[C38] Knot meter location

Chuck Finn charles at finn.ws
Fri Jan 14 11:34:09 EST 2011


I will be installing my third version of the knotmeter early this 
spring.  When I upgraded last time to Navman, I could not get the 
original (flush mount) paddle wheel out.  It seemed to be almost welded 
in and I gave up after heating and hydraulic force wouldn't budge it.  
Plus, the Signet hole was larger than the Navman.   I am currently 
installing Raymarine which of course is a different hole size 
(larger).   My research at the time was extensive, but came down to 
essentially two observations.  First was that the paddlewheel should be 
located where there is as little laminar flow disruption as possible.  
Generally this is as far away from your bow waterline as you can get, 
but not so far back as to be disrupted by the pressure wave generated by 
the keel, which is rather small and angled due to the raked design of 
our keel.  Second is the preferred location is on the centerline of the 
boat, but varying a bit to either side does not make much difference.  
So, I think your boatyard probably made the right call regarding 
location although I would have a real problem with them varying from 
explicit instructions!

I have to admit that I probably keep installing a knot meter more out of 
sentiment and for emergency purposes than usefulness these days (I still 
have a lead-line on board and actually do use it).  Frankly, it was 
quite disappointing ten years back when I first got GPS to discover how 
variable the readings are based upon boat speed, slime, etc.   My chart 
plotter is so much more accurate regarding VMG, SOG, etc. that I seldom 
refer to the knotmeter other than to get excited when we are flying 
because it invariably reads faster than we are really going.  But for 
the moment, what's the harm in believing you are doing 13 knots!  I do 
agree with Tom's point regarding tides and this can be important at 
times, but I tune with my chartplotter.

Chuck Finn
Mighty Quinn #114 (1981, with the knot meter just aft of the water tank 
right inside the access door on the vee berth)

On 1/13/2011 10:46 PM, Steve Smolinske wrote:
> Does anyone have any knowledge on the location of the knot meter on 
> the centerline?  Peregrine is in the yard having mushroom mounts 
> removed and replaced with flush mounts.  I instructed in writing to 
> the yard along with a photo to place the knot meter on centerline 33% 
> of the distance from the keel to the waterline entry point.  They have 
> placed it 1' in front of the keel and stated that is the perfect 
> location and any further forward it will be subject to disturbance 
> from the bow wave.  (yes they went ahead without contacting me 
> first)   We are going to talk about it tomorrow morning and Im hoping 
> someone has some definitive information I can use. Grrrr  Thanks
> Steve
> #312 Peregrine
> Seattle
>
>
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