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  • 1 Post By sawingknots
  • 1 Post By SlowButSteady
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  #1  
Old 10-11-2011
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A Few Knotmeter Questions...

I have a Cal21 with a swing keel/weighted center board. The keel trunk comes up through the cockpit sole for the retracting cable, leaving a large slot/"thruhull" that needs to be plugged for performance and dry feet.
The plug for all intents and purposes is a 2x4 with fancy ends in it. I would like to mount my depth sounder and knotmeter on the underside of the plug. I don't want to put any extra hole in the 40 year old fiberglass if I don't have to.

Three questions...

1. Will the knotmeter and depthsounder work properly directly behind the keel with whatever turbulence it may cause?

2. What type/brand of knot meter should I get for this location? ( I already have a DSer)

3. Because the plug is removed and stowed every time I use it...Is there a quick connect, weather proof electrical plug in you can reccommend? (I was thinking maybe an automotive one would work)

Of course I want to do it on the cheap...I consider a knotmeter a luxury rather than a neccessity for the type of sailing I do.
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Old 10-11-2011
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The knotmeter in particular should not be in the spoil of the board. These transponders are typically placed ahead of the keel for clean flow and an unobstructed 'view'. Also you may well find the ds 'fooled' by the turbulence behind the keel.

I suspect the sounder will not like trying to shoot through the narrow gap of the trunk.. it should be at the hull surface.
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Old 10-11-2011
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fast or not so fast

a knot meter only determines the speed the water is moving under your hull,present currents,tides etc cause false speed readings,a small inexpencive gps will give your true progress,it does,t matter how fast your wheels are spinning if your still standing still
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Old 10-11-2011
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Both the knotmeter and the sounder transducers should be placed forward of the keel/centerboard. As stated earlier, a knotmeter works best with unobstructed flow, and at sailboat speeds the turbulent wake of the keel/centerboard can be fairly wide. Also, the boundary layer gets thicker with distance from the "leading edge" (where the water first touches the hull). So, placing a knotmeter as far forward as possible, yet not so far forward that it will be seeing air from time to time, is best. The sounder transducer should be placed forward to prevent a "blind spot" from the keel/centerboard, and to give you a few seconds more time to react to any submerged hazard(s). One can place either or both transducers beside the keel, aft of the keel, on the transom, et cetera, but the result will be less than optimal.
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Old 10-11-2011
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Quote:
Originally Posted by sawingknots View Post
a knot meter only determines the speed the water is moving under your hull,present currents,tides etc cause false speed readings,a small inexpencive gps will give your true progress,it does,t matter how fast your wheels are spinning if your still standing still
For navigation a GPS is probably best, since it does give one a better estimate of the speed over the ground. However, for tuning sailing performance, the speed through the water is more important. As such, using a GPS as a knotmeter is often less than ideal.
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Old 10-11-2011
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For navigation, knowledge of the depth is important. You can use an inexpensive fishfinder with the transducer placed on the inside of the hull on top of a blob of toilet bowl wax. As others have said, find place that will have unobstucted view down and forward. The fishfinder can also give you some clue as to what is on the bottom (rocks, sand, weeds).
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Old 10-11-2011
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Quote:
Originally Posted by sawingknots View Post
a knot meter only determines the speed the water is moving under your hull,present currents,tides etc cause false speed readings,a small inexpencive gps will give your true progress,it does,t matter how fast your wheels are spinning if your still standing still
It matters if you are trying to trim your sails to get the most of them and know what your usual speed is under each particular trim. Like in a race.

I have both and use both regularily.
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