speedo pickup HST,

[Randy Deary]

I Know, I know, get a GPS, BUT I have a Nordskog 0-100mph pitot tube speedo (This is a big air pressure gauge calibrated in MPH and it looks it, not a very nice bezel) and I can't see the obvious place to mount the tube, the transom is so curvy and stepped in so many places. It doesn't seem right to mount it anywhere near the slipstream of the gear case so on one of the outer sponsons I'm figuring is where to go. The pick-up I have is from Rex Marine and is an adjustable tube, even at it's longest will not reach the water as the sponson is stepped as well from the bottom up to the flat mounting area, I have access to a shop and could have a bracket made. Any HST guys with an idea??
Thanks,
Randy

[CompKing]

Randy
I had mine out on the right sponson but never could get it to work very good or stay in one peice.:( Drove around most of the time without one,I vote for the gps:)

[TUFFboat]

I dont know what your transom looks like, but I think I can guess at it. Mount the tube bracket sideways in the notch (cut out). than bend the tube carefully to about 90 deg. Plan the mount and bend so that you have about an inch or so sticking down. have the tube in the water on the up stroke side of the prop so that you dont turbulant the power side of the prop swing. Your numbers wont be perfect being that high because of water tension but the alternative is too restrictive. That set-up will work for consistency, which is probably what you really want anyway.

[Capt.Insane-o]

I have an Hst, speedo mounted in the same spot as yours. It left the hull at 105. Pitots suck. Thank the man who came up with the gps. Don't drill anymore holes below the water line than neccesary.

[Boz]

with Capt.

[skidoo29]

Mine is mounted on the right sponson like comp-king's, still intact but I don't use it very often. GPS all the way. no special bracket on mine, and the way I see it the less holes drilled under the water line the better(mine was there when I bought it), Most speedos in my opinion fluctuate too much after 80 anyways.


warren

[Randy Deary]

All righty then, I'll fill the holes that were left by previous owner, not install this BRAND NEW speedo and pickup ( in the days gone by when I had my last "fast" outboard, Vector with a tweeked in-line, early 80's a Nordskog speedo was the only way to go)
and look into a GPS, where do I look? Holy smoke you guys cost me money, why didn't I find you BEFORE I bought the wrong stuff!!!! In that spirit, what do I do about steering? Twin cable same side, twin cable opposed or Hydraulic and which one?
Price is somewhat of a concern but generally I want to do the better thing ( see comments on throwing NEW oudated speedo in waste basket for better technology).
Thanks,
Randy

[Balzy]

All the way for a HST for me. Had dual single side. Converted mine and love it.........

[Capt.Insane-o]

Tried just one side with my drag motor one time. One time. Switched back to dual opposed , much mo betta. Something about having only one part keeping me straight at 100 ++ or two. I had none one time this summer at about 75. The ol' HST turns pretty darn good! About turned me out of the boat!

[rookieflyer]

From a control and instrumentation engineer's perspective, I wouldn't jump on the GPS bandwagon so quickly. I am aware of the obvious advantages that come from eliminating a trash collector from the boat, but I have more than a suspicion that GPS for speed measurement may be a bunch of hype.

I have been researching GPS methods for a design project during recent weeks, and when I took the published error tolerances of commercial GPS units, there is no way to be sure that consistent results can be achieved. While speed may be fairly accurate during one run, there's now way to be sure that an hour later or in another location that will be the case. Some high-end units employ sophisticated median filtering techniques that take a large number of position samples over a relatively long period of time to achieve greater accuracy. I did the math yesterday, and to get repeatable 3% accuracy in a MPH reading consistently, such a unit could only be able to report speed updates every minute or two, which would be way too slow. 3% is also not very good.

I think a properly designed and well calibrated pitot system would actually be easier to get accurate and consistent results from.

With what I have learned about GPS, DGPS & Military GPS technology in recent months - I will never trust a GPS speed result - unless it is taken by a military GPS unit - these use two different frequencies at once, and they also use a long "P" code to help compensate for inaccuracies caused by distortions in the ionosphere. Commercial units are just not equipped to do this.

So, until someone shows me otherwise, I am of the opinion that GPS units are NOT the best way to measure MPH on the water. I can design and build either type unit at will, and I would still invest in a good pitot assembly and installation before GPS.

[stvhelm]

Do you even own a GPS? I take mine in my car when I go on long trips to use the road maps (garmin gps3). the mphs are very consistent and are never more than 1 mph away from the speedometer. it will bounce up and down a few tenths but thats it. 3% is ridiculous. In my boat every attempt to break my topspeed is usually within a few tenths. I also have a gaffrig 120mph speedo with a stainless pitot. I notice the slightest change to the angle of the pitot reflects a change in mph. from slightly angled in to slighty angled out will throw off the reading about 5 mph. grass or debree will do that. from my experiences I find the gps to be much more accurate and consistent than the speedo / pitot.

[Balzy]

on this one. I have never and I say never in the 5 or more years that I have been using GPS's in my boats seen any flucuation in speed. Same top end #'s everytime unless I do something right with setup or props and then it's repeatable again and again. I also use one in my truck hooked up with a data cable to a laptop and have never seen any funny speeds or flucuation there either. Always the same differance to the speedometer dependent on the tire diameter I am running at the time. What it says is what it is. I agree there is an error thrown in but only IF THEY WANT AND WHERE THEY WANT. I don't think the goverment is too worried about terorist attacks in the areas most of us boat. At least not where I boat because my GPS's are and have always been spot on. I believe the GPS is the only way to get an accurate speed reading. Now if you wanna go buy a pitot tube and speedo go ahead. I still have one in my boat but the pitot is tipped up most of the time. I til it down if I wanna impress a passenger. It's an easy thing to do because when the speedo stops bouncing around and sits tight on the 100 MPH stop pin I am doing 92 MPH. After the pitot pops out of the water for a split second and the needle jumps past the 100 MPH stop peg I can push it all the way to he back of the "0" stop peg (on the wrong side of it) and then and only then will you be doing a true 100 MPH plus.

Now I have seen some Gaffrigs and a couple others that have been within 3% or so but noy any better than that.

Take your pitot tube and open up the throat a little and see what happens to you speedo readings. Too many veariables for me to trust any of them. They are all happymeters !!!!!!!!!!!!!

[skidoo29]

I agree with balzy and stv 100%. The amount of times me and my brother have used a gps whether it be in a boat, car, or riding a snowmobile, the repeatability and accuracy is amazing.

[rookieflyer]

I should have started by saying that, while as my username implies, I am not the most experienced performance boater on this forum, so while I am sure my statements concerning GPS technology are accurate, I do learn about the application of technology on the water from you guys. So, please enlighten me if I am missing something....

I think you are mostly agreeing with me, or at least supporting my explanation - but saying something else. Years ago, an old, experienced marketer told me that to sell more of anything, instead of making it perform the best, make it so it does what customers want or expect from it. To me, that's giving someone less than what they pay for.

Yes, I do own, use and design GPS systems. I also agree that the GPS I use on my boat APPEARS to be giving good numbers. But the science says that they are fudged. On my design team I have a very experienced engineer that designs satellite systems and software for the US Gov't, and is currently also doing contract engineering work for the Jet Propulsion Laboratory. Last year he completed a highly successful software project that actually involved satellite manipulation and GPS for oceanographic study. When discussing this with him, he does say that even commercial GPS CAN BE amazingly accurate, but he agrees that there is no way to guarantee the repeatability of it's results without bending the laws of physics. That's where software comes in.

Granted that my boat is not very fast (or pretty) - only about 70 MPH (hey, it's a small cuddy - which I often need for carrying test equipment, tools and SCUBA gear) - I do notice that when we go fast, we are generally going pretty straight also, and not for very long - usually for a number of minutes. That scenario does not lend itself to much GPS error because satellie angle changes are insignificant, likewise ionosphere depth and density. But take the exact same setup 500 or 5,000 miles away, and the only way you will get the same exact reading is by chance, because you'll be reading data from different satellites at vastly different angles and with potentially great differences in atmospheric conditions.

These links shows some reasons for the deviation in readings, including DOP (dilution of precision) and ground speed rounding from knots to MPH and KPH. The last couple of pages of the .edu link show nice graphs of how the actual readings come in and are displayed differently:

http://www.gmat.unsw.edu.au/cr/gmat4900/tutwk9.pdf
http://www.ct-gmbh.de/download/sirfnmea.pdf

When you use your GPS the same way in the same area most often, error should be minimal. I's very easy to make a displayed number appear to be steady - it's one of the first things a beginning programmer learns. It's also fairly simple to take readings that are outside of the norm in a string of successive readings, and discard them, leaving only the ones that fit right down the center. It's even easier to make acceleration and decel numbers appear to be smooth with soft ramping, which is a first semester skill. All this makes it look like the instrument is doing well, while only the programmer knows it's been fudged. All GPS units do this sort of thing when they lose the signal, such as on land under trees, or indoors, etc. They will plug in the previous reading and repeat it until a new set of nunmbers is received and calculated. Can you see how doing this with MPH would be nearly un-noticeable if the error only existed for a couple of readings?! Next if the s/w looks back at the past few readings and sees that they were increasing or decreasing at a certain rate, it can increment or decrement the the missing value at the same rate.

When we buy a GPS unit, we are paying for the hundreds of $200 programming hours that went into making the results appear just right spread out over the number of units sold. After all, they would only cost as much as a cheap AM radio otherwise - there are no magical or nuclear componets inside. Programming for fast and accurate raw data is simple - data manipulation and signal conditioning for consumer use isn't.

As for the + or - 1 MPH comment on land, that's 4% error at 50 mph. I have read reviews of props and other parts that brag about a .1 mph GPS speed gain. While this may be possible, I think it's pretty bogus. Sorry. The successful marketing of commercial GPS was dependent upon software that would make the resulting numbers APPEAR to be consistently accurate to technically saavy users.

Selective Availability, which was the way the US Gov't intentionally introduced errors into broadcast commercial GPS signals - was disabled back in 1995. Before then, nobody would have even thought about getting accurate speed from GPS. Nowadays, we get pure, raw data from the satellites.

It is true that physics tells us that pitot angle, orifice diameter, and even things like the elasticity of the tube or hose between the pitot and the instrument - must have significant effects on readings. I find that most pitot tubes on boats are just not engineered as part of a system. Most just seem to be attached with strength and drag as the primary concerns. When you change the angle to impress, how do you know where to put it when you want accurate readings again? What calibration method do you use? Doppler speed? digital stopwatch over a known distance? or the pitot tube speedo?

Try a different perspective. For someone who really has to get an extra .01 MPH out of his boat, or perhaps for the drag racer who wants to see extra inches...he'll do best by knowing how all the combined inputs (jack, trim, rpm, mixture, etc..) afect the output (speed result) at various points through his acceleration curve. If plotted speed against time, the steepest angle between any two points is the goal. With software fudging going on, the overall average of that curve will likely be right, but from point to point it is intentionally wrong so as to be understandable to the average user.

Guys, sorry if I wrinkeld some eyebrows, but while working on a system that will do just that - gather real-time race data fast enough to adjust these things on the fly, I looked at if from an engineering perspective. While I can design a system that will tell you what you expect, or want to see, I have never designed a system that intentionally misleads the user - even to the slightest degree and I never will. Read the manual that came with your GPS and see what accuracy they claim for the unit....I'll bet they're honest about the error tolerance too.

....Try this one, I have: run as fast as you safely can in some fairly rough water whereas for a second or two you're riding on the cav plate, then the bow comes down and slaps the water on and off for the next second or two before getting back up. Most often your GPS never changes it's displayed MPH for the second in-between when your speed slows albeit briefly, but significantly. On another occasion if the timing was just right, it might.

Perhaps to the typical performance boater these small errors are insignificant. Perhaps to others, the convenience and improvements from eliminating the clog and movement of the pitot outweigh the slower, softer response of the GPS and make GPS the method of choice......That's what I'm looking to learn.

But perhaps I'm working on a much more accurate GPS speedometer design than that which any boater will ever want...

What kind of GPS units are you guys using, anyway? What makes them the best choice out of what's available?

Please, keep educating me......:confused:

[Balzy]

GPS 12 and GPS 3+ Garmins. I agree with your error from the gov deal. I just don't think and haven't seen it affect any GPS readings I have ever gotten. Also, I have taken my GPS from WI to KY to TN and haven't seen any variations on either one. I believe the accuracy on the 12 is +/- .1 Knots up tp 999.9 Knots if I remember right. (don't have the book real handy) The 3+ says =/- 0.1 knots RMS steady state.

Even though the speed display is slow (which I understand, you couldn't read it too well if it was jumping all over the place) the max sped recall seems to always pick up the max speed.

Explain something to me. If the gov throws in an error, is it constantly changing or does it just throw the position off compared to the real long and lat? If it just throws it off and stays a constant error why would affect the speed reading? Wouldn't it just tell you that yopu are somewhere you really aren't? (So the terrorists can't pop a missle in the right building)

Good discussion here

[skidoo29]

while I agree with balzy, I will admit you make some excellent points and seem to be very educated in how a gps works whereas I have a very limited knowledge on the subject, no need to appoligize for wrinkling some eyebrows,this message board is here for everybody to learnand the last few posts have certainly taught me alot, hopefully the discussion will continue.

warren

[BarryStrawn]

rookieflyer - Are you discussing velocity accuracy of a rapidly accelerating vehicle? If so, none of the consumer units are useful. Far too slow at 1 data record per second. There are several available for motorsports use but they are expensive. Search on Real Time Kinematics as the buzzwords and you should find something useful. Been a while since I read anything but I think I remember 2cm position accuracy @ 200+ Hz. I don't remember the velocity ratings but lets just say F1 auto teams have been using them for some time to replace traditional timing equipment while testing.

Most of us use GPS as steady state speed indicators and general navigation. Some guys only use them to remember the peak speed. I use a Garmin 48 which is an older handheld unit with marine database and detachable antennae. Similar guts as a Garmin 12. Stated accuracy on this every other consumer GPS I have looked at is "0.1 knot RMS steady state". I believe they all use the same core algorithms paid for by Uncle Sam many years ago. I've never seen a bogus velocity reading on mine. Selective availablity also did not effect the velocity data - just position. Although I'm sure the DoD does all kinds of useful things with the data when they need to.

[Travis Fulton]

i just purchased a magellan gps 310 from wally world for $79.95 and it works for speed, i checked it w/ a buddys expensive model and they red identically!! they track from the same sat's hey it's a cheap speedo thats portable, just a thought, good luck!!!!

[rookieflyer]

The intentionally introduced error (SA), or selective availability, is no longer done.

Your thought on 'happy meters' is well taken. Granted, I often use mine for the same reason, even if not at your level of 'happiness'....but using any of these toys for anything else is asking more from them than what was intended.

Even w/o SA in the picture, the error does pretty much stay consistent, with only minor variations. I believe this is why the speed numbers we get on the display are so consistent.

I recently received Email from a guy who went from England to Ireland and picked up a position shift of more than 8 meters. With DGPS, it was reduced to less than 3. The complexities of how and why errors seem to step in various places and not in others is something I'll have to investigate further.

I want a design that is more than a happy meter - that will do more than just tell how fast over a period of time, but what's happening at an instant in time.....info that's more technically useful. I don't want to sell them just because they'll make people feel good....but I think you get the point....

Now, if only I had that time machiine!!!

[WATERWINGS]

Does the 310 hold MAX SPEED? Or only the speed at the time?

[ultrafast]

We just went through all of this at the Swawnee Run. Warren ran 125.0 GPS and 124.5 Radar. I would say thats pretty darn accurate.

[Cougar21]

I used a Land & Sea nosecone on my HST/225 Johnson. There is a built in pitot in the tip of the cone. It seemed to work well, even at extreme jack height.

[sho305]

Nice thread guys, I used DeLorme GPS on a laptop driving around a lot. It worked nice, and would map where you were, and save the whole route, etc. It was only a little plastic antenna you put on the dash and plugged into the laptop. The speed only updated every 1-2 seconds and was very close to the car, unless you were changing altitude of course, then only slight change. Very nice for finding places on the map, or recording places. I gather those spinny wheels on the jet boats are not accurate either?

[AQUAholic]

Am I the only one here that does'nt care about a few mph or a few percent error on my speed ??

Not trying to sound like a dick, but I just am not that worried about how fast the old STV really is.

[Techno]

Getting back to Randys question. Sell the pitot speedometer, maybe to a bass man.
A hand held GPS is roughly $100 but is a bit hard to read digits that are this size. For $400 you can get a dash mounted speedometer that is either digital ( still too small and too expensive) or a needle analog. Or a fairly high end hand held GPS with mapping and so many features you'll run on the shore looking at it. I think that the hand helds, all of them, can be dash mounted. I takes a rectangular hole and a bezel of somekind.


I think for anyone using a GPS for a speedometer it doesn't have to update every tenth of a second, they aren't watched like that. Even a few MPH error at over a hundred is just fine. They are just indicators for speed.
Someone did a test with a measured 1/4 mile with triggers, radar and GPS. Although the readings were different they were so close it don't matter.

As already mentioned the accuracy and speed of a GPS can be checked in your car. Be a passenger and keep an eye on both speedometers, speed up -slow down.

[AQUAholic]

The Garmin E-Trex lets you enlarge the number size to like 1/2" or 3/4" in height, and is only 119.00 at Bass Pro when not on sale. The GPS-12 is 145.00 or 149.99.

I think a few of the others will do this too, not sure which. I had a Garmin GPS-12 that did not have that capability.

[rookieflyer]

So it seems that all the arguments have been valid ones. There are guys like myself that just want to have fun on the water, and a little MPH error doesn't matter. There are some who really want to know if the prop or jet or injector change REALLY added 1 or 2 MPH or not, and there are the most serious racers, who need the edge competetively, or to justify an expense to a sponsor.

I think We'll have this all covered soon. In the 1st Q of '03 We should be through beta testing of a modular instrument line that can be used as a data collection system, or the individual modules can be used as stand-alone digital instruments.
The first four modules to become available are the Data Module; Tachometer; Pitot Speedometer and GPS Speedometer.
The Tach and Speedo's all have ultra-bright 1inch tall displays. The Data Module being designed now will have either a text display of ot's own &/or the ability to communicate with a Palm type PDA or a PC.

When MPH & RPM (+ a gear ratio input) are used, the data unit can plot theoretical prop slip or efficiency in real time and the data can be downloaded and graphed. This can be used for tune & test or for manual use while running.

We plan to produce a 2nd phase that includes modules for trim, jack and possibly some safety functions. The trim and jack functions will have manual and automatic capability. In automatic, either a stored profile or a dynamically created curve will provide outputs that will maximize performance or efficiency.

I am still looking for input from you all. Even though much of the hardware has been designed and built, there's still a lot of programming to do, which means the actual details of what data is collected and how it's used can still be adjusted if necessary.

BTW, the prototypes are 3 1/8 dia rounds for standard in dash mounting. Would it be wise or foolish to produce any other sizes or shapes?