View Full Version : Pitot tube installation.

08-11-2008, 04:49 PM
I am in the process up replacing my pitot pickups on my '76. One of the tubes had a leak, so I need to replace that, too.

While removing the old pitot tube, I found that the tube went from the pickup into a fitting on something behind the gas tank. It feels like a piece of tubing around 1" diameter with a barbed fitting at each end. I can't see it, not even with a mirror. I'd have to remove the gas tank to remove it, which I really don't want to do.

Does anybody know what this thing is? Do I need to use it? I was just going to run the tube from the pickup to the back of the speedometer. Can I do that???


08-11-2008, 04:50 PM
i beleive that is a thing that will keep your speedo from bouncing around at speed from pressure drops. there is a name for it but it wont come to me right now.

08-11-2008, 05:20 PM
That's called a ballast and it keeps the speedometer from being too sensitive. It uses air, not water, to move the needle. If the gauge's diaphragm leaks, water will come shooting up to the dash and either fill the gauge or leak out. The lines should be cleared when winterizing, but blowing air (low pressure) from the front and out the pitot. If the lines and ballasts aren't cleared before winter, the water can freeze and cause the ballast to burst. It's not the end of life as we know it, it just needs to be repaired. If you can't solder this, any radiator shop can. If you know a plumber, they can do it, too.

08-11-2008, 05:28 PM
I thought it might be some sort of reservoir. My last boat didn't have anything like that, and the speedo worked fine, from what I can remember...

Well, I'll have the boat out on Wednesday and see how it does. If I get needle bounce, I'll put the ballast back in the loop.

Thanks for the replies.

08-11-2008, 08:22 PM
A single speedo will not require a ballast tube, but dual speedos do, much like the H tube between the exhaust pipes just behind the headers/and mainfold pipes on a high performance car/street rod, cause it equaliizes the pressure between the 2 pipes/tubes for a more even, steady, accurate flow.

08-11-2008, 08:31 PM
I have seen dual speedo boats with a ballast on each. What is needed is based on the gauge itself.

08-11-2008, 08:55 PM
I never seen one ballast tube for 2 speedometers. I have one for each.

I say if it is impossible to hook up behind the tank and you dont want to remove the tank, go ahead and bypass it for now and see how it works. Worst case, you will have to remove tank later.

My dads old outboard never had one and had stable read outs.

08-12-2008, 11:39 AM
My dads old outboard never had one and had stable read outs.

My last boat was an outboard, and it didn't have a ballast either...

The two ballasts are not connected (nothing between them). The Port side ballast and tube were intact, so I just installed the new pickup. The Starboard side was leaking somewhere, so I replaced the tube and the pickup, bypassing the ballast for now. I'll be taking the boat out latest this weekend to see what difference, if any, there is.

For what it's worth, I was reading about some of the new speedometers, and they have something in the gauge itself that prevents needle bounce, so they don't need a ballast. My speedometers are new(er) Sammy Duvall's, but I don't know if they have that feature...

I'll know more by Monday...

08-25-2008, 05:40 PM
Sorry for the delayed conclusion...

I took the boat out and both speedometers work fine. The one with the ballast in-line rolls up to speed slower than the one without, but they both track accurately and neither bounce much. This also fixed the mysterious leak that I have had all season (hole in the old pitot tube leaking into the back of the boat).

On the bad side, I've been out three times since this was fixed and was not able to ski on any of those occasions...The fog was really bad. Had about 40' visibility. Had to wait until the wind cleared the fog out, but then it was chopped out... :mad::(