Speed vs. PSI
Nathan Zimmerman
Does anybody know the relationship between speed of the boat and
pressure (PSI) at the speedometer ?
Thanks,
Nathan Zimmerman <p...@westol.com>
David Burton
45 psi is about 50 mph at my elevation. Note the speedo
indicates the difference in pressure between the local
pressure and the pitot tube pressure. Since water density is
about the same at practical altitudes but air pressure is
not you will have different indications on the speedo at
different altitudes with the same boat speed. The speedo
adjust knob compensates if you can get a reference speed
somehow.
It seems an altitude vs. pitot tube pressure vs. boat speed
chart could be imagined. Who has a brain big enough to
figure that out?
DB
KENGIBBONS
Nathan Zimmerman <p...@westol.com> wrote:
>Does anybody know the relationship between speed of the boat and
>pressure (PSI) at the speedometer ?
I also once wondered this, and Airguide faxed me the table of speeds vs
psi. Armed with this information, I made some special adapters which
allowed me to hook up my compressed air tank and a precision pressure
guage to the speedo. In the garage, I perfectly calibrated my speedos.
Later, I took the boat to the course to validate my "scientific" work. It
was way off, and why I don't know. I was just easier to dial in off the
course gates than trouble shoot my methods, so I reset the speedos to the
course gates and was much happier.
acss...@acs.eku.edu
David Krohne
These speedos read air pressure, not water pressure. If you get a leak
where the hose connects to the speedo head which allows water to get all
of the way up to the head, therefore reading water pressure, you have a
problem, The water forced into the pitot tube compresses the air in the
lines and balance tube, which is what the speedo reads. It is nothing
more than an air pressure guage with a movement that translates the non
linear speed vs pressure relationship into a fairly linear speed dial.
This is what makes the airguide ski speedos much better than standard
boat speedos. (That and the ability to calibrate.) Unfortunatly
Airguide doesn't make as good a product as they did several years ago,
although they have improved somewhat in the past three or four years. A
good pair of speedos that are matched and accurate at both low and high
speeds are very hard to find. By the way, to be as accurate as possibe,
the speedo hoses should have a bleed valve at the head end to bleed of
the pressure caused by the speedo pickups being a few inches below the
surface when the boat is off plane. (14.7 lbs per 33 ft of depth) In
reality no one puts bleed valves on boats anymore since the effect is
only noticeable at very slow speeds.
Dave