What Type of Speedo?

[Jurgen]

Hi All, My boat currantly has a Pito Tube type speedo that's not working so have been using my GPS for speed info.

My question is what is the most accurate speedo

Pito Tube
Paddle Wheeel
GPS

Regards
Jurgen

 

[benjenbav]

Just avoid the skintight ones.

Oh, sorry, not that sort of speedo...

 

[Jurgen]

Just avoid the skintight ones.

Oh, sorry, not that sort of speedo...

Especialy when the S falls off lol

 

[gjgm]

Hi All, My boat currantly has a Pito Tube type speedo that's not working so have been using my GPS for speed info.

My question is what is the most accurate speedo

Pito Tube
Paddle Wheeel
GPS

Regards
Jurgen

well paddle wheel is going to give you speed through the water, whereas gps, over the ground.

 

[Ripster]

For traveling about I always use SOG via the GPS. I am more interested in the actual speed I am going, rather than how fast I happen to be travelling through the water. If I am testing engine performance say against RPM then I use the speed log, which on mine is a paddle wheel (if its not jammed with weed that is!).

 

[EugeneR]

Hi All, My boat currantly has a Pito Tube type speedo that's not working so have been using my GPS for speed info.

My question is what is the most accurate speedo

Pito Tube
Paddle Wheeel
GPS

Regards
Jurgen

GPS is the most accurate and gives you your actual speed over ground and/or over the chart. If your speed was much faster than the speed of the tide, then I'd probably just stick to this since the impact on fuel consumption is less.

A paddle wheel measures your speed through the water. It is less accurate because it can be influenced by physical factors such as growth on the paddle, etc. However, it reflects the amount of "work performed" by your engines and is useful in the context of fuel consumption, etc. Or just knowing what the tide is doing.

My pitot tube seems only to work at higher speeds e.g. above 10 knots and is influenced by many factors e.g. tapping on the dial, for example. But that might just be mine. I don't trust it very much and rely on GPS instead.

 

[andyo83]

GPS is the most accurate and gives you your actual speed over ground and/or over the chart. If your speed was much faster than the speed of the tide, then I'd probably just stick to this since the impact on fuel consumption is less.

A paddle wheel measures your speed through the water. It is less accurate because it can be influenced by physical factors such as growth on the paddle, etc. However, it reflects the amount of "work performed" by your engines and is useful in the context of fuel consumption, etc. Or just knowing what the tide is doing.

My pitot tube seems only to work at higher speeds e.g. above 10 knots and is influenced by many factors e.g. tapping on the dial, for example. But that might just be mine. I don't trust it very much and rely on GPS instead.

GPS isn't that accurate. It doesn't take into account the curvature of the earth and will always be innaccurate by a few mph.

 

[jhr]

... presumably also inaccurate if you're not travelling in a straight line, since it will be measuring progress between two points?

 

[jfm]

GPS isn't that accurate. It doesn't take into account the curvature of the earth and will always be innaccurate by a few mph.

You are kidding aren't you? A few millionths of a mph, maybe

I'll calculate it exactly for you, if you want.

 

[jfm]

... presumably also inaccurate if you're not travelling in a straight line, since it will be measuring progress between two points?

It measures by dividing time/distnace every couple of metres or so. It does not divide time/distnge of the whole passage. The non striaght line you are contemplating would have to be a zig zag course with the notional roadcones about 2-3m apart. your passengers would be mighty annoyed if you drove like that and your steering system (or arms) would give up halfway through the voyage

 

[Jurgen]

Think I will stick to the GPS, am replacing my gauges so can save a few pounds by not replacing the speedo, less is more?

 

[EugeneR]

It measures by dividing time/distnace every couple of metres or so. It does not divide time/distnge of the whole passage. The non striaght line you are contemplating would have to be a zig zag course with the notional roadcones about 2-3m apart. your passengers would be mighty annoyed if you drove like that and your steering system (or arms) would give up halfway through the voyage

Most systems average readings over a period of 1-5 seconds. If the distance or time between measurements is too short, it will give varying readings as you go up and down in rough seas. (Zig zag-ing every 2-3m is possible in the up/down dimension, for smaller boats at least)

Here's a question - does it make a material difference whether I am traveling along a sine wave (up and down with movement caused by the waves) rather than a flat surface? I know it is a longer line but does it impact fuel consumption or speed over ground in any way?

Or would it be a matter of energy consumed by going up cancels energy saved coming down again; and, most of the lifting being done by wave energy rather than the boat's engines. I'm not talking rough seas which cause you to come off the plane and plough, just just normal "sine waves".

 

[Jurgen]

Most systems average readings over a period of 1-5 seconds. If the distance or time between measurements is too short, it will give varying readings as you go up and down in rough seas. (Zig zag-ing every 2-3m is possible in the up/down dimension, for smaller boats at least)

Here's a question - does it make a material difference whether I am traveling along a sine wave (up and down with movement caused by the waves) rather than a flat surface? I know it is a longer line but does it impact fuel consumption or speed over ground in any way?

Or would it be a matter of energy consumed by going up cancels energy saved coming down again; and, most of the lifting being done by wave energy rather than the boat's engines. I'm not talking rough seas which cause you to come off the plane and plough, just just normal "sine waves".

Far too technical for me, am off to find some Raggies to annoy!

 

[MapisM]

Here's a question - does it make a material difference whether I am traveling along a sine wave (up and down with movement caused by the waves) rather than a flat surface? I know it is a longer line but does it impact fuel consumption or speed over ground in any way?

Here's an answer - it depends.
I mean, what do you rate as "material"? Of course it makes a difference in both consumption and SOG, AOTBE. Not so much because of the ups and downs, but rather because of the longer distance.
But whether that can be relevant... Well, I suppose that in conditions where such difference is indeed relevant, you wouldn't want to be there, to start with!

And in reply to the OP, the GPS is not comparable with the other two methods, 'cause as has been said it gives the SOG, not the STW (as the other two do). GPS is extremely reliable for SOG, whilst for STW neither paddle wheel nor pitot tubes are very reliable. Doppler systems are much better.

 

[tinkicker0]

Most systems average readings over a period of 1-5 seconds. If the distance or time between measurements is too short, it will give varying readings as you go up and down in rough seas. (Zig zag-ing every 2-3m is possible in the up/down dimension, for smaller boats at least)

Here's a question - does it make a material difference whether I am traveling along a sine wave (up and down with movement caused by the waves) rather than a flat surface? I know it is a longer line but does it impact fuel consumption or speed over ground in any way?

Or would it be a matter of energy consumed by going up cancels energy saved coming down again; and, most of the lifting being done by wave energy rather than the boat's engines. I'm not talking rough seas which cause you to come off the plane and plough, just just normal "sine waves".

With a triangulation of at least 22000 miles on a side, I don't think riding up and down a wave is going to affect the outcome much.

 

[MapisM]

With a triangulation of at least 22000 miles on a side, I don't think riding up and down a wave is going to affect the outcome much.

The way I understood the question, that's not the point.
For any given RPM/STW, the SOG will be slower when riding up and down compared to a flat cruise because, well, it actually IS slower.
Not a matter of how the height affects the triangulation, which I agree is negligible (and should be automatically taken into account by the GPS algorythm, anyway).

 

[Kilter]

Far too technical for me, am off to find some Raggies to annoy!

That's annoying!

 

[tinkicker0]

The way I understood the question, that's not the point.
For any given RPM/STW, the SOG will be slower when riding up and down compared to a flat cruise because, well, it actually IS slower.
Not a matter of how the height affects the triangulation, which I agree is negligible (and should be automatically taken into account by the GPS algorythm, anyway).

Sorry, was getting confused with the comment about the curvature of the earth.

Yes I have seen a GPS on a light aircraft when climbing to read a true ground speed significantly less than the 75 knots or so indicated on the ASI. Can't remeber the exact figures cos it a a long time ago when I had a life and lots of disposable income.

 

[jfm]

Most systems average readings over a period of 1-5 seconds. If the distance or time between measurements is too short, it will give varying readings as you go up and down in rough seas. (Zig zag-ing every 2-3m is possible in the up/down dimension, for smaller boats at least)

Here's a question - does it make a material difference whether I am traveling along a sine wave (up and down with movement caused by the waves) rather than a flat surface? I know it is a longer line but does it impact fuel consumption or speed over ground in any way?

Or would it be a matter of energy consumed by going up cancels energy saved coming down again; and, most of the lifting being done by wave energy rather than the boat's engines. I'm not talking rough seas which cause you to come off the plane and plough, just just normal "sine waves".

That's a great question. It's quite involved. If your engines are off then all the lifting work is done by the waves. If your engines are on and you do 1kts the answer is basically the same. But if you're doing say 20kts then gut feeling tells you your engines are contributing to the lift. But are they? They are causing you to lift from the trough to the peak faster than you would lift at 1knots, so that means more power is being applied, but i do not see that a greater quantity of work is being done

Can i think about it a while?

Meantime here is a related question which might help determine if more work really is done: if you put a car on a hill with no handbrake, it will roll down. This is all to do with a down-the-hill force being created from the vectoral sum of the upward force from the road surface which is perpendicular to the road surface, and the gravitational force which is vertically downwards

Now, if you had a swell on the sea, and imagine it moving very slowly almost freeze framed, and you put a cork on the water half way between peak and trough, will that cork just float still or will it tend to "roll" down the "hill" on the face of the wave. Eliminate the effects of wave crests etc, and just consider a body of water whose suface is at an angle. I think it floats stationary, and does not run down the hill, becuase I cannot see how a body of water can extert a force perpendicular to its own surface in this scenario, which means there is no analogy with the car rolling down a hill. But that's a preliminary thought and I'm happy to be found wrong. If I'm not wrong, then the answer to your question is that no work is done by the boat's engine in lifting the boat from trough to peak.

This might all be googlable. Don't have time today

 

[rafiki_]

In summary, GPS will give the most accurate measure. My paddle wheel is usuualy a couploe of knots off, despite being clean etc.