Old Aqua Meter Speedometer, what is missing?

[ohmydog]

Hi folks, I bought a neat old 1958 fiberglass runabout that is in great shape. It has an Aqua Meter Speedo on it that is not working. It looks like most of it is there except the part that goes in the water. I'm not exactly sure how it all works and what is missing, and if there is any reason to trying to find the replacement for any missing/busted parts. Hopefully some of you have some input/ideas that could help. I'll post photos. Thanks in advance for any replies.

 

[UncleWillie]

The Speedometer is actually a Pressure gauge at heart.
The dynamic pressure of the moving water against the Pitot Tube (that you are missing) on the transom, is measure and displayed as the boat speed.
The pressure is exponential. The middle of the dial will be at ~70% of the top speed, and half speed will be at the 25% point.

 

[ohmydog]

Thanks Unclewillie. I see that with most kits, the tube from the pitot gets attached straight to the speedo. Any idea why the setup on my boat is so different? It appears that the pitot (or what's left of it) is attached to a metal tube through one of the drain holes. That tube is then attached to another tube like device on the back side of the transom. Then it's attached to the rubber hose that flows to the speedo on the dash. I'm not sure what all that stuff is.

Perhaps I should get a new pitot and some tubing and bypass that other stuff?

 

[UncleWillie]

Sound like a plan!
You can test the speedometer by blowing into the tube with lung pressure. NOT compressed Air!
A set of young healthy lungs should be able to get the speedometer to indicate about 10 mph. (~2psi)
Don't go replacing everything else until you confirm that the Speedometer is working.

It is common to find the hose plug up with mud and/or insect nests.
Detaching both ends should allow a free air stream to flow.
When attached to the Speedometer, nothing will flow, but the pressure should move the needle.
----------------------------------------------------------------------------------------------------------------------
For your Reference...
1.5 psi = 10 mph
2 psi = 12 mph <<< Typical pressure produced by a set of young health lungs.
6 psi = 20 mph
13 psi = 30 mph
23 psi = 40 mph
36 psi = 50 mph
52 psi = 60 mph

Notice that the scale is exponential; twice the speed gives 4 times the pressure.

 

[ohmydog]

No Title

Ok. Blew on the end of the broken pitot. It was leaking right from the threads of where it screws into the connector that goes through the transom. It's a different set up than I can find on any speedo/pitot kit on the internet. The pitot is attached to a box that has a cavity. The box is then screwed onto a coupler that runs through the hole in the transom. It's then connected to some brass tubing and into a long cylindrical thing (I'll attach photos). Anyway, off the top of the long cylindrical thing is where the rubber tube is connected that goes to the speedo.

I removed the broken pitot and box to expose the tube coupler. I rigged up some tubing to the coupler and blew on that and got nothing. To bypass that apparatus, I cut the rubber tubing that goes direct to the speedo from just after that cylinder thing and did a blow test. I was able to get the speedo to work then.

It seems like I should just get a new pitot and some new tubing and bypass that cylinder? Thanks for all the info.

 

[Bondo]

Ok. Blew on the end of the broken pitot. It was leaking right from the threads of where it screws into the connector that goes through the transom. It's a different set up than I can find on any speedo/pitot kit on the internet. The pitot is attached to a box that has a cavity. The box is then screwed onto a coupler that runs through the hole in the transom. It's then connected to some brass tubing and into a long cylindrical thing (I'll attach photos). Anyway, off the top of the long cylindrical thing is where the rubber tube is connected that goes to the speedo.

I removed the broken pitot and box to expose the tube coupler. I rigged up some tubing to the coupler and blew on that and got nothing. To bypass that apparatus, I cut the rubber tubing that goes direct to the speedo from just after that cylinder thing and did a blow test. I was able to get the speedo to work then.

It seems like I should just get a new pitot and some new tubing and bypass that cylinder? Thanks for all the info.

Ayuh,.... You should be able to get rid of what you have at the transom, 'n replace it with a more current plastic type pitot tube affair,...

 

[ohmydog]

Yep, that's the plan moving forward. I'll update after I get the parts and install. Unless I have some questions about the install. Seems pretty simple now however. Thanks.