speeds , fact and fiction ?

[jimboooo]

hi , seems to me i either cant drive my boat , am doing something wrong , or people just plain exaggerate the speed of there rigs .i have had three boats so far over the years also did my pb2 on a rib . my first boat was 14 ft with a two stroke 40hp she did about 30knts on a very good day , a 19ft cuddy 3ltr inboard 135 hp she did about 34knts , and my current boat deep vee 21ft cuddy 150hp 4 stroke outboard , same as above 34 knts but quieter ! !the rib had twin 80s on the back and again was approx 32 knots . some of the specs and speeds quoted by users and manufacturers are surly make belive or mph on the pipet style speedos ? my speeds are recorded on gps in knots not mph, do people realise there is a diiference between a knot and mph ? just interested in other peoples takes on this .

 

[RIB_imposter]

In my experience it is very common for people to "talk up" the speed they do.

The speeds you quote don't seem slow to me. Perhaps playing with props, trim, engine height etc you might get a little extra speed but it won't be masses.

I also think it is common for people to observe a speed in mph but later report it in knots. I have seen the same done in kph-knots which creates some rather impressive talk in the bar!

 

[Elessar]

you need to start sailing 51.7 knots clicky

 

[mlines]

We had our first opportunity yesterday to see what our "40 MPH" 18ft 130hp Bowrider would do yesterday (it was the smoothest I have been in off Studland this first year of ownership).

We saw 28 Knots (32mph) on the GPS with 4 people on board along with picnic, clothing, water toys and Dog. This was without looking at trim, weight distribution, tide and wind direction. This was with the engine at 4000 rpm, the manual suggests it would have gone to between 4400 and 4800.

So I guess its conceivable that with proper thought to balance, trim, wind and tide we could see closer to 40mph - but to be frank it would have been pretty scary by then (how come a 20 knots cruise on water seems so fast compared with 23mph on roads!!)

Martin

P.S - the only way we could get the "ringo"/"Donut" to work was by laying off the back of it. Sitting on it meant the front dug into the water - is there a trick to it???

 

[BrendanS]

my speeds are recorded on gps in knots not mph, do people realise there is a diiference between a knot and mph ? just interested in other peoples takes on this .

My 21' cuddy, with 4.3l petrol inboard, is supposed on paper to do 44 knots flat out. According to the plotter, it does, and the speedo is pretty accurate, but is in mph, so I don't use it much, preferring the knots readout on the plotter.

 

[AndieMac]

P.S - the only way we could get the "ringo"/"Donut" to work was by laying off the back of it. Sitting on it meant the front dug into the water - is there a trick to it???

You need to lean back and pull the handles up at the front/bow until it pops on the plane, otherwise it becomes a submarine. If you have had a play already you will realise the most exhilarating part of the ride is the tight turns.
Of course this is the part where it is most likely you will come off, and also understand how hard and unforgiving the water surface can be at 20 knots.

 

[Gobbycat]

GPS

We saw 28 Knots (32mph) on the GPS with 4 people on board along with picnic, clothing, water toys and Dog. This was without looking at trim, weight distribution, tide and wind direction. This was with the engine at 4000 rpm, the manual suggests it would have gone to between 4400 and 4800.

QUOTE]

Don't forget the GPS speed is slightly slower than actual speed. Note this on the TomTom, doing 69mph speedo showing 74mph, have heard the excuse of the TomTom speed being quoted to the police doesn't wash!

 

[bedouin]

Don't forget the GPS speed is slightly slower than actual speed. Note this on the TomTom, doing 69mph speedo showing 74mph, have heard the excuse of the TomTom speed being quoted to the police doesn't wash!

If I had the choice between believing a GPS speed and a speedo I would believe the GPS every time.

What makes you think that your TomTom is inaccurate?

 

[tinkicker0]

Well my 20ft cuddy cabin with mercruiser 3 litre does 147knots with 12 people, 147 gallons of fuel, 300 cans of beer, 20 gallons of pee and a large shaggy dog aboard.
Verified by GPS.

Or in the real world it manages to struggle up to 34 - 36 GPS knots with 2 people and weekend gear aboard. 4000 RPM WOT

It should do 40 knots @ 4500 RPM apparently, however where I boat, the water is sans chop and therefore very sticky.

 

[oGaryo]

does 147knots, Verified by GPS.

is that before or after consuming said 300 cans of beer? lol ;-)

 

[trying]

We saw 28 Knots (32mph) on the GPS with 4 people on board along with picnic, clothing, water toys and Dog. This was without looking at trim, weight distribution, tide and wind direction. This was with the engine at 4000 rpm, the manual suggests it would have gone to between 4400 and 4800.

QUOTE]

Don't forget the GPS speed is slightly slower than actual speed. Note this on the TomTom, doing 69mph speedo showing 74mph, have heard the excuse of the TomTom speed being quoted to the police doesn't wash!

Sat Nav and GPS speeds are pretty accurate. The fact that the police are not interested in what Tom Tom says is partly because it is not a calibrated device and partly because they are not interested in anything that might get you off the speeding conviction!

 

[bedouin]

Sat Nav and GPS speeds are pretty accurate. The fact that the police are not interested in what Tom Tom says is partly because it is not a calibrated device and partly because they are not interested in anything that might get you off the speeding conviction!

Luckily it is not up to the police. If you could produce a TomTom track (do they have that facility on the car ones?) then I can't see that the police would be able to get a conviction.

 

[jimmy_the_builder]

Through the water has to be the most relevant speed for a vessel, but it's hard to measure it as accurately as speed over ground. It certainly seems to be the case on my boat (Fairline Targa 40) that the accuracy of the log varies with the speed of the vessel; these screencaps from our trip down to Dartmouth at the weekend show our speed through the water as 32.5kts, and over the ground as 31.5kts. The tide was slightly against us so I think the log is slightly over and and our true speed through the water was probably about 32kts. (At much lower speeds, I noticed a materially bigger gap between the speed through the water against sog).

Cheers
Jimmy

 

[Peanuts]

Hopefully there is a photo of my GPS showing a normal top end speed for a S28, the only problem was it clocked it whilst I was at anchor!! So don't always assume it is correct.

 

[ontheplane]

P.S - the only way we could get the "ringo"/"Donut" to work was by laying off the back of it. Sitting on it meant the front dug into the water - is there a trick to it???

Nope - that's exactly right, lean back a bit and wait for it to "pop" up.

The more acceleration you have the less you'll need to do it, but if you lean forward it will dig in....

In fairness some boats pull so hard you've got no option but to lean back!

 

[Gobbycat]

If I had the choice between believing a GPS speed and a speedo I would believe the GPS every time.

What makes you think that your TomTom is inaccurate?

Because I know the speedo is correct

 

[bedouin]

Because I know the speedo is correct

That's a circular argument if ever I heard one. How do you know that the speedo is correct - why do you have more faith in your speedo than in GPS?

 

[neale]

Because I know the speedo is correct

This is just in my experience so don't shoot the messenger.

I have compared various sat navs with various cars and the car speedo ALWAYS reads more than the sat nav. It usually goes something like this: 3mph more at 30, 4 mph more at 50 and 5 mph more at 70.

I understand that car speedos are made to over read because the alternative would cause more problems.

I regularly drive to the GPS speed, including through fixed and average speed cameras and have not been flashed yet.

As I say, all in my experience.

 

[Nick_H]

The speedo on your car performs a calculation from how many times the wheels go round multiplied by the circumference of the tyre, but the circumference of the wheels changes with tyre type, wear and pressure, so the manufacturer has to err on the side of caution and ensure it doesn't under-read, which means most of the time it over-reads. Speed cameras also have error margins built in, and most forces allow you something like 10% + 3mph over the limit, so it's quite feasible to go through a 50 mph speed trap with your speedo reading 60, without being fined. Indeed in the 50 mph average speed traps I set the cruise control to 57 mph with no fear of being penalised, though I normally end up stuck behind the car in front doing 45!?.

Gobbycat, unless there's a software fault in your GPS it will be completely accurate, far more so than a paddlewheel speedo on a boat or a speedo on a car, although on a boat of course a GPS measures SOG, and the speedo measures speed through the water.

 

[nonitoo]

My understanding is that the manufacturing tolerances for the car speedo are from 0-10%, in other words the speedo must never under read. Therefore they are either accurate or slightly over read.

That is my experience of my speedos in cars as against the GPS speed.

It also is the case that, in court, when plod says that the car he was chasing was doing 110mph you can be pretty sure that the speeders speedo was displaying substantially more (up to 120mph).

Tom