Knotmeter installation

[Danny]

I am seeking advice regarding the positioning of my speed transducer. I am concerned about getting accurate results so that I can eventually tie into a performance tracking package. I have been told that to achieve accurate and repeatable tack to tack measurements I need to place the transducer on centerline, which makes sense to me. However, I have a major structural member on the centerline of my hull forward of the keel, which precludes the possibility of achieving a forward centerline location.

1) How essential is the centerline location? Can a non-centerline location be compensated for?
2) Can the transducer be located aft of the keel? It would be structurally possible to achieve a centerline location in this configuration, but would the downwash from the keel or other hydrodynamic effects in that region render the readings inaccurate? If this is an acceptable location, how far should the transducer be placed from the keel/saildrive/rudder?
3) Any other advice or guidance in this regard?

[gonzo]

You can put two transducers; one on each side. In the fore and aft position it should be between a third aft from the bow and the middle of the hull. Further forward will get too much turbulence and aft will show less speed because of the eddies.

[yipster]

other advice may be a GPS instead fitting thrue hull speed transducers. you get accurate time, speed and position data into a performance tracking file and when using a chartplotter much more as well. an old laptop with free chart software on a GPS allready can do wonders

[Polarity]

Yes but that only gived you Speed and direction over the ground, and not actual speed/heading. How can you tell if your boat is performing to its Polars if you cant say exactly what effect current is having...

[yipster]

hi Polarity,
here a list of functions of the -not free- tsunamis chartplotter from transas, offcourse some data needs input from other than a GPS source. strange enough the older navisailor has more functions, like true wind calc, tide and currents calc, anchor alarm and (satelite) depth sounder, and much more it seems. GPS is one of the best "free" (your tax money) services around. still missing data?

[Polarity]

still missing data?

Yep.

A GPS will never tell you your speed through the water. Ie how fast the yacht is being driven. You might be trucking downwind at 8knots, but if you are doing it against a 3 knot current / surface drift your GPS will say 5 knots. Or if you are overpowered and making lots of leeway your GPS only gives you your cog and sog. You need to know your heading from a compass to measure it accurately. - hence in the list above the log and magnetic compass interface. It does not matter what you do with the numbers the GPS will not give you accurate speed through the water.

In ans to the original question if you put one on each side in the suggested position and have a small tilt switch mounted athwartships it will switch the transducers over between tacks.
All the same I am not sure how much difference a couple of inches of the CL would make.

Good sailing

Paul

[Danny]

Thanks, folks.

I agree that the GPS solution does not give me the data I want. The knotmeter gives me speed through the water which is critical to my performance analysis. GPS output is also valuable to me for distance/time/bearing to waypoint data, but not useful to me for performance data.

As for the knotmeter set-up, I like the concept of the dual transducer arrangement, and can see how it could (in theory) be set up. I imagine that there would be challenges in calibration of the two instruments to ensure that they yield similar data. Any suggestions on how to achieve this?

Danny

[Polarity]

hmm... I would assume that 2 x identical instruments in 2x identical places would give 2x the same result. This set up was in use on a Swan 651 I sailed in Alaska a few years back. The tilt switch was rigged to a b+g set up - but I'm not sure if it was theirs.I would talk to the instrument makers, as this is not an unusual problem..

Good luck with it!

[asathor]

A used thermostat switch and a $5 12volt relay from Radio Shack will do the job - pot it in glass next time you repair something on the boat and it will be marine grade to boot. I cannot imagine the lifetime precision of those little paddlewheel bearings exposed to the "scum" of the sea being so good that they need to be "matched" - such precision would only last a few days at the most.

If you tack and trim the sails the same way on both tacks a GPS will tell you oodles - A simple spreadsheet or math calculator using your GPS and compass reading can convert this into a polar plot, and you can include information like heel and wave action and wave direction and get a handle on their influences.