How accurate are pressure speed tubes? What if they're drained?

insttech1 · Jun 26, 2011

The kids lifter the speed sensor arm; it drained water. It will show almost no speed until about 15mph, and then jumps to 25 with very minor throttle.

Do they need to be lubed or something at the plunger? Should they be drained and refilled? Are they filled automatically when lowered? I just want something a little bit more reliable for tubing speeds.

Boat: 1999 regal lsr 1700 volvo penta 135hp.

thanks!

reddogg · Jun 26, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

Have you thought of getting a gps? Maybe a fishfinder with the speed attachment. Lowrance makes a decent one (x-50) and you can get depth, temp, speed, second temp if needed.

red

insttech1 · Jun 26, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

Put in a fishfinder with depth and temp; no gps. Out of the budget. If the function is already there on the boat, and a simple way to make it smoother or know not to lift it up to drain, then I need to learn that, then pass it on to kids (who lifted it again today while by the swim ladder....grrrr...)

Thanks Much!

insttech1 · Jun 26, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

Oh and I asked for a thorough manual from Regal....it did not cover such things in detail.

reddogg · Jun 26, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

My boat has the speed gage with pitot mounted to the transom. I know of no way to make it smoother, It's supposed to lift incase something hits it so just keep the kids from messing with it. It drained some water out of the tube and has to refill again before being accurate. Oh, the fishfinder I was talking about is on sale this month at overton's. I got it for $75 instead of $119, nice little unit.

red

JoLin · Jun 27, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

Slight misconception, guys- speedos work via air pressure in the tube, not water pressure. Could be once they've been lifted they take awhile to 'recalibrate' themselves, but there's no need to refill them with water. In any event, I've never had one that was worth anything in terms of accuracy.

If all you need is actual speed, the cheapest portable auto GPS will give you that. I've seen those for $50. Install a 12v accessory plug in the boat (handy for other stuff anyway) and you're golden.

My .02

H20Rat · Jun 27, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

If you have water in your speedo tubing you have a problem! It essentially is a sealed pressure system, if water is more than a couple inches up the tubing it means you have a leak somewhere in the system, either in the tubing or in the gauge itself. (you are going to have water dripping out whereever the leak is) Think of it this way, turn a glass upside down and then press it into a sink full of water. That is your speedo system. The speedo gauge is measuring the air pressure in the top of that glass.

The angle of the pitot can make a HUGE difference. Most pitot's will have lots of friction once they are in the down position, so you can pick the angle that works best. I know on mine if I drop it down all the way it is wildly inaccurate, but if I pull it back about 10 degrees it is actually fairly close to the GPS at all speeds.

Susquehanna Squid · Jun 27, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

100% agreement with smokingcrater.

Kosse · Jun 27, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

JoLin said:
If all you need is actual speed, the cheapest portable auto GPS will give you that. I've seen those for $50. Install a 12v accessory plug in the boat (handy for other stuff anyway) and you're golden.

Another option if you have a smartphone w/ GPS is to just use it. I know for android based phones there are several apps in the market that will display speed. Iphones will as well.

haulnazz15 · Jun 27, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

Agreed w/Jolin and Smokincrater. Shouldn't be any water "draining out". The kits flipping up the pitot tube shouldn't hurt anything as it's made to flip up in case you hit an obsticle on the water. Make sure it is pointed down when you back off of the trailer (sometimes trailer bunks flip the pitot up). Also, look for any cracks/splits in the pitot tubing going up the transom. Lastly, there should be a small hole in the pitot tube front side, make sure it is clear of any obstructions.

Some pitot don't read accurately, some read accurately within a specified range. Mine is actually pretty accruate at any speed above 7-8 mph.

insttech1 · Jun 27, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

OK all thanks much for the valuable education! When I lifted it, about 3 tablespoons of water came out. Therefore, I thought it was based on water pressure from the plunger. I'll check for leaks.

Maybe I don't quite understand two things though--if there's a small hole in the front of the pitot as stated, doesn't that let water in? And secondly, as for changing the angle of it as mentioned above, it's either up or down. It will not stay in any "mid" position. I don't know if there is an adjustment for it, but I will re-check that as well.

And I'll look into other methods for speed as well....it's not critical, just wanted everything to work w/ no bugs...and I don't have a lot of real estate in front me to mount much else....it was fun finding a place just to put the fishfinder...but at least it's in there and doesn't obstruct viewing aything else.

Thanks Again!

sschefer · Jun 27, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

insttech1 said:
OK all thanks much for the valuable education! When I lifted it, about 3 tablespoons of water came out. Therefore, I thought it was based on water pressure from the plunger. I'll check for leaks.

Maybe I don't quite understand two things though--if there's a small hole in the front of the pitot as stated, doesn't that let water in? And secondly, as for changing the angle of it as mentioned above, it's either up or down. It will not stay in any "mid" position. I don't know if there is an adjustment for it, but I will re-check that as well.

And I'll look into other methods for speed as well....it's not critical, just wanted everything to work w/ no bugs...and I don't have a lot of real estate in front me to mount much else....it was fun finding a place just to put the fishfinder...but at least it's in there and doesn't obstruct viewing aything else.

Thanks Again!

The small hole in front of the Pitot pickup arm lets water into the air filled (natural occurance) tube and compresses it. The compressed air is what the gauge is working off of. If you have a loose connection at the dash (not uncommon), the air will be pushed out and water will enter the gauge. The gauge will likely be unreliable from that point on.

Regarding accuracy, they are very accurate but require calibration. If the gauge is a good one, there will be a calibration screw on the back. We used to take two or three boats and run them side by side and then average the speeds were were reading. It wasn't the best but it got us in the ballpark. Today you can use gps from your cell phone as was mentioned in an earlier post. Take that data, calibrate your gauge and you'll have reasonable accuracy for speed over water. Note that the GPS is reading speed over ground so make sure you're going in a straight line when you compare GPS with your Pitot style speedo.

dockwrecker · Jun 27, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

They all quit, horribly inaccurate and they're not worth looking at most of the time. It's the least most important guage on your boat. If you want to know how fast you're really going get a GPS if bragging rights are so important. The tach, temp and oil guages are the primary concern followed by the trim guage. You'll do better learning to drive with the tach and the trim guage than a speedo. Seat of the pants planing experience is the most valuable.

insttech1 · Jun 28, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

Yeah bragging rights are definitely not the intention of this. I'm wanting to learn to drive by the tach and feel of the boat; just wanted to use speed as a correlation, and also use that to help teach the kids about correct boat operation.

Thanks again for the info on how the plunger operates. I'll check the gauge as well to ensure no water intrusion.

Boat probably won't be in again this week, but we might be able to squeeze a run in...we'll see.

Thanks!

JimS123 · Jun 28, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

They are quite accurate if properly maintained. My family boat had an issue when it was new so I sealed both ends of the tube and it reads to within 1 mph of the GPS. Its been running now for 27 years without fail.

My "old" boat's system didn't work at all, but it was a good unit so I did some troubleshooting. All it needed was a new tube and a cleaning of the pitot orifice. That unit is 59 years old, and its accuracy is right on!

Just replace your tube and blow out the orifice. Be sure not to enlarge the hole or it will read high.

insttech1 · Jun 29, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

Thanks Jim! I take it I need to remove arm/plunger to get to the orifice? And what exactly do you mean when you say you "sealed both ends of the tube"?
Thanks!

JimS123 · Jun 29, 2011

Re: How accurate are pressure speed tubes? What if they're drained?

insttech1 said:
Thanks Jim! I take it I need to remove arm/plunger to get to the orifice? And what exactly do you mean when you say you "sealed both ends of the tube"?
Thanks!

I don't know what "arms" and plungers" are. Google a marine speedometer and I'm sure you can find a pic with the proper teminology. Arms maybe, but no plungers.

Remove the "assembly" from your transom. There will be a bracket attached with 3 screws. Contained within the bracket will be the pitot tube, and it will be hinged so it will pop up if you hit something.

The top of the pitot tube will exhibit a hose barb. That's where the black rubber tube will be attached. The bottom will have a small hole (orifice), usually within a brass fitting, facing forward.

Once its off, blow thru the hose barb to be sure its clear. If needed, pass a small 5 mil nicron wire through the orifice to clear it. Don't drill it out or anything.

Pull the old tube through the boat, with a new one attached. Put marine underwater sealant on the barbs before you connect the new tube - one end to the pitot, the other end to the back of the dashboard gage. Be sure not to get sealant inside the tube.

How accurate are pressure speed tubes? What if they're drained?

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