"My airguide was great but after 10 years it failed", "Why did it break", "Why is it filling with water"
Airguides are very reliable and can easily last thousands of hours over decades of use.There are only three reasons I've ever seen or heard of airguides failing.
Ruptured due to water freezing inside the gauge
Rusted due to long term water inside the gauge
Ruptured due to excessive pressure eg doing 55+mph or pickup blasted with an air compressor with gauge still connected.
Water should never get inside the gauge but will if there is an air leak in the line.
Water pushes against a pickup (actually called a pitot) and this compresses the air in the lines going to the gauge. This air pressure makes the gauge read. This is the way almost all boat speedometers work.
If there is a leak in the line, all the air is pushed out and water makes its way to the location of the leak plus a bit further as water pressure pushes it past. If the leak is too close to the speedo, water will get into the speedo. Eventually you will have problem 1 or 2 above.
Pitots/Pickups and Installation advice:
Practically any pitot will be fine also. Customers recommend this model http://www.skidim.com/prodinfo.asp?number=769B which costs $13.95 at the time of writing. Alternatively you can buy them from almost any boat shop, even smaller local ones. They are a very common item. Or search Overtons or any other marine supplier for "Pitot".
The best pickups are the ones built into an outboard leg, so use that if you have an outboard but be careful not to go faster than 50 mph/80kph.
For tube I use the thin flexible hose normally used for drippers in a garden watering system. Available from any hardware store it is cheap and does the job perfectly.
Airguides are not too picky about what type of tube you run from the
pitot to the speedo. The pressure at 50mph/80kph is 30 psi, within the range of normal tap water pressure.
To replace an existing speedo, I simply cut 10mm (say 1/2 inch) off the existing hose to ensure a good fresh seal. I also use a cable tie to further clamp it sealed. As long as it is tight to put on it will seal fine. Replacing an existing speedo (almost all brands) is a 5 minute job. The hole in the dash will 99% certain be the same size (85mm to 95mm hole size is fine).
What about those Brass Tubes:
Many boats use brass equalizer tubes. These are just chambers which store lots of air in the system. They should be installed vertically and somewhere along or very near the back of the boat. The pitot line goes into the bottom and the line to the speedo comes out the top. As the pitot hole is small, it takes time for enough water pressure to get in to change the air pressure in the equalizer tube. The tube simply slows down the speedo's response. You can reduce this effect by putting some water into the equalizer tube or filling them completely by running the boat with the speedo hose off of the tube. Once water comes out the top it is full and you clamp the speedo hose back on, while the boat is still running. This speeds the speedo up greatly but in this case you are much better off without the tube at all (just put a joiner in instead or don't cut the hose in the first place).
For example, if you have the equalizer tube installed and tow a barefooter with a reasonably snappy boat, your speedo will lag behind and you will overshoot the desired speed. In the slalom course, the speedo will tend to read a set speed and not vary much even though the boat speed does. For example "you pulled me back 2mph as we went through the gates and the boat held the same speed for the rest of the course". No it didn't hold speed, the tube averaged out the speedo reading and the speed kept varying by about that 2mph.
I much prefer boats without the tubes and many people have removed them and thanked me for the suggestion. Nobody I've spoken to has put them back on after trying without.
What about Blockages?
If junk such as weeds etc gets into the pickup and blocks it, then your speedo can't work. You can get partial blockages which make it slow to go up and down.
Most people rarely get blockages but if it happens to you here is how to fix it:
There are two simple ways to clear a blockage.
Use a small drill bit and turn it slowly as you put it into the Pitot hole BY HAND. Ensure the bit is the right size and turn it in the forwards direction so any junk gets spit out rather than pushed in. Cork on the end will make it float if you drop the bit while doing this in the water.
Remove the hose from the
back of the speedo and blow compressed air from an air compressor through it. Give it say 15 seconds to blow all the junk and water out. Ensure a correct seal when you put it back on the speedo per installation instructions above. Also be aware that if you blow into the pickup with the speedo still hooked up you WILL destroy the speedo instantly.
Generally the pickups on an outboard leg are least prone to being blocked. The cheaper aftermarket pickups are
less liable to block than the airguide weedmaster ones (which I don't have any more). Some boats are fitted with a thru hull pickup towards the centre of the boat. These work great also.
If you have an old type airguide or want to restore the unit you have, www.classicboatwork.com have been very well recommended to me. If you have a 2025 series airguide, while I still have them, replacement is a more economical option however if you have a different model and want to stick with your old one talk to Classic Boatwork. Email them via their web site contact section for a price for rebuilding - expect their price to be more than a replacement 2025 speedo but still good considering the quality of their work and how involved the
task is. http://www.classicboatwork.com/classicparts/gauges/airguide/airguide.html
Modifications for fast boats
It is possible to use a 50MPH Airguide on a faster boat if you install an air
pressure limiter. The best ones shut off the speedo at 30psi and drop
it to zero until you slow back down again then the speedo springs back to
life once speeds have dropped to a safe level.
1. The limiter should be at the back of the boat, not near the
speedo (to make sure there is plenty of hose between there and the
speedo so no water can get up there)
2. DO NOT get a blowoff valve type as this will let water all the way up to the valve and a bit past. Not good for your speedo
3. You could get a 30psi limiter/regulator but you want to be sure
it works and that air pressure does not exceed 30psi. You do not want to
think it is working when in fact your speedo is being subjected to huge
pressure.
At this stage I do not sell these but normal air handling/hydraulic shops should be able to help.
For approximate reference, 5psi = 20mph, 11 psi = 30mph, 19 psi = 40mph, 30 psi = 50mph. Note just how low the pressure at and below 20mph. This is why the Airguide Contralog Movement is so important to "magnify" these low pressures so you can see them on your gauge.
How to winterize your speedo when stored in a freezing climate
Winterize your airguide when you winterize your boat. This is done by removing the hose from the airguide (the airguide, NOT the pickup), blowing air down the line (NEVER into the speedo). Blow out any water etc until the hose is dry. Next season, cut a few millimetres off the hose to ensure a snug fit, press it on and zip it up with a cable tie.