
__________________
S/V One With The Wind
'85 Baba 35
"You and I may agree, but we could both be wrong."
RBEmerson is offline   Reply With Quote
Old 17-02-2009, 15:53   #40
Registered User

Join Date: Sep 2007
Location: Colombo
Posts: 1,059
I am very familiar with how they work - was actually just wondering what was used to fill all the space in the long tubular part of the body of the Raymarine (and previous incarnations of the same company) ones, whether just potted with something to fill the tube between the top and the paddle wheel end (if so what?) , or empty, or what?
__________________
MidLandOne is offline   Reply With Quote
Old 17-02-2009, 16:26   #41
Registered User

Join Date: Apr 2007
Posts: 718
They are fully potted. Not sure exactly what material, some sort of plastic/epoxy. There's 5 wires including the shield drain wire. Red-pos to hall effect device, shield-neg, green-square wave output from hall device for speed, white-pos for temperature sensor, brown-temp sensor output.

Eric
__________________
fairbank56 is offline   Reply With Quote
Old 17-02-2009, 17:13   #42
Registered User

Join Date: Sep 2007
Location: Colombo
Posts: 1,059
Thanks for that.

From the weight I assumed they had something filling them up but have never had a dead one to chop up to see what it was.

John
__________________
MidLandOne is offline   Reply With Quote
Old 17-02-2009, 17:39   #43
Long Range Cruiser
 
MarkJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2007
Location: Australian living on "Sea Life" currently in England.
Boat: Beneteau 393 "Sea Life"
Posts: 12,828
Images: 25
Quote:
Originally Posted by MidLandOne View Post
When you guys knocked the top off the log sensor what did it look like inside or did you not get beyond the plastic top? - is a bit hard to see in the photo. Am interested as I have the same one.

Thanks

John
Hi John,

We only drilled down through the top plastic coverings. It seems all the top part is hollow and designed for the ring pull. So with as much of thast removed as possable (leaving a small amount for the ring pull, or making another hole for the ring pull) you get quite another centimeter or so to work with.

We could not see where the wires ended up i.e there they were attached too. So if the breach was deeper in it may be even more difficult.

But there is really no alternative. If it doesnt work you either give it a bash fixing it, or buy a new one. They are cheap in the USA, expensive here!

Nic's dad was very good with the soldering iron but still took a few tires and some very delicate work.

Theres nothing to lose if you have a broken one, don't chuck it out till you chop right into it

At least we know the manufacturers bit about not being able to solder the wires together is BS!

Mark
__________________
Notes on a Circumnavigation.
OurLifeAtSea.com

Somalia Pirates and our Convoy
MarkJ is offline   Reply With Quote
Old 18-02-2009, 12:17   #44
Senior Cruiser
 
senormechanico's Avatar

Cruisers Forum Supporter

Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
Posts: 5,827
RBE,

Thanks for the pinout of the paddlewheel assy.
My temp sensor hasn't worked since I bought the boat.
That will give me something to add to the list of stuff I need to fix...


Steve B.
__________________
senormechanico is offline   Reply With Quote
Old 18-02-2009, 12:37   #45
Registered User
 
RBEmerson's Avatar

Join Date: Jan 2008
Location: SE PA
Boat: Baba 35 - One With The Wind
Posts: 392
Er, I sent a pinout? Looking over my earlier notes and the comments from fairbanks56, I missed the leads used by the temperature sensor. Any pinout I sent is ...ah... somewhat lacking?

Anyway, here's a comment from Chris Martin at Raymarine on how to troubleshoot "temperature reported but no speed reported":
Quote:
Originally Posted by Chris Martin, Raymarine Tech Support
If you have an O scope you could see a wave pattern when you turn the paddle wheel. You could also use an ohm meter across the shield and green wire from the transducer and you should see a "short" every quarter rotation and infinity between the 4 readings.
__________________

__________________
S/V One With The Wind
'85 Baba 35
"You and I may agree, but we could both be wrong."
RBEmerson is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
FP Tobago Transducer Lazy Cat Marine Electronics 3 09-11-2011 18:31
Transducer Compatability salty_dog_68 Monohull Sailboats 6 31-03-2010 14:36
Speed transducer placement - Oops?! Weyalan Marine Electronics 10 25-07-2007 16:26
Transducer Charlie Marine Electronics 6 02-03-2007 22:06
Protecting the Speed/Log transducer Weyalan Construction, Maintenance & Refit 6 16-01-2006 21:06



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 23:52.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.