Sailing question

[Dave.B]

I've just spent a very enjoyable weekend sailing with my neighbours. However, one thing that puzzled us a lot is how the "log" works. For the uninitiated, this is basically a speedometer that measures the boat's speed (in knots) through the water. It works both while under sail and while motoring.

So, we are sitting in Salcombe harbour, moored to a buoy, and the tide is moving towards and past us at an estimated 2-3 knots, yet the log is firmly pegged on zero. However, upon arriving in Salcombe earlier that day, it quite clearly showed our speed as 2 knots (hence not a low threshold on the instrument).

While motoring, and while under sail, the GPS unit usually confirms that our speed over the ground is different from that shown on the log (as you would expect due to tidal streams) - so it seems very odd that those very same tidal streams are not picked up when the boat is stationary.

So, my question is, does anyone know how this device works?

BTW, all three of us on the boat are engineers (Civil (me) & Mechanical (the other 2)), and we can't figure it out!

[Clanad]

Having some familiarity with the device, I checked arctic corsair web site for confirmation to the following:

Modern ships use a pitot-static tube, which functions by sensing the difference between static and dynamic water pressures. Static pressure depends on the depth and density of the water; dynamic pressure is proportional to speed. Projecting through the bottom of the ship is a tube with a hole at its forward end to record the dynamic pressure and two other holes at right angles to record the static pressure. When the ship is motionless, the dynamic and static pressure are equal; when the ship moves, dynamic exceeds static. The difference varies as the square of the ship's speed.

[john91]

don't take offence, but was it switched on? most leisure craft would have an impellor type log, this is a little paddle wheel that sticks through the hull and revolves as water passes over it, generating an electrical signal which is read on a cockpit display,in harbour most electrical items are switched off to save battery power, also the impellor should be withdrawn into the hull to protect it against fouling by ropes, seaweed etc, perhaps this was done, or even the impellor was fouled by weed which dislodged when you started sailing.

[Dave.B]

Clannad, thanks for the reply - that makes a lot of sense.

John, no offense taken - we did think about this, but the instrument is not switched on or off at all.

[Dave.B]

Actually, having read clannad's answer a second time...

There seems to be no difference between a stationary boat with water moving past it, and stationary water with the boat moving through it.

How can the pitot tube make the differentiation between the 2 cases?

[Clanad]

Dave.B, a number of thoughts about your response...the pitot tube device under discussion is the same type of device as used on aircraft for airspeed measurement.  The airspeed read out on the instrument panel is indicated airspeed (actually calibrated, but not pertinent to thiss discussion) and takes no account of whether the aircraft is flying into or away from the wind.  A small aircraft tied down on the ramp in a fairly high wind, say 30 to 40 knots, will indicate a reading on the airspeed indicator, depending on the trheshold indicated speed on the instrument. So the Teleflex Type A (a standard brand) pitot mast speed log should also read a tide of sufficient force. I suspect your description of being moored to a bouy with the estimated tide of 2 to 3 knots may be contributing to the error, since the indicator does have a threshold speed.  The mast must be directly  into the direction of the ship or tide to be accurate.  Additionally, your description of using the GPS may not take into account the calibration necessary for accuracy.  This is especially true if WAAS is not being used or unavailable.  Lastly, it's only my assumption that the device is of the pitot mast type.  It could be a paddlewheel device, which has numerous errors if mounted incorrectly, most proununced at low hull speeds....

[Tim Baxter]

Additionally there are other, different type of sensor for the log.  There is the dopler which measures true speed over the ground and the electro/magnetic which measures speed through the water.  All have their draw backs but all can be accurate in their reading, each type has to be calibrated throught still or moving water and when speed is to be calculated the water vector has to be factored in.  Obviously if the water is too deep the dopler log cannot get a reading from the sea bed.  Ocean going ships now have both the log and Satnav, Satnav is always more accurate.

The original log was just that, a log thrown over the side and then timed to see how long it toook to travel past the length of the ship thus giving the speed of the ship throught the water (not over the ground)