Hw do you measure true boat speed?

[odes20]

Just wondering how we should measure boat speed at any revs or WOT.

I read some claims on here of speeds in boats with my eyebrows raised as I can't see those speeds being possible with some of the motors/ configurations etc.
Is it that some measure speed by the speedometer on the dash of their boat? As for me I use my GPS /Plotter

I have found speedometers on boat very inaccurate, Is this true?
Any thoughts?

Cheers
John

[davez104]

Only trouble with GPS is it gives you speed over ground, it doesn't take into account the speed of the water you are on. Your speed relative to the water would be your true boat speed. I guess a paddle wheel or the pitot type speedo would be the only ones that would measure true boat speed, but I don't know how accurate they are.

Dave.

[BrettoM]

John,
I have just got a new 440 hornet trophy with 60HP four stroke and have been interested in speed at various revs. What I have noticed is the reading on my speedo is only ever 1 or 2 Km/h off what the GPS is saying. Hence I get some comfort given they are very close of a reasonably accurate speed reading.
Cheers
Brett

[odes20]

Only trouble with GPS is it gives you speed over ground, it doesn't take into account the speed of the water you are on. Your speed relative to the water would be your true boat speed. I guess a paddle wheel or the pitot type speedo would be the only ones that would measure true boat speed, but I don't know how accurate they are.

Dave.
Dave my Garmin has Speed accross Ground as well SOG? Would this be as accurate as you could get?
John

[davez104]

Not sure what the difference would be but it still wouldn't take into account if you where running with or against the tide, could make a few knots difference to your speed over ground.

For most purposes the GPS would be accurate enough I guess.

Dave.

[black runner]

Yep my speedo is within a knot of the gps unless I am in strong current which adds another +/-2 to the difference depending on the direction of travel.

As far as speed measurement goes, some tend to stretch their performance figures for reasons best known to them, and in these cases are usually using a hand held instrument rather than speedo or GPS :grin:

Cheers

[odes20]

Yep my speedo is within a knot of the gps unless I am in strong current which adds another +/-2 to the difference depending on the direction of travel.

As far as speed measurement goes, some tend to stretch their performance figures for reasons best known to them, and in these cases are usually using a hand held instrument rather than speedo or GPS :grin:

Cheers
Arrr!! To funny black runner! but I suspect you are right!

Cheers
John

[Angla]

You just know you are faster if you get there before the next boat.

Gps for my money. It's a bit of an average anyways as we are not speed boats. Wind and tide would make a bit of difference either way, but who really cares.

Cheers
Chris

[STUIE63]

I reckon use the gps but do a run upriver then a run downriver and take the average and that should be true speed over water . i have seen a water wheel read 80 km/h on a tiller steer 40hp this would be at least 20% out I never saw the tinny's speed checked with a gps though
Stuie

[trueblue]

you have to run both ways to tell the difference, ie, if you are wind or water assisted accross ground, but yeah, who cares

Its all within cooee anyway, and the only thing that really matters is if your boat is faster than the other boat!

[Dicko]

I have a big wedged block of wood with a long rope tied to it. In that rope there is a knot every 47ft.

You throw the wood out while travelling, then count how many knots move through your hands in 30 seconds, thus giving your speed in 'knots'.

I used to use a 30 second hourglass to time this, but having now moved to the modern era, I use my watch instead.

[STUIE63]

you have to run both ways to tell the difference, ie, if you are wind or water assisted accross ground, but yeah, who cares

Its all within cooee anyway, and the only thing that really matters is if your boat is faster than the other boat!

I have had readings as far as 4 nautical miles different eg 31nm and 35 nm so i would say that boat did 33nm

[Noelm]

GPS is the only proper reading consistently accurate, a speedo only needs a bit of weed or a few grains of sand jammed up the pickup hose and it is way out, a paddle wheel is kind of OK, until you take into account chop and wash causing incorrect readings, but then as mentioned, who the hell cares, unless you are in a slow zone (which is very slow) I have yet to see a 100KPH sign out near the shelf, or yet to see many days when the speed is relative to anything except getting smacked down on chop, in which case, who cares how fast you were going when you mate was tossed overboard because your boat can do 60KPH so thats how fast we should be going!

[FNQCairns]

I have a big wedged block of wood with a long rope tied to it. In that rope there is a knot every 47ft.

You throw the wood out while travelling, then count how many knots move through your hands in 30 seconds, thus giving your speed in 'knots'.

I used to use a 30 second hourglass to time this, but having now moved to the modern era, I use my watch instead.


47 feet? Dicko you dinosaur, get with the times! today we have new knot tying technologys available that can double your speed and almost halve your fuel consumption:cheesy:

[Jarrah Jack]

I have a big wedged block of wood with a long rope tied to it. In that rope there is a knot every 47ft.

You throw the wood out while travelling, then count how many knots move through your hands in 30 seconds, thus giving your speed in 'knots'.

I used to use a 30 second hourglass to time this, but having now moved to the modern era, I use my watch instead.

Very funny Dicko, think I needed a laugh. Hows the sextant going?

[Noelm]

I wonder how you would go with a wind vane speedo on your car, you get pulled up by a radar cop, and you tell him you were only doing 50K but it was with the wind? OK against the wind though, then you would be slower than the speed indicated, so by that, the only true and repeatable accurate speed measuring device is a GPS. sure it MAY be out at sometime by a knot/kph because of the tide, but as a general rule, speed over ground and distance over ground is what counts, try navigating by speed/distance through water rather than over ground and try telling the reef that you just ran into it should not be close yet!

[odes20]

I agree that it does not matter that much in the grand scheme of things. I was just wondering if there was much degree of innacuracy from boat speedos as compared to what one would hope GPS is more accurate.
Seems like most posts so far have the speedos reading fairly close to the GPS.
The exageration factor is probably the most likely then to make some rigs do speeds they are physically incapable of !

Cheers
John

[Noelm]

some speedos are not so good, especially when in the lower and upper range, some are OK in the mid/cruising range, but being such as primitive way of obtaining reference, as in a tube that can get blocked, kinked, subject to bubbles and turbulence or leak at any time, then a speedo is to be considered a "fun" instrument only, probably the last thing I would ever fit, along with a trim gauge.

[Bill_Corten]

G'day guys. Actual boat speed between two given points can vary considerably in the same boat. This is dependent on how the boat is trimmed, whether running with or against the tide, with or against the wind, whether the boat is full of fuel or just lightship, whether the clears are up or down and the number of persons on board and the sea conditions.
Working on the speed over the ground is what determines how long it takes to travel between those two start and finish points. GPS will give you this very accurately.
Some people like to skite about their boats performance but their tale needs to be qualified to all of the above factors to enable anyone to compare apples with apples.
These days so many people don't know the difference between knots and miles per hour and will tell you their boat does X knots when it may only have been measured in miles per hour.

Cheers
Bill

[Geoff-]

True boat speed is speed over water, water is the medium the boat is travelling through. Measured by a paddle wheel or a pitot tube, both of which are susceptible to inaccuracy but when in good condition should be fairly accurate.

The most accurate reference speed will be speed over ground (as given by GPS), does not account for current but unless you are in a strong tidal area the difference should be minimal.

As others have said you can work out your gauge accuracy by doing a run upstream and downstream and average them, compare that with your GPS speed.

I only have 2 speeds stopped and bloody quick :D

[suzygs1000]

I have a big wedged block of wood with a long rope tied to it. In that rope there is a knot every 47ft.

You throw the wood out while travelling, then count how many knots move through your hands in 30 seconds, thus giving your speed in 'knots'.

I used to use a 30 second hourglass to time this, but having now moved to the modern era, I use my watch instead.


I might try that, Dicko.

Tell me tho, do you need to have gloves on? Wouldn't the knots hurt your hands?

Dave.:undecided: