Seastoke
Well-Known Member
Ok out today doing 2000 revs speed across ground 13 knots what should my log say ,cheers roy
Speed from sat nav against log
- Thread starter Seastoke
- Start date
dunmor
Well-Known Member
which way was the currant going??
lisilou
Well-Known Member
Depends on tide direction and speed.
L
(knew this DS theory would come in handy hehe)
L
(knew this DS theory would come in handy hehe)
superheat6k
Well-Known Member
Date, tide times, port and time of departure, arrival, weather info ...
Seriously you need to see what the tide us doing. Stop the boat in the water and in open water and see what direction and speed the boat travels at on the GPS. Then go to speed desired, but at approx 90 degrees to the GPS static in the water tide direction. Although there will be a small vector error, the log and GPS should more or less agree.
benjenbav
Well-Known Member
Seastoke
Well-Known Member
Thanks guys fgot to say the log aint working so am going off sat just trying to work out , so is the sat nav right in real terms as with tide or against ,so if iwas with tide and it was 13 knts and against tide it was 13 knts ,maybee more revs thats the distance covered .is true
superheat6k
Well-Known Member
GPS will give you distance / speed over the ground, that's all. Your engines are using fuel to propel the boat through the water, so if the water is going the same way you will either gain more speed / distance for the same fuel or same distance / speed for less fuel. Vice versa if you are pushing the tide.
l'escargot
Well-Known Member
The GPS gives you speed over the ground, the log gives you speed through the water.
Simply put, in zero tide they should both read the same, in 2 knots of tide against you the log should read 2 knots faster than the GPS and with 2 knots of tide behind you the log should read 2 knots slower than the GPS.
Engine revs don't come into it. The two instruments will just tell you what speed over the ground or through the water you are doing at certain revs, at a particular time, but that can vary with other factors such as wind and whether the boat has a fouled hull not...
Simply put, in zero tide they should both read the same, in 2 knots of tide against you the log should read 2 knots faster than the GPS and with 2 knots of tide behind you the log should read 2 knots slower than the GPS.
Engine revs don't come into it. The two instruments will just tell you what speed over the ground or through the water you are doing at certain revs, at a particular time, but that can vary with other factors such as wind and whether the boat has a fouled hull not...
Jim@sea
Well-Known Member
The person I bought my boat off advertised that it would do 28knots. (with a paddle wheel log) My GPS says different.(and that was on Windemere)
Searush
Well-Known Member
LOL
You mean the seller was telling you porkies & you believed him!!
Speed attained depends on the weather, water conditions, fuel, hull state, & hull loading (people, fuel, tools etc etc) prop fitted - the list goes on. Does it really matter? If so get a bigger motor (if the hull will carry it).
vlazyjack
Member
Are these calculations still valid for a planing hulled boat?
sailorman
Well-Known Member
what ever your boat is it is still moving through a moving body of water.
think areoplanes in wind
vlazyjack
Member
Thanks, I've always known this, but just wanted to see if anyone out there thinks there is something I missed, following a heated discussion at the weekend.
henryf
Well-Known Member
Imagine you drove your car along the wagons of a train. If the train is doing 30 mph and you remain stationary your speedo (the log) will indicate 0 mph but the gps would show 30 mph. Drive in the same direction as the train at 30 mph on the speedo (log) and your speed over the ground (GPS) is 60 mph.
If you turn the car around and stop the speedo (log) will read zero mph but your speed over the ground (GPS) will be 30 mph backwards. If you drive the car forwards at 30 mph on the speedo (log) it will have a relative speed over the ground (GPS) of zero.
Henry
If you turn the car around and stop the speedo (log) will read zero mph but your speed over the ground (GPS) will be 30 mph backwards. If you drive the car forwards at 30 mph on the speedo (log) it will have a relative speed over the ground (GPS) of zero.
Henry
Hurricane
Well-Known Member
Imagine you drove your car along the wagons of a train. If the train is doing 30 mph and you remain stationary your speedo (the log) will indicate 0 mph but the gps would show 30 mph. Drive in the same direction as the train at 30 mph on the speedo (log) and your speed over the ground (GPS) is 60 mph.
If you turn the car around and stop the speedo (log) will read zero mph but your speed over the ground (GPS) will be 30 mph backwards. If you drive the car forwards at 30 mph on the speedo (log) it will have a relative speed over the ground (GPS) of zero.
Henry
Ahh - Henry
I always have this argument with the kids - not 30 MPH though.
Imaging sitting in a stationary train - how fast is the beam of light coming out of the front of the engine?
Yep - correct - the speed of light
Right now move the train forwards at 30MPH
How fast is the beam of light now traveling as it leaves the engine?
Einstein recons that it is still traveling at the speed of light.
This is where I recon that Einstein is wrong
The kids recon I'm wrong and that Einstein has proved his case.
However, I still stand by my theory that the beam of light is now traveling at the speed of light + 30MPH
Sorry guys - I know this is totally different - just wanted to change this silly discussion into something sillier.
tico
Well-Known Member
Dunno... I'd have to ask the cat, but it's sitting in the fridge (i think)
superheat6k
Well-Known Member
Einstein seems to have proven this one, but for this to be true then as the distance the light travels over a given time, which in simple terms differs for the two examples given, for the principle that the speed of light is constant, regardless of the moving speed of the observer, then the other factor of the speed equation, distance over time, then it is time that must alter, relative to the speed of one observer relative to another. This is Einsteins Special Theory of Relativity, but not sure this is the right forum for this one.
I have tried to read and understand the theory, but put in the class of things basically unfathomable, like fast cars, women, labour governments, etc.
taz
Well-Known Member
Yes, but if you fit black bulbs in your lamps does it go dark when you switch them on?
LittleSister
Well-Known Member
In principle, yes. I have doubts as to whether a hull mounted log would read very accurately when the boat is planing, but as I consider anything over 5 knots as roaring along, it's not something I've ever had to worry much about.
LittleSister
Well-Known Member
If you want to get really silly, then factor in that the train doing 30mph is on the rotating surface of the Earth, itself flying through space at a couple of million mph. Where does that leave you?
