GPS Speedos... Good Idea or Not?

[AbunDiga909]

Today I saw a lot of boats, a lot of fast boats, and a lot of expensive boats. To name a few, a lot of the Donzis, Bahas, Fountains, and others, their speedos that lets say go up to 120MPH, are GPS Speedos. What do you think of the idea of Nautiques, and the inboard ski market, using GPS Speedos? It seems that whenever anyone goes to calibrate their speedo, they have to go out and buy a GPS system anyway, which isn't cheap. The Paddlewheels is a good start, but still, you cannot beat GPS... What's your oppinion...

[surroundsound64]

I think they are a little overkill for a ski boat...

Also, it'd be pointless for anyone useing it in a river where there is current, or even some lakes!

And I think it'd just make CCs, or any other boat, that much more expensive.

Just personal preference though.

[AbunDiga909]

Well then there's personal preference, when you bring up current. When you're going for a joy ride, and are timing how far it is to someplace, for example, you'll want your true speed, GPS. But when you're boarding and just want your resistance speed, then the paddlewheel is right for you, I'd assume. I just thought it'd be nice to drive down the lake the same speed with the speedo showing the same reading throughout.

[Mikeski]

Surround,

Actually the GPS system would work better than any boat based unit in a river with current for slalom use. The speed of the water traveling by the bouys just adds an undesireable variable to the skiing and driving. The GPS speedo would eliminate one of the aspects of that variable. I have been in a few boats with portable GPS speed readouts and I don't think they respond quick enough for our needs, maybe the expensive units would?

Mike

[surroundsound64]

Surround,

Actually the GPS system would work better than any boat based unit in a river with current for slalom use. The speed of the water traveling by the bouys just adds an undesireable variable to the skiing and driving. The GPS speedo would eliminate one of the aspects of that variable. I have been in a few boats with portable GPS speed readouts and I don't think they respond quick enough for our needs, maybe the expensive units would?

Mike

I'm confused, the GPS wouldn't calculate your speed relative to the water. Lets say the current is moving 3mph aginst you. For a slalom skiier it doesn't so much matter what your forwards speed is so much, it is your relative speed through the water, or the speed of the ski over the water that matters. So, if you want to pull the skiier at 32mph (or through the water), your "true" speed needs to only be 29mph. When the 29 is added to the 3mph of current moveing opposite you, you have the relative speed.
The GPS has no way of knowing, unless it was plugged in, what the speed of the current is.

Like in a plane with the wind. If a plane needs 50mph of airspeed to create enough lift over the wings, and the wind is moving at 25mph aginst him, his true airspeed only needs to be 25mph in order to get off the ground.

Unless I'm confused about something, which is always a possibility. :grin:

[ag4ever]

Surround is right, except for the plane analogy, his GROUND speed would need to be 25 MPH to take flight, because his airspeed would be 50 MPH with a 25 MPH head wind.

But Mikeski is right on the slalom. The course would have to be floating with the current for the water based speedos to be accurate. The course is timed from when you enter the first gate to exit the last, and if they are stationary, then the GPS would be better, but then for free skiing, the boat based would be better.

[surroundsound64]

Surround is right, except for the plane analogy, his GROUND speed would need to be 25 MPH to take flight, because his airspeed would be 50 MPH with a 25 MPH head wind.
.

Ah! Thanks for the correction!

[Mikeski]

My post was directed toward a slalom course situation, the basis for the speed control invention in the first place.

In reality speed is irrelevant for wakeboarding. You only want something to use as a reference for the wake development. Apparent speed works well for that which is what our water based speedos display. GPS displays actual speed.

[jthooker]

Quoting from the AWSA rule book, Section 8.05, re: Boat Speeds:

All speeds are “speeds over the bottom”; compensate in all cases where a current exists.

GPS provides “speed over the bottom” and is not affected by current so compensation would not be required as with pitot tube or paddle wheel pickups. However SOtB is really only relevant to slalom and jump since speed is a skier’s choice for tricks and wakeboard.

Strictly speaking, boat speed isn’t constant in slalom and jump. It’s a calculated value based on time versus distance measurements through the buoys. The instantaneous boat speed varies depending on the pull of the skier at any given point in the course. Most skiers would find that a “rock solid”, unvarying speed (if it were even possible) would be uncomfortable and unforgiving.

A good GPS speedo might help a driver to maintain a more constant speed if it was more responsive and stable than the paddlewheel or pitot tube speedos. But even Perfect Pass does not use a speed feedback loop to control boat speed for slalom and jump events. It controls engine RPM based on baseline time/distance parameters and on variable parameters that are entered for each skier. In that respect, it mimics the pull of a good throttle man behind the wheel. With the addition of a slalom (or jump) switch it mimics the pull of a very good throttle person who slightly anticipates the skier’s actions.

Now if you hooked the GPS into the Perfect Pass … ?

If you want to read a lively discussion about speed control, check out this thread on Skifly:

http://p205.ezboard.com/fwaterskiing...cID=1075.topic

Whew …. having said all of that, I think that a GPS speedo with some of the other GPS goodies would be a good option. If nothing else it would add some more gee-whiz techno stuff to play with between sets.

[j2nh]

We use Lavorsi GPS speedos in our Hydrodyne outboards that are used for pulling show ski acts.
GPS speedos are great and do not foul. Cost is around $250.

BUT

Response is slow. Depends on what you are doing of course but when driving you do have to compensate for the lag during acceleration.
Having drive both the paddle wheel is by far the better system and the only one you could incorporate into a speed control system.

[siuski]

I don't have PP in my boat, but I do have a GPS. The GPS works well enough to pull my wife jumping in practice. She doesn't feel much of difference between the PP and the GPS. However for wakeboarding, I only calibrate the speedo's once in a while with the GPS.

I'm confused with jthooker's post of the AWSA rule...does that mean that if your in a river that has a 3mph current, going upstream on a corse that is fixed on the bottom, that to meet the course boat times from gate to gate the skiier would actually be skiing 3 mph faster? How are those tournaments run? The only tournament that I can think of on a river is the MOOMA masters...I'm sure more are out there.

I also question the accuracy of the paddle wheel...how does the direction of the flow of water affect it? The water flow over the bottom of the boat is angled out on a v-bottom boat...then factor in the different attitidude angles at various speeds. Has anyone correlated the speed of the paddle wheel vs. a GPS? Not that matters or it isn't accurate enough for wakeboarding or tricking...just this post got me thinkg.

[ajskier]

I think that jthooker is right on. The rule the way I interpert it is, that if you have a current that the speed needs to adjust accordingly to the speed of the current of the river. I ski on a river all of the time in Columbus. Say the current is 3 mph and we are going up stream against the current, the boat would have to run about 39 mph to give me a pull at 36 mph and if I was going down stream I would only have to have the boat run at 33 mph.

We don't use PP on the river, but I am assuming that it would work very similar to that, just because the PP is still trying to maintain that speed. This would be very similar to the set-up on PP that allows you to adjust for head and tail winds.

Just a thought, but for people using PP on rivers maybe Perfect Pass(the company) should make an adjustment for river current. I don't know if that is something that they have thought of or not.

I am just taking a guess at this, but it seems to make sense in my head.

After reading this, it makes me think of the thread that is going on over on the skifly site. They have been talking about speed and how is affects jumpers and slalom skiers. People have mentioned using a GPS, but they also say that it is not fast enough to keep up with the speed swings in these events, but for wakeboard and trick, I think that it would work because the speed is to remain constant.

[magene]

Part of my job involves precise positioning of ships at sea using GPS receivers and thought some information might be useful. As usual, you get what you pay for. Generic, recreational GPS receivers can only measure position and calculate speed by taking the difference between two position measurements and dividing by the time between measurements. These receivers have an accuracy of about 15 feet (10 feet or so if you have a differential GPS) and can measure a position about once a second. A ski boat at 34 mph travels about 50 feet per second (if I did the math right) so a 15 foot error is pretty significant when calculating speed unless you average over several measurements, hence the slow update rate for velocity.

You can get pretty good velocity accuracy if you are willing to spend $25-50 grand or so for survey quality GPS equpment and are able to locate a GPS base station on shore at a fixed location near the course. You can get position accuracies on the order of inches and update rates of 20 to 100 times a second. This kind of setup would probably be significantly more accurate than a paddle wheel speedo. As a side benefit, you would also get an accurate track of the boat through the course.

If you really wanted to spend some time and cash, you could use this approach to provide the speed input to a perfect pass system instead of the paddle wheel. Whether it would be better than a stock perfect pass or an experienced driver would be an interesting experiment.

Hope this wasn't too boring

Gene

[bkhallpass]

It will be intersting to see how waterskiing evolves. Many argue at 43 off, the skiers are
now reaching the physical limitations of what a normal sized (under 6'6") individual
can do. There's just not much length available. What will they do next. Perhaps
using the expensive systems will come into play so that the person who breaks the
record with the truest speed is the winner? Perhaps they'll just increase the speed?

BKH

[ScarletArrow]

You can get pretty good velocity accuracy if you are willing to spend $25-50 grand or so for survey quality GPS equpment and are able to locate a GPS base station on shore at a fixed location near the course. ....If you really wanted to spend some time and cash, you could use this approach to provide the speed input to a perfect pass system instead of the paddle wheel. Whether it would be better than a stock perfect pass or an experienced driver would be an interesting experiment.

Hope this wasn't too boring

Gene

No, not boring at all...and I think $25 - 50 grand on a GPS Speedo is a reasonably priced assesory. :mrgreen: