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Old 21-04-2016, 02:51   #1
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Water speed sensor

The "speed through the water" sensor does not appear to work on our newly acquired Lagoon 380. This means that true wind speed indications are inaccurate. The sensor must be on the hulls somewhere - does anybody know where they are located.
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Old 21-04-2016, 02:56   #2
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Water speed sensor

I am not sure if it's the same setup as the L450 but check the hull port side in the forward berth under floor panel, if the wheel is clogged that may be your issue.

I hope so as the new sensor is not cheap...


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Old 21-04-2016, 03:18   #3
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Re: Water speed sensor

Ah thanks for that - I assumed it would be on the outside of the hull somewhere. I have heard cases of sensors on the hull being damaged during haul-out. We will be back on the boat in May so I will have a look where you suggest. Many thanks, Paul
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Old 21-04-2016, 06:27   #4
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Water speed sensor

Here's a photo showing mine inside the hull and the plug you need to put in immediately once you remove it, it is simple enough to do but you will get a little salt water in the hull during the process.

Make sure your plug is ready to fit in or you could have a big issue with water....

Paul if you boat is not used frequently the Raymarine guys in Singapore recommend removing the unit and leaving it inside the hull and replace it with the plug so you don't get any growth on the sensor.


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Old 21-04-2016, 22:24   #5
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Re: Water speed sensor

The sensor is a paddle wheel log, located in front of the port keel. They clag up with marine growth every month so a swim and unclagging is the easiest fix if the water is warm enough
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Old 21-04-2016, 22:41   #6
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Re: Water speed sensor

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Originally Posted by TOT View Post
Make sure your plug is ready to fit in or you could have a big issue with water....
Best bet is to keep the plug tied to the fitting in the same manner you have an emergency bung tied to the fitting and every other skin fitting in the boat.
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Old 22-04-2016, 03:49   #7
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Re: Water speed sensor

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The sensor is a paddle wheel log, located in front of the port keel. They clag up with marine growth every month so a swim and unclagging is the easiest fix if the water is warm enough
does your speedo shows correct speed ?

Mine is like this

in 4 kn understates 0.6kn
in 5 kn understates 0.8kn
in 6 kn understates 1kn
in 7 kn understates 1.2kn

I check against GPS and motor in flat water and drive full circle and get average GPS speed in circle that should equal speed in water.

It is annoying because of true wind & true angle. And cant find any instructions for speed calibration.
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Old 22-04-2016, 04:23   #8
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Re: Water speed sensor

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Originally Posted by arsenelupiga View Post
does your speedo shows correct speed ?

Mine is like this

in 4 kn understates 0.6kn
in 5 kn understates 0.8kn
in 6 kn understates 1kn
in 7 kn understates 1.2kn

I check against GPS and motor in flat water and drive full circle and get average GPS speed in circle that should equal speed in water.

It is annoying because of true wind & true angle. And cant find any instructions for speed calibration.
All speed logs i have seen have an adjustment/calibration screen on the display unit. Research your model/manufacturer and you should be able to find the instructions.
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Old 22-04-2016, 04:52   #9
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Re: Water speed sensor

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All speed logs i have seen have an adjustment/calibration screen on the display unit. Research your model/manufacturer and you should be able to find the instructions.
ok thanks. had another look and found it in manual.
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Old 22-04-2016, 07:01   #10
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Re: Water speed sensor

Yours sounds similar to ours. I think I recalibrate by 20% across the board. Having accurate STW log is indispensable for determining current as well as TWS and TWA. It will never be prefect and usually different on opposite tacks, but it's nice to get it in the ball park.
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Old 22-04-2016, 07:49   #11
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Re: Water speed sensor

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Originally Posted by monte View Post
The sensor is a paddle wheel log, located in front of the port keel. They clag up with marine growth every month so a swim and unclagging is the easiest fix if the water is warm enough

It is more than just a paddle wheel as the depth sensor is included in the same unit.


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Old 23-04-2016, 17:48   #12
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Re: Water speed sensor

The one on our 440 needs cleaning every 3-4 days as the water temp is 32c and the growth it incredible. I use a small stiff brush and do it from underneath. Saves mopping up. Ours is a three way. Speed, depth and temp.

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Old 24-04-2016, 05:02   #13
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Re: Water speed sensor

STW is obviously critical for determining current. However, for TWS wouldn't you want SOG? Just curious.

Since I'm not sailing, I've given up on STW because every paddle wheel I've every used clogs up quickly and tends not to be very accurate. In the Northeast US you don't go in the water unless it's an emergency! Pitot tubes only seem to work at higher speeds (I suspect that's why they're common on outboards) and they, too, clog up.
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Old 24-04-2016, 05:17   #14
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Re: Water speed sensor

Yep, my DST800 paddle wheel clogs in a few months. Worse, the temperature part has failed 3 times, the last time has it outside the warrantee from the original purchase date.

How about the CS4500? Can be antifouled, accurate speed, won't clog so easily. Downside is expensive and only speed and temperature so need another depth sounder.


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Old 29-04-2016, 15:40   #15
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Re: Water speed sensor

Capt Tom you are correct. To get accurate True Wind you need SOG. Speed through the water gives a vector enabling a calculation to be made but it can only be accurate when you have no current. And that doesn't happen too often.

For some strange reason some manufacturers continue to use speed through water, when SOG is also available via GPS.

So I don't even bother with my paddle wheel. Inaccurate even when working properly, frequently clogged up, and doesn't give accurate true wind calculation anyway.




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