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Old 20-04-2017, 14:40   #1
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Speed / Depth Placement

I've got two holes in my hull, aligned fore and aft with several inches between them, for the speed and depth transducers. I'm replacing both, but I can't remember which one goes forward of the other!

The theory might be interesting, but I'm concerned with what works. If your transducers / sensors are aligned fore and aft of each other, and they work properly, can you please tell me which one is in front - speed or depth?

Colin
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Old 20-04-2017, 16:36   #2
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Re: Speed / Depth Placement

I'd put the speed sensor in front. The depth transducer may set up some extra turbulence in the near-hull flow stream, and this could affect accuracy of speed measurement.

Further, the mfgs of some depth transducers suggest not antifouling their faces. If that practice is followed, growth is likely, and this will damn sure mess up the flow to a speed paddlewheel mounted aft of it! Guess how i know about this!

Jim
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Old 20-04-2017, 18:28   #3
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Re: Speed / Depth Placement

Thanks, Jim. I think that's how it was installed originally. However, I've been doubting myself because the depth transducer also likes smooth flow, although I'm guessing it can handle the small disturbances caused by the paddle.
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Old 20-04-2017, 19:55   #4
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Re: Speed / Depth Placement

Not sure how fast your boat is, but you want the speedo where there's the least chance of it being affected when the boat surfs, or the keel starts cavitating. As both such occurrences make your speedo all but useless. And these things are of little importance to your depth gauge, since when you really need it, you'll be going slow anyway.
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Old 21-04-2017, 07:57   #5
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Re: Speed / Depth Placement

Quote:
Originally Posted by cfarrar View Post
Thanks, Jim. I think that's how it was installed originally. However, I've been doubting myself because the depth transducer also likes smooth flow, although I'm guessing it can handle the small disturbances caused by the paddle.
I'm not usually the advocate of tech here, but I would seal over the speed paddle hole and just go with GPS. On my boat, I have a handheld or two, a MFD and an VHF, all of which tell me speed. In addition, I've never had much trouble knowing our speed by looking, or by chucking a chip over the bow and counting the seconds until it passed the stern. I've also been given a Walker log for the taffrail, again, if I really want to know.

Smooth flow over the depth transducer, however, is quite important to me, so I would prioritize that. Just my .02.
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Old 21-04-2017, 08:07   #6
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Re: Speed / Depth Placement

Have you thought of moving one of the transducers maybe to the other side of the hull?
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Old 21-04-2017, 08:57   #7
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Re: Speed / Depth Placement

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Originally Posted by S/V Alchemy View Post
I'm not usually the advocate of tech here, but I would seal over the speed paddle hole and just go with GPS. On my boat, I have a handheld or two, a MFD and an VHF, all of which tell me speed. In addition, I've never had much trouble knowing our speed by looking, or by chucking a chip over the bow and counting the seconds until it passed the stern. I've also been given a Walker log for the taffrail, again, if I really want to know.

Smooth flow over the depth transducer, however, is quite important to me, so I would prioritize that. Just my .02.
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Old 21-04-2017, 09:18   #8
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Re: Speed / Depth Placement

Quote: "I've never had much trouble knowing our speed by looking, or by chucking a chip over the bow and counting the seconds until it passed the stern."


Bingo! Only way to fly :-) Work you numbers in advance so you don't have to do it every time. Then remember them. There ain't many. Chuck you chip, even a piece of paper, count your elephants, and bingo - you know your speed. e.g. five elephants = 3 knots.

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Old 21-04-2017, 09:22   #9
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Re: Speed / Depth Placement

I'd put the depth sensor in front. No bubble stream or turbulence issues for the depth.
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Old 21-04-2017, 12:21   #10
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Re: Speed / Depth Placement

Quote:
Originally Posted by S/V Alchemy View Post
I'm not usually the advocate of tech here, but I would seal over the speed paddle hole and just go with GPS. On my boat, I have a handheld or two, a MFD and an VHF, all of which tell me speed.
Thats funny, I would say seal over the depth hole, and use a transducer that mounts inside the hull with no hole. Why have a hole at all if one is not needed?

GPS speed is ok, but it's speed over ground. Often its good to know the speed through the water. Especially when there's current or leeway involved. Also, a speed instrument is easier to leave ON all the time than a GPS. And finally, the LOG function of the speed instrument is handy for recording daily, weekly, season, and trip total distances.

Also, GPS speed is based on your previous position, so its an average, and has a time lag. Only a speed instrument can tell your actual instantaneous speed. And this is vital for tweaking sail trim or steering in a race.
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Old 21-04-2017, 14:16   #11
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Re: Speed / Depth Placement

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Originally Posted by Cheechako View Post
I'd put the depth sensor in front. No bubble stream or turbulence issues for the depth.
I'd point out that many runabout type motor boats have their depth transducer mounted at the transom, a point with huge turbulence and disturbed flow in general. They work just fine there as I understand it.

I don't think that the amount of turbulence (very little) generated by a paddle wheel sensor will bother a depth sounder in the slightest. Doesn't seem to on my boat,,,

Jim
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Old 21-04-2017, 14:33   #12
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Re: Speed / Depth Placement

Quote:
Originally Posted by TrentePieds View Post
Quote: "I've never had much trouble knowing our speed by looking, or by chucking a chip over the bow and counting the seconds until it passed the stern."


Bingo! Only way to fly :-) Work you numbers in advance so you don't have to do it every time. Then remember them. There ain't many. Chuck you chip, even a piece of paper, count your elephants, and bingo - you know your speed. e.g. five elephants = 3 knots.

TP
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Old 21-04-2017, 18:05   #13
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Re: Speed / Depth Placement

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Originally Posted by S/V Alchemy View Post
I'm not usually the advocate of tech here, but I would seal over the speed paddle hole and just go with GPS.
SOG <> STW.

In many situations, it can be very useful to know the difference.
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Old 21-04-2017, 18:38   #14
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Re: Speed / Depth Placement

I write software for a marine electronics company. The depth transducer will detect the disturbance of the paddle wheel as excessive noise. This makes detecting the bottom, difficult in shallow water, which is where I am most concerned about depth. The accuracy of the paddle wheel is also rather poor. The turbulence created from the transducer hole would not have any real effect on accuracy. That said, I disagree with eliminating the paddle wheel in favor of GPS. The paddle wheel can tell you things about current that the GPS knows nothing about.
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Old 21-04-2017, 20:14   #15
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Re: Speed / Depth Placement

Quote: "Marpol"

Oh dear :-0)!
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