Hi all
On Friday we went for a sail down the river in the 349 and on the way beating back up in 15 to 20 knots of breeze from approx 1.30 pm to 3.30 pm the log was reading roughly 1 to 2 knots faster than the speed over ground reading (most of the time the log was reading 6 to low 7 and SOG at the same time was just over 5)
The tide on Friday was a low at 11.29 am of 0.42 m followed by a high at 6.04 pm of 1.15 m so the tide was on its way in
Also looking at the river plot there was about 1000 megalitres a day flowing down the river upstream of where we were so we were going against that
The section of river we were sailing in was approx 1.3 nm wide with average depth of approx 60 to 80 feet and we were able to sail the 9 nm length of the river without tacking
Any thoughts would be appreciated
Regards Don
Hey Don,
Maybe I'm misinterpreting what you wrote, but your boat speed has to be higher running against the tide than your speed over ground.
if you're doing 20knots into a 10knot current, your speed over ground will be 10 knots.
Is this what you had, or am I reading you wrong?
Hi Shaggy
In theory we were going with the tide flow (we started at the mouth of the Huon and were sailing towards upstream towards Port Huon) and the tide times i gave were for Port Huon, but against the flow of the river so if you take the river flow out of the equation the tide should have been helping out SOG
I guess my question might be does the 1000 megalitres of water per day flowing down the river counteract the incoming tide
Regards Don
Ahah! I thought I had Mr Donk backwards! Sorry mate , I shall watch with interest to those smarter than I to assist.
Q=va
1000ML x 1000 / width (m) x depth (m) = v (m/s )
If you work it out the flow velocity induced by the 1000 megalities will be approximately sfa. Hope that helps
Sounds like if you anchored in the middle of the river your log would say 1 to 2 knots boat speed although you sog would be zero.
Select to expand quoteTrek said..
Sounds like if you anchored in the middle of the river your log would say 1 to 2 knots boat speed although you sog would be zero.
Hi Trek
That is pretty much correct if the log is reading correctly
Regards Don
The fresh water runs out over the top of the salt water in some rivers, saltwater is more dense than fresh, the tide pushes in from underneath.
Donk, has the log ever been accurate, and has it ever been calibrated? I currently have my wind speed showing a much higher reading than it should - say 15 kts when it would be lucky to be 15 kph. It can be calibrated, but that will involve standing-on next to a 'trusted' anemometer to compare readings. In my case, on southern Port Phillip bay, I could use the South Channel Fort and wait for the half-hourly update from BOM. Logs and wind instruments are tiny mechanical devices that use rotation speed to 'estimate' a speed reading, and so have inherently only modest reliability.
Lets run the numbers. A river 1300m wide by 10 m depth has a cross section of 13,000 sq m. A magalitre is a million litres, or 1000 cubic metres. So if the river flows one m you have moved 13 Megalitres (Ml). 1000Ml would therefore span about 77m of river length. And its got all day to flow that 77m, so the runoff flow velocity is negligible.
Select to expand quoteriverider said..
The fresh water runs out over the top of the salt water in some rivers, saltwater is more dense than fresh, the tide pushes in from underneath.
Quite true. In our local system there can be a flood with the water moving down the river at 4 or 5 knots but the incoming tide still moves up under it.
Shags...there is no one smarter than you...you bought the Pogo...remember?.
Yara...or should I say Professor Yara. Gotta love your analysis. Wish I could understand it.
