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Old 19-02-2017, 07:17   #1
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Another hole in the hull for pitot tube, is it worth it?00

Im fixing an older wooden boat. It has a speedometer that uses a pitot tube through the hull to measure pressure differential. The plank that the tube goes through needs to be replaced. With next to no personal sailing experience how usefull is this measurement when I could install a GPS speedo and have no hole through the hull.
I know the two measuements would be different. GPS style is speed over land and pitot is speed through water.
FWIW I intend to be mostly coastal cruising in the waters around Nova Scotia.
Holes for the pitot tube would be small, two holes about 10mm (3/8").

Is it worth putting two holes in the new plank?
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Old 19-02-2017, 08:03   #2
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Re: Another hole in the hull for pitot tube, is it worth it?00

No personal experience, but as a wooden boat owner and lover, I will be looking forward to some intelligible responses.

Welcome aboard!
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Old 20-02-2017, 14:30   #3
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Re: Another hole in the hull for pitot tube, is it worth it?00

Knowing the speed over ground (SOG) and the speed through water (STW) makes it possible to estimate the current in the area, by neglecting leeway. This information is always useful because the actual current is often different from the values mentioned on charts or in tidal stream atlases.

But a pitot log isn't sensitive at low speed because the pressure is proportional to STW squared.

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Old 20-02-2017, 14:39   #4
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Re: Another hole in the hull for pitot tube, is it worth it?00

Thanks for the feedback people. Ya I am just learning that the pitot tube style speedo is not to useful at lower speeds. What other options are there for a STW type of measurement because as I think about it, the waters around Nova Scotia are very tidal and I am going to have to be aware and in tune with the tides for safe navigation.

Still on the fence about reinstalling the original pitot sensor but fortunately the boat will not likely see water until 2018 and that's only going to happen if I get off this computer!

Take care, always learning, lots of great advice here.

Jeff
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Old 20-02-2017, 14:52   #5
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Re: Another hole in the hull for pitot tube, is it worth it?00

The cheapest way to be "in tune with the tides" is to buy a tidal stream atlas and a nautical almanac with tide tables! It is usefully complemented by attentive observation: lobster or crab pot buoys give a good feeling for the speed and direction of current.

At present, a *calibrated* paddlewheel log is a good compromise of price and accuracy. This requires a wider hole in the hull than a pitot.

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Old 20-02-2017, 16:03   #6
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Re: Another hole in the hull for pitot tube, is it worth it?00

Don't think I'd reinstall the pitot tube knotmeter. Really old technology and probably unrepairable if something goes wrong. Do really like knowing how fast the boat is going through the water so would reccomend a newer paddle wheel knotmeter. I want to keep the boat moving to the best of my ability and a knotmeter is the only way to get feedback. It's also nice to have a log function just in case you are thrown back on DR for navigation.

GPS will give speed over the bottom but no indication of the boat speed through the water. The GPS speed lags way to much to use to measure the effectiveness of changes in sail trim or other performance tweaks.
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Old 20-02-2017, 16:20   #7
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Re: Another hole in the hull for pitot tube, is it worth it?00

Just for old sake airplanes still use pilot tubes but as mentioned move much faster than a sailboat can creating a higher differential across it. Unless you have a really good reason like refurbishing boat to original show room standards, ditch the old unreliable equipment and install a paddle wheel system. In fact if you still keep the pilot tube system install the paddle wheel anyway. Much more accurate but depending on speed, tack etc. Also has some inaccuracy. Get one you can pull out inside for occasional cleaning. you'll have to clean occasionally.

Happy sailing.
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Old 20-02-2017, 21:26   #8
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Re: Another hole in the hull for pitot tube, is it worth it?00

Pitot tubes also need to be into the current (wind or water) to accurately measure fluid movement. How are they placed on boats? Easy systems to troubleshoot, though. I haven't seen how they work in water, but I imagine it's the same principle: fluid acts on a bellows with a calibrated resistance against a static fluid, static port provides a base pressure. I would imagine the likelihood of something clogging it would be greater than in the air though. Plus static port clogging. Still relatively easy to troubleshoot though. I'd go with a more current method.
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Old 21-02-2017, 13:29   #9
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Re: Another hole in the hull for pitot tube, is it worth it?00

A pitot tube protruding under the hull is also perfect for catching weeds, floating lines, etc.

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Old 21-02-2017, 15:48   #10
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Re: Another hole in the hull for pitot tube, is it worth it?00

Thanks for the input everyone, I'm glad I asked the question.

At this point I think it's safe to say that another hole in the hull is worth it to have an indication of 'Speed Through Water' but also it's become quite clear that using the existing pitot tube system is probably not really worth it.

So now I'm on the hunt for a more up to date solution. I am only starting my research on this and today a nice hill and a waxed snowboard got in the way of my research but it looks like ultrasonic sensors might be the way to go but so far all I've seen are the sensor end, they seem to require a nav system?

Anyway looking for suggestions for a modern STW sensor and readout, I'll be googling and learning.

Thanks again,
Jeff
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Old 21-02-2017, 18:11   #11
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Re: Another hole in the hull for pitot tube, is it worth it?00

Picked up parts for a Raymarine ST60 Plus knotmeter/log/temp. Got the transducer, new but shopworn, for $65 from the consignment store Blue Pelican in Alameda, CA. Got the NOS instrument head from Ebay for $225 with shipping. Have the ST60 plus on my other boat and it's worked fine for over 6 years including a sail to Hawaii.

Might try and contact Blue Pelican as they had a number of Tridata, speed/log/depthsounder, heads and may have had some transducers as well for pretty good prices. I was looking for a knotmeter/log/temp so didn't explore further once I found the transducer for that instrument.

If you don't want to spend that much money, look for a transducer for the instrument you want and install that while the boat is hauled. Pickup the display head when you're more flush.
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Old 21-02-2017, 18:33   #12
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Re: Another hole in the hull for pitot tube, is it worth it?00

If you have a depth transducer you might consider replacing it with a depth/temp/speed transducer. You would need to add a new readout but you can avoid putting another hole in your bottom. You didn't mention what brand of instrumentation you have but most major manufacturers offer such a transducer though you will probably have to spring for a new readout device. If your electronics are modern enough to support 2000 you cab even buy a "smart transducer that will display the depth and speed through your chartplotter.
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Old 07-03-2017, 06:31   #13
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Re: Another hole in the hull for pitot tube, is it worth it?00

Ahhh, a subject of my name sake ( Pete O Static )

I would eliminate a Pitot system for sure. Too many things to go wrong. You have the Pitot side of the instrument ( dynamic ) and you also have the Static side ( atmospheric ) which must always be kept free from plugging.

The Pitot tube will be prone to sea weed, barnacles and other growth and debris while the static side, will always be prone to spiders nests and dirt. Just not a great idea on a boat.

You are in Tidal waters in Nova Scotia so I would certainly use a paddlewheel. Then you can accurately determine the tidal stream when comparing to the GPS SOG.
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