Are marine speedometers designed to function with a certain orifice size in the pitot tube, or is just pressure sensing due to speed and the opening size isn't critical?

I'm wondering if my Airguide speedometers will work with other brands of pitot tubes since pitot tubes for my speedos are no longer available.

Pitot tubes have two ports on them. Not just one. The one on the end of the tube senses the dynamic flow and there is one machined flush to the tube and it senses static pressure.

You need both to get a proper measurement.

Apparently the marine pitot tube is a simple pressure gauge tube. As velocity increases pressure increases. Pressure gauge scale is labeled in speed units.

Aircraft pitot tubes require a pressure differential measurement.

Have you considered upgrade the speedo to a GPS unit, would be way more accurate as to the correct speed since your model is discontinued.

I have no experience with the ones at the link, it's just what I found with google, might be a good time to just upgrade the boat..

Link

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Originally posted by 4MUL8R:

Aircraft pitot tubes require a pressure differential measurement.

The pitot tube itself only has the forward facing opening. The gauge needs a static pressure source which could be incorporated into the pitot tube housing, but doesn't have to be. Heated pitot tubes will have a tiny drain hole for flight through rain and icing conditions, but that isn't a static port.

My 1958 Cessna had what looked like 1/4" copper water line sticking out from the wing for the pitot tube. The static port was mounted on the side of the fuselage. None of it was heated. It still had vacuum tube radioes when I bought it, and they worked.

quote:

Originally posted by pbslinger:
Are marine speedometers designed to function with a certain orifice size in the pitot tube, or is just pressure sensing due to speed and the opening size isn't critical?

In theory it is just a simple ram pressure being measured, so any size orifice gives the same result. But idk anything about boat stuff.

quote:

Originally posted by HRK:

Have you considered upgrade the speedo to a GPS unit, would be way more accurate as to the correct speed since your model is discontinued.

Two different measurements.

Pitot type speedometer measures speed of boat relative to water.

GPS shows vector sum of boat speed and water motion (current).

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Originally posted by V-Tail:

quote:

Originally posted by HRK:

Have you considered upgrade the speedo to a GPS unit, would be way more accurate as to the correct speed since your model is discontinued.

Two different measurements.

Pitot type speedometer measures speed of boat relative to water.

GPS shows vector sum of boat speed and water motion (current).

Nonetheless they both measure speed, would that not satisfy the requirement to know how fast, or slow you are moving, is there a reason you'd want one over the other presuming the GPS will be the more accurate reading...

Water skiing and wakeboarding where there’s tides and/or currents.

quote:

Originally posted by HRK:
Have you considered upgrade the speedo to a GPS unit, would be way more accurate as to the correct speed since your model is discontinued.

I have no experience with the ones at the link, it's just what I found with google, might be a good time to just upgrade the boat..

Link

I have a classic tournament water ski boat, a 1984 Ski Nautique I bought new. They came with dual water pressure sensing speedometers, redundant in case one fails during a timed ski run. The actual speed was calculated from stop watch timing between buoys, the speedos just helped you get there and were calibrated before competitions.

I want to keep my boat original, and like two working water pressure sensing speedos. Some with old boats have GPS guts put in one of the speedometers. I may do that someday.

If deposits were to create a lack of expected water pressure to the gauge itself, at the actual dynamic water pressure, that speedo reading might be off.

I'm going to keep it real simple and straight forward. Mount any marine pitot tube that fits your hull. Preferable with the same mounting holes so all you need to do is press silicone sealant into the holes before screwing it down. Hook it up to your speedometer and then use a GPS to test how acturate your speed is. If you need to tow a skier at exactly 36 mph than use a small bit of white pinstriping tape on the glass for the speedometer to line the needle up with when the GPS says 36 mph. Note calm water is sort of essential for this calibration.

BTW I still have the EP ski I used to use. When I was young I could just manage 2 buoys on a 75 foot line, at the 3rd I just didn't have enough grip strength left. Now at 71 I don't even have the leg strength to get up. BTW I always came up on one leg, tried 2 leg and never managed to master it.

quote:

Originally posted by Scooter123:
BTW I still have the EP ski I used to use.

Finally let my EP X-2 64" go two house moves ago. Even as a teen that was way too much ski for me.

For ships it's called a pit log.

https://en.wikipedia.org/wiki/Pitometer_log

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I have a classic tournament water ski boat, a 1984 Ski Nautique I bought new.

Awesome boats. Glad you keep it going.

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Originally posted by Georgeair:

quote:

Originally posted by Scooter123:
BTW I still have the EP ski I used to use.

Finally let my EP X-2 64" go two house moves ago. Even as a teen that was way too much ski for me.

Mine is 165cm (65 inch) and I bought it after trying my sisters 62 inch Comp 1. BTW she still has hers and still ski's, in part because she's 5'1" and only weighs about 105 lbs. She has always popped out of the water like a cork.

quote:

Originally posted by Fly-Sig:
My 1958 Cessna had what looked like 1/4" copper water line sticking out from the wing for the pitot tube. The static port was mounted on the side of the fuselage. None of it was heated. It still had vacuum tube radioes when I bought it, and they worked.

My ‘48 Cessna 140, had the same, but the tubing was aluminum. It had a cute little flapper cover that (normally) was automatically raised by dynamic air pressure on takeoff. Taking off with zero airspeed was kinda interesting the one time it stuck.

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BTW I still have the EP ski I used to use.

I jumped on my EP jump skis at Nationals in 2023. I bought them about 1988 or so. Skied on them a couple of months ago.

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Originally posted by slosig:
My ‘48 Cessna 140, had the same, but the tubing was aluminum. It had a cute little flapper cover that (normally) was automatically raised by dynamic air pressure on takeoff. Taking off with zero airspeed was kinda interesting the one time it stuck.

Mine had the same flapper which never failed. I'm 99.9% sure the tube was copper.

My uncle had a beautiful C140. The artificial horizon was driven by an external venturi derived vacuum. As I recall he had a couple of nice comm and nav radios, but everything else was original configuration.