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Author Topic:   Mercury Pitot Tube
derf posted 11-15-2005 03:50 PM ET (US)   Profile for derf   Send Email to derf  
Please see pic from my 1995 Mariner 115.
What in the world is this?

I think it might be for a speedometer. Whatever it is, can I plug it and/or just pull the tube off and point the discharge down?

fishgutz posted 11-15-2005 04:05 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
Its the tube for a speedometer. There is a very small hole that feeds it on the front of your gear case. The thing is called a pidot. Pronounced [Deleted non-standard pronouciation]. It should be plugged if not used, but I don't think its really important.
kingfish posted 11-15-2005 04:09 PM ET (US)     Profile for kingfish  Send Email to kingfish     
That would be "pitot", actually.
LHG posted 11-15-2005 04:22 PM ET (US)     Profile for LHG    
I've owned Mercs for some time now, including a pair of 115's, and I have never seen a cobbed up installation like that from the factory. Somebody did that on their own.

In the factory configuration, the tubing is inside the gear case and exits vertiically near the shift shaft, where the external tube for the speedo is then hooked on. My guess is that it was somehow damaged, and they rigged this up to avoid tearing down the gearcase to fix it.

Has that 115 been aftermarket extended to 25"? That could be another reason

derf posted 11-15-2005 04:28 PM ET (US)     Profile for derf  Send Email to derf     
I dunno about aftermarket extensions. I will look and see if anything looks "extended". Will plug the pitot.
Boat doesn't have a speedo, so I guess this motor was on another boat at some time.
bsmotril posted 11-15-2005 08:29 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
On a late nineties Merc, that tube was tucked back into the hole from which it emerges from the gearcase. There was a black nylon cable tie that secured the two pieces of tubing together right above the hole. That is the way they came right from the factory if not rigged to a speedo.
jimh posted 11-15-2005 10:55 PM ET (US)     Profile for jimh  Send Email to jimh     
[Changed TOPIC; was "WITW Is This? Mysterious(To Me) Outboard Part." The use of unusual abbreviations was a mystery to me and many others.]

For guidance on pronounciation, see:

Tom W Clark posted 11-16-2005 09:16 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The pitot tube in the gear case of a perfect example of a great idea that didn't work. It was actually an OMC innovation that emerged around 1980. I remember when it came out I thought ", so simple so clean. No more broken plastic pitot tubes and extra holes in the transom."

The reality quickly became apparent with my first motor so equipped, a 1982 Johnson 90 hp. The very first chunk of driftwood hit rendered the pitot inoperative. It took a lot of work to dig out those little wood fibers and return the speedometer to service.

No sooner would I do so that it would clog again. This pattern repeated itself over and over until I realized it was just the way it was going to be. Discussing this with my dealer he advised me to "just install a plastic pitot tube, they won't clog."

OMC stopped using the built in pitot after just a few years. I was surprised when I bought my Revenge 25 this spring with its 1989 Mercury 150s. They both had built in pitot in the gearcases, and wood chunks clogging them.

The easiest way to plug the hose is to bend it double and cinch it tight with a zip-tie. You do not want it left open or it may squirt water up at your motor when running (if it isn't already clogged ;-)

LHG posted 11-16-2005 02:43 PM ET (US)     Profile for LHG    
Tom - I have used this detail for years,(since 1984) with only rare occasions of blocking, which seem to clear themselves. I guess it depends on boating water. The Great Lakes are almost entirely debris free.

What many people don't know, is that if not using a Merc speedo, when buying aftermarket, such as Teleflex, a specially calibrated model is necessary. This is because the gearcase pickup is more efficient than the plastic pitot pickup, and transmits a higher air pressure to the instrument. Otherwise, the speedo will read considerably higher than actual speed

Tom W Clark posted 11-16-2005 08:07 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

You still use a pitot tube speedometer? They have these things now called GPS...

LHG posted 11-16-2005 08:17 PM ET (US)     Profile for LHG    
I am old fashioned, and just love the look of the analog gauges. Actually, each boat has three sources of speed, the speedo, sonar/gps paddle wheel, and GPS.

Actually, Tom, look at the latest Mercury Smartcraft BW is rigging on the Whalers, and you will see two or three big analog round gauges, both tach(s) and speedo. Then all the other Smartcraft functions appear digitally on these gauges. JimH just told us that Evinrude is about to introduce the same thing, probably subcontracted to Teleflex. The analog speedo gauge is alive and well.

PeteB88 posted 11-17-2005 12:58 AM ET (US)     Profile for PeteB88  Send Email to PeteB88     
So how accurate are these pitot tubes? I have one on my Yam 40 1992 that I hooked up to the old speedo. It works most of the time but not sure of accuracy and I do not have a GPS on board yet.


sosmerc posted 11-17-2005 01:10 AM ET (US)     Profile for sosmerc  Send Email to sosmerc     
Standard pitot tube speedos normally read "happy"....meaning, they generally read on the fast side. GPS is pretty darn accurate...maybe within 1/2 mph.
The paddle wheel speedo is very accurate at non-planing speeds. It's pretty neat to have all 3 setups for comparison.
LHG posted 11-17-2005 01:45 AM ET (US)     Profile for LHG    
Doesn't the new Mercury Smartcraft Speedometer gauge, and essential part of the Smartcraft package, pick up it's speed reading from the engine gearcase pitot hole?

How is the new Evinrude Tach/Speedo pair of Smartcraft gauges going to pick up it's reading?

I have found a need for all three speed sources. At low speeds, the calibrated paddle wheel is the most accurate over the water speed, and not affected by current. But above 50 or so, she floats and becomes worthless. At medium and higher speeds, both the pitot and GPS seem to be more accurate. The shortcoming of the GPS is that is can't accomodate currents, tidal flow, etc. Here the pitot and paddle wheel excel. They say DGPS is more accurate for speed than plain GPS.

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