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Old 21 April 2002, 03:25   #1
Country: UK - England
Town: cambridge
Make: Valiant DR600
Length: Six
Engine: Merc 4 st. EFI 115hp
Join Date: Dec 2000
Posts: 24

I have a Tachometer and a fuel guage fitted, but would like to fit a Speedometer, and any other instruments thought useful.
Any advice on what and where to buy would be appreciated. Also how to fit a speedo. The Mercury handbook is in Spanish, and a bit hard to follow, but it looks as if there is a 'pick up' on the engine which is an internal impeller?
There is plenty of console space and it is a Mercury 115 4 stroke.

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Old 21 April 2002, 09:14   #2
Country: UK - Scotland
Town: Highlands
Boat name: Quicksilver
Make: Quicksilver
Length: 3m +
Engine: Mariner 15hp
Join Date: Aug 2001
Posts: 1,771
Hi John

Speedo = GPS I would have thought.

Keith (but what do I know anyway) Hart

Small boat - BIG truck
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Old 21 April 2002, 10:28   #3
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Country: UK - Isle of Man
Town: Peel, IOM
Boat name: Saffron
Make: Scorpion
Length: 8m +
Engine: I/B Diesel 315hp
Join Date: Nov 2000
Posts: 2,197
Are we missing something here?
As Keith says a GPS/Plotter will be a lot more accurate (not to metion a lot more use) than any speedo.
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Old 21 April 2002, 10:30   #4
Country: UK - England
Town: Portishead, Bristol
Boat name: "
Make: Ribcraft, Cowes Mari
Length: 5m +
Engine: Mercury 90hp 4-strok
Join Date: Aug 2001
Posts: 600
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A few ways of giving your speed at sea

The old way was to tie a log to a piece of rope with knots in - spaced at a given interval - and count the number of knots that pass in a given period - hence "knots"

These days however there are 2 main ways

GPS - This will give you true speed "over the ground" - ie if you are going along a river it will give your speed relative to the bank. The GPS gets its information from the satalites and is highly acurate. Much more acurate than your speedo in you car - try it on the motorway and see how un-acurate your speedo is! The GPS will display your speed on the screen in Knots, MPH or Kmph.

Fishfinder (or other speed sensor) - This will give you speed "Through the water" A lot of fishfinders have a small paddlewheel attachment that goes on the transom of your boat. As you go through the water the paddle wheel spins giving the speed reading

Examples between "through the water" and "over the ground"
If there is a current (flow/tide) of say 5 knots and you boat is traveling in the same direction of the tide at say 10 knots, then we would say the spped through the water was 10 knots and the speed over the ground was 15 knots.
If you were going into the tide, the speed through the water would still be 10 knots but your spped over the ground would be 5 knots.

I can't see any real way of taking a reading off the engine - apart from going out with a GPS on a lake (no tide/current) and making a note of your speed at different engine revs.

Obviously a light boat will go fater at the same revs as the same boat heavily laden. When you accelerate from standstill your engine will have high revs but little speed.

The "pick up" in the engine, I would have thought, would be for the Tachometer.

Best solution - get a GPS!
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Old 21 April 2002, 13:13   #5
Country: UK - England
Town: Upavon, Wiltshire
Boat name: Dromedary
Make: Ribtec
Length: 6.55
Engine: Honda 130
Join Date: Apr 2001
Posts: 183
The "pickup" you are talking about is a small hole in the front of the leg which picks up the water preasure caused buy the foreward motion of the boat this saves you fitting the one which is screwed to the transom which comes with the speedo, they are not very accurate below 10MPH but do not cost much. you should find a small plastic tube near the top of the leg connect this to the tube from the speedo and all should work. the same system is used on aircraft (pitot tube) to measure air speed.

Hope this helps

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Old 21 April 2002, 14:51   #6
Country: Ireland
Town: ireland
Make: lencraft rib
Length: 5m +
Engine: yamaha 80hp four str
Join Date: Feb 2001
Posts: 42

Hi John,
As well as a GPS I have fitted a Faria speedo to my 5m Rib which
is another instrument to fill up my console!It has a plastic sensor to the transom and is easily fitted.I agree with the experts about the gps etc.
check out
Hope it helps!
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Old 21 April 2002, 15:25   #7
Country: UK - Scotland
Town: Highlands
Boat name: Quicksilver
Make: Quicksilver
Length: 3m +
Engine: Mariner 15hp
Join Date: Aug 2001
Posts: 1,771
Hey, you're going to have a gps anyway right?

Keith (I've got one) Hart
Small boat - BIG truck
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Old 21 April 2002, 16:45   #8
Country: UK - N Ireland
Town: Bangor
Make: Shakespeare
Length: 7m +
Engine: O/b 225
Join Date: Jan 2001
Posts: 188
The only "problem" with using a GPS for speed is if you do a lot of "raking" about - a GPS only gives an average speed in an average direction - if you go at 30 knots around in a circle it would read zero - ish. A log (proper name for a boat water speed & distance instrument) will give you the actual speed through the water. The "pitot" type instruments have advantages - they work instantaneously and use no power, so they are good for a quick, easy reading without bothering to switch on a GPS and let it get a fix. The pressure produced by a pitot tube (including the "hole" in the front of the outboard leg) is proportional to the square of the speed, hence the funny spacing on the speedo dial, and hence the crap accuracy at low speeds. Providing you get a pitot-type speedo that has a max speed close to that of your boat it will be quite accurate over the top half of speed range.
If you really want accurate water speed you can always fit one of the digital-type speed logs - these use a paddle wheel on the transom to give a very accurate, linear speed display. You can even get small, battery powered ones at around £80.
[25 years of instrumentation experience was good for something after all..][useless information...]
Of course, if you make the pitot tube big enough you have a built-in shower on your Rib (see Spirit of Cardiff for details).
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Old 22 April 2002, 02:57   #9
Country: Canada
Town: Newfoundland
Length: no boat
Join Date: May 2000
Posts: 2,097


If you're Merc is the latest version one option would be to have the Mercury Smartguages fitted. Basically two instruments - Rev and Speedo with analogue and digital displays. Give you everything from speed, litres per hour burned, total burned, distance run, atmospheric pressue, water temp, oil temp, inside leg measurement, etc etc. Can be interfaced with GPS and fuel gauge to give range etc.

Bloody marvellous bit of kit IMHO. We have them on our 150 Opti. Best thing is the total litres burned ... makes working out the shares of the fuel bill at the end of the day so much easier!!

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Old 22 April 2002, 03:00   #10
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Country: UK - England
Town: Brighton
Length: 3m +
Join Date: May 2000
Posts: 6,681
if you go at 30 knots around in a circle it would read zero - ish
Only if it was a very, very small circle. Small enough so you were just spinning around going nowhere (which is why you speed would show as zero!)

Why do you want to know the speed through the water? Wouldn't you rather know how fast you are going from A to B? In any case, at ribbing speeds the difference isn't very significant:

30 knots RIBspeed plus or minus 3 knots of tide (27-33kt) vs
6 knots of stick-boat speed plus or minus 3 knots of tide (3-9kt)

Conventional speedos fitted to RIBs tend, in my experience, not to be very accurate. Especially those fitted by the manufacturer which seem to have a strange tendency to indicate a higher than actual top speed . . .

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