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Old 15-03-2017, 06:53   #1
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Replacing Knotmeter impeller while the boat is out.

I am sure this will be a quick answer for somebody who has dealt with this. Having our Hunter 380 pulled, the slings broke off some impeller legs. I haven't made it back up to the boat to inspect and order the kit but was wondering: Is there a lower o-ring/seal that would allow you to retract the unit the small amount needed while in the slings and simply push back down once it is out of the slings. Otherwise I just need to find the plug or order a new one.

Thanks in advance

Ron
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Old 15-03-2017, 07:17   #2
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Re: Replacing Knotmeter impeller while the boat is out.

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Originally Posted by Dallasron View Post
I am sure this will be a quick answer for somebody who has dealt with this. Having our Hunter 380 pulled, the slings broke off some impeller legs. I haven't made it back up to the boat to inspect and order the kit but was wondering: Is there a lower o-ring/seal that would allow you to retract the unit the small amount needed while in the slings and simply push back down once it is out of the slings. Otherwise I just need to find the plug or order a new one.

Thanks in advance

Ron
What kind of Speedo? If a Standard Horizon, I have a spare dummy plug we no longer need that you can have.
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Old 15-03-2017, 07:30   #3
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Re: Replacing Knotmeter impeller while the boat is out.

OOPS,,,,,Raymarine ST60
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Old 15-03-2017, 15:26   #4
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Re: Replacing Knotmeter impeller while the boat is out.

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OOPS,,,,,Raymarine ST60
It incorporates an internal flap which reduces water flow to a dribble.
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Old 15-03-2017, 22:40   #5
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Re: Replacing Knotmeter impeller while the boat is out.

There is an o ring on the transducer near the paddle wheel end. However I'm not sure how much you can loosen the inside nut and still have it secured in place as you describe. As stated previously, there should be an internal flap designed to swing closed when removing in water to install dummy plug. On our boat it slows the incoming water down but I'm amazed how much water will come in while pulling transducer to install the dummy plug. When you find your dummy plug that fits the fitting, tie it on a string next to the through hull fitting same as the emergency tapered plug at every fitting.

Raymarine fittings have a tab that lines up with an arrow on transducer pointing towards bow. This needs to be aligned or the nut securing the transducer in place will not seat properly and leak.

1st rule of boating is to keep water on the outside. Happy sailing.
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Old 16-03-2017, 08:47   #6
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Re: Replacing Knotmeter impeller while the boat is out.

You can change the speed transmitter unit while the boat is in the water.
The Plug is good to have, so you can clean the paddle wheel when needed.
Try to find the plug!
However, you can won't have any use for the plug when changing the unit.
Remove the old transmitter, and just slide the new transmitter in its place.
Each time you pull the transmitter, expect to get some water in the boat.
You should have a flapper the slows the water, but water flow will not stop.
Make sure you are ready! If yiou have never done it, ask a friend for a demo.
Looks about like this, unless the flap doesnt close. A previous boat didnt have a flap, but didn't foul as often.
I end up cleaning mine several times a year, in the Chesapeake.
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Old 16-03-2017, 09:03   #7
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Re: Replacing Knotmeter impeller while the boat is out.

Quote:
Originally Posted by Dallasron View Post
I am sure this will be a quick answer for somebody who has dealt with this. Having our Hunter 380 pulled, the slings broke off some impeller legs. I haven't made it back up to the boat to inspect and order the kit but was wondering: Is there a lower o-ring/seal that would allow you to retract the unit the small amount needed while in the slings and simply push back down once it is out of the slings. Otherwise I just need to find the plug or order a new one.

Thanks in advance

Ron
I deal with this issue on a yearly basis. I have two boats: one has the transducer located where it is not an issue but the second one has a transducer located right where the slings go.
For a few years, I did the dummy plug thing, but then I discovered that I could retract the sensor far enough in the tube that the paddle wheels were inset far enough that the slings can't get them. Not a drop of water gets into the boat. I pull the retaining pins and retract the transducer JUST BEFORE lifting and push it back down IMMEDIATELY after launch. Right at the same time as I check for leaks.
Just to be on the safe side, I fabricated a mechanical stop so that the transducer can't totally pop out due to water pressure (pretty low but still a possibility), but that has never been an issue. The big problem is remembering to remove/retract the sensor before lift-out ...

edit: You only have to back it up about 3/4" and the paddle wheel fins are totally protected.
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Old 16-03-2017, 09:31   #8
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Re: Replacing Knotmeter impeller while the boat is out.

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OOPS,,,,,Raymarine ST60
Order the new speed transducer and use the broken one as adummy plug.

Don't recall when the airmar flaps were introduced, so depending on age, the original may not have one.

My boat transducer (not installed by me) is also where the slings contact the hull. On haulout move the boat into the lift area and swap the dummy plug just before they snug the slings up. On put in, after the slings are loosened, but still in place, put in the speed transducer. This way, should you run into trouble during the swap (they can lift the boat before it sinks). The first time will be scary, after a couple more times a cake walk.

PS if you just loosen the transducer the transducer, the water pressure will push it out and you will have a gusher the height the hole is below the waterline.

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Old 16-03-2017, 09:32   #9
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Re: Replacing Knotmeter impeller while the boat is out.

I had the same type knotmeter and impeller. I took it out and glassed over the hole. One less hole to maintain.
I use the GPS speed from my chartplotter. It gives only speed over ground but that takes away guesswork for navigation.
That's my 2 cents worth.
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Old 16-03-2017, 10:49   #10
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Re: Replacing Knotmeter impeller while the boat is out.

I believe Raymarine uses the impeller knot meter to calculate​ True Wind Speed. You only get Apparent Wind Speed with no impeller. I may be wrong but it's pretty easy to check...
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Old 16-03-2017, 12:57   #11
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Re: Replacing Knotmeter impeller while the boat is out.

Does Raymarine sell just the 4-legged knotmeter wheel? We have the same issue, 3 legs do not give an accurate water speed, and epoxy does not seem to hold on any plastic one I've tried to attach. PS we pull and replace ours every time we haul-our fwd strap goes right over the throughhull!
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Old 16-03-2017, 14:12   #12
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Re: Replacing Knotmeter impeller while the boat is out.

Hi, folks,
Here in the Gulf Coast of Florida, my transducer vanes would foul quickly with marine growth, even though my diver (who scrapes the hull monthly) would clean it. Finally on my last pull-out, I removed it altogether and glassed over the hole. Like someone else said, one less hole in my boat!

I had read an article in Good Old Boat about using an over-the-side fishfinding transducer as a depthsounder. They created a "chimney" on the inside of the hull, filled it with oil (like cooking oil) and suspended the transducer in the chimney and covered the top, and voila, a depthsounder without a though-hull hole.

So I did a variation of that: created a little chimney of 4 in PVC pipe, cut the bottom edge to the interior slope of the hull and "glued" it in place with silicone sealant. Instead of oil, which seemed messy, I used Ultrasound gel (easy to buy on line). Works wonderfully, though about every 6 months I have to replace the gel as it is water-based and slowly "evaporates" (gets gummy and stops working).
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Old 16-03-2017, 15:18   #13
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Re: Replacing Knotmeter impeller while the boat is out.

Found the plug!!!,,,will strap it in the bilge. Pulled the meter and waiting on the rebuild kit. I've done hot taps on pressured water systems so I guess a little water spewed about is just a good start to a needed cleaning

I do like the idea of a little line with a snap hook sized to let the sensor back out enough to clear the slings. Take a couple minutes to glass down or glue a tiny padeye next to the thruhull.

Thanks all!!
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Old 16-03-2017, 16:19   #14
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Re: Replacing Knotmeter impeller while the boat is out.

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Originally Posted by sailcrazy View Post
Does Raymarine sell just the 4-legged knotmeter wheel? We have the same issue, 3 legs do not give an accurate water speed, and epoxy does not seem to hold on any plastic one I've tried to attach. PS we pull and replace ours every time we haul-our fwd strap goes right over the throughhull!
+Most hunters I have worked on have AirMar units supplied with RayMarine goods. Check Defender Marine, talk to Cherly Gerkin,--knows her stuff.
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Old 17-03-2017, 05:52   #15
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Re: Replacing Knotmeter impeller while the boat is out.

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Originally Posted by boom23 View Post
I believe Raymarine uses the impeller knot meter to calculate​ True Wind Speed. You only get Apparent Wind Speed with no impeller. I may be wrong but it's pretty easy to check...
This is the industry standard for calculating apparent wind speed; boat speed readings must be water referenced vs. ground referenced (e.g. if the wind and water are both moving in the same direction at 5 knots, you can't sail...take the analytics from there).

If you don't have a paddlewheel (or sonic reader of STW), you're not going to have wind speed readings.

As far as paddlewheel go, If my boat is going to sit anywhere for more than a few days, I pull the paddlewheel and put in the plug. Takes less time than cleaning the damn thing. And I always put in the plug before hauling the boat, even though if they put the slings where I tell them to it would be clear of them. Peace of mind.
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