wind speed / boat speed, using wind meter

Hello. A physics question:
Using the hand-held floating ball Dwyer wind meter.

If you use the dwyer wind meter on a moving boat (pointed in the direction of movement), will it show the speed of the boat?

or is there a mix of wind and boat speed and it will not indicate true boat speed?

or does it show the speed of apparent wind?

and would it mater which point of sail is underway? as I can imagine straight downwind would be accurately measurable with the hand held meter.

but it seems other points of sail would not be measurable with the meter, do to a mix of wind and boat speed.

any help? Scott

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It will show you the speed of the apparant wind only which, by definition, is the vector sum of the boatspeed plus the true windspeed. The accuracy of the reading will only be dependent on the accuracy of the meter, has nothing to do with the boat's point of sail.

Think about if you did the same experiment using a car. Drive the car at 30mph in zero wind and your meter will read 30mph airspeed. Then a 10mph gust of wind hits from straight ahead and your meter jumps up to 40mph, but your car is still only going 30mph. Then the wind switches 180 degrees and a 10mph gust hits from behind, the meter drops to 20mph and still your car is going the same 30mph.

The meter can only detect the airspeed that flows past it, just like the sails on your boat.

You can not directly determine boatspeed from the airspeed of the apparant wind unless you also know the true windspeed and direction and either the boat's direction or the aparant wind direction (you need to know two sides of the velocity triangle to calculate the third).

sm

I re-read the first part of your post where you said the meter is pointed in the direction of the boat's movement.

If the meter is intended to be pointed directly into the wind (some meters just require the meter to be held into the airflow) and you don't do so (i.e. the wind is comming from 45 deg off the bow) then I think you'll just end up with an inaccurate reading that has no meaning. For example, if you were sailing along on a broad reach so the apparant wind was comming at 90 degrees, but your directional windspeed meter was pointed straight ahead, it would get a reading of 0 which would be an incorrect measurement of both the boatspeed and the apparant windspeed.

Point being that a windmeter is only going to be able to accurately tell you the apparant windspeed on your boat.

sm

GPS works great.

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Bodhisatfa

Yea, so do stop watches, radar guns, and mechanical speedometers, but that wasn't the question....

sm

Guess you got your answer, wind meter won't work, as for your suggestions, they cost to much and or require more than one person, gps is cheapest easiest and you can do it solo. Have fun.

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Bodhisatfa

You need a compass.

While your boat is stopped use your wind meter to measure the true wind velocity.
Use your compass to measure the direction of the true wind.

Next take the apparent wind velocity and direction while you are sailing.

Find the angle between the true wind and the apparent wind.

Take your apparent wind velocity and multiply it by the sine of the angle to get some number (call it Y).

Take your apparent wind velocity and multiply it by the cosine of the angle to get some number, then subract the true wind velocity from that number to get another number (call it X).

Add X to Y and then take the square root of that number. The result is you boat speed.

I think it will be hart to get good measurements but it should be a fun project.

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http://hobiebob.blogspot.com/

Hey Bob,I think there is something wrong with those calculations.If I understand correctly you are breaking the vector into it's vertical and horizontal components.You would then need to square X and Y add them up and then take the squre root(pythagorean theorem).Maybe I am wrong a diagram would be usefull.

Yes of course X and Y need to be squared before adding and taking the square root.
Thanks for the correction.

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http://hobiebob.blogspot.com/