[C320-list] Speed paddle

[Graeme Clark]

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⛵️⛵️🙏😀✌️
> 
> 
>