How good are the experts.

[coliholic]

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?

 

[byron]

LOL! When was the question and answer posed.

ô¿ô

 

[coliholic]

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.

 

[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

 

[kimhollamby]

Okay okay. Daggers back in scabbards please, point made. Definitely a mistake or old advice but I'll get it changed...

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[oldgit]

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]

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 ?

 

[kimhollamby]

Yes, that's exactly the way boat tests are conducted, either with radar or these days thanks to differential and/or no SA, GPS.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[trev]

I would have thought 20 knots rather ambitious for 32s, fourteen on a good day and the arrived at twelve for old engines aint too bad.

Trev

 

[boatone]

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]

Yes, but only if running in same direction as tide.

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

 

[hlb]

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

 

[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

 

[Guest]

Re: Last post ment to be to Oldgit

Last post forgot to change title to read oldgit

cneighbour

 

[tcm]

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]

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]

It's still there. I'm checking regularly. Just to make sure you're not falling down on the job;-)