speedometer

[marshan162]

Maybe this will make it easier to understand

If you walked at 3 MPH along a conveyer belt that is travelling at 3MPH. Your speed relative to any fixed point(GPS) would be

1. In one direction 6MPH
2. and if you walked back in the opposite direction 0MPH
3. and if you just stood still 3MPH

[Jon Fuller]

I think it needs to be simpler than that!

[larby]

Originally Posted by Burty

best way to go is with a radar gun as they dont need to compensate for wind and tide

That would depend on whether the radar gun is on the water or the shore!

It's amazing how much of this we get over here. People do a run with the tide down the 'river' at the back of herm, and amazingly they're boat is 8kts quicker!!! Magic!!!

[OCRDA]

On your Etec you can input and display both water and GPS Speed on same guage.
How do you get on with the trim guage mine seemed to give all sorts of odd readings.

Bob

[Jon Fuller]

Originally Posted by larby

T People do a run with the tide down the 'river' at the back of herm, and amazingly they're boat is 8kts quicker!!! Magic!!!

You have some wicked tidal speed over there! Not surprising with a 40' rise & fall on springs. A perfect example of the simple statement I'm trying to make. A land based radar would have the same errors as gps because it measures GROUND SPEED. Sure, you can perform a set of two way runs with gps to acertain your exact speed, but a simple good quality pitot will make it all a lot easier, and at a glance, you'll know how fast you're going, or be able to make a good comparison with previous speeds attained. It may not be acceptable for record breaking, but for prop testing, or just wanting a non bullshit speed indication that's as spot on as you could need, they're perfic.

With the courses/legs Bob does, I'm surprised his gps gets a chance to settle down & decide what direction he's going, let alone give an accurate speed.

I'll bet you all them 'Herm River' racers quote that +8 knt speed as their best top speed, which takes us back to what I said in my first post.

[larby]

Originally Posted by Jon Fuller

I'll bet you all them 'Herm River' racers quote that +8 knt speed as their best top speed, which takes us back to what I said in my first post.

Nah, they all add at least 10mph after the tidal increase!!
There's a ring 20 with a 200 rude over here that does 95mph I'll have you know!! A while back i told someone my cat did just under 80 whilst prop testing and i got told 'my mate's ring 16 goes waaaay faster than that with a 75!' Yeah, sure it does mate!

James

[Jon Fuller]

Originally Posted by larby

A while back i told someone my cat did just under 80 whilst prop testing and i got told 'my mate's ring 16 goes waaaay faster than that with a 75!' Yeah, sure it does mate!

James

That evidence suggests neither pitot or gps were involved, just a jackanory-ometer.

[marshan162]

If you want to check or calibrate your Pitot Speedo use this spread sheet
you will need a known good pressure gauge on a compressed air supply

Just enter the speed in MPH and it will show the pressure reading you should need

[Jon Fuller]

That's a useful spreadsheet, thanks.

I have a chart with the stagnation pressure curve given to me by Bill Maloney, but that wee jobbie is cool. interesting that the mph/psi crossover at 70 happens to be at that magic number that can be so hard to pass.

[marshan162]

I think these £5 digital tyre pressure gauges are supposed to be around 0.5 PSI accurate so can be used as a reference.

No gauges have linear error so the best plan is to calibrate the gauge at the speed you are most interested in, and i guess that would be 70 - 90 for most of us

[marshan162]

Originally Posted by Jon Fuller

That's a useful spreadsheet, thanks.

I have a chart with the stagnation pressures given to me by Bill Maloney, but that wee jobbie is cool. interesting that the mph/psi crossover at 70 happens to be at that magic number that can be so hard to pass.

Thanks I'll smarten it up a bit and add a choice of salt or fresh water for the more pedantic amongst us. I only slung it together for this post

[marshan162]

Originally Posted by Jon Fuller

interesting that the mph/psi crossover at 70 happens to be at that magic number that can be so hard to pass.

Another interesting observation is that at 317 MPH it produces nearly 1500 PSI. More than a few tie raps for the plumbing on Bluebird i think.

Shows the stress that any underwater protrusions, fins etc, are under at serious speed.

[Jon Fuller]

We don't really know if Bluebird went over 300. But Mr Warby certainly did. although his record is 317, I wonder what his best one way speed was. Those numbers are massive eh. And I think his boat was timber!. The rudder stock must have had some good long screws holding it on.

[Hydrostream]

That's interesting!
So, how do you set the speedo then?

A

[marshan162]

Originally Posted by Hydrostream

That's interesting!
So, how do you set the speedo then?

A

If you have screws on the back you are in luck otherwise you have to break into it and again if there are no screws you can pull the needle off and repostion it. This means zero and low numbers will be out but who cares.
If you get it bang on at say 70, it is still going to be pretty good at 60 and 80

Next option is to remark the dial or finally just make a chart of the errors for tranlating later. Lets face it the only reason for any accuracy is to make comprisons at the top end, so an accurate 10 MPH band at the top is probably sufficient