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Old 10-03-2021, 15:34   #1
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Do I need my speed transducer?

James on Zingaro recently had his speed transducer break and pop out, almost flooding his boat. He plans to remove it entirely and glass in that hole, which had me thinking, that seems like a good idea. Mine has been broken since I bought the boat this summer and growing up on my family boat, we never had one. Is there any reason to keep/fix it, other than being able to have an idea of how strong a current is? Does this data integrate in any way with my other instruments, chart plotter, radar, etc?

I'm also interested in replacing my thru-hull depth transducer with an in-hull device, but I have a cored hull, so that wouldn't work. But if I remove my speed transducer, I could glass in that hole and mount an in-hull depth transducer on that solid area, right?
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Old 10-03-2021, 15:55   #2
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Re: Do I need my speed transducer?

Depending on your instrument configuration, you may not get true wind speed or direction absent the reference to boat speed.

One freak accident usually isn’t a reason to abandon a knotmeter that is ubiquitous on almost all boats but your choice.
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Old 10-03-2021, 16:06   #3
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Re: Do I need my speed transducer?

Wouldn't true wind speed be calculated SOG from the gps? How would speed through the water matter in calculating TWS?

Any opportunity to reduce the number of holes in my hull are interesting to me.
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Old 10-03-2021, 16:19   #4
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Re: Do I need my speed transducer?

I removed mine and glassed over the hole. I never used it and when I thought about checking speed through the water, it was always fouled. In my mind, all that really matters is SOG, which I get from my GPS. I think racers will have a different take though.
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Old 10-03-2021, 16:27   #5
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Re: Do I need my speed transducer?

Quote:
Originally Posted by S/V Illusion View Post
Depending on your instrument configuration, you may not get true wind speed or direction absent the reference to boat speed.
Ah, I did a quick google dive into Apparent Wind Speed vs True Wind Speed vs Ground Wind Speed, and I see why that would be necessary for TWS. I guess the question is, do I need TWS.

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Originally Posted by Copacabana View Post
I removed mine and glassed over the hole. I never used it and when I thought about checking speed through the water, it was always fouled. In my mind, all that really matters is SOG, which I get from my GPS. I think racers will have a different take though.
This is sort of how I'm thinking right now.
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Old 10-03-2021, 16:29   #6
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Re: Do I need my speed transducer?

I had always a lot of problems on various boats with speed transducers. More often than not they had problems with the paddle-wheel, either because barnacles liked to make it their home, algae got stuck in it, cavitation or other strange flow effects gave bad readings or the wheel simply didn't work properly. On my boat I only have the SOG and COG from the GPS. This allows correct calculation of the true wind. I don't really miss the speed through water given by the paddle wheel. I'd never bother installing one again. Having one less hole in the hull is a bigger benefit than this measure. I think this will go the way of the dodo.

In case my GPS breaks, the classic measure with a bottle-log and a chronometer every hour is a fair enough approximation for dead-reckoning. This how it was done for centuries. It's fun to do when bored on a passage.
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Old 10-03-2021, 16:45   #7
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Re: Do I need my speed transducer?

My knotmeter and depth finder is one unit. As above, always have problems with the knotmeter not working. Last time there was a family of little crabby things living on it.
Abe
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Old 10-03-2021, 17:01   #8
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Re: Do I need my speed transducer?

Like all things sailing, the answer is "It Depends." (FWIW, I think that would make a great sailing blog name...)

If you are a casual sailor doing local day sails, of course you don't need to measure speed through the water (STW). Speed Over Ground (SOG) from your GPS is good enough. Most navigation suites will let you calculate true wind speed and angle from SOG as well as from STW. The error introduced here is small, and not likely to change your sailing decisions.

If you are making long ocean passages, or even long coastal trips, having SOG and STW is extremely valuable navigation information. Knowing the local tide set and drift can really help you keep moving toward your destination as fast as you can.

Just by way of example, if your navigation region includes the US East coast, knowing when you are in, or out, of the Gulf Stream is key to getting to where you are going as fast as possible.

Lots of people don't understand this, or know how to use it, but that doesn't mean it lacks value to the knowledgable navigator.

Is it "necessary"? Of course not. Would I choose to sail without it? NO WAY.
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Old 10-03-2021, 17:25   #9
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Re: Do I need my speed transducer?

I’ve had the same problem with the paddle fouling far too often.
I’m thinking of glassing in the hole and getting a Knotstick to read speed through the water when necessary, as the trolling unit doesn’t have to “live” in the water thus eliminating the fouling problem.


http://www.knotstick.com/
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Old 10-03-2021, 17:40   #10
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Re: Do I need my speed transducer?

I've never understood why boat speed (speed through water) is important for calculating true wind.

I get that-

-the anemometer measures apparent wind.
-motion of the boat causes the apparent wind to vary from true.
-variations between boat speed & SOG give an indication of current
-heading & COG differences indicate leeway and current. (Without a log that
measures lateral as well as forward motion, separating the two data points
is by estimation not calculation)


To illustrate my ignorance.

Sailing at a given angle to windward two scenarios

6 kts SOG & 9 kts boat speed.
Instrument shows difference between true & apparent wind

6 kts SOG & 6 kts boat speed.
Instrument shows same true & apparent wind.

In both cases the speed through the air is the same.

Is reliance on boat speed for true wind calculation a hangover from and era where GPS did not exist?
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Old 10-03-2021, 17:42   #11
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Re: Do I need my speed transducer?

Took ours out years ago and never looked back. Yes I would like to know the difference between speed through the water and speed over ground. But in the tropics you need to pull the unit every week to clean and even then I didn’t trust the data. Perhaps in cold clear water it’s worth it but not my experience. One less hole.
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Old 10-03-2021, 17:47   #12
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Re: Do I need my speed transducer?

I removed mine and glassed the hole about 10 years back. One less sink your yacht hole and one less thing to keep clean and calibrated. SOG is fine for me and my rocket ship.


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Old 10-03-2021, 18:05   #13
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Re: Do I need my speed transducer?

I pull the Airmar transducer (DST810) when not underway.

It lives in a capped PVC tube containing white vinegar.
Goes back in clean no scrubbing required.

IF you are removing a combination paddle wheel with depth do not let it run dry or you risk overheating & failure if the rest of the system is powered up for an extended time.
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Old 10-03-2021, 18:27   #14
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Re: Do I need my speed transducer?

Quote:
Originally Posted by Rucksta View Post

Is reliance on boat speed for true wind calculation a hangover from and era where GPS did not exist?
No, not at all...

In MOST cases the difference is small, but not in all. The difference is that "TRUE WIND" as calculated with Speed Through the Water is relative to the water surface, which is where (hopefully!) your boat lives. If you use GPS to do the calculation you actually end up with GROUND WIND, i.e., wind relative to the unmoving ground. If there is no current they are the same... but...

Let's put together a simplistic hypothetical designed to make the math easy and avoid trigonometery...

A boat with a heading of 000º, in a current with a set of 000º, and a drift of 5 knots. The wind is from 180º at 8 knots. Speed through the water is 0 kts.

Apparent wind is 3 kts with an AWA of 180º.

A calculation of TWS and TWA using STW would tell you your TWA is 180º and your TWS is 3 kts. Which is the right answer, because that is what you really have to sail with.

A calculation of TWS and TWA using SOG gives very different answers. It would give a TWA of 180º and a TWS of 8kts. While this is great data if you are sitting still on the ground, you don't actually have 8 knots of wind to sail with.

Just as an example, my boat sails fine downwind (slow--but fine) with 8 knots TWS. With a TWS of 3 kts, our sails would just hang there. So this is a real difference, in the real world.

You might decide this is not relevant to you if you sail in a place with no currents, or that you don't really care. That's fine. But the difference can matter to your boat even if you decide it doesn't matter to you.

It is not required information, but it does help sail a boat efficiently.
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Old 10-03-2021, 18:33   #15
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Re: Do I need my speed transducer?

Can you give an example of when you would want TWS and TWA over AWS and AWA?
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