Everyone's Favorite Word "Pitot" (Speedometer Question)

[bds85466]

So my speedometer will barely register anything. I posted earlier in someone elses thread regarding the physics of the gauge but got no answer. The speedo gauge relies completely on a pressure drop (technically, it's a relabeled pressure gauge?...therefore this post shouldn't be in the "Electronics" section) so there's no electronic signal it needs to function besides a backlight for visibility? If so, it would also need a static pressure inlet to establish the baseline and difference in pressure, right? My question is, is the pitot supposed to be open on the end, and where then is the static inlet? I found what could be everyone's favorite cleaning tool (a nail?) in the end of my pitot. Haha. Can someone guide me through some of this? This could be causing a problem right? I've got pictures.

 

[JAL51974]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

I think you have the physics of the speedo correct, it is measuring pressure. I have to look at mine again but I think it has a small hole on the underside that is on the plastic stem but the hole faces the stern of the boat. The place you show with the metal object is normally there, again I will check mine later this evening and let you know. If there is any kind of kink in that rubber hose going to the gauge on the dash, or if it is cut anywhere or even just pinched it won;t register pressure correctly.

 

[bds85466]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

I guess it makes sense that the inlet should be facing the direction of the flux across it, not on the back side. I'll have to look at it again. I was just looking for a hole on the thing and there happened to be a metal thing that looked like the head of the nail. Putting two and 1 together I got 4 I guess. If someone could let me know the common locations of the inlets, I would appreciate it.

 

[bruceb58]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

If so, it would also need a static pressure inlet to establish the baseline and difference in pressure, right?

You are thinking of an aviation type measuring sytem where the pressure difference is smaller. That is why you need a static port. You don't need that on a boat type system.

The opening is on the front.

 

[bds85466]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

So like on the opposite side of where that metal peice is? Where can I blow into or try to clean out without damaging anything?



ps...I like your location. Not to start a poilitcal conversation, but it sadly looks more and more like communism every day! I still love enjoying the fruits of my labor.

 

[Silvertip]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

Pivot the pitot down (operating position). There is a tiny hole at the leading edge that when the boat is at speed, water enters that hole, compresses the air in the tube leading to the gauge and a Bourdon tube tries to unwind under pressure which makes the needle move. You looked all over the pitot except in the front or "business end". Remove the tube at the gauge and blow backwards through the tube and the pitot to clear it. Otherwise a needle or tiny drill bit will work if something really stubborn is lodged there. Do not use air pressure from the back forward or you will damage the gauge.

 

[bds85466]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

Thank you for the advice. Kindof like a mullet: "business in front, party in the back".

When you say stick something in there, do you mean on the rear side of the boat (pitot opening) or near the gauge?

And one more question. In my 2nd picture, any idea what that hole in the middle of the pitot bracket is for?

 

[External Combustion]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

Silvertip is dead nuts on. A boat speedometer that uses the pitot system is the exact same as the aircraft type in that it is simply a measure of the water pressure as you move through it. The aircraft type has a static port that is lead to the outside of the aircraft to make sure cabin pressure does not affect the instrument. The boat type has it's static port in the case.

The front side of the pickup is where as Sivertip said "the business end" is located. Clean it out, but don't enlarge it as you will increase the psi that the guage senses and increase the speed reported as compared to the actual speed.

If your speedo reports low, see if you can get some instrument oil and lubricate the gear train. Check with your local watch maker and he will gladly sell you a bit of oil. He might even do standard maintenance on you instruments for under a sawbuck. The last time that I had to do this my watchmaker friend required that I brought him some bass for dinner.

 

[bds85466]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

I'm going out to look at it now. I'll let you know what I find.

My speedo was going between 0-20 (we were probably maxing around 45mph) and weakly shaking...seems to me there probably is something stuck in either the tube or the gauge is sticky.

 

[bds85466]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

Well, I didn't see anything stuck in the thing. I disconnected the hose and blew through the pitot and that is now for sure resistance free. I took the hose off the gauge and blew threw the hose. I thought I was blowing all the way through, but you know how it feels when you're blowing on a really long tube. I couldn't quite tell if my breath was making it all the way through because the other end is so far away. Without another person to listen it'd be tough to tell. Once I get it out on the water I guess I'll know if it did anything. I do have a can of compressed air, that might work well? Otherwise could I test the pitot by blowing compressed air several inches away from it? Obviously direct pressure could blow the gauge(?).

 

[ziemann]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

bds85466- I responded to your original question in Don Dickinson's thread- sorry didn't notice you had started this thread until I had responded to you in the other one....

It looks like you have your answer...

 

[rgag081499]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

NEED HELP!!!! Im having the same issue but I dont have a pitot, but I have a hole on my lower unit that faces towards the stern of the boat is this where i hook the tubing up too? Thanks

 

[seamegr]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

Ah Grasshopper (bds85466), in the world of Smartcraft the pitot tube from the stern drive connects to a transducer (100psi for up to 80mph or 200psi for above 80mph) to get the signal to drive the electronic speedo. So it is in the right forum.

rgag081499, if you have a Mercruiser the pitot pickup maybe in the drive like my Bravo 1. Don't know about other models of drive. Makes sense tho.

Greg

 

[pete58]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

Hi everyone
This is a great forum and I am glad I found it.
I am pretty new to boating etc as I haven't had my boat for long and still try to learn about it.
One of my problems is the same kind of pitot speedometer. I replaced the existing Suzuki one in the boat and reconnected it to the little tube. When I followed the tube back from the dash I found that it just ends somewhere inside the boat bunched up with other electrical cables.
Now I am trying to find out if this is correct or is the tube supposed to be connected to a transducer or simply hang down the stern of the boat to pick up the difference in pressure when the boat moves.
Can someone please give me an answer in simple terms?
Thanks

 

[Silvertip]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

If the tube is not connected to anything at the back of the boat it cannot possibly send any meaningful pressure to the gauge. There are only two connection possibilities at the back of the boat. 1) a plastic pitot(a pickup) that screws to the hull and hangs in the water. These are usually white and sell for about 10 bucks or less at nearly any store that sells boating supplies. It is a universal device and can be used with any pitot style speedometer. 2) Your engine may have the pitot built into the lower unit. In that case there is usually a small tube that exits the engine somewhere around the mid section. Connect the tube to that tube. There is a third style of speedometer but it is electronic and uses a paddle wheel type of transducer much like those used with fish finders.

 

[a70eliminator]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

Here's waht I did with my pitot, I didn't have any watch oil or anything and didn't know i could lube the gauge, but I took my air compressor and experimented with blowing air at the leading edge of the pitot and yes it got the speedo moving, I then took the tube off the pitot and just shoved the tube onto the tip of my blowgun it took about 80PSIG to get the speedo gauge to peg, I pegged it a couple times slow and steady just to exercise the mechanics in the MPH ranges never before reached, don't know if helped or hurt anything but it seems to work fairly accurate.

PS: bds, I find myself trying to enlarge your avatar to get better look at that engine!

 

[luv2b0at]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

LOL!!! I did the same thing!! Nice motor!! Maybe she could help clean you pitot?

 

[Saskatoon2005]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

How do you pronounce "pitot"? Is it peeto or pit-tot

 

[luv2b0at]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

pee-dot
That's what we called them on the helicopters. They were the air speed indicatorsa and they heated up to prevent ice build up. Got damn hot too...

 

[Docknocker]

Re: Everyone's Favorite Word "Pitot" (Speedometer Question)

Still waiting for the close-up of the '72 Johnson motor stand...