Mechanical Speedo Range

[rcarduino]

Hi,
With so many entry level sports boats topping out around 45Mph, why is it impossible to find a mechanical gauge in this range or can someone suggest a 45 Mph/40 Knot gauge ?

Thanks

Duane.

 

[GA_Boater]

Re: Mechanical Speedo Range

Plenty of speedometers here on iboats that range 50-80 at the top end. speedometer - iboats

 

[rcarduino]

Re: Mechanical Speedo Range

Thanks, I will have another look, but they all seem to be 35Mph, 55Mph or higher with nothing in the 45 Range. I am going to end up going for 55 and will put up with the two trivial issues -

1) The exponential range between 45 and 55 that takes about 20% of the dial will never be used on my boat :-(
2) The lower range between 0 and 30 is very compressed, I don't want the wife trying to hard to see 22Mph while I am 70 foot behind the boat and she is steering/timming/throttling.

The GPS Would be perfect, but the signal doesn't hold well in our area.

Thanks

Duane.

 

[JimS123]

Re: Mechanical Speedo Range

For what you want, you're going in the wrong direction. Any analog dial will present readability issues. Plus, the analog instruments use a pitot tube and are nothing more than a pressure gauge. Right off the bat you completely lose the 0-10 mph range.

A digital paddle wheel speedometer would suit your requirements. The speed range is infinite and it can be deciphered without any issues. I set my speed at 1.1 mph and that's what it reads. If I'm at WOT at 45.2 mph that's what it says.

Analog pitot tube instruments (if properly installed) are accurate to within 1 mph within the operating range of the dial. My paddle wheel is spot on up to about 6 or 7 mph; at the 45 mph range its only accurate to maybe within 3 or 4 mph. If that's close enough for you, then that's what you need.

 

[MH Hawker]

Re: Mechanical Speedo Range

Do you use all of the 120 range in a car.

 

[Chris1956]

Re: Mechanical Speedo Range

if you are looking for an accurate speed reading, get a GPS. None of the other kinds of speedometers are accurate.

 

[JoLin]

Re: Mechanical Speedo Range

if you are looking for an accurate speed reading, get a GPS. None of the other kinds of speedometers are accurate.

Even a $50. portable car unit will display accurate speed, if that's all you need it for.

 

[H20Rat]

Re: Mechanical Speedo Range

The GPS Would be perfect, but the signal doesn't hold well in our area.

if you are looking for an accurate speed reading, get a GPS. None of the other kinds of speedometers are accurate.

OP's location: Dubai, UAE

GPS is an american system, and the DOD often screws with the signal when the satellites aren't over US territory. Might work one day, might be way off the next, and there might be no signal on the third.

 

[JimS123]

Re: Mechanical Speedo Range

if you are looking for an accurate speed reading, get a GPS. None of the other kinds of speedometers are accurate.

Read the original post - that's not an option. Pitot tube speedos are dead spot on accurate if installed properly and maintained. But the analog readout didn't suit his requirements either. GPs is OK in a calm invironment, but if you're operating in a current they are totally worthless.

 

[metriccrescentwrench]

Re: Mechanical Speedo Range

Do you use all of the 120 range in a car.

Hell yeah !!!!!

GPS is an american system, and the DOD often screws with the signal when the satellites aren't over US territory. Might work one day, might be way off the next, and there might be no signal on the third.

You're thinking of what was called "Selective Availability". That was a type of encoding that provided for a high level of accuracy for US military GPS receivers with the proper decoder, and a lower level of positional accuracy for anyone else. The SA didn't mess with speed measurements, only position (even in civilian receivers). Furthermore, the DOD has had SA turned off for several years now.

OH, and as far as boat speedometers, I have an 80 MPH speedometer in my boat right now and manage to hold speed good enough for pulling skiers and wake boarders. I'll admit that that area around 20 is pretty compressed. I had a 55 mph speedometer on my last boat and had zero problem holding speed for watersports with that one.

 

[rcarduino]

Re: Mechanical Speedo Range

I tend to do most of my boating in Umm Al Qwain its a hour away from Dubai and has a large area of mangrove flats.

https://maps.google.com/maps?q=Umm+Al+Quwain+-+United+Arab+Emirates&hl=en&ll=25.586575,55.64249&spn=0.069827,0.110378&sll=37.0625,-95.677068&sspn=31.426353,56.513672&oq=umm+al+quwa&hnear=Umm+Al+Quwain+-+United+Arab+Emirates&t=h&z=13

I use google earth sattelite views to mark the deeper channels and potential fishing spots and then upload this to the GPS, its a bit of a process but works well.

The problem with GPS in this area is that the signal often drops once the boat is underway. It could be engine noise getting transmitted through the wiring loom, or the coast guard transponder that we have to have fitted to our boats is too close, either way GPS is not as useful/reliable as I would have liked.

I am going to get a Faria 55mph gauge which give a little more visibility in the 0-30 range than my current 70mph gauge.

The paddle wheel is an interesting option, I could use the electronic output to drive a series of indicator leds mounted in a ski mirror. Im thinking red, amber, green to indicate too fast, sweet spot, too slow and a display and dial on the back to set the speed for the sweet spot.

Thanks

Duane.

 

[UncleWillie]

Re: Mechanical Speedo Range

If you would like a gps that the admiral can read easily and the most common range of speeds is big and clear...
A GPS Water-Sports Speedometer is just what you need.
50% of the gauge covers 15 to 35mph.

 

[Chris1956]

Re: Mechanical Speedo Range

Pitot tube speedos are real inaccurate, because they clog or partially clog, boat hulls ride differently at different speeds, such that the pitot may get different water or air bubbles at different speed, and finally a lot of them depend on the unfolding of a brass coil, due to water pressure to move a mechanical gear and then the needle. These pivot points wear and corrode, at least in salt water.
I submit I can adjust that speedo pitot tube to read nearly whatever speed you would like, simply by using an awl to modify the intake orifice. Imagine what sand and other debris do to it.

 

[Chris1956]

Re: Mechanical Speedo Range

In addition, although I have seen a GPS get confused on speed and direction, when in rough water, that was only one model. All the rest of them were rock steady in terms of speed and course. If Selective availability were turned off, that should only reduce the GPS accuracy from 3meters to something greater. It has been on for a number of years during this peace time. In war time, it is likely the GPS Signal can be encoded, such that only the US and her Allies can use it.

 

[haulnazz15]

Re: Mechanical Speedo Range

Pitot tube speedos are real inaccurate, because they clog or partially clog, boat hulls ride differently at different speeds, such that the pitot may get different water or air bubbles at different speed, and finally a lot of them depend on the unfolding of a brass coil, due to water pressure to move a mechanical gear and then the needle. These pivot points wear and corrode, at least in salt water.
I submit I can adjust that speedo pitot tube to read nearly whatever speed you would like, simply by using an awl to modify the intake orifice. Imagine what sand and other debris do to it.

I think we are confusing the terminology. The pitot tube is extremely consistent, but it's accuracy will vary from boat to boat. It may be off by 3mph at 30mph, but it will consistently be off every time. The problem with GPS in a watersports scenario is that you are measuring ground speed, not speed across the water. So going upstream into a 5mph current would cause the GPS to read 30mph, but the actual speed of the water under the skis/hull would be 35mph. The pitot or paddlewheel corrects for that issue. Lots of options, it just depends on what type of "accuracy" you really want/need.

 

[JimS123]

Re: Mechanical Speedo Range

Pitot tube speedos are real inaccurate, because they clog or partially clog, boat hulls ride differently at different speeds, such that the pitot may get different water or air bubbles at different speed, and finally a lot of them depend on the unfolding of a brass coil, due to water pressure to move a mechanical gear and then the needle. These pivot points wear and corrode, at least in salt water.
I submit I can adjust that speedo pitot tube to read nearly whatever speed you would like, simply by using an awl to modify the intake orifice. Imagine what sand and other debris do to it.

I have a 30 year old pitot speedo and a 52 year old one. Both are dead on at all speeds within the readout. As I stated before, "if installed correctly and maintained". Sure, if sand opens the hole, the pitot must be replaced. If the line spings a leak and won't hold pressure it needs to be sealed up. The only time I have an issue is if a weed catches it (stop, back up and it clears itself), or when the pitot kicks up from hitting a BIG weed (stop and push it back down).

Yessir, making the hole bigger will give you any speed you want, albeit not the correct speed. back in the '50's we all drilled out our pitots on purpose!!. The girlfriends were quite impressed when they saw us doing 60 mph with the ole' tower of power.....LOL.

The pitot is oem in the center of my dash and it tells me water speed at cruise and WOT. The paddle wheel is located to the left and I use it to set trolling speed to an accuracy of 0.1 mph. The GPS is off to the right and it gets me home in the fog. The speed option is used to merely check the other 2 when I'm in a wind free, current free location.

 

[UncleWillie]

Re: Mechanical Speedo Range

I think we are confusing the terminology. The pitot tube is extremely consistent, but it's accuracy will vary from boat to boat. It may be off by 3mph at 30mph, but it will consistently be off every time. The problem with GPS in a water-sports scenario is that you are measuring ground speed, not speed across the water. So going upstream into a 5mph current would cause the GPS to read 30mph, but the actual speed of the water under the skis/hull would be 35mph. The pitot or paddle-wheel corrects for that issue. Lots of options, it just depends on what type of "accuracy" you really want/need.

+1
The Pitot is a very simple, accurate and reliable device.
The speedometer may need to be calibrated but the Pitot tube never does.
If you have ever flown in a commercial airliner, Your Life depended on at least one of them.

... Yessir, making the hole bigger will give you any speed you want, albeit not the correct speed. back in the '50's we all drilled out our pitots on purpose!!. The girlfriends were quite impressed when they saw us doing 60 mph with the ole' tower of power.....LOL...

The Pitot measures Dynamic Ram Pressure of the water flow.
The is NO flow inside of the Pitot Tube or in the hoses attached.
The size of the opening has NO effect on the pressure inside of the tube or the Speedometer reading.
The pressure inside of the tube is a static pressure(No Flow) and any partial blockage in the hoses will not effect the readings;
It will be slow to respond but it will eventually get to the correct number.

However....
Anything that redirects the water flow external to the Pitot tube IS going to effect the reading.
A piece of debris on the Tube, or excess growth on the hull in front of the tube that slows down, speeds up, or deflects the water flow,
IS going to change the indicated speed.

 

[Fleetwin]

Re: Mechanical Speedo Range

The pitot driven speedometer is an INDICATOR-not completely accurate.

Using the example above, sure, Commercial Aircraft use a pitot for airspeed. However, airspeed is relative to headwind/tailwind, etc. Ground speed is the true indicator as to how fast, or slow, you'll get somewhere.

With all of that said, if you like to ski/tube/wakeboard at a certain speed, a pitot tube speedometer can help. Realize, it is an INDICATOR. Find what speed (indicated) that you like to go with and work from there. Accuracy is moot once you "dial in" your preferred speed.. You may be actually taveling at 26 mph BUT 22 is "indicated". Understand?

By the way, water current can effect a pitot tube speedo. Not an issue on lakes but can be on rivers.

 

[haulnazz15]

Re: Mechanical Speedo Range

The pitot driven speedometer is an INDICATOR-not completely accurate.

By the way, water current can effect a pitot tube speedo. Not an issue on lakes but can be on rivers.

Absolutely. That's why the pitot-static system on aircraft shows you Indicated Air Speed, not true air speed. If you can grab a GPS and a body of water with no current (small lake), you can actually compare the pitot and GPS speeds to come up with a Calibrated speed for the pitot to account for any inaccuracy at a given speed. Kind of overkill for most boaters, but not impossible. The dilemma between these different speed monitoring systems is the reason there has been 3 or more tournament speed systems in use over the past decade. ZeroOff, PerfectPass, PerfectPass Stargazer, etc have all tried different ways of measuring speed in order to generate a consistent watersports speed through the course.

 

[rcarduino]

Re: Mechanical Speedo Range

Interesting that there is no consensus on the best approach for competition.

My wife has a lot to learn about throttle and trim control before I need to worry about that level of accuracy though.

Thanks all

Duane.