[C320-list] Speed paddle

[Graeme Clark]

Scott

You are of course correct, I meant ground wind.

However as far (and it seems many other cruisng sailors) are concerned the paddle wheel log is so inaccurate (or more correctly, in constant need of recalibration as it fouls up) as to make it virtually useless for assessing true wind! I have a friend who has some sophisticated doppler speed transducer that will measure t within 0.1 of a knot. If my paddle wheel is within 1 knot of my real speed I reckon I am doing well!

Indeed the SOG will not directly interface with an instrument that wants STW but Warren (i think) refrred to a device that will effectively rewrite teh SOG as STW and send it onwards, disguised like that.

IMHO SOG is a better bet than STW unless the currents are very strong indeed!


But I take your point

Graeme
> On 28 Apr 2017, at 18:12, Scott Thompson <surprise at thompson87.com> wrote:
> 
> Graeme, I dispute your characterization of "a more accurate true wind reading" in the example. You are confusing true wind with ground wind. The difference usually is minor, but not in your example, or any other time there is a lot of current relative to wind speed.
> 
> If SOG and apparent wind speed are identical, as they are in your example, then ground wind is zero, which I gather is what you mean when you assume "no wind." However true wind is usually defined as the wind that you perceive when you are drifting with the current, i.e. with zero boat speed through the water (STW). So in your example the correct value of true wind speed is 2 knots. However, since SOG and apparent wind speed are identical, the calculated true wind speed will be zero if one incorrectly uses SOG in place of STW.
> 
> It may seem unnatural to define "true" wind speed relative to the water rather than relative to the Earth, but this makes a lot of sense when you think about it. Sailors routinely check true wind by going head to wind to check direction and then wait until STW is zero and then take apparent wind speed to measure true wind speed. (True wind and apparent wind are the same at this point.) But this is not the same reading you would get for either wind direction or wind speed if you measured instead from an adjacent boat anchored in current. The anchored boat's apparent wind is the same as ground wind, not true wind. True wind measured this way is the wind that is relevant for interpreting polar diagrams. It is also most relevant for thinking about the physics of sailing. Sailboats derive their energy from the relative movement of wind and water, and this is precisely what true wind measures.
> 
> BTW, my wind instrument will not display true wind unless it is receiving STW data. It ignores the SOG even when that data is available via the Seatalk bus. (I learned this the hard way a few years ago when my boat speed display failed and stopped outputting STW data.) My chartplotter can display calculated ground wind or true wind or both. It also calculates current, which can be very useful for deciding if you should try to sail into or out of a current, or for making layline calls to tack or gybe. (My chartplotter will display predicted laylines adjusted for current, for example.) I value the STW measurement for these reasons independently of any differences between true and ground wind direction. Those differences are small, after all, if there is much wind relative to current.
> 
> Scott Thompson
> Surprise, #653 (launching next week!)
> 
> 
> On 4/28/2017 3:24 AM, Graeme Clark wrote:
>> Bev
>> 
>> It's the other way round! In order to calculate true wind the instrument needs to know how fast you are going through the water
>> 
>> Think of if there was no wind but you were motoring at 5 knots. The apparent wind would be 5 knots and the instrument can only work out the true wind if it knows you are moving at 5 knots
>> 
>> If you are motoring at 5 knots but there's a 2 knot current against you, your apparent wind is 3 knots because wind is determined by speed through air not speed through water
>> 
>> So in fact the GPS derived SOG ( speed over ground) used to fake a speed through water STW input to the instrument would give a more accurate true wind reading than the paddle wheel speed
>> 
>> But as you note, the current can also be worked out if you know SOG and STE. I'm my example of motoring at 5 against current of 2 the SOG is 3 so current is STW less SOG
>> 
>> if your plotter also knows the direction the boat is heading (from compass) and the direction it is actually going (track or COG, course over ground) from GPS it can work out the total "set", which is the effective direction and speed of the current, although it has no means of separating leeway from this so even if sailing in slack water it would still show a "set" equivalent to leeway
>> 
>> These sort of problems are often more easily understood when you work out what is happening when a particular value falls to zero or becomes ridiculously high, or if wind is directly ahead or astern!
>> 
>> Hope this helps and that I haven't got it wrong! Open to corrections!
>> 
>> Graeme
>> England
>> 
>> Sent from mobile: please excuse typos etc.!
>> 
>> 
>>> On 28 Apr 2017, at 00:38, Bev Wright <bev.wright at verizon.net> wrote:
>>> 
>>> Warren and Richard,
>>> 
>>> I have hull #15 and I find the same thing true - gets fouled easily, inaccessible under the v-berth and water shoots out, no flapper, so we don't end up using it unless it miraculously starts working.  However, for me as a club racer, while I have a GPS for SOG, I believe that the primary reason we should have it operational is to see how much current is with us or against us, i.e. speed through the water. I'm not so much interested in true wind even if I should be. I have a wind instrument that has a true wind setting but I thought that was calculating the difference between our speed and the wind speed/direction. Are you saying that this instrument you mention will calculate the speed through the water?
>>> 
>>> Sorry, I'm not an engineer, although my sailing/racing father is, so I should ask him as well.
>>> 
>>> Bev Wright
>>> s/v Whoosh
>>> Edgewater, MD
>>> 
>>> -----Original Message-----
>>> From: C320-list [mailto:c320-list-bounces at lists.catalina320.com] On Behalf Of Warren Updike
>>> Sent: Thursday, April 27, 2017 6:01 PM
>>> To: C320-List at Catalina320.com
>>> Subject: Re: [C320-list] Speed paddle
>>> 
>>> We have stopped using the speed transducer because, here on the Chesapeake Bay, the damn thing doesn't last a week in the Summer before some critters decide to take-up residence in it. As a result, it stops working. And, as our older hulls don't have the transducer with the shutoff flapper, and the transducer is under the V-berth, it's a real pain to get at it to remove it. Further, the in-flow you get when you do pull it has no direct path to the bilge (all issues on the older, shallow bilge design.) All reasons why we no longer use it. If you don't have a wind instrument and you have a GPS, why would you need Speed-thru-the-water (STW?) Gauging set and drift, perhaps?
>>> 
>>> There is, however, a solution for racers, those with wind instruments, and others who want/need true wind display. This is a device from New Zealand, for about $100, that will take an input of GPS speed-over-ground (SOG, as a NMEA-0183 message from the Sea Talk network on-board,) and output an analogue STW to the Wind Instrument. Problem solved. I've seen it said that SOG is a better factor in calculating true wind than STW.
>>> 
>>> Oh, BTW, we haven't yet installed this thing. But, if anyone is interested, message me off-list and I'll dig out the info for you.
>>> 
>>> Warren and Pattie Updike
>>> 1994 C320 "Warr de Mar" #62
>>> Middle River, Chesapeake Bay
>>> 
>>> -----Original Message-----
>>> From: Richard Wotruba [mailto:wotrubadick at yahoo.com]
>>> Sent: Wednesday, April 26, 2017 7:40 PM
>>> To: c320-list at lists.catalina320.com
>>> Subject: [C320-list] Speed paddle
>>> 
>>> Thanks for everyone's help regarding salt water under engine.Looks like it is raw water pump.I ordered new one from tr diesel hope it works.Next challenge does anyone know where to purchase new paddle wheel for speed instrument.     WindSong,hull 3,1993 thanks for support⛵️⛵️🙏😀✌️
>>> 
>>> 
>>> 
> 

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