Outdrive1

This is brutal, I think he was asking since he was running on top of a 4mph current how fast is his boat really moving? We'll on the earths surface it's still moving 85 mph but if you subtracted the current out of that I guess your boat is only moving 81mph on top of the water. If you turned around and ran against the current did your speed change?

Stormrider

85mph +/-10%

Wobble

I think he got the response he was looking for

Edward R. Cozzi

If he was measuring the speed with a speed wheel or air pressure speedometer, then the current is a factor in this situation. A GPS is not influenced by those conditions. The vessel might go slightly faster WITH the current, but the GPS will read faster.

Emaginashun

NO SIMPLE ANSWER - the GPS is always accurate, but the question can be open to interpretation depending on the view.

Currents, tides and wind are factors, but the GPS Does Not measure the tide, currents or wind so there is no difference between water, land, direction, etc.

Using a Kayak for example.
So, let's say you are paddling at 3 1/2 knots (on GPS) and there is a current of 1 knot in the direction you are paddling (ex 1) and returning, the current is directly against you (ex 2).

In the first situation, your "Paddling Speed" is
2 1/2 knots
and the second example your "Paddling Speed" is
4 1/2 knots.

The GPS would give you the correct speed, but not your correct paddling "Stroke Speed". And yes, there is a difference.

If you had 4 miles to paddle and you figured your speed at 4 1/2 knots but the tide is now against you, the calculated "Arrival Time" based on the GPS would be inaccurate.

You would have calculated about 50 minutes and now it might be 1 hour and 15 minutes due to the current.

We must be aware of all factors. The speed on the GPS is accurate, but your "Measured Speed" is only averaged.

FLYWITHME

I actually talked to Werner Von Braun...beleive it or not...the rocket scientist... about your question...before he passed away...since, as we all know, he invented the GPS...and what he told me..was very interesting.

He said...that if the GPS read 85 MPH...then that's exactly the speed you were moving at...relative to everything else around you. However....the actual speed...or shall we say...performance that your boat is actually doing...is directly related to...not only the speed of the current...but also the wind speed. And that...will depend on which way the wind was blowing...the angle of the wind...in relationship to the centerline of your boat....and the direction of the current...and it's angle of direction...to the centerline of the boat.

Simply put...it means that the speed and direction of the current and the wind will either cause your boat to go slower or faster. If you are going with the current...and the wind is behind you...your boat will go faster. The reverse of this is obvious...your boat will go slower. Also...the relative humidity will also affect you speed...since wet humid air is thicker than dry air...so..in humid air..you go slower...and in dry air..you go faster.

Also...if you want to know the best performance of your boat...it has to be tested at sea level...since engines are less efficient...and produce less horsepower...the higher up you go above sea level.

All the information I've given you...is the same for airplanes. It's the only way a pilot can determine his actual speed...at any one time...using...what is called...dead recogning.

Except for saying I talked to Von Braun about your question...and him inventing the GPS...everything else I've said is factual....but I did actually meet him in person...at Cape Canaveral....a long time ago. Allways like to get some humor in there. It's nice that we can have some fun at OSO.

OldSchool

I think that 85 is an obtainable speed for Nort's Formula.......as long as there is a strong wind!!!

blume

That was Mike. DA

Edward R. Cozzi

Just before he passed away, I posed that very question to Dr, Carl Sagan, who attended many Bar-B-Ques at my home. Carl said that due to the recessive interaction of the "billions and billions" of stars in the galaxy, each having its own effect on the Earth's gravitation pull, that the modern method of reading the velocity of an offshore boat in a 4 knot current is directly proportionate to the amount of squeeze on the throttles by the driver.
Now, Latham and Kiekhaefer controls, while they share a similar specific gravity, factor in differently than the Gaffrig/Livorsi units. Another variable is how the trim switch in the controls is wired. If the hot wire going to the switch is closer to the palm of the driver, then the differential is going to be significantly higher due to the magnetic field that we all know surrounds any electrical device, i.e. how your compass, (magnetic), is affected by turning on the cabin lights switch in a 35' Fountain Lightning while the vessel is underway.

Anyhow, after factoring in all the above data, Carl agrees that it is very likely this boat is travelling at 85 MPH, (or pretty close).

enticer

Troutly
You are way off....... it is 85