How good are the experts.

coliholic

New member
Joined
11 Dec 2001
Messages
3,969
Location
Cambridge
Are they allowed to be totally wrong?

Just been reading the Ask The Experts Forum and there's a question there from a chap who's had his AQD32 engines rebuilt in his Princess 32 but still only getting 12 knots as against his anticipated 20knots.

Pat Manley advises caution in how the actual true boat speed is measured. His comments are

< By the way, GPS is a very poor tool for measuring speed due to the imposition of 'selctive availability by the US military. Decca is much better. So that you don't have to take into account the tide, a calibrated speed log is the best method.>


Am I missing something here? Or maybe Pat is the ideal person to sell all those old Decca's to?
 

coliholic

New member
Joined
11 Dec 2001
Messages
3,969
Location
Cambridge
Have a look yourself Byron, PBO right at the bottom of the Navigation Bar thingy, right at the bottom of your screen where all the different mag names are. (Must admit I only just found it by accident, didn't know it was there). Follow link to Ask the Experts and it's the question about AQD32's. No date on any of it. Great innit?

Perhaps you could ask Pat how to get your Decca working again. Tell him it's "just stopped working". Be interesting to see the reply.
 
G

Guest

Guest
Can't imagine when the question and answer was posed, but judging by Pats photo on the PBO site it
can't be that long ago. It maybe time for dear ole Pat to retire. He took me on my VHF course last
year, upon which subject he knows his stuff. However if he thinks Decca it more accurate than
GPS. he clearly has lost the plot. Has he not heard of selective availability being dropped
or about the demise of Decca. Whats tide got to do with a GPS reading SOG ?


cneighbour
 

oldgit

Well-known member
Joined
6 Nov 2001
Messages
27,330
Location
Medway
Surely if going with tide can add 2/3/4/ knots to speed or subtract if going other way.Unless of course in jump across titchy size river like thames with no flow to speak off above locks(All now closed for repairs because of Thames boaters bang crash into gates etc).
 

Col

New member
Joined
14 Oct 2001
Messages
2,577
Location
Berks
It was speed over the ground they were talking about, not speed through water.
The speed through water is the proper way of measuring speed of boat, you could use SOG off GPS, just take a run in opposite directions, then take the average SOG to determine boat speed through the water. Does this make sense ?
 

boatone

Well-known member
Joined
29 Jul 2001
Messages
12,842
Location
Just a few cables from Boulters Lock
I also have a Princess 32 with twin AQD32's and 270 legs but havent a clue how fast it will go as I'm based on the non tidal Thames!
Some literature I picked up from a boat directory publication says the P32's were available with single or twins ranging from 75 to 140 HP with smaller engines giving max 10 knots single 18-20 with twins and twin 140's up to '25knots or more' .
I have recently been doing some capacity sums and the combined weight of full fuel and water tanks on mine is some 1600lbs - roughly equivalent to 10 people. So I guess full tanks would make quite a difference to performance!
I have just had my props refurbed and the pitch altered slightly to let me run slightly higher revs for river speeds.


TonyR
boatone@boatsontheweb.com
 

Bergman

New member
Joined
27 Nov 2002
Messages
3,788
Yes, but only if running in same direction as tide.

If doing runs across tide flow will not give true answer.
 

hlb

RIP
Joined
16 May 2001
Messages
26,775
Location
Any Pub Lancashire or Wales
Well That just knacked the latest anouncment Kim.
Quote'ish. " Dont answer thread unless you know the answer and are clever and have experience." Theres a fat lot of sense in trying to get this lot to give clever advice if the experts cant.

Haydn
 
G

Guest

Guest
Point is tides are academic to GPS sats as they track you cos they record SOG as it happens. well
almost, just slight delay in transmission, but non the less speed over ground. The satalite does'nt
know your on the water.

cneighbour
 
G

Guest

Guest
Re: Last post ment to be to Oldgit

Last post forgot to change title to read oldgit

cneighbour
 

tcm

...
Joined
11 Jan 2002
Messages
23,958
Location
Caribbean at the moment
Re: but...

Incidentally, differential or non-diff gps does not make any difference to accuracy of speed readings, merely to accuracy of lat/long position. All gps devices that work and have batteries etc...will show 0.00 knots at standstill and will be similarly accurate SOG at any other speed.
 

adarcy

New member
Joined
31 Aug 2001
Messages
844
Re: How come?

Surely all GPSs measure speed by distance moved between lat/long position v time therefore any variability in the accuracy of the fix will affect the speed estimated.
Our computer and nav guru has (or had) a program on his laptop which followed the fix from the GPS and drew the "wander" as the fix varied when we were anchored or tied up. It has certinly got much tighter since SA has been switched off but not gone altogether ( we do not have DGPS). But if the fix varies how else is a GPS going to calculate speed? Doppler against the satellite - I don't think so.

Anthony
 

coliholic

New member
Joined
11 Dec 2001
Messages
3,969
Location
Cambridge
It's still there. I'm checking regularly. Just to make sure you're not falling down on the job;-)
 
Top