speedometer

[Pash 1]

Originally Posted by Pash 1

I did its distance over time......

and you answered it for me with your interpretation of the equation..

[Jon Fuller]

Rob, to get back to your original question, my personal opinion is that a decent quality pitot (water stagnation pressure) speedo is the most stable, repeatable and bullshit free way to measure your boat speed.

You could go for GPS only, but for prop testing etc, they don't give the repeatability you idealy want. Your indicated speed will vary with the body of water in which you're moving, so it's impossible to get a good handle on your speed without doing at least a 2 way run. they also exhibit spurious high (and low) readings for no apparent reason.
We've heard there are high voltage reasons, both positive & negative for using them, I think they come into their own for navigating, where they are without doubt the absolute bollox.

My vote if you're looking for a cheap option, check out ebay for a used Gaffrig, or advertise on here, maybe a gps convert will sell you his.

Jon

[baj666]

sorry to butt in here but i just cant get my head around this...is speed not measured by point to point??..if i had a speedometer working by the air pressure then the true speed of an object would be whats above the water and not below..and why dont marine gps have compensators below the water line to give a reading allowing for water body speed

[Jon Fuller]

If it were that easy to have an accurate 'compensater' below the waterline, why would you need the gps?

And if you were using an air speed indicator, running with a 30knt tail wind, and you had a 25knt boat, you'd indicate minus 5knts. a lorra use that, almost as good as a gps.

Rob asked for an opinion, I've given mine, as have a few others, some informed, some not. Rob can take who's advice he likes.

[OCRDA]

I have used a Garmin GPS 111 for many many years now and can honestly say it is spot on (point to point) . If you want to be ultra critical you can add a Yamaha Digital Speedo to take a comparison with.

GPS 111, 1V, or V often on ebay and I could sell you a new Yam Speedo cheap. How accurate do you want to be ?? I could also fix you up with a data logger that has input for water and gps speed.


Good Luck

Bob

[Jon Fuller]

So Bob., if you're out boating, and you run into the tide (lets say a spring, just for arguments sake) and then run with the tide, does your gps read the same speed both ways?

[baj666]

Originally Posted by Jon Fuller

If it were that easy to have an accurate 'compensater' below the waterline, why would you need the gps?

And if you were using an air speed indicator, running with a 30knt tail wind, and you had a 25knt boat, you'd indicate minus 5knts. a lorra use that, almost as good as a gps.

Rob asked for an opinion, I've given mine, as have a few others, some informed, some not. Rob can take who's advice he likes.

not saying your wrong or offering advice ime just trying to get my head around this speed thing and looking,reading and thinking out aloud

[baj666]

Originally Posted by Jon Fuller

If it were that easy to have an accurate 'compensater' below the waterline, why would you need the gps?

And if you were using an air speed indicator, running with a 30knt tail wind, and you had a 25knt boat, you'd indicate minus 5knts. a lorra use that, almost as good as a gps.
Rob asked for an opinion, I've given mine, as have a few others, some informed, some not. Rob can take who's advice he likes.

works for aircraft

[Jon Fuller]

I suggest you get yourself an air speed indicator then.

[baj666]

Originally Posted by Jon Fuller

I suggest you get yourself an air speed indicator then.

[BluFin]

Out of intrest what is the units used for instrument on the pressure system, is it gallons per hour because it cant be miles or knots as the are set distance.

This far too static can we bring in Wind effect as well

[BluFin]

Originally Posted by baj666

works for aircraft

NO No No.... not much good below 60 and gross inaccurate at the lower range. Air is worse that water at least in basic Physics you cant compress the wet stuff.

[Jon Fuller]

Originally Posted by BluFin

is it gallons per hour

Gallons per hour? There's no flow as such Johnny boy, the whole point is that it's the stagnation pressure at the pitot that's measured.

Next you'll be telling me that drilling the hole bigger in the pitot will increase indicated speed.

[baj666]

Originally Posted by BluFin

NO No No.... not much good below 60 and gross inaccurate at the lower range. Air is worse that water at least in basic Physics you cant compress the wet stuff.

so i guess theres no room for friday night banter...sarcasim or any other kind of tongue in cheek comments light hearted or not without sounding like
1...your out to piss people off
2...your a complete idiot who should'nt be in charge of a watercraft
3...your not worthy of a humourous reply back
now what was the thread about??

[larby]

Originally Posted by baj666

so i guess theres no room for friday night banter...sarcasim or any other kind of tongue in cheek comments light hearted or not without sounding like
1...your out to piss people off
2...your a complete idiot who should'nt be in charge of a watercraft
3...your not worthy of a humourous reply back
now what was the thread about??

Seems to be the case generally here lately!

[BluFin]

If I could give an example & use the coming weekends race at Torquay as an example.

You have two identical 6 ltr cats one with a GOOD quality Pitot system & the other a GPS there is a 5 kt tide running & the wind is 15 kts over the tide which boat would get to the finish line first......................





Easy..... the fastest one over the race.
Conclusion
It don’t matter what’s fitted the type of instrument don’t matter but the quality of the instrument does. Cheap pressure ones are inaccurate a decent one is better but then approach the price of a GPS one. Livorsi are more expensive than Nordskog. I went for what I did as it’s a multi function instrument & ltd dash panel area was available. Speedo seems accurate; RPM is a very easy read so is the compass - volt meter thrown in as well. Even if its functions are there as a backup – they are there just in case one of the others goes tits up. Ya pays ya money ya makes ya choice. A good book on navigation basics is well worth the money too. Covers all the wind tide correction stuff.

[BluFin]

Originally Posted by Jon Fuller

Next you'll be telling me that drilling the hole bigger in the pitot will increase indicated speed.

You know thats an Airbus trade secret

[OCRDA]

Originally Posted by Jon Fuller

So Bob., if you're out boating, and you run into the tide (lets say a spring, just for arguments sake) and then run with the tide, does your gps read the same speed both ways?

No it reads the speed that will get you from A to B and if you are remotely interested in how fast the water is going you can compare it to the water one.
On record runs at coniston the GPS was very accurate both with and against the tide and wind.

UIM Circuit rule book around the 600ish rules tells you all about speed and how to measure it.

Bob

[Jon Fuller]

Oh, is Coniston tidal now then?

Never mentioned the effect of wind, not really relevant, other than maybe our friend using an air speed indicator.

I don't need to read a little book to understand the relationship between speed over ground and water speed.

[Burty]

our new rude came as standard with a gps speedo i dont use it to see the actual speed but to see the difference trim and water ballast makes. both measurements are as good as each other but as you say you need a 2 way run to confirm, best way to go is with a radar gun as they dont need to compensate for wind and tide