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30-07-2009, 07:25 PM
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#21
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Registered User
Country: England
Location: Hampshire
Occupation: Credit Crunched
Interests: Rally Driving
Boat name: Double 2 Shirts
Boat make: Planatec 41
Engines: 2x625's+ From Uncle Sam
Cruising area: Solent
Join Date: May 2007
Location: Hampshire
Posts: 2,522
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Quote:
Originally Posted by Pash 1
I did its distance over time......
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and you answered it for me with your interpretation of the equation..
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31-07-2009, 08:25 PM
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#22
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numbskull
Country: United Kingdom
Location: South
Occupation: none
Interests: none
Boat name: Leviathan
Boat make: Phantom 28
Cruising area: South Coast
Join Date: Mar 2004
Location: South
Posts: 15,959
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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
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31-07-2009, 08:39 PM
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#23
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Registered User
Join Date: Jun 2009
Posts: 670
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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
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31-07-2009, 08:44 PM
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#24
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numbskull
Country: United Kingdom
Location: South
Occupation: none
Interests: none
Boat name: Leviathan
Boat make: Phantom 28
Cruising area: South Coast
Join Date: Mar 2004
Location: South
Posts: 15,959
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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.
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31-07-2009, 08:53 PM
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#25
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Senior Member
Country: UK
Location: Devon
Occupation: Garage Proprietor
Interests: PowerBoat Racing
Boat name: If Only
Boat make: Bernico F3 OCR, Bernico Prototype Inboard, and some Ribs
Engines: Yamaha Pro V 115
Cruising area: UK, France
Join Date: Jan 2005
Location: Devon
Posts: 3,095
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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
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If Only
National Outboard Immersed Propeller Mono Record 103mph
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31-07-2009, 08:57 PM
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#26
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numbskull
Country: United Kingdom
Location: South
Occupation: none
Interests: none
Boat name: Leviathan
Boat make: Phantom 28
Cruising area: South Coast
Join Date: Mar 2004
Location: South
Posts: 15,959
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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?
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31-07-2009, 08:59 PM
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#27
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Registered User
Join Date: Jun 2009
Posts: 670
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Quote:
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.
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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
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31-07-2009, 09:04 PM
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#28
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Registered User
Join Date: Jun 2009
Posts: 670
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Quote:
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.
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works for aircraft
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31-07-2009, 09:10 PM
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#29
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numbskull
Country: United Kingdom
Location: South
Occupation: none
Interests: none
Boat name: Leviathan
Boat make: Phantom 28
Cruising area: South Coast
Join Date: Mar 2004
Location: South
Posts: 15,959
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I suggest you get yourself an air speed indicator then.
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31-07-2009, 09:14 PM
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#30
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Registered User
Join Date: Jun 2009
Posts: 670
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Quote:
Originally Posted by Jon Fuller
I suggest you get yourself an air speed indicator then.
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31-07-2009, 09:16 PM
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#31
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Traveling
Country: UK
Location: Alderholt
Occupation: Aerospace
Boat name: T/T D2S
Boat make: Midas 27' Cat, Argo 16 Cat. Avon Rib Thingy
Engines: Merc 280-ROS -JSRE,65Xs, 75 Stinger, Yam 60
Cruising area: Any Seedy Bar
Join Date: Sep 2004
Location: Alderholt
Posts: 4,225
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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
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31-07-2009, 09:19 PM
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#32
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Traveling
Country: UK
Location: Alderholt
Occupation: Aerospace
Boat name: T/T D2S
Boat make: Midas 27' Cat, Argo 16 Cat. Avon Rib Thingy
Engines: Merc 280-ROS -JSRE,65Xs, 75 Stinger, Yam 60
Cruising area: Any Seedy Bar
Join Date: Sep 2004
Location: Alderholt
Posts: 4,225
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Quote:
Originally Posted by baj666
works for aircraft
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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.
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31-07-2009, 09:24 PM
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#33
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numbskull
Country: United Kingdom
Location: South
Occupation: none
Interests: none
Boat name: Leviathan
Boat make: Phantom 28
Cruising area: South Coast
Join Date: Mar 2004
Location: South
Posts: 15,959
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Quote:
Originally Posted by BluFin
is it gallons per hour
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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.
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31-07-2009, 09:28 PM
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#34
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Registered User
Join Date: Jun 2009
Posts: 670
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Quote:
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.
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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??
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31-07-2009, 09:31 PM
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#35
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Registered User
Country: Guernsey
Occupation: Engineering
Boat make: None, boo!
Engines: Turbines mainly!
Cruising area: The inside of my workshop!
Join Date: Mar 2008
Posts: 2,646
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Quote:
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??
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Seems to be the case generally here lately!
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31-07-2009, 09:48 PM
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#36
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Traveling
Country: UK
Location: Alderholt
Occupation: Aerospace
Boat name: T/T D2S
Boat make: Midas 27' Cat, Argo 16 Cat. Avon Rib Thingy
Engines: Merc 280-ROS -JSRE,65Xs, 75 Stinger, Yam 60
Cruising area: Any Seedy Bar
Join Date: Sep 2004
Location: Alderholt
Posts: 4,225
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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.
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31-07-2009, 09:53 PM
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#37
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Traveling
Country: UK
Location: Alderholt
Occupation: Aerospace
Boat name: T/T D2S
Boat make: Midas 27' Cat, Argo 16 Cat. Avon Rib Thingy
Engines: Merc 280-ROS -JSRE,65Xs, 75 Stinger, Yam 60
Cruising area: Any Seedy Bar
Join Date: Sep 2004
Location: Alderholt
Posts: 4,225
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Quote:
Originally Posted by Jon Fuller
Next you'll be telling me that drilling the hole bigger in the pitot will increase indicated speed.
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You know thats an Airbus trade secret
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It is impossible to lick your elbow.
Team25Racing.com
At least 75% of people who read this will try to lick their elbow!
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31-07-2009, 09:53 PM
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#38
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Senior Member
Country: UK
Location: Devon
Occupation: Garage Proprietor
Interests: PowerBoat Racing
Boat name: If Only
Boat make: Bernico F3 OCR, Bernico Prototype Inboard, and some Ribs
Engines: Yamaha Pro V 115
Cruising area: UK, France
Join Date: Jan 2005
Location: Devon
Posts: 3,095
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Quote:
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?
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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
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National Outboard Immersed Propeller Mono Record 103mph
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01-08-2009, 12:00 AM
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#39
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numbskull
Country: United Kingdom
Location: South
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Boat name: Leviathan
Boat make: Phantom 28
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Join Date: Mar 2004
Location: South
Posts: 15,959
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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.
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01-08-2009, 01:03 AM
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#40
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Registered User
Country: UK
Location: Weston Super Mare
Occupation: Electrical Engineer
Join Date: Mar 2004
Location: Weston Super Mare
Posts: 6,351
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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
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