[Rhodes22-list] Toys for Rhodes

[Roger Pihlaja]

Jeff,

You are asking questions about a very complex subject.  I will try to give
simple & short answers.  The type of sensors you are refering to are called
"pitot tubes" & there are versions intended for marine use.  Pitot tubes
operate by measuring the differential pressure between an orifice which is
pointing into the flow stream and another orifice which is at the same
vertical height and exposed to the same fluid.  The velocity as measured by
this type of sensor is proportional to the square root of the differential
pressure.  As such, pitot tube sensors are very inaccurate at the low
velocities usually encounted on sailboats.  These types of speed sensors are
very common on planing speedboats and of course on aircraft because of the
greater velocities.  The only common sensor, (Meaning sufficiently
inexpensive for most of us to afford to buy!)  which is sufficiently
accurate at the low velocities encountered on sailboats, is the paddlewheel
turbine.  The paddlewheel turbine type sensor needs to be installed near the
front of the boat, far enough back from the bow to not be affected by the
bow surging up and down, not so far back that the laminar flow has separated
into turbulence, and sufficiently deep that the transducer is always
submerged on either tack.  The prefered location on a Rhodes 22 is on the
port side just in front of the shoal draft keel.

The depth transducer has two possible locations on a Rhodes 22.  One is on
the starboard side, approximately amidships as close as possible to the
shoal draft keel.  This places the transducer basically in front of the ice
box underneath the cabin sole.  Usually, a screw-in inspection plate is
installed in the cabin sole above the transducer to facilitate installation
and maintenence.  The other location is in the lazarette compartment as far
forward as possible and several inches to starboard of the centerline
cockpit drain.  However, in the lazarette compartment, the backside of the
transducer and the cable must be protected against impact and chafing from
all the heavy gear normally carried back there.

If you want any more details than that, I suggest you talk to Stan at GBI.

Roger Pihlaja
S/V Dynamic Equilibrium

----- Original Message -----
From: <JSusanj at aol.com>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Tuesday, May 18, 2004 11:57 AM
Subject: Re: [Rhodes22-list] Toys for Rhodes


> How much does the hull shape affect the measurments?  I know that flight
test aircraft put sensors out on a boom to get readings in air that is not
disturbed by flowing around the fusilage.  Since it would be difficult to
stick something out front, it would seem better to trail something behind
rather than attaching it to or through the hull.
>
>
> Jeff S.
>
>
> In a message dated 5/17/2004 9:07:07 PM Eastern Daylight Time,
cen09402 at centurytel.net writes:
>
> >
> > Jeff,
> >
> > There are transom-mounted transducers for the knotmeter/log/water
> > temperature and also some installations of the depth sounder transducer
> > wherein the sensor is bonded to the inside of the hull with epoxy.
Without
> > going into the technicalities of these installations, neither type works
> > particularly well.  To the best of my knowledge on the subject, both
types
> > of transducers will give the best accuracy & sensitivity (greatest
measured
> > depth) if installed in properly located and oriented thru-hulls.  A
> > below-the-waterline thru hull is no big deal.
> >
> > Roger Pihlaja
> > S/V Dynamic Equilibrium
> >
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