speedos

[95glas]

normal for them not to register til near top speed? i have pilot tube style. but i am thinking of upgrading my gauges.. which best cost per effectiveness regular, liquid or gps?

 

[Philster]

Re: speedos

Regular pitot air pressure is ok.

GPS is best, but can still be off as it measures speed over land, not speed over water.

There usually isn't a need for anything beyond a pitot type. But, if you invest in a GPS system, you get the speed reading with it.

I have a GPS speedometer wired to my Garmen GPS. I wasted money. No need for it.

 

[Mntom]

Re: speedos

GPS is best, but can still be off as it measures speed over land, not speed over water.

Um.... GPS units satellites for measurements. They don't care if you are over land, water, or air. They are as accurate they can be no matter where they are.

 

[95glas]

Re: speedos

how accurate are regular speedos? mine doesn't register anything til near top speed. i remember reading in one of the boating mags speedos on new boats are usually off 10 mph on the high side- so on my 1995 boat..

 

[mogfisher]

Re: speedos

Um.... GPS units satellites for measurements. They don't care if you are over land, water, or air. They are as accurate they can be no matter where they are.

I think what he is getting at is that a gps cant compensate for water currents and wont give an accurate reading of water speed. Not a problem for most, but for trollers fishing in a strong current it might come into play. Otherwise, yes, a gps will give a very accurate measurement of speed no matter what.

 

[Hank496]

Re: speedos

From a navigation point of view (and getting an estimate of how long it will take you to get somewhere) speed from a GPS is best. It shows SOG (Speed Over Ground) in this case 'ground' means surface of the earth.

A pitot tube measures speed of the boat though the water. This is where this type of speedometer can get inaccurate because it can not measure how fast the water is going. For example, a boat is moving upstream in a river and the pitot speedo says it is doing 25 mph at 3500 rpm. But the river is moving at 10 mph. As a result the net speed of the boat going against the river current is 15 mph. Now the driver turns the boat around and goes down stream at 3500 rpm. Speedo still reads 25 mph, but actual speed of boat is 35 mph.

 

[ziggy]

Re: speedos

gps is best....

a gps measures sog (speed over ground) it comensates for everything.

if on a river and i go up river at 3k rpms and speed is 25 mph. that's how fast i'm going for sog up river.
if on a river and i go down river at 3k rpms and speed is 35 mph. that's how fast i'm going for sog down river.
these are the true speeds i'm going at. the speed over ground. the river is moving at 5mph. that don't change, but my speed does because i'm either going with the current or against it.

any other devise to measure speed, pitot tube, paddle wheel will not register the proper speed. it will not compensate for speed of river current.
if i were stationary with either of the above devises and facing up river. the speedo would read 5 mph since that's the speed of the water moving underneath me. but i would be stationary and doing 0 mph. a gps would read 0 mph.

 

[Teammuir1]

Re: speedos

ZIGGY
I AGREE 110%
besides.. you can go with a GPS that has other features that
maybe your depth finder fish finder might not have....
but with the Technology as it is today... GO WITH GPS.... just
my 2 cents.

 

[Philster]

Re: speedos

Um.... GPS units satellites for measurements. They don't care if you are over land, water, or air. They are as accurate they can be no matter where they are.

In the canal, with the tide barreling out....

My boat's top speed is 69, according to my GPS.

If I turn around it's 56 according to my GPS.

My boat's top speed is really 63.

 

[metriccrescentwrench]

Re: speedos

normal for them not to register til near top speed?

No, this behavior is not normal. Something is wrong, either a restriction in the line between the pitot and speedometer, a restriction in the pitot, or the speedometer is just plain messed up. I've got both the speedometer and gps (by virtue of it being incorporated into my fishfinder) and in my case the speedometer is fairly accurate up to around 25. After that the speedometer starts reading a little high, at top speed it reads 48 and gps says 45 (still not bad).

IMHO, unless you have some compelling reason to spend a lot of money, when you replace your gauges I'd get a new speedometer (along with a new pitot and tubing).

 

[95glas]

Re: speedos

No, this behavior is not normal. Something is wrong, either a restriction in the line between the pitot and speedometer, a restriction in the pitot, or the speedometer is just plain messed up. I've got both the speedometer and gps (by virtue of it being incorporated into my fishfinder) and in my case the speedometer is fairly accurate up to around 25. After that the speedometer starts reading a little high, at top speed it reads 48 and gps says 45 (still not bad).

IMHO, unless you have some compelling reason to spend a lot of money, when you replace your gauges I'd get a new speedometer (along with a new pitot and tubing).

thanks was going to go the gps route but don't think good dollar/value return ..maybe if had something with more power/ needed record ability .. biggest reason wanted was to make sure made best decision on a prop - but i think alot easier just to use a handheld for that if necessary

 

[Silvertip]

Re: speedos

Quote:
In the canal, with the tide barreling out....

My boat's top speed is 69, according to my GPS.

If I turn around it's 56 according to my GPS.

My boat's top speed is really 63.

Those are indeed true over the earth speeds because you have three different dynamics. Tide going out carries you at tide speed plus boat speed. Going into the tide your speed is boat speed minus tide speed. No tide your speed is 63. All three are accurate GPS numbers and represents your true speed in whatever direction you happen to be going. Put your boat on water with no current but a 40 MPH wind. Going with the wind the GPS will show true OTG speed because the wind is giving you a push (obviously not 40 MPH worth). Going into the wind the GPS is showing true OTG speed because the wind is working against you. With no wind or current your true speed is 63 as the GPS indicates.

 

[a70eliminator]

Re: speedos

how accurate are regular speedos? mine doesn't register anything til near top speed. i remember reading in one of the boating mags speedos on new boats are usually off 10 mph on the high side- so on my 1995 boat..

I've responded a couple times now to this same question.
The speedometer in my Chaparral starts with O then 20, then 30, after 30 it then begins to show 1 mph increments<so no they aren't very accurate below planing speeds, at least mine isn't.