FourPoints, I believe your statement is misleading: you do not need to substract or add any current velocity if looking at GPS speed !
absolutly not true, in the context of determining if the boat is going fast or slow. If someone complains that they are only going 4kts according to the GPS, but they are heading into 1.75kts of current, they are actually sailing the boat at 5.75kts as far as anything relating to sail trim or engine RPM is concerned.
ALL discussions of speed (when discussing through the water speed, not race tatics or course strategy) must only consider "boat speed", aka through the water speed. You don't measure your speed performance under power off GPS, it's measured off the paddle wheel, because hull speed and similar calculations ONLY care about the speed the boat is moving relative to the water. At no point does the speed of the water factor into that discussion.
Since GPS calculates your SOG (speed over the ground) it does not care whether there is current or wind helping or slowing the boat !
My point exactly! the OP complained that his GPS was reporting him to be going slower than he expected / wanted. Whenever you are determining the boat performance under sail or power, you don't use GPS speed because it doesn't factor in wind or current (you most certainly should use GPS speeds for any navigation purpose because that's what the boat is doing relative to the chart).
And it is dead accurate !
While at the same time being dead irrlevant without also knowing what the current is doing under the hull.
Boat speed as obtained through any type of in-the-water instrument however (i.e. paddle wheel type or other) has to have current velocity pushing or holding the boat added or substracted only if you want to know distance you will cover. So if you have both type of instruments on board and you're reading 8 knots on the GPS and 6 knots on the speed/log instrument, it means your engine or sails are pushing the boat at 6 knots through the water but with the help of the current you'd really be covering 8 miles in that hour !
If you were against that same current, your in-the-water boat speed would still show 6 knots, but your GPS would only show 4.
EXACTLY! if you take the 2nd example you used, and then the skipper complains they are going too slow because they are only seeing 4kts on the GPS, it doesn't mean they are sailing the boat any better or worse, or the engine is pushing the boat any different, the boat is still moving at 6kts in anything that matters when discussing performance, all the while the GPS only shows 4kts.
If the OP was asking about navigation advice then we would be advising him to use the GPS reported speed not the paddle wheel, because the paddle wheel speed doesn't take into account current to show you true speed over ground.
YVRguy, if you're looking at your GPS take a look at the chart to find out whether your have any current for or against and of course check the situation of the tide where you are as both the ebb or flow will impact your GPS boat speed. FWIW, my 34 reaches 6.5 on both the GPS and the speed/log instruments when in perfectly calm and still waters at 2650 RPM (Yanmar 27, 3-blade 15 X 11 prop). However, GPS speeds drops down to 4 when traveling up the 2.5 knot current of the St. Lawrence river.
after argueing against my point, you make the same argument as me? I'm begining to think you actually do understand this, you just didn't read my post very clear before responding.
