Thanks for all the replies! The "home made pitot tube" got me
looking. It seems that you can use any cheap pitot tube, or that open
pipe end facing forward, and then you attach that to an open ended
clear tube pointing up, with the height/scale marker's 'zero' at the
water line. The faster you're going, the higher the water will rise
in the tube. The design of the pitot makes no difference.
At www.EngineeringBoolBox.com there's graphs and a table. The height
the water rises to is essentially proportional to the square of the
speed. At 10 miles/hour the water will rise just over a meter; at 5,
1/4 meter, etc. At 20 water will spout out the top of a 4 meter tube,
which is why they make the speedometers.
(I think I want to bring the tube over the transom and inside so I
don't have to lean out the back of the boat and fall overboard to get
a reading!)
I'll make a mount for this, figure out where the markings go on the
stick, and take the boat out again to get some speed readings when
it's ready!
---
I also liked the "Knot Stick". GPS ("General Purpose Speedometer"?)
would be good as long as you're going far enough to take good
readings and the water isn't moving. I doubt its efficacy for quick
speed tests in a limited area. First GPS I saw was on the weathership
Vancouver in 1976. It took up about 16" of a 6' tall 19" equipment
rack, evidently reduced from the previous one that took up the entire
rack!)
Craig
=====
>Hi
>
>There was a home made pitot in wooden boat magazine a while ago.
>
>Basically a plastic fish tank tube with plastic straw ends in the water.
>One end cut at a diagonal to face the water flow. The other straight.
>The "display" consists of an inverted loop, in the tubing, containing
>coloured water. Once it is calibrated at two speeds you can make your own
>numeric scale.
>
>Index and back issues on their web site.
>
>Regards
>
>Captain Kerry Thomas
> >kerryjthomas@
>
>-----Original Message-----
>From:
>electricboats@yahoogroups.com
>[mailto:electricboats@yahoogroups.com]
>On Behalf Of Craig Carmichael
>Sent: Wednesday, 20 February 2013 9:17 a.m.
>To: electricboats@yahoogroups.com
>Subject: [Electric Boats] Measuring boat water speed for low speed & cost
>
>In my e-outboard tests I couldn't measure the speed of the boat. I bought a
>pitot tube, but the speedometer I found for it goes from
>0-60 MPH and the first marking is at 10 MPH. The next one is at 15.
>(From 40 to 60 each MPH is widely spaced - exactly the opposite of what's
>needed.)
>
>Blowing into the pitot tube, it takes all the pressure I can make to move
>the needle just a little.
>
>There must be some more sensitive transducer that can be used with a pitot
>tube? Or is there some other way to measure speed that doesn't cost a lot?
>
>Cheers,
>Craig
>
>=====
>
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