How do knotmeters work

Ninefingers · #1 (**permalink**) 10-19-2011

Are they orientated so they will only read speed if the hull is moving forward, or will they read sideways drifting speed, say when you're hove to.

Sabreman · #2 (**permalink**) 10-19-2011

There is a sensor in the shaft of the transducer that counts the number of paddle wheel rotations. The paddlewheel has a small magnet in the paddle and as it passes the sensor, the rotation is noted. Since the paddle wheel is a static size, then number of rotations of a given size wheel in a given time directly translates to speed. Thus, the transducer works by water causing the paddlewheel to rotate.

Whether the transducer works in reverse depends on the design. our Datamarine does.

You need a GPS chartplotter or other GPS device to gather lateral movement. A knotmeter will not do that.

CorvetteGuy · #3 (**permalink**) 10-19-2011

Take a rope, tie a knot every metre, drop the rope at the bow of the boat and count the knots that disapear,,,,,,,how many knots???

Jd1 · #4 (**permalink**) 10-20-2011

There are all kinds of ways to sense speed. Probably the cheapest is the paddle wheel that Sabreman explains. They can work with a magnet or even sense the individual paddles of the paddle wheel with an inductive pickup. One can also measure speed many other ways such as a pressure pickup and probably ultrasonic through the hull (never seen one though). I am sure that there are many other, exotic ways to sense speed.

jackdale · #5 (**permalink**) 10-20-2011

Picture is worth ...

The white bits are the paddles.

celenoglu · #6 (**permalink**) 10-20-2011

They will not read sideways speed whatever the model is. Only GPS can calculate speed (sog) whatever the direction is, because it calculates speed depending on where you were a second ago and where you are now.

overbored · #7 (**permalink**) 10-20-2011

Airmar makes ultra sonic speed sensors. they will fit in the same well as the paddle wheel type. I installed one and will never go back. always dead on never gets dirty and can't be broken at haul out. it is flush with the hull. it also will only read forward or back

MY36 · #8 (**permalink**) 10-20-2011

Wow, these ultra sonic sensors seem too good to be true. Has anyone else had any luck with them? My paddle wheel gets jammed up a week or two after a cleaning and I'm too chicken to take it out for a cleaning while the boat is in the water.

svHyLyte · #9 (**permalink**) 10-20-2011

Quote:

Originally Posted by Ninefingers

Are they orientated so they will only read speed if the hull is moving forward, or will they read sideways drifting speed, say when you're hove to.

A typical Paddle Wheel knot meter measures the speed of the water parallel to the yacht's centerline if the sensor is oriented parallel to the centerline. If the yacht is making leeway, the Paddle Wheel measures the vector component of total speed parallel to the centerline (again, if the sensor is parallel to the centerline).

As an (exaggerated) example, if a yacht is making 6 knots through the water but at a leeway angle of 15º with respect to the water (i.e. the yacht's course varies from it's heading by 15º), the knot meter will indicate a speed of about 5.8 knots. In fact, however, most sailing yachts tend to have much smaller leeway angles through the water although the water itself will have a set and drift that will effect the yacht's speed and direction over the ground.

jstroup · #10 (**permalink**) 10-20-2011

Quote:

Originally Posted by overbored

Airmar makes ultra sonic speed sensors. they will fit in the same well as the paddle wheel type. I installed one and will never go back. always dead on never gets dirty and can't be broken at haul out. it is flush with the hull. it also will only read forward or back

Was the ultrasonic a quick swap out for the paddlewheel or did you need to purchase a new instrument? I currently have Raymarine ST60 instruments and would love to make this change.